There are several parameters which are used to determine the performance of an engine which are generally known as Engine Performance Parameters. These parameters include several efficiencies, ratios of different values etc. let us discuss each of these parameters one by one.

## Indicated thermal efficiency (η ith)

Indicated thermal efficiency is the ratio of indicated power (ip) and energy in fuel per second.

η ith= ip (KJ/s)/ energy in fuel per second (KJ/s)

Indicated power is defined as the sum of friction power and brake power

## Brake thermal efficiency (η bth)

Brake thermal efficiency can be defined as the ratio of brake power (bp) to input fuel energy in appropriate units.

η bth= bp (KJ/s)/ energy in fuel per second (KJ/s)

## Mechanical efficiency (η m)

Mechanical efficiency can be defined as the ratio of brake power to indicated power

η m= bp (KJ/s)/ ip (KJ/s)

Indicated power is defined as the sum of friction power and brake power

## Volumetric efficiency (η v)

Volumetric efficiency can be defined as the ratio actual volume of air-fuel mixture sucked by the engine to the actual volume of cylinder.

η v = actual volume of air-fuel mixture sucked by the engine/ actual volume of cylinder

## Relative efficiency or efficiency ratio (η rel)

Relative efficiency or efficiency ratio can be defined as the ratio of actual thermal efficiency to air- standard thermal efficiency.

η rel= actual thermal efficiency / air- standard thermal efficiency

## Mean effective pressure (mep)

Mean effective pressure can be defined as the ratio of area of indicator diagram to length of indicator diagram.

Mean effective pressure = area of indicator diagram / length of indicator diagram

## Mean piston speed

Mean piston speed is the product of twice of length of stroke and rotation speed of crankshaft.

Mean piston speed= 2LN

L = length of stroke

N = rotation speed of crankshaft in rpm

## Specific power output

Specific power output is defined as the power output per unit piston area.

## Specific fuel consumption (sfc)

Specific fuel consumption (sfc) can be defined are the fuel consumption per unit time per unit power produced by the engine.

sfc = fuel consumption per unit time/ power produced by the engine

## Equivalence ratio

Equivalence ratio can be defined as the ratio of actual fuel-air ratio to the stoichiometric fuel-air ratio.

Equivalence ratio = actual fuel-air ratio/ stoichiometric fuel-air ratio