Technical Terms (part 2)

bore
The diameter of an engine cylinder.

stroke
The distance the piston moves from bottom dead center to top dead center.

displacement
The measurement of an engine's size. It is equal to the number of cubic inches the piston displaces as it moves from bottom dead center to top dead center, multiplied by the total number of cylinders.

Displacement = A x S x N

A = area of the piston (in square inches) S = stroke (in inches) N = number of cylinders

compression ratio

The extent to which the combustible gasses are compressed within the cylinder. It equals the volume existing within the cylinder with the piston at bottom dead center divided by the volume within the cylinder when the piston is at top dead center. Compression Ratio = X/Y

engine efficiency
The ratio of power obtained to power supplied. Many losses take place in gasoline engines going to things like cooling, exhaust, and engine friction. Usually only about 15% ends up being useful power.

mechanical efficiency
The relationship between brake horsepower and indicated horsepower. It is about 85% in typical scenarios.

Mechanical Efficiency = BHP/IHP

BHP = brake horsepower IHP = indicated horsepower

brake horsepower
The power available to propel a vehicle. It is equal to the indicated horsepower minus the remaining power after losses due to friction and to run the fan, water pump, and other components.

engine torque
Torque is turning effort, and in an automotive engine, this is produced by the pressure from the crankshaft on the pistons. Engine torque is relatively low at idle speeds, but increases at a high rate as engine speed increases. Torque is maintained at a high level while an engine is running, but decreases once engine speeds reach a certain point.

rated horsepower
The horsepower based on a formula developed in the early days of the industry, which assumes a mean effective pressure of 67.2 psi and a piston speed of 1000 rpm. The engines of today operate at much higher pressure and speeds, so the formula is no longer accurate. However, this rating is still used for licensing motor vehicles.

indicated horsepower
The method of rating an engine that is based on the actual power developed by the engine in an indicator diagram. This reading includes the power necessary to overcome engine friction. The indicator diagram is produced by an oscilloscope that makes a drawing of the events occurring within the cylinder. It records the pressure existing at each instant of a complete engine cycle from the time the combustible mixture is drawn into the cylinder until the completion of the exhaust stroke. The area of the diagram is proportional to the power generated.

IHP = (PxSxAxC)/33,000

IHP = indicated horsepower P = Mean effective pressure (in psi) S = Stroke (in feet) A = Area of the cylinder (in sq. ft.) N = Number of power strokes per minute C = Number of cylinders

friction horsepower
The power required to overcome the friction from within an engine.