5 types of Engine Efficiencies you need to know »

Engine efficiency refers to how well an engine converts the energy in its fuel into useful mechanical power.

Usually, we break engine efficiency down into several different measures:

Thermal Efficiency
Indicated Thermal Efficiency
Brake Thermal Efficiency
Volumetric Efficiency
Mechanical Efficiency
Overall Engine Efficiency

Also read: Factors affecting Efficiency of an engine.

1. Thermal Efficiency

Represents the fraction of heat added by the fuel that’s converted into mechanical work.

What it means: How effectively the engine turns the fuel’s chemical energy into work inside the cylinders (before mechanical losses).

Measures how well the engine converts the heat energy from fuel combustion into mechanical work.

Internal combustion engines (ICEs) usually have 25–40% thermal efficiency, while advanced diesel engines can reach ~45–50%.

For real engines we split this into indicated (inside the cylinder) and brake (at the crankshaft), below.

Higher compression ratios typically enable greater thermal efficiency.
Diesel (typically 35–45%) converts more than petrol (typically 25–35%)

2. Indicated Thermal Efficiency

Represents the ratio of power produced by combustion in the cylinders (before mechanical losses) to the total energy provided by the fuel.

Indicated Thermal Efficiency is the ratio of the indicated power (IP) developed inside the engine cylinder to the energy supplied by the fuel.

It shows how effectively the engine converts the chemical energy of the fuel into power inside the cylinder, before mechanical losses (like friction and pumping) are considered.

where:

IP = Indicated Power (kW) → power developed inside the combustion chamber
mf˙ = Fuel mass flow rate (kg/s)
CV = Calorific value of the fuel (kJ/kg)

3. Brake Thermal Efficiency

Represents the ratio of power available at the output (after mechanical losses) to the total energy provided by the fuel.

Brake Thermal Efficiency is the ratio of the brake power (BP) (the actual usable power available at the crankshaft) to the energy supplied by the fuel.

It tells us how efficiently the engine converts the chemical energy in fuel into usable mechanical work at the output shaft.

where:

BP = Brake Power (kW) → power available at the crankshaft
mf˙= Fuel mass flow rate (kg/s)
CV = Calorific Value of the fuel (kJ/kg)

4. Volumetric Efficiency

Represents how much air enters the cylinder during each intake stroke — normally measured against its theoretical maximum.

Indicates how effectively the engine can fill its cylinders with fresh air (or air–fuel mixture).

Higher volumetric efficiency = better combustion and higher power output.

Forced induction (turbocharging, supercharging) improves this.

Higher volumometric efficiency means more air-fuel mixture can be introduced for combustion — yielding greater power.

5. Mechanical Efficiency

Represents the ratio of power output (at the crankshaft) to the power developed within the cylinders.
This measures losses due to friction, bearings, and other resistance.

Ratio of the brake power (actual usable power at the crankshaft) to the indicated power (power developed inside the cylinders).

Accounts for mechanical losses due to friction, pumping, and auxiliary loads.

Usually falls in range from 80–90%.

6. Relative Efficiency:

Relative efficiency (also called efficiency ratio) is the ratio of the actual thermal efficiency of an engine to the air-standard efficiency (ideal cycle efficiency).

It shows how close a real engine performs compared to its ideal thermodynamic cycle (Otto cycle for SI engines, Diesel cycle for CI engines).

where:

η_actual = Actual thermal efficiency of the engine (Brake or Indicated)
η_air-standard= Efficiency of the corresponding air-standard cycle

7. Overall Engine Efficiency

Overall Engine Efficiency (sometimes called fuel conversion efficiency) is the ratio of the useful power output of the engine (brake power) to the total energy supplied by the fuel.

It is essentially the same as Brake Thermal Efficiency (BTE), because it represents the net efficiency of the whole engine system (from combustion → piston → crankshaft output).

Overall engine efficiency is basically the net efficiency at the output shaft, equivalent to Brake Thermal Efficiency (BTE) — it shows how much of the fuel’s energy is actually turned into usable work.

Overall, for internal combustion engines:

✅ Petrol (SI) typically falls in range 25–35%.
✅ Diesel (Compression Ignition) typically falls in range 30–40%.
✅ Large, heavy-duty diesel engines can reach up to 50%. with advanced technologies.

FAQ Section:

1. What is engine efficiency?
Engine efficiency is the ratio of useful output (mechanical work at the crankshaft) to the energy input from fuel. It shows how effectively an engine converts chemical energy into usable work.

2. What is thermal efficiency in engines?
Thermal efficiency measures how much of the heat energy released from fuel combustion is converted into work. Internal combustion engines typically have 25–40% thermal efficiency.

3. What is the difference between indicated thermal efficiency and brake thermal efficiency?

Indicated Thermal Efficiency (ITE): Based on power developed inside the cylinder (before friction).
Brake Thermal Efficiency (BTE): Based on actual usable power at the crankshaft (after friction and losses).

4. What is mechanical efficiency?
Mechanical efficiency is the ratio of brake power (usable output) to indicated power (developed inside cylinder). It accounts for frictional and mechanical losses.

5. What is volumetric efficiency?
Volumetric efficiency measures how effectively the cylinder fills with fresh charge (air/fuel mixture). Turbocharging and supercharging improve it.

6. What is combustion efficiency?
Combustion efficiency indicates how completely the fuel is burned. Poor combustion reduces power and increases emissions.

7. What is overall engine efficiency?
Overall efficiency (essentially Brake Thermal Efficiency) is the net efficiency of the engine, showing how much of the fuel’s energy becomes useful work at the output shaft.

8. What is relative efficiency?
Relative efficiency is the ratio of actual thermal efficiency to ideal air-standard cycle efficiency (Otto/Diesel cycle). It shows how close an engine is to theoretical limits.

9. Why is engine efficiency less than 100%?
Because of inevitable losses such as: