Pedicab with Fluid Power Regenerative Braking

Benjamin Koch, B.S.

Institute of Technology University of Minnesota

Master of Science In

Mechanical Engineering

William Durfee, Ph.D., Adviser

May 2010 Minneapolis, MN

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Table of Contents

Abstract ........................................................................................................................................... 2 Acknowledgements ......................................................................................................................... 3 1.0 Introduction ............................................................................................................................... 4

1.1 Background ........................................................................................................................... 4 1.2 Commercial Opportunity ...................................................................................................... 6 1.3 Problem Definition................................................................................................................ 8 1.4 Previous Work ...................................................................................................................... 8 1.5 Hydraulic Circuit ................................................................................................................ 11 1.6 Original Design Issues ........................................................................................................ 15 1.7 Design Description.............................................................................................................. 16

2.0 Analysis................................................................................................................................... 17 2.1 Modeling ............................................................................................................................. 17 2.2 System Losses ..................................................................................................................... 22 2.3 SimHydraulics Simulation Results ..................................................................................... 23 2.4 Braking Results ................................................................................................................... 27

3.0 Design Changes ...................................................................................................................... 28 3.1 Directional Control Valve ................................................................................................... 28 3.2 Operator Control ................................................................................................................. 28 3.3 Pump/Motor ........................................................................................................................ 29

4.0 Recommendations ................................................................................................................... 31 4.1 Directional Control Valve ................................................................................................... 31 4.2 Hydraulic Accumulator ....................................................................................................... 32 4.3 Hydraulic Pump/Motor ....................................................................................................... 32 4.4 Pedicab Chassis and Drive System ..................................................................................... 33

5.0 Conclusion .............................................................................................................................. 34 Appendix A - Commercial Pump/Motor and Accumulator Options ............................................ 37

Parker ........................................................................................................................................ 37 Sauer-Danfoss ........................................................................................................................... 37 Eaton ......................................................................................................................................... 37

Appendix B – Charging Sequence Analysis ................................................................................. 38

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Abstract Many forms of human transportation have a similar basic operational challenge: starting in

physically demanding conditions. The focus of this project is the starting movement of a

pedicab, a bicycle-driven pedestrian taxi, from a complete stop with a full load of passengers. In

starting a pedicab, the driver pedals out from the complete stop, providing the entire source of

energy. To assist in the initial movement of the pedicab, a hydraulic system was designed and

installed into a pedicab to collect energy dissipated during braking for use during start up. To

improve efficiency and operation, attention was focused on a hydraulic pump/motor and a

hydraulic accumulator

The original hydraulic assist system designed in the spring of 2008 was updated with a lower

fixed-displacement (0.5 in3) Eaton pump/motor to provide the desired assist effect during

operation.

The controls of the hydraulic system were reviewed and improvements were made. A lever

control was installed on each handlebar to replace the single lever originally placed between the

operator’s legs. The new controls were installed similar to traditional bicycle brake levers. An

electric control valve was installed to aid in operator control. The hydraulic charging circuit is

controlled by the right cantilever and the hydraulic discharging circuit by the left.

Replacing the large displacement (4 in3) pump/motor with a smaller displacement option (0.5

in3) resulted in a pedicab that is easier to pedal and an assist system that is easier to charge and

store energy. The update to the control system provides an intuitive control option for novice

bike riders.

Recommendations include the installation of a larger hydraulic pump/motor (1.2 in3). This will

increase the energy collected during the charging sequence and the energy output during the

discharging sequence, while maintaining the ease of use. A lighter accumulator is also

recommended. Suggestions include the carbon fiber accumulator developed by Parker or the

strain-based accumulator technology under research at Vanderbilt University. Finally, replacing

the electric control valve with a mechanical option would create an entirely mechanical system.

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Acknowledgements

Many thanks go out to all advisers, contributors and colleagues on the pedicab project. Specific

thanks go to:

Prof. Will Durfee and Mike Gust for their patience and insight into the Center for Compact and

Efficient Fluid Power and overall hydraulic knowledge.

Mr. Bill Byrd for his contributions to the pump/motor discussion and for his recommendations.

Mr. Forrest Price and Mr. J. Newlin of the Science Center of Minnesota for their conversation

and ideas.

Prof. Eric Barth and Alex Pedchenko of Vanderbilt University for sharing their research on strain

based accumulators.

The previous research team (Eric Burgett, Richard Dreyer, Keith Jackson, Kevin Kysylyczyn

and Jeffrey Lai) for their thorough and comprehensive project report and deliverables.

The “Research Experience for Teachers” team (Jeff Givand, Jake Schreifels and Melissa

Schreifels) for their labor, testing and ideas.

All of these individuals added to my knowledge of the pedicab, hydraulic systems and

components, and to my overall engineering design and research experience.

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1.0 Introduction

1.1 Background

Pedicabs, also known as cycle rickshaws, are a common mode of transportation in many areas of

the world. They are traditionally found in high numbers in Eastern culture including

Bangladesh, Burma, India and Indonesia. According to Wheeler and I’Anson (1998), the city of

Dhaka, Bangladesh, has approximately 100,000 officially registered pedicabs in the city and

likely two or three times as many unregistered pedicabs (p.56). Wheeler and I’Anson also noted

other large cycle rickshaw populations in Rangoo, Burma (7,000) (p. 141); Agra, India (5,000)

(p. 11); and Yogyakarta, Indonesia (5,000) (p. 161). Rajvanshi (2002) estimated that overall in

India there are more than 2 million cycle rickshaws that carry passengers 6-8 billion kilometers

per year (p. 703). While extremely popular in the East, the pedicab population in the West is

growing (Chan & Confessore, 2005). A Google search for the terms “pedicab USA” returns

over 45,000 websites including small businesses that have emerged offering pedicab sales and

support, advertising opportunities and tourism ventures around the United States. In New York

City, the pedicab population doubled from 2003 to 2005 (Chan & Confessore) and is estimated

to be around 500 vehicles (Spielman, 2009). In Chicago, the pedicab population is

approximately 200 vehicles (Spielman). In the Twin Cities, companies such as Como Pedicab,

Peterson’s Pedicab and Cycle Seven offer taxi service or rentals (Mozer, 2009).

Pedicabs come in different models, including varying passenger seating and operating options

(pedal type, operator’s seat and handlebars). For example, some models place the passenger seat

in front of the operator, others behind and still others operate as a sidecar. Other pedicab options

and amenities include derailleur gearing, braking options (caliper rim brakes, disc brakes, etc),

seating sizes, and passenger amenities (umbrella or shade cover, passenger hand railing, etc).

