Engine Measurements

1
Engine Measurements

• Horsepower
• Torque
• Thermal efficiency
• Mechanical efficiency
• Volumetric efficiency
• Displacement
• Compression ratio

2
Horsepower

• Power the rate at which work is done
• Work distance x weight
• 1 horsepower 33,000 ft. lb. per minute
• Brake horsepower measure of actual usable
  horsepower delivered at crankshaft
• Gross horsepower max. hp. developed by engine
  equipped with accessories needed for operation
• Net horsepower max. hp. developed with all
  accessories normally installed in vehicle

3
Torque

• Torque twisting force
• Force x distance
• Measured in pound feet
• Engine torque is actual measurement of engines
  true power
• Peak torque is reached at engines maximum
  volumetric efficiency

4
Thermal Efficiency

• A measurement of how much energy provided by
  burning fuel, is used to produce horsepower
• Heat from burning fuel is lost to
• Cooling system
• Lubrication system
• Exhaust system
• Thermal efficiency of average engine is about 25

5
Mechanical Efficiency

• Relationship of power developed within engine and
  power delivered at crankshaft
• Mechanical efficiency is lost to internal
  friction and losses increase with speed
• Most engines have mechanical efficiencies of
  about 90

6
Volumetric Efficiency

• Measurement of engines ability to draw air and
  fuel into cylinders
• Determined by ratio of actual amount drawn in and
  actual cylinder volume
• As engine speed increases, volumetric efficiency
  decreases
• Affected by engine speed, intake design,
  temperature, valve size and position, exhaust
  design, atmospheric pressure

7
Displacement

• Total number of cubic inches or liters of space
  in a cylinder when piston moves from TDC to BDC
• Calculated by bore x stroke x number of cylinders
• More displacementmore power
• Bore diameter of cylinder
• Stroke distance piston travels from TDC to BDC

8
Displacement

• Piston speed decreases as stroke is decreased
• Cylinder, piston and ring wear is decreased as
  stroke is decreased
• Torque is increased when stroke is increased
• Most engines use a stroke that is slightly less
  than the cylinder bore

9
Compression Ratio

• Relationship between cylinder volume when piston
  is on TDC to cylinder volume with piston on BDC
• Average compression ratio for gasoline engine is
  8.51 (normal production engines can vary from
  81 to 111)
• Diesel engine compression ratios vary from 17.51
  to 22.51
• Increased compression ratios in gasoline engines
  result in detonation or self-ignition of fuel