Georgia Institute of Technology | Marquette University | Milwaukee School of Engineering | North Carolina A&T State University | Purdue University | University of California, Merced | University of Illinois, Urbana-Champaign | University of

Minnesota | Vanderbilt University

Fluid Power Innovation & Research Conference

Minneapolis, MN | October 10 - 12, 2016

Free Piston Engine Based

Off-Road Vehicles

Chen Zhang, Research Assistant (presenter)

Keyan Liu, Research AssistantUniversity of Minnesota

Advisor: Prof. Zongxuan Sun

Photo

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Outline

• Project Overview

• Piston Motion Control of FPE

• Trajectory Based Combustion Control

• Independent Pressure and Flow Control

– Concept introduction

– Performance demonstration through simulations

• Summary and Future Work

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Project Overview At a Glance Major Objectives or Deliverables

Next Steps

• What are your research goals?

Investigate the design, control and testing of

the FPE based off-road vehicles.

• How does this project fit into the CCEFP’s

overall research strategy?

Increasing energy efficiency

Improving energy storage capabilities

Reducing environmental impact

• What is the original contribution?

Controlling the hydraulic FPE in real-time to

produce the required pressure and flow rate

Independently.

Designing appropriate hydraulic actuation

systems accordingly for both linear and rotary

motion to reduce or remove throttling losses.

• What is its advantage over competing

technology?

Possesses shorter response time

More compatible with mobile applications.

Plan for next step

Explore the performance of FPE as power

source in actual hydraulic circuits through

simulation of different working cycles.

Investigate more sophisticated trajectory

synthesizing methods

How can industry help / contribute?

Providing operating duty cycle for off-highway

vehicles.

Providing industrial guidance on modeling,

experimental system design and applicability

of this technology

Control of the FPE to provide required

pressure and flow rate independently.

Design of efficient hydraulic actuation systems

for modular and digital fluid power sources.

Evaluation of the FPE based off-road vehicles

and comparison with conventional vehicles.

Project Overview

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Control of FPE:

The FPE at UMN• Opposed Piston Opposed

Cylinder (OPOC) Design

• Direct Fuel Injection

• Uniflow Scavenging

Variable compression ratio• Advanced combustion strategy

• Multi-fuel operation

Reduced frictional losses

Higher power density

Fast response time

Internally balanced

Modularity

Exhaust Ports

Intake Ports

IntakePorts

ExhaustPorts

Check Valves

Servo Valve

On-off Valve

On-off Valve

LP

HP

Outer Piston Pair

Inner Piston Pair

Hydraulic Chambers

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• System Modeling– Combustion model

– Hydraulic model

– Gas dynamics

– Piston dynamics

• Hardware Improvement– Sensor calibration

– Pre-charge system

– DAQ and control system

– Moog valve and Lee valves

– Ignition control

– High pressure DFI system

– Supercharger system

• Implementation of Control– Virtual Crankshaft design

– Engine motoring tests

– Engine combustion tests

The developed robust repetitive controller

acts as a virtual crankshaft that would

force the piston to follow the reference

signal through the hydraulic actuator.

Engine start

Misfire recover

Real time frequency and CR control

Control of FPE:

Virtual Crankshaft

Experiment set-up in UMN test cell

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Control of FPE:

Motoring Test

In-cylinder gas pressure, hydraulic chamber

pressure and piston tracking performance

Virtual crankshaft is able to

actively regulate the piston

motion of the FPE to track any

prescribed trajectory reference. [1]

Feedforward controller is also

developed to further improve the

performance of the virtual

crankshaft mechanism. [2]

[1] Li, K., Sadighi, A. and Sun, Z. (2014). Active motion control of

a hydraulic free piston engine. IEEE/ASME Transactions on

Mechatronics, volume (19), pp. 1148-1159.

[2] Li, K, Zhang, C. and Sun, Z., "Precise piston trajectory

control for a free piston engine." Control Engineering Practice,

34 (2015): 30-38.

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Piston motion, combustion chamber pressure,

hydraulic chamber pressure and control signal

Continuous combustion

operation is achieved.

Each fuel injection

results in a strong

combustion.

Virtual crankshaft is

able to maintain engine

operation even with

cycle-to-cycle

combustion variation

Control of FPE:

Continuous Combustion test

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Virtual Crankshaft

Applications

Virtual crankshaft mechanism

Trajectory based

combustion control

• Improved thermal efficiency [3]

• Reduced emissions [4]

• Optimal trajectory based on load requirement and fuel property [5]

Independent pressure and

flow rate control

• Producing the required flow rate at different pressure in real time

• Fast response time to load variation

[3] Zhang, C., Li, K. and Sun, Z., “Modeling of Piston Trajectory-based HCCI Combustion Enabled by a Free Piston Engine”, Applied Energy, vol. 139, pp. 313-326, 2015. [4] Zhang, C. and Sun, Z., “Using Variable Piston Trajectory to Reduce Engine-out Emissions”, Applied Energy, vol.170, pp. 403-414, 2016.[5] Zhang, C and Sun, Z., “Optimization of Trajectory-based HCCI Combustion”, DSCC 2016.

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Independent

Pressure and Flow Rate Control

Virtual Crankshaft

Hydraulic FPE

IPFC

Output flow rateat load pressure

Piston Position

Servo valve signal

Reference

Measured load pressure

Required flow rate

Fuel injection Amount

• The key component is the Independent Pressure and Flowrate Controller (IPFC),

which is able to synthesis a unique trajectory reference for the hydraulic FPE,

and derive the corresponding fuel injection amount, according to required flow

rate and measured load pressure.

• The synthesized trajectory reference is then sent to the virtual crankshaft, which

ensures accurate piston motion tracking by adjusting the opening of the servo

valve through different servo valve signal.

