Engine Size and Performance Measurements

Engine Size Measurement

  • Engine size is determined by:

    • Number of cylinders

    • Cylinder diameter

    • Piston travel per stroke

  • Changing any of these variables alters engine size.

  • Engine size information is used for:

    • Ordering parts

    • Measuring wear during major engine repairs

Bore and Stroke

  • Cylinder bore: The diameter of the engine cylinder, measured across the cylinder, parallel with the top of the block.

    • Typical range: 3-4" (75-100 mm).

  • Piston stroke: The distance the piston moves from top dead center (TDC) to bottom dead center (BDC).

    • Controlled by the amount of throw (offset) built into the crankshaft rod journal.

    • Typical range: about 3-4" (75-100 mm) in automotive engines.

  • Shop manuals provide bore and stroke specifications (e.g., 4.00" x 3.00").

    • Bore is always the first value, and stroke is the second.

  • Generally, a larger bore and a longer stroke make an engine more powerful.

    • Allows more fuel and air to be drawn in during the intake stroke, resulting in greater combustion pressure.

  • Trend is toward smaller, more efficient engines to improve fuel economy and reduce emissions.

Piston Displacement

  • Piston displacement: The volume the piston displaces as it travels from BDC to TDC.

    • Determined by cylinder diameter and piston stroke.

    • A large cylinder diameter and large piston stroke produce a large piston displacement.

  • Formula for piston displacement:

    • pistondisplacement=bore2×3.14×stroke4piston displacement = \frac{bore^2 \times 3.14 \times stroke}{4}

  • Example: Engine with a 4" bore and 3" stroke:

    • pistondisplacement=(4")2×3.14×3"4=37.68cu.in.piston displacement = \frac{(4")^2 \times 3.14 \times 3"}{4} = 37.68 cu. in.

Engine Displacement

  • Engine displacement (engine size): The volume displaced by all the pistons in an engine.

    • Calculated by multiplying piston displacement by the number of engine cylinders.

  • Units of engine displacement:

    • Cubic inch displacement (CID)

    • Cubic centimeters (cc)

    • Liters (L)

    • 1 liter = 1000 cc

  • Engine displacement is usually matched to the vehicle's weight.

    • Heavy vehicles need larger engines for more power.

    • Light vehicles need smaller, low-power engines.

Force, Work, and Power

  • Force: A pushing or pulling action, measured in pounds or newtons.

  • Work: Done when force causes movement, measured in foot-pounds or joules.

    • Formula for work: work=distance×forcework = distance \times force

  • Example: Lifting a 400 lb engine 3 feet:

    • work=3′×400lb=1200ft⋅lbwork = 3' \times 400 lb = 1200 ft \cdot lb

  • Power: The rate 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×forcetimepower = \frac{distance \times force}{time}

  • Example: Hydraulic lift raises a 3000 lb car 8 feet in 30 seconds (0.5 minutes):

    • power=8′×3000lb0.5min.=48,000ft⋅lbminpower = \frac{8' \times 3000 lb}{0.5 min.} = 48,000 \frac{ft \cdot lb}{min}

Compression Ratio

  • Engine compression ratio: Compares the cylinder volume with the piston at TDC to the cylinder volume with the piston at BDC.

    • Indicates how much the air-fuel mixture is pressurized during the compression stroke.

  • Compression ratio is expressed as two numbers (e.g., 9:1).

    • 9:1 means the maximum cylinder volume is nine times larger than the minimum cylinder volume.

  • Maximum cylinder volume occurs at BDC; minimum occurs at TDC.

  • Example for a gasoline engine:

    • Volume at BDC: 40 cu. in.

    • Volume at TDC: 5 cu. in.

    • Compression ratio: 40 / 5 = 8:1

  • Engines in the 1960s had high compression ratios, designed for high-octane leaded gasoline (e.g., 11 or 12:1).

  • When lead was phased out, compression ratios were reduced to improve driveability, but this resulted in lost power and gas mileage.

  • Modern high-tech engines are designed with relatively high compression ratios again (e.g., 11 or 12:1 for naturally aspirated engines).

  • Supercharged and turbocharged engines have lower compression ratios (about 7 or 8:1) than naturally aspirated engines.

    • The supercharger or turbocharger forces air into the engine above atmospheric pressure.

    • Lower compression ratio allows a larger volume of air (oxygen) to be forced into the combustion chambers.

    • In effect, the supercharger or turbocharger raises the compression ratio and pressure in the engine cylinders to provide greater fuel-charge compression.

  • Diesel engines have very high compression ratios compared to gasoline engines (e.g., 17:1 to 25:1).

    • Diesel engines are compression-ignition engines, so the fuel charge must be pressurized until it is hot enough to burn.

    • No spark plug is used; compression pressure alone must start combustion.

Compression Pressure

  • Compression pressure: The amount of pressure produced in the cylinder during the compression stroke.

    • Measured in pounds per square inch (psi) or kilopascals (kPa).

  • A compression gauge is used to measure compression pressure.

    • The gauge is screwed into the spark plug, injector nozzle, or glow plug hole.

    • The ignition or injection system is disabled, and the engine is cranked over.

  • Typical compression pressures:

    • Gasoline engine: 130-180 psi (900-1200 kPa)

    • Diesel engine: 250-400 psi (1700-2800 kPa)

    • Turbocharged or supercharged engine: 110-150 psi during a compression test

  • Service specifications give the exact compression pressure for a new, unworn engine.

  • Compression pressure is an indicator of engine condition.

    • Low compression pressure indicates air leakage due to bad rings, burned valves, or a blown head gasket.

Engine Torque

  • Torque: A turning or twisting force.

  • Engine torque: A rating of the turning force at the engine crankshaft.

    • Combustion pressure pushing the piston down applies a strong rotating force to the crankshaft.

    • This turning force is sent to the transmission/transaxle and then to the wheels.

  • Engine torque specifications are given in a shop manual (e.g., 450 ft lb @ 3000 rpm).

  • In metrics, engine torque is often stated in newton-meters (N·m).

Horsepower

  • Horsepower (hp): A measure of an engine's ability to perform work (power).

    • Originated when James Watt compared the work done by a steam engine to that done by horses.

    • One horsepower is approximately equal to the amount of work done by a strong horse in one minute.

  • A 300 hp engine could theoretically do the work of 300 horses.

  • 1hp=550lb⋅ftsecond=33,000lb⋅ftminute1 hp = 550 \frac{lb \cdot ft}{second} = 33,000 \frac{lb \cdot ft}{minute}

  • Formula for horsepower:

    • hp=work33,000=distance×weight33,000hp = \frac{work}{33,000} = \frac{distance \times weight}{33,000}

  • Example: Small engine lifting 500 lb a distance of 700' in one minute:

    • hp=700′×500lb33,000=10.6hphp = \frac{700' \times 500 lb}{33,000} = 10.6 hp

Factory Horsepower Ratings

  • Automobile makers rate engine horsepower at a specific engine speed (e.g., 400 hp @ 5000 rpm).

  • Different methods of calculating engine horsepower:

    • Brake horsepower (bhp): Measures the usable power at the engine crankshaft.

      • Measured using a prony brake or an engine dynamometer (dyno).

    • Chassis dynamometer: Measures the horsepower delivered to the drive wheels.

      • Indicates the amount of horsepower available to propel the car.

    • Indicated horsepower (ihp): Refers to the amount of power formed in the engine combustion chambers.

      • Measured using a special pressure-sensing device in the cylinder.

    • Frictional horsepower (fhp): The power needed to overcome engine friction.

      • A measure of the resistance to movement between engine parts.