According to Wheeler, the pedicab evolved from the rickshaw, a human-powered vehicle

popular in late 1800s Japan. An operator who runs in front of the passenger area pulls the

rickshaw. The cycle rickshaw or pedicab emerged sometime in the 1920s in Singapore and

Thailand. The evolution from rickshaw to cycle rickshaw is credited to the shortage of fuel

during World War II. The lack of other public transportation, taxis and buses, furthered the

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innovation of the pedicab as a viable human-powered transportation option (Wheeler & I’Anson,

1998, pp. 180-183).

In Asia, the pedicab is common, transporting people around urban areas quickly where standard

taxicabs can be hampered by increased traffic. Pedicab operators are often classified as lower

class citizens, who often own nothing more than the pedicab itself (Rajvanshi, 2002, p. 704). In

other cases, the pedicab operator will rent the vehicle on a daily basis from the owner of a small

fleet of pedicab cabs (Wheeler & I’Anson, 1998, pp. 62-63).

In the United States, pedicabs exist scattered across many larger cities where they are operated as

a tourism venture. In New York City, the Planet Green company pitted a pedicab against a

taxicab to determine which mode of transportation was faster around the city. In their video

presentation, they note that over the course of four locations and approximately 1.7 miles around

central Manhattan, the pedicab was faster by approximately six minutes in New York City

traffic. They also note that the pedicab does not require fuels or emit CO2 (Main Street Pedicabs,

2009b). During President Obama’s inauguration in 2009, pedicabs were used for transporting

visitors and tourists around Washington D.C. offering a “quicker commute,” according to CBS

News (Smith, 2009).

Pedicabs are also used for transporting materials and parcels, where the passenger seating area is

replaced by cargo space. These cargo pedicabs are seen around universities, such as the pedicabs

used by the University of Minnesota’s Facilities Management Land Care Division, and in large

factory or warehouse spaces transporting materials and supplies. Pedicabs allow quick

transportation between tasks with out the use of fuels or electric motors.

Due to the physical exertion required to operate a pedicab, assist systems have been developed in

the form of electric motors (EcoSpeed, 2010a). These electric assist systems run off a battery

pack and are typically non-regenerative. Two types of electric assists are available, a hub motor

model installed in the driving wheel and a mid-drive model installed in series with the drive

system of the pedicab. An example of a hub motor model is the electric assist system available

from Main Street Pedicabs that utilizes a Heinzmann (2009) electric hub motor attached to the

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front wheel of the pedicab. A handlebar control allows the user to set the amount of power

delivered to the motor that drives the attached wheel. EcoSpeed (2010a) offers a mid-drive

model that is integrated with the drive system of the pedicab, providing power directly to the

chain of the drive system.

1.2 Commercial Opportunity

While pedicabs are not popular in the United States at this time, there is an increase in their use

and visibility in major cities (Chan & Confessore, 2005). In cities like New York and Chicago,

where population density is high and owning a personal car for shorter daily trips is not practical,

residents are known for their use of public transportation (buses, trains and taxicabs).

Considering society’s movement towards cleaner forms of transportation, it is understandable

that the public will be looking for new ways to transport themselves. Owning a personal car

might not be cost effective. Transportation options such as bicycles, in-line skates and public

transportation are logical to fill that need. Pedicabs used as a means of public transportation are

an attractive option because they could be used in the same way as a taxicab. Pedicabs are also a

unique transportation option in the United States and their novelty could prove to be an enticing

option for passengers such as tourists. For operators this could prove to be a major selling point.

One challenge to the operator of a pedicab is that continued starting and stopping is exhausting.

As with taxi cabs in major cities, frequent stops and starts are common as customers enter and

leave the taxi. For a pedicab operator, the largest amount of energy expended during the

operation of such a vehicle would be in beginning motion from a complete stop with a full load

of passengers. Once moving, the momentum of the pedicab makes operation easier. In many

cases, this effort requires frequent rest periods for an operator. The assist would potentially

allow the operator to use the pedicab for longer periods, by minimizing the exertion normally

used to begin motion with a heavy load. Rajvanshi proposed an electric assisted pedicab model

that would allow the operator to approximately double the distance traveled by the vehicle in a

single shift (Rajvanshi, 2002, p. 705).

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Current electric assist systems are powerful and sufficient for easy operation; however, the

systems noted in this paper only offer rechargeable battery options, which require an external

power source and time to recharge the drained batteries. According to EcoSpeed (2010b), their

longest lasting battery pack (15Ah, 38V Li-Fe-PO4) offers power for up to 25 to 50 miles under

normal stop-and-go and hill climbing operation. A battery pack weighs roughly 10.5 lbs, the

expected time to charge would be one hour, the estimated lifetime of such a battery would be

2,000-3,000 recharge cycles, and the cost of such a battery would be approximately $1,400

(EcoSpeed, 2010b).

The proposed hydraulic assist system would perform similarly to an electric system during an

assisted start but would regenerate power during the braking sequence rather than during a

battery-recharging period. As Rajvanshi (2002) noted, many Indian pedicab operators are poor,

in some cases only owning the pedicab itself (p. 704). A rechargeable battery assist would not

be a viable option, as many would not have the means to recharge the batteries. A regenerative

hydraulic assist would not require access to an electric power source and would be self-

contained. The regenerative nature of the system would also allow for a longer lifetime of the

initial investment, as rechargeable batteries demonstrate a specific lifetime and are expensive.

Alternatively, hydraulic fluid is inexpensive, $19.87 per gallon (Drill Spot, 2009). The volume

of the reservoir installed in the assist system is one gallon.

EcoSpeed (2010c) does reference the option for regenerative braking and argues against its use

in bicycle and pedicab electric assists. Their reasoning compares the weight and speed of a

bicycle with that of a hybrid electric car that uses regenerative braking. By comparison, a

lightweight, low speed bicycle does not have as much momentum and thus does not regenerate a

large amount of energy as a full size car would. They conclude that the benefit versus cost is not

worth the investment (EcoSpeed, 2010c). In the pedicab’s case, there is additional weight of the

added hydraulic components and passengers that would generate larger momentum. This

additional momentum would translate into regeneration of larger amounts of energy.

Additionally, this added weight would be more difficult to move from a stop and the assist would

help to overcome that difficulty.

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Eaton Corporation is marketing one example of a publically available hydraulic assist system.

Their Hydraulic Launch Assist for refuse or garbage trucks works similarly to the pedicab assist,

although it is scaled up to work with large trucks. It includes a pump/motor assembly, a low-

pressure reservoir and a high-pressure accumulator. Their system claims to improve fuel

efficiency by 15 to 30%. Of all the documentation available, no mention is made as to the use of

a fixed-displacement or variable-displacement pump/motor. Available documentation is

attached in Volume II, Section 1.5.

1.3 Problem Definition

This project’s objective was to develop an efficient, fluid powered regenerative assist system for

a pedicab. Providing an assist to the starting movement of a fully loaded pedicab will make the

operation of a vehicle much easier for the operator. Therefore, identifying appropriate hydraulic

components to provide high efficiency is important. Integrating a simple and intuitive control

system for the operator will also improve the ease of use. By incorporating a simple user

interface and efficient fluid power components, a system was created that improves the

performance of the pedicab operator.

