Engine Performance Parameter

Indicated Power (IP)

The power developed inside the cylinders.
\[W_{net} = \frac{P_m V_s}{cycle}\]
\[IP = = \frac{P_m V_s}{cycle}\frac{N_{cycle}}{60}\]

• \(IP =  \frac{P_m ALNk}{60}\)                       for 2 stroke Engine

• \(IP =  \frac{P_m ALNk}{120}\)                       for 4 stroke Engine

Pm : Mean pressure 

Vs: swept volume =  AL

L : Stroke Length

N: rpm and k is the number of cylinders

Brake Power (BP)

Power available at output shaft.

\[BP = \frac{2 \pi N T}{60}\]

Frictional Power (FP)

FP = Indicated Power -  Brake Power

Mechanical Efficiency 

\[η_th = \frac{BP}{IP}\]

Thermal Efficiency 

• Indicated thermal efficiency  \(η_{ith} = \frac{IP}{m_f CV}\)

• Brake thermal efficiency  \(η_{bth} = \frac{BP}{m_f CV}\)

mf = mass flow rate (kg/s)

CV = Calorific value (J/Kg)

Indicated Specific Fuel Consumption(ISFC)

\[ISFC = \frac{m_f}{IP}\]

Volumetric Efficiency

It represents the breathing capacity of cylinder. It is the ratio of actual volume of the air entering the cylinder per cycle to the swept volume.

\[η_v = \frac{m_a}{ρ_a V_d}\]

$m_a$ : entered air (Kg/s)

Vd : displaced volume 

$ρ_a$ : density of air 

Morse Test (multi cylinder Engines)

The test consists of making inoperative in turn each cylinder of the engine and note the reduction in brake power developed. 

\[BP_{all} = Σ(IP_i - FP_i)\]

\[IP_i = BP_{all} - BP_{with-ith-cylinder-cut-off}\]

Willan's Line Test

A graph connecting fuel consumption (vertical axis) and  brake power (horizontal axis) at constant speed is drawn and is extrapolated on the negative axis of brake power. The intercept of the negative axis is taken as the friction power of the engine at speed. This method is also known as fuel rate extrapolation method.

In SI engine the variation of pumping loss from no load to full load very high. So power loss is not constant at different loads; Willan's test is not valid for SI engines.