How much energy reaches the flywheel (or dynamometer) compared to how much theo­retically could be released is a function of three efficiencies, namely, the following:

1. Thermal efficiency

2. Mechanical efficiency

3. Volumetric efficiency

Thermal Efficiency

Thermal efficiency can be quoted as either brake or indicated. Indicated efficiency is derived from measurements taken at the flywheel. The thermal efficiency sometimes is called the fuel conversion efficiency because it is defined as the ratio of the work produced per cycle to the amount of fuel energy supplied per cycle that can be released in the combustion process. The available fuel energy is obtained by multiplying the mass of fuel supplied by the heating value of the fuel, as


Tit =

MfQhv yfQhv


Wc = work per cycle mf = mass of fuel inducted per cycle Qhv = heating value of the fuel P = power output Ґ = fuel mass flow rate

Because specific fuel consumption (SFC) can be expressed as


SFC = -1 P

The equation for r|t can be rewritten as

SFC x Qhv SFC(g/kWh) x Qhv(MJ/kg)

Because Qhv for gasoline = 43.5 MJ/kg. Therefore, the brake thermal efficiency is 82.76.

Mechanical Efficiency

The mechanical efficiency compares the amount of energy imparted to the pistons as mechanical work in the expansion stroke to that which actually reaches the flywheel or dynamometer. Thus, it is the ratio of the brake power delivered by an engine to the indicated power

Brake power






Gross indicated power


Imep gross



Brake thermal efficiency = Indicated thermal efficiency * Mechanical efficiency

Volumetric Efficiency

The parameter used to measure the ability of an engine to breathe air is the volumetric efficiency, tiv. It is defined as the ratio

_ Mass of air inducted per cylinder per cycle

Mass of air to occupy swept volume per cylinder at ambient pressure and temperature

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