1.4 Previous Work

An early hydraulic assist pedicab prototype was designed and fabricated by a team of

undergraduate students at the University of Minnesota in the mechanical engineering capstone

design course, ME 4054 (Burgett, Dreyer, Jackson, Kysylyczyn & Lai, 2008). Their work

included the purchase and assembly of the pedicab, designing the original hydraulic circuit and

the selection of the original components.

The pedicab chassis that was purchased for this project was bought online from the EBay

website from a private seller.

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Figure 1.1 - Pedicab

The pedicab has a 1.5:1 gear ratio with a single drive rear wheel (right of the driver) and a

coasting rear wheel (left of the driver). The braking options included a front wheel caliper brake

and a rear wheel parking brake.

The ME 4054 team made modifications to the drive system to link the assist system to the drive

system. The rear wheel parking brake was removed. An additional sprocket was added to the

drive axle and an accompanying chain drive was added connecting the pump/motor directly to

the new sprocket.

The components of the assist system designed by the ME 4054 team included a steel shell

accumulator, hydraulic pump/motor, reservoir, directional control valve and other assorted

valves. Details of these components are listed in Table 1.1.

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Table 1.1 - List of Components Included in Original Design

Hydraulic Component Model Specifications

Pump/Motor Parker TorqMotor

TE Series Displacement: 4 in3

Accumulator Parker ACP Series

4 inch Bore

Gas Valve

Capacity: 122 in3

Working Pressure: 4000 psi

Port Size: Female SAE #12

Reservoir Parker 165 Series Capacity: 1 Gal

Plastic

Directional Control Valve Parker Series DV1G Actuator: Cam Lever

Daman Subplate – SAE 12 ports

Pressure Relief Valve Parker Series RD102

Rated Flow: 10 GPM

Max Inlet Pressure: 3600 psi

Max Pressure Setting: 3000 psi

Max Tank Pressure: 3000 psi

Pressure Gauge Central Hydraulics Range: 0-2000 psig

Check Valve Parker Series CVH103P -

Metering Valve Parker Series MV1200S -

Hydraulic Fluid Exxon Mobil DTE-25 -

The components were installed below the passenger seat, with the pump/motor installed

underneath the pedicab chassis as shown in Figure 1.2.

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Figure 1.2 – Pedicab with installed hydraulic assist system installed below bench seat

1.5 Hydraulic Circuit

The hydraulic circuit designed by the ME 4054 team had eight fluid power components. The

circuit had three settings: neutral, charging and discharging.

Assuming that the neutral setting is used directly after the discharge sequence, the hydraulic fluid

has all been returned to the reservoir. The neutral setting (Figure 1.3) draws fluid from the

reservoir through connection T and on to the directional control valve (DCV). The fluid leaves

connection B to the pump/motor. From the pump/motor, the fluid is sent to the directional

control valve through connection A. The directional control valve then directs fluid back to the

pump/motor. The accumulator is not utilized which offers minimal resistance during normal

operation of the pedicab.

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Figure 1.3 - Hydraulic assist circuit in neutral position

The charging setting (Figure 1.4) draws fluid from the reservoir through connection T and pumps

it through the directional control valve. The pump/motor draws the fluid from the directional

control valve through connection B and then back through connection A. The fluid then pumps

to the hydraulic accumulator through connection P. As more fluid pumps to the accumulator, the

pressure builds, storing energy. As the pressure builds and the accumulator fills, this provides

resistance through the pump/motor that can be used to slow the vehicle.

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Figure 1.4 - Hydraulic assist circuit in charging position

The discharging setting (Figure 1.5) releases the pressurized fluid from the accumulator to the

directional control valve through connection P. The valve then directs the fluid through the

pump/motor driving the motor utilizing connection B and A. The fluid then directs back the

reservoir through connection T. The driven motor is the assist that helps to launch the pedicab

from a stop.

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Figure 1.5 - Hydraulic assist circuit in charging position

A pressure relief valve was installed between the pump output and the directional control valve

to ensure maximum pressure acceptable by the individual components is not exceeded. If the

pressure rises to the pre-determined level, the pressure relief valve will trigger and any remaining

fluid being pumped is bypassed to the reservoir. A check valve was installed between the

directional control valve and the motor inlet to ensure that the fluid does not backflow through

the circuit. A bleed-off valve was installed between the accumulator and the reservoir, allowing

the user to manually release the accumulator pressure. A pressure gauge was installed at the

accumulator to monitor the stored pressure. The read out is displayed on the handlebars

convenient to the operator.

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Figure 1.6: Hydraulic Assist Circuit - Neutral Setting (Arrows indicate the flow of the fluid through the circuit)

Figure 1.6 shows the fixed displacement system installed in the pedicab below the bench seat

along with annotations of selected components and the direction of the fluid flow during neutral

operation. The pump/motor is installed underneath the chassis and is not visible in Figure 1.6

The fluid used in the pedicab system is DTE-25 by Exxon-Mobil (Burgett et al., 2002, pp. 59-

62).

1.6 Original Design Issues The pump/motor that was implemented into the system designed by the ME 4054 team was

oversized and did not function as intended. The pump required too much power to effectively

charge the accumulator. This resulted in quick stops with few rotations of the input shaft and

thus little fluid charged into the accumulator. When pressure was stored in the accumulator, the

oversized motor drained the fluid rapidly and supplied a surge of energy rather than an even

assist for the vehicle.

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Additionally, the installed directional control valve did not have a neutral center, making normal

operation of the pedicab difficult by providing added resistance.

The original control method was a lever and linkage installed to the vehicle frame between the

operator’s legs. The linkage passed behind the operator and through the passengers’ leg space to

the directional control valve. This proved to be uncomfortable for the operator and problematic

should the passengers interfere with the linkage system running at their feet. By moving the

lever forward or backward, the position of the control was changed and the charging and

discharging functions were engaged. While easy to use, this setup required the operator to

remove their hands from the handlebars to operate the system, a potential safety concern.

In order to provide a functional, easy-to-use assist system these components needed to be

replaced.

1.7 Design Description

During this project, computer simulations were conducted to determine the feasibility of a

variable-displacement hydraulic pump/motor to replace the original fixed-displacement

pump/motor. At the end of that process, a plan and recommendation was in place to improve the

efficiency of the hydraulic assisted pedicab.

A variable-displacement pump/motor (Parker Company model V12 series) was selected to

replace the fixed-displacement predecessor and plans were made to replace the heavy steel

accumulator. However, these plans were abandoned due to cost and limited component

availability.

The control system was updated to include a more intuitive design that is similar to that of a

traditional bicycle. The new control system implemented bicycle brake levers to control the

directional control valve. One lever is connects to the charging sequence, while a second is

connects to the discharging sequence.