• The variable opening of the servo valve can also affect the output flow rate at

different load pressure produced by the FPE

Basic Concept

+Error

-

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Chamber 1&3

Chamber 2

Load

100% To Load

100% From Tank

100% Flow Output+60%

Chamber 1&3

Load

60%

Net flow

Chamber 2

60%

40%

40%

-40%

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Independent

Pressure and Flow Rate Control

1

3

2

Working Principle

+20%

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Switching Point+

-

+

-

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Independent

Pressure and Flow Rate control

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Net Hydraulic Force

Left Gas Force Right Gas Force

The piston motion is subject to the

Newton second law, where the force

consists of gas force and hydraulic

force.

While the gas force is subject to the

ideal gas law, the hydraulic force is

the net force from all hydraulic

chambers, which can be controlled

by valve timing.

Consequently, the valve switch time

has a direct and unique influence on

the hydraulic force and therefore the

trajectory, thus forming a one-to-one

mapping. 𝑥 = −

𝐹𝑔_𝑙 − 𝐹𝑔_𝑟 ± 𝐹ℎ𝑦

𝑚

𝐹𝑔_𝑙 𝐹𝑔_𝑟

𝐹ℎ𝑦

𝐹𝑔_𝑙: Ideal gas law,

Instantaneous combustion model

𝐹𝑔_𝑟: Ideal gas law

𝐹ℎ𝑦 = (𝑃𝑙𝑜𝑎𝑑 − 𝑃𝑡𝑎𝑛𝑘) × 𝐴𝑝𝑖𝑠𝑡𝑜𝑛, Constant

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Independent

Pressure and Flow control

Mapping from output flow rate to valve switch time,and then to Piston Trajectory

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Different piston trajectory leads to various flow rate at a specific load pressure.

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Independent

Pressure and Flow control

FPIRC16

Virtual Crankshaft

Hydraulic FPE

IPFC

Output flow rateat load pressure

Piston Position

ServoRef

Measured load pressure

Required flow rate

Combustion Info

+Error

-

Thermo-dynamics

Combustion

Hydraulic Dynamics

Piston Dynamics

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Independent

Pressure and Flow control

FPIRC16

Piston Tracking Performance, Hydraulic

Pressure, Flow Output (Top to Bottom)

The synthesized reference can be

properly tracked by the virtual

crankshaft.

The servo valve switches at the

designed time, as indicated by the

hydraulic pressures.

The friction loss and oil leakage

can be automatically compensated

by the virtual crankshaft.

The actual output flow rate is

slightly less than the reference

case, mainly due to tracking error

and servo valve dynamics.

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Independent

Pressure and Flow control: steady state performance

FPIRC16

The required output flow

rate changes from a high

value to a low value at

3.036s.

The corresponding piston

trajectory reference is also

varied at the same time.

There exists A smooth

transient performance when

the required output flow

rate is changed and the

output flow is re-stabilized

within ~0.1s.

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Independent

Pressure and Flow control: transient performance

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Summary and future work

• With proper reference trajectory and fuel injection

strategy, the FPE can work as a digital fluid power

source capable of independently controlling the output

flow and pressure.

• Simulation results show that the unique mapping

among the piston trajectory, servo valve switch time and

output flow rate of the FPE can be achieved and the

working principle is thus verified.

• Simulation results also demonstrate the FPE, under this

control method, has a relatively short response time

(~0.1s) to realize variable output flow rate demands.

• Next, we will combine the model of the FPE with an

actuator side model to further verify the proposed idea.

• Better trajectory synthesizing methods and transient

control strategies will also be investigated

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Questions?

Back up slides

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Hydraulic FPE vs. Digital Pump

𝑄𝑜𝑢𝑡 = 𝑄𝑙𝑜𝑎𝑑 𝑄𝑜𝑢𝑡 = 𝑄𝑙𝑜𝑎𝑑 − 𝑄𝑑𝑟𝑎𝑖𝑛𝑄𝑜𝑢𝑡 = 𝑄𝑙𝑜𝑎𝑑

FPIRC16

Trajectory Generation

Independent Pressure and

Flow control

Load

Fraction of displacement

Start

Set the left chamber at the TDC

Set valve timing according to Dx

Adjust fuel amount so that the piston returns

to the same TDC

Numerically Calculate the trajectory

Calculate corresponding

flowrate

End

Record the fuel amount and trajectory

Off-line Sweeping

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Desired

actuator speed

Actuator

pressure

Valve opening cmd

Hydraulic

actuator

+

+ PIDFree Piston

Engine Actuator

speed

Flow

Fraction of

displacementValve

Hydraulic plant

Hydraulic pressure source

Fluid

capacitor

_

Delta

pressure +

+

Source

pressure

PID

_

Combining FPE with the actuator

Control scheme of hydraulic controls for the FPE and actuator

Virtual Crankshaft

Hydraulic FPE

IPFC Piston Position

Servo valve signal

Reference

Fuel injection Amount

+Error

-

Output flow rateat load pressure

Measured load pressure

Required flow rate

Independent

Pressure and Flow control

FPIRC16

Case study: wheel loader working hydraulic circuit

S

C

R

PM2

PM1

ICE

Final

drive

Lift

cylinder

Tilt

cylinder

Working hydraulic system

Hydraulic power split drivetrain

LA LB TA TB

LS

compensator

Pressure

limiter

Loading-sensing pump

PM1–Pump/motor1

PM2–Pump/motor2

Planetary

gear set

LA – Lift chamber A

LB – Lift chamber B

TA – Tilt chamber A

TB – Tilt chamber B

Working hydraulic circuit

Drivetrain

Independent

Pressure and Flow control

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Independent

Pressure and Flow control