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Chapter 1

Development of Off-Road Vehicles

Types of Off-Road Vehicles

Early off-road vehicles were powered by steam enginesThe first tractors were large and cumbersome

Used for plowing and threshingLater adaptations allowed for use as a motor cultivator

General-purpose tractor was then created to perform major farm operations

Descriptive Classification

CrawlerStandard row-cropHigh-clearanceUtilityOrchardLawn and gardenMultipurpose

Power tillerTree skidderSkid-steer loaderFour-wheel drive with smaller front steering wheelsFour-wheel drive with equal-sized wheels and articulated frame steering

• Based on steering method, arrangement of frame, and traction members

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Lawn Mower

Riding Tractor

Utility Tractor

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Bull Dozer

Tracked Tractor

Skid Steer Loader

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Chapter 2

Engine Performance Measures

Combustion Engines

Two types: Spark Ignition (SI- gasoline), Compression Ignition (CI- diesel)Power in fuel

Power from burning fuelHeating values measured by burning a sample of fuel in a calorimeterLess than ½ of fuel equivalent power is available at engine flywheel

Power Flows in an Engine

Engines may be rated by power, but power varies greatly depending on where it is measured.

Tractors rated by their take-off (PTO) powerIf no PTO, then use drawbar power

Large 4WD tractors

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Power Distribution

Pfe is the fuel equivalent powerPi is the indicated powerPf is the power lost to frictionPb is the brake powerNit is the thermal efficiencyNm is the mechanical efficiency

Srivastava et. al., 1993, Eng. Principles of Agric. Machines, ASAE

Fuel Equivalent Power

Power embodied in the fuel:Pfe = mf * Hg / 3600

Where Pfe = fuel equivalent power (kW)mf = fuel consumption rate (kg/h)Hg = gross heating value of the fuel (kJ/kg)

Use when fuel consumption is measured on a mass basis

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Fuel Equivalent Power

When fuel consumption is measured volumetrically, use:

Pfe = (qf * ρf * Hg) / 3600Where qf = fuel consumption rate (L/h)ρf = fuel density (kg/L)

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Srivastava et. al., 1993, Eng. Principles of Agric. Machines, ASAE

Facts:

Burning of the fuel produces high pressure on each piston which, when multiplied by the piston area, generates a force. Although the pressure varies throughout the piston stroke, it is possible to calculate an indicated mean effective pressure (pime)

Facts

Combustion is a complex process where fuel must vaporize and mix with air to form a combustible gaseous mixtureBurning fuel-air mixture creates exhaust, raises temperature and increases the pressure to drive the pistons.

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Combustion Process

Assumptions in chemical reactionAll hydrogen links with oxygen to form H2OAll carbon converts to CO2 and COAll gases in air are ignored except O, and NAtmospheric air contains 3.76 moles of N2 for every mole of O2

Combustion Reaction

C16H34 + 24.5O2 + 92.12N2 > 92.12N2 + 16CO2 + 17 H2O

fuel air

Atomic weight: C=12, H=1, O=16, N=14

Note: there is no CO in this reaction. It is a chemically correct reaction, since the air to fuel ratio is stoichiometric.

Combustion Reaction

Air/Fuel Ratio

Equivalence Ratio

9.141*3412*16

)28*12.9232*5.24(≡

++

≡FA

icStoichmetrAF

ActualAF

≡Φ

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Equivalence Ratio Φ

Φ >1 Rich: More fuel than O2, CO in exhaust

Φ = 1 Stoichiometric

Φ < 1 Lean: More O2 than fuel, O2 in exhaust

Exhaust Gas on Dry Basis

92.12N2 + 16CO2 + 17 H2O

dry wet

Note: Dry basis ignores the H2O

%2.85100*1612.92

12.92% 2 ≡+

≡N

%8.14100*1612.92

16% 2 =+

=CO

Indicated Power

Pi = (pime*DeNe) / 120,000Where Pi = indicated power (kW)pime = indicated mean effective pressure (kPa)De = engine displacement (L)Ne = crankshaft speed (rpm)

The number in the denominator is the product of 2 revs/cycle*60 sec/min*1000 W/kW (4-stroke engine)This equation will also work for a two-stroke engine, substituting 60,000 for 120,000 in the denominator.

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Engine Displacement

De = ApLn / 1000Where Ap = top area of piston (cm2) = π(d2)/4

d = piston diameter (cm)

L = stroke length (cm)n = number of cylinders in the engine

Two-Cycle Engines

Increasing Indicated Power

Three possible ways:1. Increase engine size (De)2. Increase engine speed (Ne)3. Increase the internal pressure and consequently the

stress on the engineNote: indicated power is not immediately available for useful work- must first be transmitted to the flywheel of the engine. Interest is in the flywheel power (brake power, Pb) of the engine, first measured by a prony brake. The derivation of the brake-power equation begins with a consideration of how work per crankshaft revolution relates to crankshaft torque.

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Brake Power

Pb = (2πTbNe) / 60,000Where Pb = brake (flywheel) power (kW)Tb = engine brake torque (N*m)Ne = engine speed (rpm)

Prony Brake Dynamometer

Goering et. al., 2003, Off-road Vehicles, ASAE

Friction Power

Pf = Pi – PbWhere Pf = friction power (kW)

By definition, friction power includes any part of the indicated power that is not delivered to the flywheel.

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Mean Effective Pressurep-v diagram

intake

Compression

Combustion Power

Exhaust

Goering et. al., 2003, Off-road Vehicles, ASAE

Measurement of the pime

A pressure transducer continuously measures the cylinder pressure while an encoder measures the crankshaft rotational position. The data are fed into a computer that calculates pime.

Brake Pressure and Torque

By combining Pb = (2πTb*Ne) / 60,000 and pbme = (120,000*Pb) / (De*Ne) and simplifying, the following equation can be derived:

Tb = (De*pbme) / 4π (brake torque)

Brake mean effective pressure:pbme = (120,000*Pb) / (De*Ne)

Where pbme = brake mean effective pressure (kPa)

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Friction Mean Effective Pressure

pfme = (120,000 Pf) / De*NeWhere pfme = friction mean effective pressure (kPa)

For a compression-ignition (CI) engine, the pfme is primarily a function of engine speed:

pfme = A0 + A1(Ne/1000) + A3(Ne/1000)2

Note: A0, A1 and A3 are engine specific constants

Indicated, Mechanical, and Brake Thermal Efficiencies

Indicated thermal efficiency:eit = Pi / Pfe

Mechanical efficiency:em = Pb / Pi

Brake Thermal Efficiency:ebt = Pb / Pfe

Specific Fuel Consumption

Fuel consumption, either in kg/h or L/h, is not a good measure of engine performance; such fuel consumption depends not only on the engine efficiency, but also on the engine displacement and load. Therefore, specific fuel consumption (SFC) has been developed as a measure of engine performance.

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Specific Fuel Consumption

XSFC = mf / Px (kg/kWh)

XSFC = 3600/(Hg*ex)

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Where:X is replaced by I, B, D, P depending on whetheryou are interested in Indicated, Brake, Draw-Bar, or PTO based specific fuel consumption.

i.e. : ISFC represents the indicated specific fuel consumption

Engine Torque Generation

Tb = Ti –Tf ; Where, Tb = brake torque (N*m)Ti = indicated torque (N*m)Tf = friction torque (N*m)

Ti and Tf are defined as Ti = ((Hg * eit)/0.06π)*(mf / 2Ne)Tf = (De * pfme) / (4π)

The indicated torque is nearly proportional to the amount of fuel used per engine cycle. The friction torque varies proportional with the friction mean effective pressure that is a function of engine speed.

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