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The hydraulic circuit was updated with a new directional control valve. The new directional

control valve employed a neutral center option. This is important because the assist system is

continuously engaged, pumping fluid during the duration of operation. A neutral center allows

an operator to pedal the pedicab without the resistance of the moving pressurized fluid to the

accumulator.

A smaller fixed-displacement pump/motor (0.5 in3, Eaton Char-Lynn M-0J-05) was installed

replacing the 4 in3 model. This pump/motor is a better option for the assist design because it

allows the pedicab to slow at a reasonable rate, while also pressurizing the accumulator. The

smaller displacement option does not offer a quick braking action as the large model, but does

provide an even assist over a longer period of time, rather than expending the stored energy

quickly.

2.0 Analysis

2.1 Modeling

To begin the redesign process, a computer simulation of the hydraulic system was developed to

discover opportunities for efficiency gain and optimal characteristics and settings for new

hydraulic components. The analysis of the system included predicting power efficiency, system

losses and pressure in the system. By comparing simulations of different hydraulic circuit

arrangements, the most efficient option can be determined.

The simulation of the hydraulic pedicab assist was completed using the hydraulic simulation

software, MATLAB’s SimHydraulics. By modeling the hydraulic assist circuit with

customizable MATLAB block components, simulations can approximate the operation and

performance of the hydraulic circuit. Using this software, simulations analyze the benefits of a

variable-displacement pump/motor over a fixed-displacement pump/motor. Figures 2.1, 2.2, and

2.3 show the fixed-displacement pump and variable-displacement pump hydraulic circuits

designed for use in the simulations and the pressure sensing subsystem used in the variable-

displacement pump simulation.

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Figure 2.1 - Fixed-displacement Hydraulic Circuit in SimHydraulics

Figure 2.2 - Variable-displacement, Pressure Sensing Hydraulic Circuit in SimHydraulics

Figure 2.3 - Pressure Sensing Subsystem included in Figure 2.2

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The SimHydraulics model in Figure 2.1 contains a velocity source, pump, hydraulic pipe,

pressure relief valve, directional control valve, accumulator, reservoir and hydraulic fluid. The

model in Figure 2.2 is nearly identical except for the replacement of the fixed-displacement

pump/motor with a variable-displacement pump/motor and the addition of the pressure sensing

subsystem. In each of these figures, the weight of the pedicab and passengers is modeled using a

rotational friction block and an inertia block attached to the output of the motor.

Figure 2.3 shows the inner workings of the pressure sensing system. This subsystem has a

hydraulic valve actuator that measures the pressure in the accumulator and adjusts the pump’s

displacement proportionally. The reading from the actuator is subtracted from 0.009 to adjust

the displacement of the pump. This dimensionless constant relates directly to the maximum

output of the hydraulic valve actuator. Therefore, the pump/motor is set to maximum

displacement when there is no pressure across the directional control valve. As the pressure in

the accumulator rises, the displacement of the pump lowers. Also shown in the figures are

reference blocks (MRR) that provide a ground reference for the system, conversion blocks (S-

PS) that translate a physical signal to a SimHydraulics signal, signal builder blocks that supply

the input to the system, and solver configuration that allows the SimHydraulics software to

simulate our model.

In the simulations, separate pump and motor components were used. In the actual fluid power

circuit, a reversible pump/motor fills both roles. To represent the dual operation of the

pump/motor, the pump and the motor were given identical properties.

Figure 2.4 shows the signal to the directional control valve (DCV) for a run that simulated the

pedicab stopping and starting. A signal of zero provides a neutral setting of the DCV allowing

fluid to bypass the accumulator as it runs through the circuit. A positive signal indicates that

fluid directs to the accumulator, thus charging the system. A negative signal indicates that the

accumulator provides pressurized fluid to the motor. The magnitude of the signal the DCV

indicates how far to open the valve, but in this simulation the valve was instantaneously switched

between positions. The initial 30 seconds of the simulation indicates normal pedaling. At 30

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seconds the system charges the accumulator for 37.5 seconds or the time needed to decelerate

from 75 rpm to rest at an average deceleration of 2 rpm per second. The ME 4054 team used a

speed of 75 rpm to develop their design (Burgett et al., 2002, p. 96). The charging period and

magnitude of deceleration was used to ensure a fully charged accumulator. The system then sits

at rest for 22.5 seconds and then enters the discharging sequence. The discharge period was over

estimated to ensure the accumulator is completely evacuated.

Figure 2.4 – Simulated directional control valve signal

A rotational friction block and an inertial force block were added after the motor to mimic the

rolling resistance of the weight and wheels on the ground and the resisting inertial force to keep

the pedicab at rest. The rotational friction block incorporated a friction force equal to the rolling

frictional force. For simplicity the rolling friction value was calculated using the following

equation from Giancoli (1998, pp. 96-97).

Nf FF ⋅= µ (2.1)

The Ff term is equal to the rolling frictional force, µ is equal to the dimensionless coefficient of

friction and FN is equal to the normal force. The coefficient of friction between rubber and an

ideal dry roadway is 0.015 (Gillespie, 1994, p. 117) and the weight of the pedicab and passengers

is assumed to be 772 pounds. It is assumed that the normal force is equal to the weight of the

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complete system and passengers. By equation (2.1) and the assumption of normal force, the

rolling resistance is equal to 11.6 pounds. Noting that the rear wheels support most of the weight

and assuming that the rolling resistance is the maximum amount of force to roll the pedicab, the

required moment to move the pedicab is calculated by the following equation.

rFM fR ⋅= (2.2)

MR is equal to the required moment, Ff is equal to the rolling friction force from equation (2.1)

and r is the radius of the rear wheel. Knowing that the radius of the rear wheels is approximately

10 inches, the friction moment is calculated to be 9.64 lbf-ft by equation (2.2). The inertial force

is calculated using the following formula for moment of inertia from Norton (2004, p 530) where

m is equal to the mass of the system and r is the radius of the wheels. 2rmI ⋅= (2.3)

The result of equation (2.3) is a moment of inertia equal to 535.8 lb-ft2.

Because the proposed variable-displacement pump/motor is sensitive to the pressure of the

system, a new variable is introduced: the preload pressure. The preload pressure is the system

pressure at which the pressure compensator adjusts the displacement of the pump to save energy.

The preload pressure setting is set by a spring inside a pressure compensator, providing

resistance against the fluid pressure. As the pressure in the system reaches its preload pressure

and surpasses the spring force, the displacement lowers proportionally to lessen the amount of

fluid pumped to the accumulator, thus saving energy. To determine the optimal pressure setting

for a pressure sensitive variable-displacement motor a simulation was created using the same

components as the fixed-displacement circuit except for the pump/motor. The settings were

unchanged except for the preload pressure setting.

