© 2012 Delmar, Cengage Learning Engine Size and Measurements Chapter 17

© 2012 Delmar, Cengage Learning

Engine Size and Measurements

Chapter 17


Objectives• Describe various ways of measuring engine size• Understand the effects of engine compression

ratio• Explain the principles of engine power and

efficiency• Relate torque to horsepower• Understand the variations in operation of the

different types of dynamometers


Introduction• This chapter provides an understanding of:

– Various engine size and performance measurements

– Methods of understanding and measuring engine power output


Engine Size Measurements• Engine’s size

– Determined by volume of air pistons displace in cylinders

• Cylinder bore size– Diameter of the cylinder

• Usually three and a half to four inches

– Average engine has a three and a half to four inch stroke


Engine Size Measurements (cont’d.)

• Displacement– Piston or cylinder displacement

• Volume displaced by the piston• Bore2 x Stroke x 0.7854

– Engine displacement• Bore2 x Stroke x 0.7854 x number of cylinders




Compression Ratio• Determines how much air and fuel are

compressed – Compression ratio is 8:1

• Higher compression ratio increases power and fuel economy

• Increases exhaust emissions• Each point of change is said to be worth about

four to six percent change in horsepower

• Compression pressure – Pressure made by piston moving up in cylinder

• Gasoline engines typically produce 125-175 psi


Compression Ratio (cont’d.)• Effective compression ratio

– Determined by point at which intake valve closes


Physical Principles of Work• Force: any action that tends to change the

position of something– Measured in pounds or Newtons

– Push, pull, or lift

• Work: when an object is moved against a resistant or opposing force– Lifting or sliding

– English measurements: foot-pounds or watts

– Metric measurements: Newton-meters or joules

– Force x Distance = Work



Physical Principles of Work (cont’d.)

• One foot-pound: one pound is moved for a distance of one foot

• Energy: ability to produce motion against resistance

• Inertia: tendency of a body to keep its state of rest or motion– Larger masses are affected more by inertia

• Momentum: product of the body’s mass and speed

• Power: how fast work is done


Torque• Torque

– Tendency of a force to rotate a body on which it acts

• Amount of turning force exerted by crankshaft

– Engine torque varies with rpm• High at lower speeds

• Heat – Measured in Btu

– One Btu is the amount of heat required to heat one pound of water by one degree



Horsepower• Measurement of an engine’s ability to perform

work in a specified time– One horsepower equals 33,000 foot-pounds of

work per minute

• Amount of work required to life 550 pounds one foot in one second– One horsepower equals 0.746 kilowatts

• Gross horsepower – Power produced at crankshaft

• Several measurements



Horsepower (cont'd.)• Accessories that rob power (absorb about 25%)

– Alternator

– Air conditioning

– Water pump

– Cooling fan

– Power steering

– Smog pump

• Net power is what remains• Power is also lost through friction


Dynamometer• Measures engine output

– Engine dynamometer• Horsepower coming out of the engine

– Chassis dynamometer• Horsepower available at vehicle’s drive wheels

• Maximizing horsepower and torque– Valve overlap

• Period when intake and exhaust valves are open

– Scavenging • Vacuum draws in fresh air and fuel through

open intake valve




Dynamometer (cont’d.)• Types of dynamometer power absorption units

– Automotive dynamometer power absorption units • Electromagnetic and water brake

• Measuring torque and horsepower– Horsepower: (Torque x rpm) / 5250

– Torque readings: made at every 500 rpm

• Horsepower correction factors and comparisons– Correction factors: compensate for high-altitude

air or hot air

– Comparisons: done with same dynamometer



Dynamometer Safety Concerns• Engine dynamometer

– Concerns: fire, part failure, and noise

• Chassis dynamometer – Concerns: carbon monoxide, keeping the vehicle

secured and connected to rollers, part failure, and noise

• Other dynamometer types– Towing dynos

– Cycle dyne


Engine Efficiency• Engine efficiency measurements:

– Mechanical efficiency• Describes all ways friction is lost in the engine• Engine output divided by engine input

– Volumetric efficiency

– Thermal efficiency

• Amount of loss– Difference between efficiency measurement and

100%



Volumetric Efficiency• Compares volume of airflow entering the engine

with theoretical maximum– Determines maximum torque output

• Changes with:– Temperature

– Engine speed

– Load

– Throttle opening

• Increased speed and closing the throttle– Lowered efficiency


Thermal Efficiency• Ratio of how effectively an engine converts a

fuel’s heat energy into usable work– Gasoline’s thermal energy: 19,000 Btu per pound

• Brake thermal efficiency: more useful– (Brake HP / fuel’s heat input) x 100

• Spark ignition: only one-fourth of the energy from burning fuel is converted to work

• Diesel fuel: higher heat energy and engines have a higher compression ratio– Better fuel economy



Mean Effective Pressure• Pressure within the cylinder

– Increases during compression stroke• Highest after ignition

• Peak cylinder pressure – Should occur between ten and 20 degrees past

TDC

• Combustion pressure moves the piston down– Pressure drops as cycle continues

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© 2012 Delmar, Cengage Learning Engine Size and Measurements Chapter 17