chapter 16 engine size and performance measurements

© Goodheart-Willcox Co., Inc. Permission granted to reproduce for educational use only

PublisherThe Goodheart-Willcox Co., Inc.Tinley Park, Illinois

by

Russell Krick


q Engine size measurementq Force, work, and powerq Compression ratioq Engine torqueq Horsepowerq Engine efficiency


q Engine size is determined by the number of cylinders, the cylinder diameter, and the amount of piston travel per stroke

q Engine size information is used when ordering parts and when measuring wear during major repairs


Cylinder Bore

q Diameter of the engine cylinderqMeasured across the cylinder, parallel with the top of the block

q Cylinder bores vary in size, but typically range from 3–4" (75–100 mm)


Piston Strokeq Distance the piston moves from top dead center (TDC) to bottom dead center (BDC)

q Crankshaft journal offset (throw) controls the piston stroke

q Stroke varies from about 3–4"(75–100 mm)


Bore and Stroke


Piston Displacementq Volume the piston displaces as it travels from BDC to TDC

q Found by comparing cylinder diameter and piston stroke

q Piston displacement formula:

piston bore squared × 3.14 × strokedisplacement 4

=


If an engine has a bore of 4" and a stroke of 3", what is its piston displacement?

piston (42) × 3.14 × 3displacement 4

16 × 3.14 × 34

150.724

37.68 cu. in.

=

=

=

=

Piston Displacement


Engine Displacementq Volume displaced by all the pistons in an enginem piston displacement multiplied by the number of cylinders

q Units of engine displacement:m cubic inch displacement (CID)m liters (L)


Engine Displacement


Engine DisplacementIf one piston displaces 25 cu. in. and the engine has four cylinders, what is the engine displacement?

25 cu. in. × 4 = 100 cu. in.If one piston displaces 500 cc and the engine has six cylinders, what is the engine displacement?

500 cc × 6 = 3000 cc= 3.0 L


Force

q Pushing or pulling actionqMeasured in pounds or newtonsqWhen a spring is compressed, an outward movement and force is produced


Workq Occurs when force causes movementqMeasured in foot-pounds or joulesq Formula for work:work = distance moved × force applied

For example, if you use a hoist to lift a 400 pound engine 3 feet in the air, how much work has been done?work = 3' × 400 lb

= 1200 foot pounds (ft lb)


Powerq Rate, or speed, at which work is doneqMeasured in foot-pounds per second or per minute

q The metric unit for power is the watt or kilowatt

q Formula for power:power = distance × force

time


PowerIf an engine moves a 3000 pound car 1000 feet in one minute, how much power is needed?power = 1000' × 3000 lb

1 minute= 3,000,000 ft lb

min.


Compares the cylinder volume with the piston at TDC to the cylinder volume with

the piston at BDC


This engine has eight times the volume at BDC, producing an 8:1 compression ratio

Compression Ratio


q Formula for compression ratio:compression cylinder volume at BDCratio cylinder volume at TDC

q Use of high compression ratio:m increases engine fuel efficiency and power

m increases exhaust emissions (NOx)m increases risk of detonation

=

Compression Ratio


Compression Ratio(Gasoline Engine)


Compression Ratio(Diesel Engine)


Compression Pressureq Amount of pressure in the cylinder on the compression stroke

qMeasured in pounds per square inch (psi) or kilopascals (kPa)

q Gasoline engine compression pressure:m 130–180 psi (900–1200 kPa)

q Diesel engine compression pressure:m 250–400 psi (1700–2800 kPa)


Compression Gaugeq Used to measure compression pressure

q Using a compression gauge:m screw the gauge into the spark plug, injector, or glow plug hole

m crank the enginem note compression gauge readings

q Gauge readings are a good indicator of engine mechanical condition


q Rating of the turning force at the engine crankshaft

qWhen combustion pressure pushes the piston down, a strong rotating force is applied to the crankshaft


Engine Torque Specifications

q Given in a shop manualq Torque specification example:

m 450 ft lb @ 3000 rpmm engine would be capable of producing a maximum of 450 ft lb of torque at an operating speed of 3000 revolutions per minute

q The metric unit for engine torque is newton-meters (N•m)


qMeasure of an engine’s ability to perform work (power)

q At one time, one horsepower was the approximate strength of a horse


Horsepower

One horsepower equals 33,000 ft lb of work per minute


Horsepower

q Formula for horsepower:horsepower = work (ft lb)

33,000Or

horsepower = distance (ft) × weight (lb)33,000


HorsepowerFor a small engine to lift 500 pounds a distance of 700 feet in one minute, about how much horsepower would be needed?

hp = 700' × 500 lb 33,000

= 10.6 hp


Factory Horsepower Ratings

q Given in a shop manualq Automobile makers rate engine power at a specific engine speed

q Horsepower example:m 400 hp @ 5000 rpm


Brake Horsepower (bhp)

qMeasures usable power at the engine crankshaft

q Horsepower was first measured with a prony brake

q Braking was applied to the engine crankshaft, causing a force on a scale

q Horsepower was read from the needle deflection


Prony Brake

Measures engine horsepower


Engine Dynamometerq Used to measure the brake horsepower of modern car engines

q Either an electric motor or a fluid coupling is used to place a drag on the engine crankshaftm allows power to be determined


Engine Dynamometer

Using a dynamometer to measureengine performance


Chassis DynamometerqMeasures the horsepower delivered to the rear wheels

q Indicates the amount of horsepower available to propel the car

q Accounts for any power consumed by the drive train


Chassis Dynamometer


Indicated Horsepower (ihp)

q Refers to the amount of power formed in the engine combustion chambers

q Pressure-sensing equipment is placed in the cylinder

q Pressure readings are used to determine the indicated horsepower


Frictional Horsepower (fhp)

q Power needed to overcome engine friction

q Indicates the resistance to movement between engine parts

q Reduces the amount of power left to propel the car


Net Horsepowerq Also called SAE net horsepowerqMaximum power developed when an engine is loaded by all accessoriesm alternator, water pump, fuel pump, air injection pump, air conditioning, and power steering pump

q Indicates the amount of power available to move the car


Net Horsepower

Available horsepower with the engine operating all accessories


Gross Horsepower

Power available with only basic accessories installed–alternator, fuel

pump, and water pump


Taxable Horsepowerq General rating of engine sizeq In many U.S. states, it is used to find the tax placed on a car

q Taxable horsepower formula:

thp = bore squared × number of cylinders × 0.4


q Ratio of usable power at the crankshaft (brake horsepower) to the power supplied to the engine (heat content of fuel)

q By comparing fuel consumption to engine power output, you can find engine efficiency

qMost engines are about 20% efficient


Volumetric Efficiency

q Ratio of air drawn into the cylinder and the maximum amount of air that could enter the cylinder

q Indicates how well an engine can “breathe” on its intake stroke


Volumetric Efficiencyq Engines are capable of only 80–90% volumetric efficiency

q Restrictions in the ports and around the valves limit airflow

q Volumetric efficiency formula:

volumetric volume of air taken into cylinderefficiency volume of cylinder

=


Mechanical Efficiencyq Compares brake horsepower and indicated horsepower

qMeasurement of mechanical frictionqMechanical efficiency of 70–80% is normal

q 20–30% of the engine’s power is lost to friction (frictional hp loss)


Thermal Efficiencyq Found by comparing the horsepower output to the amount of fuel burned

q Indicates how well an engine uses the fuel’s heat energy

q Engine thermal efficiency is about 20–30%

q 70–80% of the heat energy is absorbed by the metal parts of the engine and blown out the exhaust

chapter 16 engine size and performance measurements

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