1. Engine Size and Measurements
Chapter 17

2. 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

3. Introduction This chapter provides an understanding of:
Various engine size and performance measurements
Methods of understanding and measuring engine power output

4. 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

5. Engine Size Measurements (cont’d.)
Displacement
Piston or cylinder displacement
Volume displaced by the piston
Bore2 x Stroke x
Engine displacement
Bore2 x Stroke x x number of cylinders

8. 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 psi

9. Compression Ratio (cont’d.)
Effective compression ratio
Determined by point at which intake valve closes

10. 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

12. 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

13. 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

15. 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 kilowatts
Gross horsepower
Power produced at crankshaft
Several measurements

17. 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

18. Dynamometer Measures engine output Maximizing horsepower and torque
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

21. 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

23. 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

24. Engine Efficiency Engine efficiency measurements: Amount of loss
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%

26. 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

27. 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

29. 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