Because the hydraulic system is intended to not only function as a driving assist for the pedicab

operator, but also as a brake, further analysis on the charging sequence of the accumulator was

conducted using spreadsheet described in Appendix 5.2 and further described in Section 2.4.

This analysis included estimations of distance to a stop under linear deceleration and volume of

fluid pumped to the accumulator. Various pump displacement options were modeled to

determine a viable braking option.

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2.2 System Losses

Losses develop from hydraulic components, flow drag through hydraulic hose and friction within

the fluid itself. In the assist system designed by the ME 4054 team the largest contributors to

system losses were the pump/motor, directional control valve and hydraulic hose. One cause of

loss results from the hydraulic hose between components. The length of hose and turns in the

hose will cause system loss due to friction within the pipe and in the fluid itself. The losses

caused by the turns in the hose are estimated by (The MathWorks Inc, 2009; Crane Valve

Group):

QQd

Kloss ×××

=42

81 π

ρ (2.4)

where K is the coefficient of loss for the component, Q is the fluid flow rate (in3/s), ρ is the

density of the fluid (0.032 lb/in3) and d is the diameter of the hose is 0.75 inches. Coefficient of

loss values vary between different fittings and bends in the house. The hydraulic assist circuit

contains the equivalent of seventeen 90° bends. Since the coefficient of loss for a 90° elbow is

0.3 (The Engineering ToolBox, 2005), the coefficient of loss for the entire circuit is determined

to be 5.1. Using the simulation results of the variable-displacement pump/motor for flow data,

the maximum loss during charging and discharging contributed by the bends in the hose and

varying fittings is approximately 50 psi by equation (2.4).

Loss is also calculated based on the length of hose between components. The pressure loss in the

hose is estimated using the Hagen-Poiseuille equation (Burgett et al., 2008, pp. 97-98):

CGdLQ

loss××

××××= 4

128π

νρ (2.5)

where Q is the fluid flow rate (in3/s), ρ is the density of fluid (0.032 lb/in3), ν is the kinematic

viscosity of the fluid (0.0685 in2/s), and GC is the gravitational constant (386.4 in/s2). The

diameter (d) of the hose is 0.75 inches and L is the pipe length (70 inches). Using the simulation

results to estimate fluid flow rate Q, equation (2.5) predicts the maximum loss during charging

and discharging contributed by the hose and is 0.560 psi, which is negligible compared to the

losses caused by hose bends.

23

Flow resistance in the directional control valve contributes to system losses. The valve contains

channels directing the hydraulic fluid to various parts of the circuit depending on the situation:

charging, discharging or neutral. Fluid traveling through the valve as well as fluid changing

directions through the valve will contribute to energy loss in the hydraulic circuit. Tests were

performed by Givand, J. Schreifels and M. Schreifels (2009, slide 13) on the directional control

valve to determine pressure loss through the directional control valve. The measurements show

a 50 psi drop through the valve during the charging sequence.

2.3 SimHydraulics Simulation Results

The SimHydraulics simulations provided pressure and flow rate data that was used to calculate

the efficiency of the hydraulic circuit designed by the ME 4054 team and the efficiency of the

new circuit with the proposed changes. Equation (2.6) was used to calculate the power where P

is power, p is pressure and Q is flow.

QpP *= (2.6)

The power was integrated over the charging and discharging periods respectively. These values

are the total work in and out of the system respectively. The total work value from discharge is

compared with work during charging to determine the efficiency value using equation (2.7),

where η is the efficiency and WCharging and WDischarging are the total work values, respectively.

ingCh

ingDisch

WW

arg

arg=η (2.7)

First, are the results for a fixed-displacement pump/motor was simulated using a pump

displacement of 4 in3, a maximum pressure of 1500 psi, a linearly decelerating input from 75

rpm to zero rpm and a charging time of 37.5 seconds (Figure 2.1). This displacement and

maximum pressure matched the hardware that was installed by the ME 4054 team (See Section

2.0). The simulation calculation for this case predicted an efficiency of 64.21%. The

corresponding power and pressure plot is shown in Figure 2.5.

24

Power and Pressure

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100 120

Time (s)

Pow

er (h

p)

-300

0

300

600

900

1200

1500

1800

Pres

sure

(psi

)

Power in during charging Power out during discharging Pressure in the accumulator Figure 2.5: Power and accumulator pressure

when using a fixed-displacement pump/motor

Next are the results for a system that included a pressure-sensing variable displacement

pump/motor (Figure 2.2). The inputs to the variable displacement simulation were kept the same

with the maximum displacement of the pump/motor set at 4 in3, the maximum pressure kept at

1500 psi, the speed at 75 rpm and a charging period of 37.5 sec. After repeated simulations

adjusting the pre-load pressure incrementally, the pressure setting just before the maximum

system pressure is reached, 1,499 psi offered the greatest efficiency under the given conditions.

This pressure setting resulted in the efficiency of 91.73%. The resulting power and pressure plot

is shown in Figure 2.6. The pressure-sensitive variable-displacement pump/motor configuration

resulted in an efficiency gain of approximately 27.5% over the fixed-displacement pump

configuration.

25

Power and Pressure

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80 100 120

Time (s)

Pow

er (h

p)

-300

0

300

600

900

1200

1500

1800

Pres

sure

(psi

)

Power in during charging Power out during discharging Pressure in the accumulator Figure 2.6: Power and accumulator pressure when using a variable-

displacement pump/motor and a preload pressure setting of 1499 psi

The pre-load pressure setting of 1499 psi allows a maximum amount of energy to be stored

within the accumulator before reducing the displacement in the pump/motor significantly. This

can be seen by comparing efficiencies resulting from various preload pressure settings (Table

2.1). This assembly delivered the same output power as the previous fixed-displacement option,

with less input work. Figure 2.7 shows the displacement of the variable-displacement pump

along with the input and output flow of the pump/motor and the system pressure. As the

pressure reaches its preloaded limit, the displacement is lowered to reduce the amount of flow

into the system. By equation (2.6) this results in less power generated while maintaining a full

accumulator. The resulting power out of the system is left unchanged resulting in a higher

efficiency calculation by equation (2.7).

26

Table 2.1

Preload Pressure (psi) Efficiency (%)

1000 76.04

1100 73.81

1200 80.44

1300 71.49

1400 68.05

1425 72.82

1450 72.65

1475 86.65

1480 65.17

1490 66.03

1499 91.73

Fixed displacement 64.21

-2

0

2

4

6

8

10

12

0 20 40 60 80 100 120 140

Time (s)

Dis

plac

emen

t (in

3 ) and

Flo

w (i

n/s)

-300

0

300

600

900

1200

1500

1800

Pres

sure

(psi

)

Pump/motor displacement Flow in during charging Flow out during discharging Pressure in the accumulator Figure 2.7: Displacement, flow and accumulator pressure when using a

variable-displacement pump/motor with a 1499 psi preload pressure

27

Using measurements from Givand et al. (2009, slide 16), the actual energy efficiency of the

pedicab using a 0.5 in3 displacement pump/motor is approximately 9% by equation (2.8).

