How to calculate thermal efficiency of IC engine? [with Pdf]

The thermal efficiency of the IC engine is the ratio of work output to heat supplied to the engine.

To calculate the thermal efficiency of an IC engine, it is necessary to have power produced by the engine and the heat supplied to the engine. But before starting, see types of thermal efficiency and decide which type of thermal efficiency you want to find.

There are two types of thermal efficiency of IC engines, which are as follows:-

  1. Indicated thermal efficiency
  2. Brake thermal efficiency

Let’s see the meaning and how to find these two types of thermal efficiencies in IC engines,

1] Indicated thermal efficiency:-

The indicated thermal efficiency is the ratio of the indicated power (Power produced by burning fuel inside the engine cylinder) to the heat energy supplied to the engine.

Indicated thermal efficiency shows that how much heat supplied is engine converts into the indicated power.

The Indicated thermal efficiency of the engine is always less than 100% due to the losses of energy produced.

The indicated thermal efficiency is given by,

ITE = `\frac{\text{Indicated power}}{\text{Heat energy supplied}}`

Heat energy supplied = Mass of fuel (m) x Calorific value of fuel (Cv)

ITE = `\frac{IP}{m\times Cv}`

Where,
IP = Indicated power in KW
Mass of fuel = m in Kg/s
Calorific value of fuel = Cv in Kj/Kg

The indicated power is given by,

Indicated Power = `\frac{P_{m}.L.A.n.K}{60}` KW

Where,
L = Stroke length (m)
A = Cross-sectional area of piston
n = Number of power strokes per minute
= `\frac{N}{2}` for 4 stroke engine
= N for 2 stroke engine
`P_{m}` = Mean effective pressure inside cylinder (Kpa)

2] Brake thermal efficiency:-

The brake thermal efficiency is the ratio of the brake power (Power available at the engine crankshaft) to the heat energy supplied to the engine.

Brake thermal efficiency is also known as the Overall thermal efficiency.

The brake thermal efficiency is given by,

BTE = `\frac{\text{Brake power}}{\text{Heat energy supplied}}`

Heat energy supplied = Mass of fuel (m) x Calorific value of fuel (Cv)

BTE = `\frac{BP}{m\times Cv}`

Where,
BP = Brake power KW
Mass of fuel = m in Kg/s
Calorific value of fuel = Cv in Kj/Kg

Brake power is given by,

Brake power = `\frac{2\pi NT}{60}` KW

Where,
N = Speed of crankshaft (Rpm)
T = Torque of a crankshaft (Nm)

Hence using these formulas we can easily find the theoretical thermal efficiencies of the engine.

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