7 Measuring Engine Performance

Learning Objectives Define engine performance. Define and compute bore and stroke. Understand the concept of energy and differentiate between kinetic and potential energy. Understand the concepts of force and pressure. Explain the concepts of work, power, and torque. Understand how levers and belt-and-pulley, chain-and-sprocket, and gear systems provide mechanical advantage.

Learning Objectives Calculate an engine’s displacement and compression ratio. Differentiate between the various types of engine horsepower. Define and calculate engine torque. Explain volumetric efficiency, practical efficiency, mechanical efficiency, and thermal efficiency.

The Engine Small gas engines classified as: Heat engines Internal combustion engines Reciprocating engines Engine operation relies on inertia

Basic Terminology Performance Engine bore Stroke Top dead center (TDC) Bottom dead center (BDC) Crank offset Square Over square Under square

Basic Terminology

Science of Engine Performance Energy Force Pressure Work Power Torque

Energy Capacity to perform work Different classifications Potential energy Kinetic energy Different classifications Mechanical, chemical, thermal, electrical Cannot be created or destroyed Can be transformed

Force Pushing or pulling of one body on another Centripetal force Centrifugal force Many forces at work in engine Tensile stress

Pressure Measured in psi Formula: Formula for area of circle Pressure = Force/Area Formula for area of circle r2 or .7854D2 r = radius D = diameter

Example of Calculating Force from Pressure Cyl. pressure=125 psi Piston diameter=3 Piston area=7.0686in2 Total Force = 125 psi  7.0686in2 = 883.575 lb

Work Formula: If force produces no movement, no work done Work = Force  Distance If force produces no movement, no work done Measured in ft-lbs

Levers and Mechanical Advantage Change distance between force and load More work done per unit force Used to open and close valves Formula: MA = ED/RD MA = mechanical advantage ED = effort distance RD = resistance distance

Leverage

Power Rate at which work is performed Formula: Power = Work/Time Standard unit is horsepower 550 ft-lb/1 sec 33,000 ft-lb/min

Torque Ability of force to cause rotation Formula: Torque = Force  Radius Measured with Prony brake

Prony Brake

Wheel and Axle Similar to lever Provides mechanical advantage Formula: ME = R1/R2 ME = mech. advantage R1 = radius (load) R2 = radius (driven)

Pulley, Gear, and Chain Drive Systems Can increase speed or torque Ratio of teeth on gears Ratio of sizes of pulleys Ratio of teeth on sprockets Small driving large increases torque Large driving small increases speed

Measurements of Engine Performance Three ways of quantifying performance Power Torque Efficiency

Engine Displacement Measure of cylinder volume Cylinder displacement formula: 0.7854  D2  Stroke Engine displacement formula: Cylinder displacement  Number of cylinders

Compression Ratio Comparison of cylinder volume at TDC and BDC Typically 5:1 or 6:1 As high as 9:1 or 10:1 in some engines

Compression Ratio (Briggs and Stratton Corp.)

Engine Horsepower Formula: Different types of horsepower Horsepower = Rate of Work / (550 ft-lb/sec) Different types of horsepower Measured in different ways

Brake Horsepower (bhp) Measured at crankshaft Prony brake Dynamometer Increases with engine speed Drops off at very high speeds

Calculating Brake Horsepower Formula: bhp = (R  L  W)/(5252 ft-lb/min) R = engine speed L = length from center of flywheel to scale W = force registered on scale

Indicated Horsepower (ihp) Mean effective pressure (mep) Formula: ihp = PLANK/33,000 P = mep (psi) L = piston stroke (ft) A = cylinder area (in2) N = rpm/2 (4-stroke) K = number of cylinders

Frictional Horsepower (fhp) Frictional loss Depends on engine design Varies with engine speed Generally about 10% Formula: fhp = ihp  bhp

Corrected Horsepower Standard brake horsepower Dry air, 60F, 29.92 Hg Conditions affect horsepower 3.5% loss per 1000 above sea level 3.5% loss per 1 Hg drop in pressure 1% loss per 10F increase in temperature 10% loss if head temp increases by >200F

Rated Horsepower Determines maximum load for continuous operation Typically 80% of maximum bhp (Briggs and Stratton Corp.)

Correction Factor Three corrections multiplied together Temperature correction Pressure correction Humidity correction Corrected horsepower formula: Corrected hp = correction factor  test hp

Temperature and Pressure Correction Ideal temp: 60F Ideal press: 29.92 Hg Correct for test conditions Test temp: 90F Test press: 28.5 Hg (Go-Power Corp.)

Humidity Correction (Go-Power Corp.)

Engine Torque Increases with engine rpm Reaches maximum volumetric efficiency Occurs at different speed in different engines Piston moves faster than cylinder can be filled Drops as speed continues to increase Drops off at lower rpm than horsepower

Torque and Horsepower (Clinton Engine Corp.)

Volumetric Efficiency Theoretical volume vs. actual volume Changes with engine speed Increases with rpm to max, then falls off quickly Can be affected by many factors

Volumetric Efficiency

Practical Efficiency How efficiently engine uses fuel Take into account all losses

Mechanical Efficiency Typically about 90% Frictional losses about 10% Formula: ME = bhp/ihp ME = mechanical efficiency bhp = brake horsepower ihp = indicated horsepower

Thermal Efficiency Percent of heat used to drive piston down Heat losses Cooling Exhaust Lubrication Typically about 20% to 25%

Thermal Efficiency Formula: TE = (bhp  33,000 ft-lb/min)/C  FHV  W TE = thermal efficiency C = 778 Btu/ft-lb FHV = fuel heat value W = weight of fuel burned/min

What does the term engine performance refer to What does the term engine performance refer to? How efficiently an engine performs work.

An single cylinder engine has a bore of 3. 5 and a stroke of 2. 625 An single cylinder engine has a bore of 3.5 and a stroke of 2.625. What is the engine displacement? Displacement = .7854  (3.5 in)2  2.625 in = .7854  12.25 in2  2.625 in = 25.2555 in3

Is expanding combustion gas an example of potential energy or kinetic energy? Kinetic energy

At the top of the compression stroke, cylinder pressure is 80 psi At the top of the compression stroke, cylinder pressure is 80 psi. If the engine has a bore of 3.25, what is the downward force on the piston? Force = pressure  area Force = 80 lb/in2  .7854(3.25in)2 Force = 80 lb/in2  .7854(10.5625in2) Force = 663.663 lb

An engine lists a 200 lb load a distance of 50 ft An engine lists a 200 lb load a distance of 50 ft. What additional information is needed to calculate the power of the engine? The amount of time it took to lift the weight.

The output gear on an engine’s output shaft has 10 teeth The output gear on an engine’s output shaft has 10 teeth. It drives a 30-tooth gear on the implement’s input shaft. What are the speed and torque of the implement’s input shaft in relation to the engine’s output shaft? Speed of the input shaft is 1/3 of the output shaft, and torque of the input shaft is 3 that of the output shaft.

An engine’s cylinder volume at BDC is 27 in3 An engine’s cylinder volume at BDC is 27 in3. If its compression ratio is 6:1, what is its cylinder volume when the piston is at TDC? 4.5 in3 (or 27 in3/6)

Which type of horsepower rating is based on the theoretical maximum power the engine can develop? Indicated horsepower

Why does engine torque begin to decrease after the engine reaches a certain speed? The piston moves so quickly that there is not enough time to completely fill the cylinder with air-fuel mixture.