2

2

212

1

in

outEnergy

Vm

Vm

⋅⋅

⋅⋅=η (2.8)

Their measurement of velocity before regenerative braking (Vin) was 10 mph (168 rpm) and

velocity using the assist without pedaling input (Vout) was 3 mph (50.4 rpm) (Givand et al., 2009,

slide 16). Recreating this trial in our simulation, results in an energy efficiency of 2.28e-12 or

effectively zero. The maximum output rpm in this case was found to be 2.53e-4.

2.4 Braking Results

To take the full advantage of the fluid power assist, the accumulator should reach maximum

internal pressure during the stopping and charging sequence. This will allow for maximum

power output during the discharge period. The ME 4054 team assumed a deceleration value of 2

meters/sec/sec for normal operation of the pedicab. Stopping from an operating speed of 4.5

meters/sec (10 mph) would therefore take an average of 2.25 seconds. Using the methods listed

in Appendix 5.2, the approximate volume of the accumulator was determined for different

starting speeds and deceleration periods. Assuming linear deceleration, zero system losses, a

fixed-displacement pump (4 in3) and a starting speed of 168 rpm, a stopping time of 2.25 seconds

would only fill 10.37% of the accumulator volume. This does not generate a large accumulator

pressure, which in turn will not generate much power during the discharge sequence. In the

SimHydraulics simulation, the accumulator is 68.88% filled before the maximum system

pressure of 1500 psi is reached and the pressure relief valve is triggered.

Using the numerical analysis detailed in Appendix 5.2, it is determined that the time to slow to a

complete stop from a speed of 10 mph and reach the necessary accumulator volume for

maximum pressure would be approximately 17 seconds, assuming linear deceleration and no

outside forces. The pedicab would then reach a complete stop in approximately 120 feet under

the listed conditions referencing the circumference of the pedicabs wheels. This stopping

distance would require the operator of the pedicab to be incredibly aware of their surroundings

and would not be considered safe under certain conditions.

28

The pedicab’s true stopping distance was tested with a 0.5 in3 displacement pump/motor installed

in the assist system (Givand et al., 2009, slide 16). The pedicab was driven to an approximate

speed of 10 mph without passengers. Once the desired speed was achieved the charging circuit

was engaged and the pedicab coasted to a stop. The distance from that point to the final resting

point was measured to be 87 feet. Without the charging circuit engaged, the pedicab coasted to a

stop without brakes in 135 feet. The accumulator did not fully charge.

3.0 Design Changes

3.1 Directional Control Valve

A neutral center directional control valve was purchased and installed into the system. The new

valve is an electric solenoid controlled device, Northman SWH-G02-C4-D12-10 (Volume II,

Section 1.2). The neutral center allows fluid to pass through the pump during normal pedaling.

Without a neutral center there would be significant additional resistance to the normal pedaling.

3.2 Operator Control

To make the control more intuitive and similar to operating a bicycle, the lever and linkage

system was removed and replaced with bicycle brake cantilevers and a brake cable. Because the

directional control valve requires movement in two directions, forward and back, two brake

levers were installed on the pedicab’s handlebars. The left handle reverses the motion of the

control valve and initiates the charging of the accumulator. The right handle moves the control

valve forward to trigger the discharging circuit. By using the handles simultaneously, the

position of the control valve adjusts to its neutral center for normal pedaling operation.

This method of control should be more intuitive for the operator and does not interfere with the

passenger space. This method of control does not require the operator to remove his or her hands

from the handlebars to control the assist system.

29

As mentioned in Section 3.1, the original control valve was replaced with an electric solenoid

controlled device. Due to this change, the brake cable control for the mechanical valve was

removed to accommodate the electric switches and wiring for the electric valve. This change

also resulted in the charging circuit being wired to the right lever and the discharging circuit to

the left lever. The new electric lever control is pictured in Figure 3.1.

Figure 3.1: Lever control (discharge switch)

3.3 Pump/Motor

An objective of this project was to determine if a variable-displacement pump/motor would be

more efficient than a fixed-displacement pump/motor in the pedicab application. After

simulating both options, it was determined that there was a distinct advantage in efficiency by

using a pressure sensitive variable-displacement pump/motor. The simulations determined that a

suitable pump would be the Parker Company model V12 series axial piston pump/motor, V12

080 M S S H S X D S X/X HPS 01 I HP L01. The price of this pump is $3,700, which is too

high for a pedicab application.

Although a variable-displacement pump would be ideal to improve efficiency, to keep costs low,

a lower (0.5 in3) fixed-displacement pump/motor (Eaton Char-Lynn M-0J-05) was purchased for

use in the pedicab. More information on this motor is found in Volume II, Section 1.3. The

original 4.0 in3 displacement pump/motor used by the ME 4054 team required too much torque

30

during the charging sequence resulting in a quick stop with little fluid pumped to the

accumulator. During discharge, the original pump used the available charged fluid quickly

resulting in inadequate movement assist. The new 0.5 in3 displacement component performs

well and offers a sufficient movement assist.

One note about using a variable-displacement pump as a motor is that while a pressure sensitive

variable-displacement pump has efficiency advantages during the charging of the accumulator,

when used as a variable-displacement motor during discharge, there are no efficiency benefits.

When charging the accumulator using a pressure sensing variable displacement pump, once the

pressure in the system reaches the maximum setting, the pressure relief valve triggers. The fluid

then bypasses the accumulator and returns to the fluid reservoir. Because you are no longer

charging the accumulator, utilizing the full displacement of the pump is wasteful. This is an

opportunity to improve the circuit efficiency by reducing the input of energy to the system as the

system reaches its pressure limits. Conversely, during the discharge of the system, the system

pressure is decreasing and the pressure relief valve never triggers, thus resulting in full

displacement throughout the entire discharge.

Additionally, when discharging the accumulator to a variable-displacement motor, there is a

finite amount of pressure and fluid available in the accumulator to power the motor. Starting

movement from a dead stop requires a significant amount of starting torque, considering the total

weight of a fully loaded pedicab (772 lbs). As shown in Section 2.1, the required torque to begin

motion of a fully loaded pedicab from rest is 9.64 lbf-ft. Increasing the displacement of the

motor would be beneficial as this would produce more torque for a fully loaded pedicab. As the

pressure decreases in the accumulator during discharge, using maximum available torque from

the remaining fluid would be ideal. This calls for the maximum displacement from a variable

motor. Because the adjustable nature of a variable displacement motor would not be utilized

during discharge, a fixed-displacement motor utilizing a larger displacement would be ideal for

the discharging assist sequence.

Using the hydraulic system as a regenerative brake also calls for a pump that would use a

moderate torque. A pump that runs at lower speeds and requires more torque would slow the

31

movement of the pedicab and serve as a consistent brake option. Too little torque during braking

and the pedicab will not decelerate at a safe rate. Transportation options such as this would

require a responsive brake. This illustrates the need for a larger displacement as well.

Conversely, by using a displacement that is too large, the momentum of the pedicab will not be

sufficient to drive the pump and charge the accumulator. In order to determine an ideal

displacement for the weight and thus the torque required in our application, the calculations in

the ME 4054 design report (Burgett et al., 2008, p 64) to determine pump size were redone.

Those calculations assumed that the maximum speed and maximum pressure would be constant

during a short deceleration period. A better assumption would be a starting speed of 10 mph, a

linear deceleration and a deceleration period of 5 seconds, resulting in an average decelerating

speed for equation (3.1) of 5 mph.

hps

mphlbst

vmP 235.05

)5(772 22

122

1=

××=

××= (3.1)

In equation (3.1), P is power, m is mass, v is velocity and t is time. Using a wheel diameter of 20

inches, the average deceleration speed is 83.625 rpm. This value can then be input into equation

(3.2) to determine torque.

inlbrpmhp

rpmPT −==×

= 5.1089625.83235.0

2π (3.2)

321.1900

5.10892 inpsi

inlbp

TD =−

=×

=π (3.3)

Equation (3.3) determines the needed displacement value, D, by the relation of torque and

pressure. In equation (3.3), it assumes that the average pressure during deceleration is 900 psi.

Sizing the pump using the revised calculations resulted in a pump displacement of 1.21 in3, as

shown in equation (3.3).

4.0 Recommendations

4.1 Directional Control Valve The addition of the electric control valve and control system (Section 3.2) adds a battery into this

hydraulic system. It would be preferred to return to the mechanically controlled option, to keep

32

the entire system mechanically operated. With a mechanically operated control, batteries are not

required to operate the system, which would be a benefit.

4.2 Hydraulic Accumulator It is important to minimize the weight of the vehicle to make pedaling as easy as possible. The

addition of hydraulic components increases the overall weight of the pedicab, so selecting

lightweight components is important. The heaviest component in the design by the ME 4054

team is the accumulator (Parker Model Number ACP10AA200E1KTD). This accumulator has a

steel shell and weighs 35 pounds. One option to reduce weight would be to use a composite

shell accumulator from Parker Company, for example, the CFA series piston accumulator.

Another option would be to use a strain-based accumulator. The strain-based accumulator is

made of an elastic material, which means it is much lighter than a traditional accumulator is.

However, it will be some time before this accumulator will be available as it is currently a

research project in the Center for Compact and Efficient Fluid Power.

4.3 Hydraulic Pump/Motor The recommendation for the pump/motor is to keep a fixed-displacement model. The best

option in all aspects of braking, charging and discharging is a fixed-displacement pump/motor

with a displacement of 1.21 in3. For ease of installation the Eaton Char-Lynn M-0J-12

pump/motor is recommended. This pump/motor weighs 4.8 lbs. (Additional details about this

pump are available in Volume II, Section 1.3.)

The hydraulic pump charging system cannot be used as a primary braking option (Section 2.4).

A high displacement pump requires a large amount of torque, which is good for stopping the

pedicab. However, the pedicab would stop before the accumulator fills with an appropriate

charge. As shown in Section 2.0, charging a fully pressurized accumulator (1500 psi) with a 4

in3 fixed-displacement pump requires approximately 24 revolutions of the pump or, based on a

wheel circumference of approximately 63 inches, approximately 120 feet while linearly

decelerating from a speed of 10 mph over a period of 17 seconds. This calculation does not

33

include considerations for friction or the effect of a partially full accumulator on the speed of the

pedicab.

A lower displacement pump charges the pedicab sufficiently but does not have the stopping

power desired and the time and distance to charge the accumulator will increase. The measured

stopping distance of the pedicab is 87 feet with a 0.5 in3 displacement pump, which is

unacceptable for use as a primary brake. This was determined experimentally without

passengers in the pedicab as noted in Section 2.4. Observations of the pedicab in operation and

numerical analysis show that the hydraulic system should only act as a secondary braking option

due to the increased time and distance to a complete stop. The regular caliper brake must be

used in conjunction with the hydraulic system to bring the pedicab to a complete stop during

most situations. This means that it is not feasible to capture all of the braking energy, unless the

operator determines that a long stopping distance is available.

4.4 Pedicab Chassis and Drive System A disadvantage of the pedicab chosen for this project is that the rear passenger bench is 31

inches wide, not wide enough for two adult passengers. Additionally, the operating space is

small for an average adult male. Another limitation is the single rear wheel drive axle. Using a

single wheel drive limits the power output during the discharging setting. A dual-wheel drive

system will offer twice the amount of tire surface area doubling the potential traction to the road

surface and therefore potentially providing more torque to drive the pedicab. Two driving

wheels would also provide an even torque to both sides of the pedicab. When transporting more

than of 600 lbs, increased torque would be beneficial. Another limit of the drive system is the

single pedaling gear. The lack of a derailleur gearing system limits the performance during

normal pedaling operation. A gearing system would not have an affect on the pump sizing or the

assist operation. The pump/motor connects directly to the drive shaft of the vehicle and would

not connect through gear system. Other pedicab models offer derailleur gears; a dual rear wheel

drive axle with differential; a wider, more comfortable passenger seat (~50 inches) and

adjustable operating space (Main Street Pedicabs, Inc, 2009a). In order to make the complete

pedicab system an attractive option for regular human-powered transportation, a new modern

pedicab would be preferred for marketing in the United States.

34

Adding a clutch would be another improvement to the pedicab. A clutch would allow the

operator to disengage the pump from drive axle and pedal uninhibited by the hydraulic system

drag. This idea is outlined in Figure 4.1. In the figure, clutch #3 would be disengaged and

clutch #1 and #2 would engage. In this example, the pump/motor does not connect to the drive

axle or pedals. Another clutch option could allow the operator to pedal directly to the pump and

charge the accumulator without movement of the entire pedicab. As shown in Figure 4.1, clutch

#1 and #2 would be disengaged and clutch #3 would engage. This clutch option would let the

operator charge up the accumulator while at a stoplight or during other stationary periods, which

would result in a larger launch assist when engaged.

Figure 4.1 - Clutch concept

5.0 Conclusion The fluid power assist system with regenerative braking does offer many benefits to a pedicab

operator. The system successfully stores energy typically dissipated during braking. This stored

energy does supply power to assist the operator in moving the pedicab during otherwise tiring

conditions. The system achieves these objectives in a self-contained system without the need for

a tertiary power source, as compared to a rechargeable battery option. Further improvements and

fine-tuning would offer increased efficiency and performance, however, the current system does

offer a sound example of fluid power as an alternative to electric assist and hybrid vehicles.

