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PowerPoint for
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Chapter 16
Engine Size and Performance Measurements
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Contents

• Engine size measurement
• Force, work, and power
• Compression ratio
• Engine torque
• Horsepower
• Engine Efficiency

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Engine Size Measurement

• Engine size is determined by the number of
  cylinders, the cylinder diameter, and the amount
  of piston travel per stroke
• Engine size information is used when ordering
  parts and when measuring wear during major repairs

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Cylinder Bore

• Diameter of the engine cylinder
• Measured across the cylinder, parallel with the
  top of the block
• Cylinder bores vary in size from 34 (75100 mm)

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Piston Stroke

• Distance the piston moves from top dead center
  (TDC) to bottom dead center (BDC)
• Crankshaft journal offset (throw) controls the
  piston stroke
• Stroke varies from about 34(75100 mm)

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Bore and Stroke
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Piston Displacement

• Volume the piston displaces as it travels from
  BDC to TDC
• Found by comparing cylinder diameter and piston
  stroke
• Piston displacement formula

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Piston Displacement

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

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Engine Displacement

• Volume displaced by all the pistons in an engine
• piston displacement multiplied by the number of
  cylinders
• Units of engine displacement
• cubic inch displacement (CID)
• liters (L)

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Engine Displacement
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Engine Displacement

• If one piston displaces 25 cu. in. and the engine
  has four cylinders, what is the engine
  displacement?
• 25 cu. in. x 4 100 cu. in.
• If one piston displaces 500 cc and the engine has
  six cylinders, what is the engine displacement?
• 500 cc x 6 3000 cc 3.0 L

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Force, Work, and Power
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Force

• Pushing or pulling action
• Measured in pounds or newton
• When a spring is compressed, an outward movement
  and force is produced

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Work

• Occurs when force causes movement
• Measured in foot-pounds or joules
• Formula for work
• work distance moved x 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 x 400 lb 1200 foot pounds (ft lb)

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Power

• Rate, or speed, at which work is done
• Measured in foot-pounds per second or per minute
• Metric unit for power
• watt or kilowatt
• Formula for power
• power distance x force time

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Power

• If an engine moves a 3000 pound car 1000 feet in
  one minute, how much power is needed?
• power 1000 lb x 3000 1 minute
• 3,000,000 ft lb min.

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Compression Ratio

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

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Compression Ratio

• This engine has eight times the volume at BDC,
  producing an 81 compression ratio

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Compression Ratio

• Formula for compression ratio
• compression cylinder volume at BDCratio
  cylinder volume at TDC
• Use of high compression ratio
• increases engine fuel efficiency and power
• increases exhaust emissions (NOx)
• increases risk of detonation

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Compression Ratio(Gasoline Engine)
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Compression Ratio(Diesel Engine)
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Compression Pressure

• Amount of pressure in the cylinder on the
  compression stroke
• Measured in pounds per square inch (psi) or
  kilopascals (kPa)
• Gasoline engine compression pressure
• 130180 psi (9001200 kPa)
• Diesel engine compression pressure
• 250400 psi (17002800 kPa)

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Compression Gauge

• Used to measure compression pressure
• Using a compression gauge
• gauge is screwed into the spark plug, injector,
  or glow plug hole
• engine is cranked over
• gauge measures compression pressure
• Gauge readings are a good indicator of engine
  mechanical condition

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Engine Torque

• Rating of the turning force at the engine
  crankshaft
• When combustion pressure pushes the piston down,
  a strong rotating force is applied to the
  crankshaft

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Engine Torque Specifications

• Given in a shop manual
• Torque specification example
• 78 ft lb _at_ 3000 rpm
• engine would be capable of producing a maximum of
  78 ft lb of torque at an operating speed of 3000
  revolutions per minute
• Metric unit for engine torque
• newton-meters (Nm)

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Horsepower

• Measure of an engines ability to perform work
  (power)
• At one time, one horsepower was the approximate
  strength of a horse

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Horsepower

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

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Horsepower

• Formula for horsepower
• horsepower work (ft lb) 33,000
• Or
• horsepower distance (ft) x weight
  (lb) 33,000

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Horsepower

• For a small engine to lift 500 pounds a distance
  of 700 feet in one minute, about how much
  horsepower would be needed?
• hp 500 lb x 700 33,000
• 10.6 hp

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Factory Horsepower Ratings

• Given in a shop manual
• Automobile makers rate engine power at a specific
  engine speed
• Horsepower example
• 300 hp _at_ 5000 rpm

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Brake Horsepower (bhp)

• Measures usable power at the engine crankshaft
• Horsepower was first measured with a prony brake
• Braking was applied to the engine crankshaft,
  causing a force on a scale
• Horsepower was read from the needle deflection

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Prony Brake

• Measures engine horsepower

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Engine Dynamometer

• Used to measure the brake horsepower of modern
  car engines
• Either an electric motor or a fluid coupling is
  used to place a drag on the engine crankshaft
• allows power to be determined

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Engine Dynamometer
Fig 6

• Using a dynamometer to measureengine performance

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Chassis Dynamometer

• Measures the horsepower delivered to the rear
  wheels
• Indicates the amount of horsepower available to
  propel the car
• Accounts for any power consumed by the drive train

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Chassis Dynamometer
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Indicated Horsepower (ihp)

• Refers to the amount of power formed in the
  engine combustion chambers
• Pressure-sensing equipment is placed in the
  cylinder
• Pressure readings are used to determine the
  indicated horsepower

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Frictional Horsepower (fhp)

• Power needed to overcome engine friction
• Indicates the resistance to movement between
  engine parts
• Reduces the amount of power left to propel the car

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Net Horsepower

• Also called SAE net horsepower
• Maximum power developed when an engine is loaded
  by all accessories
• alternator, water pump, fuel pump, air injection
  pump, air conditioning, and power steering pump
• Indicates the amount of power available to move
  the car

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Net Horsepower
Fig 8

• Available horsepower with the engine operating
  all accessories

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Gross Horsepower

• Power available with only basic accessories
  installedalternator, fuel pump, and water pump

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Taxable Horsepower

• General rating of engine size
• In many U.S. states, it is used to find the tax
  placed on a car
• Taxable horsepower formula
• thp bore squared x number of cylinders x 0.4

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Engine Efficiency

• Ratio of usable power at the crankshaft (brake
  horsepower) to the power supplied to the engine
  (heat content of fuel)
• By comparing consumption to engine power output,
  you can find engine efficiency
• Most engines are about 20 efficient

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Engine Efficiency
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Volumetric Efficiency

• Ratio of air drawn into the cylinder and the
  maximum possible amount of air that could enter
  the cylinder
• Indicates how well an engine can breathe on its
  intake stroke

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Volumetric Efficiency

• Engines are capable of only 8090 volumetric
  efficiency
• Restrictions in the ports and around the valves
  limit airflow
• Volumetric efficiency formula

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Mechanical Efficiency

• Compares brake horsepower and indicated
  horsepower
• Measurement of mechanical friction
• Mechanical efficiency of 7080 is normal
• 2030 of the engines power is lost to friction
  (frictional hp loss)

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Thermal Efficiency

• Found by comparing the horsepower output to the
  amount of fuel burned
• Indicates how well an engine uses the fuels heat
  energy
• One U.S. gallon of gasoline has about 19,000 Btu
  (British thermal units) of heat energy
• One horsepower equals about 42.4 Btu of heat
  energy per minute

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Thermal Efficiency

• Thermal efficiency formula

• Engine thermal efficiency is about 2030
• 7080 of the heat energy is absorbed by the
  metal parts of the engine and blown out the
  exhaust