35

6.0 References

Burgett, E., Dreyer, R., Jackson, K., Kysylyczyn, K., & Lai, J. (2008). Pedicab with fluid power assist. Minneapolis, MN: University of Minnesota.

Chan, S., & Confessore, N. (2005, 1/2/2005). Not so merrily, they roll along: Pedicabs vie for midtown riders. New York Times.

Crane Valve Group. Flow of fluids through valves, fittings, and pipe No. Technical Paper No. 410M). Shenandoah, TX: Crane Valve Group.

Drill Spot. (2010). Mobil DTE 25 premium hydraulic oil. Retrieved 11/29, 2009, from http://www.drillspot.com/products/297225/Mobil_DTE_25_Premium_Hydraulic_Oil

EcoSpeed. (2010a). EcoSpeed, worlds best electric assist. Retrieved 11/29, 2009, from http://www.ecospeed.com/prodemdu.html

EcoSpeed. (2010b). EcoSpeed, worlds best electric assist. Retrieved 11/29, 2009, from http://www.ecospeed.com/prodbatt.html

EcoSpeed. (2010c). EcoSpeed, worlds best electric assist. Retrieved 11/29, 2009, from http://www.ecospeed.com/emddet.html

Giancoli, D. (1998). Physics: Principles with applications. Upper Saddle River, NJ: Prentice Hall.

Gillespie, T. (1994). Fundamentals of vehicle dynamics Society of Automotive Engineers.

Givand, J., Schreifels, J., & Schreifels, M. (2009). Regenerative braking hydraulic assist pedicab. PowerPoint presentation.

Heinzmann USA. (2009). How it works. Retrieved 11/29, 2009, from http://www.heinzmannusa.com/how_it_works.html

Main Street Pedicabs. (2009a). The boardwalk pedicab is a new family favorite! Retrieved 11/29, 2009, from http://www.pedicab.com/pedicabs-boardwalk-pedicab.html

Main Street Pedicabs. (2009b). Pedicab vs. taxicab NYC video | pedicab & rickshaw blog. Retrieved 11/29, 2009, from http://www.pedicab.com/wordpress/2009/01/16/pedicab-vs-taxicab-nyc-video

Mozer, D. (2010). Pedicabs, cycle rickshaws: USA directory. Retrieved 11/29, 2009, from http://www.ibike.org/economics/pedicab-usa.htm#MN

36

Norton, R. (2004). Design of machinery. New York, NY: McGraw-Hill.

Parker Hannifin Corporation. (2009). Parker - CARBON FIBRE HYDRAULIC PISTON ACCUMULATOR. Retrieved 11/29, 2009, from http://www.parker.com/portal/site/PARKER/menuitem.7100150cebe5bbc2d6806710237ad1ca/?vgnextoid=f5c9b5bbec622110VgnVCM10000032a71dacRCRD&vgnextfmt=EN&vgnextdiv=00TEST&vgnextcatid=6575350&vgnextcat=CARBON%20FIBRE%20HYDRAULIC%20PISTON%20ACCUMULATORS

Rajvanshi, A. K. (2002). Electric and improved cycle rickshaw as sustainable transport system for India. Current Science, 83(6), 703.

Smith, S. (2009). Pedicabs beat D.C. gridlock. Retrieved 11/29, 2009, from http://www.cbsnews.com/stories/2009/01/19/national/inauguration09/main4734776.shtml

Spielman, F. (2009, 5/13/2009). Pedicabs are coming to downtown Chicago. Chicago Sun Times.

The Engineering ToolBox. (2005). Minor loss coefficients in pipes and tubes components. Retrieved 7/21, 2009, from http://www.engineeringtoolbox.com/minor-loss-coefficients-pipes-d_626.html

The MathWorks Incorporated. (2010). SimHydraulics – documentation. Retrieved 7/21, 2009, from http://www.mathworks.com/access/helpdesk/help/toolbox/physmod/hydro/ref/elbow.html

Wheeler, T., & I’Anson, R. (1998). Chasing rickshaws. Melbourne, AUS: Lonely Planet Publications Pty Ltd.

37

Appendix A - Commercial Pump/Motor and Accumulator Options

Parker The Parker Hannifin Corporation supplied many of the hydraulic components used in the project.

Parker has a broad offering of hydraulic components ranging from accumulators to valves.

Parker offers many variable-displacement pump/motor models that were investigated, however,

none was able to offer the correct combination of size or weight, control options and

displacement size. Most were both too large and heavy or did not include options for pressure

sensitive control. For accumulators, Parker offers a composite accumulator that would be a

substantially better accumulator in terms of weight and performance. Information was limited on

this accumulator option.

Sauer-Danfoss The Hydrogear product line from Sauer-Danfoss has a number of hydraulic components

including pumps, motors and valves. A variable-displacement pump was considered from Sauer-

Danfoss (model BDP), however, the idea was scrapped after the displacement was thought to be

too small for the required power and torque of the pedicab (only 0.61 in3).

Eaton Eaton provides a full line of hydraulic components. A medium-duty variable-displacement

piston pump (Model 72400) was investigated for use in the pedicab. This motor has a

displacement of 2.5 in3 with hydraulic remote control resulting in a good match to the

application. This option would allow the inclusion of a separate pressure control valve, which is

desirable. A pressure control valve would allow the circuit to sense pressure changes in the

circuit and adjust the flow of the circuit to save input energy and increase efficiency. The

benefits of a pressure sensing circuit are documented in Section 2.3. The disadvantage to this

model is that this pump weighs 60 pounds, which would be a noticeable weight increase during

pedaling.

38

Appendix B – Charging Sequence Analysis Using Microsoft Excel, a numerical analysis was performed to calculate the fluid moved by

fixed-displacement hydraulic pump. Using 240 equal time increments, the amount of fluid

moved by the pump was calculated starting at a speed of 168 RPM and linearly decelerating to

zero with the deceleration occurring over a period of 17 seconds. The first few rows of the Excel

calculations are listed below.

A B C

1 RPM Time Revolutions

2 168 17 =A2/60*B$2/240

3 =A2-A$2/240 =B2-B$2/240 =A3/60*B$2/240

4 =A3-A$2/240 =B3-B$2/240 =A4/60*B$2/240

Which is the same as

24016823 −= AA

2401723 −= BB

24017

6033 ×=

AC

The total number of revolutions over 17 seconds was calculated and found to be 23.89

revolutions. The number of revolutions was then translated into amount of fluid moved through

the motor by using the fixed-displacement of the motor (4 in3) with the result being 95.60 inches3

or 1.57 liters of fluid. The accumulator’s capacity is 2 liters, which means charging under these

circumstances would fill 78.34% of the available volume. Converting the number of revolutions

to distance traveled is calculated using the circumference of the pedicab’s rear tires, 5.235 feet,

and results in 125.1 feet.