In this article, you’ll learn what are different engine performance parameters. Explained in detail with definitionexample, and equations.

And the PDF of this article is provided at the end of it.

What is Engine Performance Parameters?

This article describes different terms relating to engine measurement and performance these include not only the physical measurements such as cylinder diameter, length of the piston stroke, cylinder volume, and so on but also the engine rating, efficiency, heat balance, etc., to study the engine performance.

Engine Performance Parameters

Following are the different parameters of engine performance:

  1. Work
  2. Energy
  3. Power
  4. Horse Power
  5. Torque
  6. Bore and Stroke
  7. Piston displacement
  8. Engine displacement
  9. Compression Ratio
  10. Indicated Horse Power (I.H.P)
  11. Brake Horse Power (B.H.P)
  12. Frictional Horse Power (F.H.P)
  13. Indicated Thermal Efficiency
  14. Brake Thermal Efficiency
  15. Mechanical Efficiency
  16. Volumetric Efficiency
  17. Relative Efficiency
  18. Mean Effective Pressure
  19. Mean Piston Speed
  20. Specific Power Output
  21. Specific Fuel Consumption
  22. Air-Fuel Ratio
  23. Calorific Value of the Fuel

#1 Work

When an object is moved by the application of a force, work is said to be done. It is measured by the product of the distance the body moves and the force applied to it. Thus,

Work = Distance X force,

If the distance is measured in meters, and force in kilogram, the unit of work will be m-kg.

#2 Energy

Energy is the term for the ability or capacity to do work. This is done on an object, energy is stored in that object. If a 10-kilogram weight is lifted by 5 meters, the weight will store 50 m-kg of work. Similarly, if the spring is compressed, energy is stored in it. and it can do work.

#3 Power

Power is the rate of doing work. The work can be done slowly or rapidly. The rate at which the work is done is measured in terms of power. A horse can do the work slowly, while a machine can do a great amount of work in a short time.

#4 Horse Power

A horsepower (h.p.) is the power of one horse, or a measure of the rate at which a horse can work, A 10 h.p. engine, for example, can do the work of 10 horse.

A horsepower is 75 m-kg/sec or 4500 m-kg/min shown in figure. As illustrated, the horse walks 50 meters in one minute, lifting the 90 kg weight. The amount of work done is 50X90-4500 m-kg/min.

Horsepower is the bigger unit of power. Power is generally expressed in horsepower

1 Horse Power = 75 m-kg/sec = 4500 m-kg/min.

#5 Torque

Torque is the twisting or turning effort to the product of the force and its perpendicular distance to the point of rotation. This may or may not result in speed. Power is something else again. It is the rate at which work is being done, and this means that something must be moving.

Torque is measured in kg-m (not to be confused with m-kg of work). If a 20 kg force is applied on the shaft through a crank radius of m to rotate a wheel, you would be applying 20x ½ = 10 kg-m of torque to the crank.

You would be applying the torque regardless of whether or not the crank was turning, and so long as you continued to apply the 20 kg force to the crank handle.

#6 Bore and Stroke

It is the inner diameter of the cylinder, which is known as a bore. Stroke is the distance the piston travels from the bottom dead center to the top dead center. The size of an engine cylinder is referred to in terms of the bore and stroke.

#7 Piston Displacement

Piston displacement is the amount displaced by the piston as it moves from its top dead center to the bottom dead center position in the engine cylinder. It determines the size of the piston in cubic centimeters. This volume depends on the cylinder bore and the piston stroke.

#8 Engine Displacement

Engine displacement has been defined as the total volume displaced by all the pistons as they move from their top dead center to bottom dead center position. It determines the size of an engine in cubic centimeters (cc). This volume depends on the cylinder diameter (bore). piston stroke and the number of cylinders. Thus,


  • D = Ddiameter of cylinder in cm
  • L = length of stroke in cm
  • N = No. of the cylinder.

#9 Compression Ratio

It is the ratio of the volume of the charge in the cylinder above the piston at the bottom dead center and the volume of the charge when the piston is at the top dead center.

Since the volume above the piston at the bottom dead center is the displacement of the cylinder plus the clearance volume and the volume above the piston at the top dead center is the clearance volume, the compression ratio can also be started as,

#10 Indicated Horse Power (I.H.P.)

The power actually developed inside the engine cylinder by the combustion of the fuel is called indicated horsepower. It is given by the relation.


  • P = mean effective pressure in kg\cm2 absolute.
  • L = It is the length of stroke in meters.
  • A = Area of a cross-section of the piston on cm2.
  • N = No. of revolutions of the crankshaft.

(In a 2-stroke engine, no. of explosions = no. of r.p.m. of the crankshaft)

(In a 4-stroke engine, no. of explosions = 1/2 x no. of r.p.m. of the crankshaft)

#11 Brake Horse Power (B.H.P.)

The power which the engine actually delivers to do the outside work is called brake horsepower. It is usually 70 to 85% of its indicated horsepower. It can be measured by some measuring instruments like prony brake or dynamometer, and is given by the following relation,


  • D = It is diameter of the brake drum = 2R, in meters
  • N = It is the no. of relvolutions/min of the crankshaft
  • W = Brake load, in kg.
  • S = Reading of Spring balance, in kg.

#12 Frictional Horse Power (F.H.P.)

Output power (or B.H.P.) of an engine is always less than the input power (or I.H.P.) because some power is lost in overcoming the friction between the moving parts.

The power lost in friction in the engine mechanism is called frictional horsepower. It is equal to the difference between the I.H.P. and B.H.P. Thus,

F.H.P. = I.H.P. – B.H.P.

#13 Indicated Thermal Efficiency

The quantity of power developed by the combustion of fuel in the cylinder is called the indicated power. And The actual amount of energy stored in the fuel = mass of fuel × calorific value of the fuel is known as fuel energy. The ratio between the Indicated Power to Fuel Energy is termed Indicated Thermal Efficiency.

#14 Brake Thermal Efficiency

The amount of power produced by a crankshaft is called brake power. The ratio between the Brake power to Fuel Energy is called Brake Thermal Efficiency.

#15 Mechanical Efficiency

Mechanical efficiency is defined as the ratio of power output to the power developed in a cylinder. In other words, it is the ratio of brake power to the indicated power.

#16 Volumetric Efficiency

The volume of the air-fuel mixture drawn into the cylinder at atmospheric pressure during the intake stroke compared to the volume of the cylinder is known as volumetric efficiency.

Volumetric efficiency for an engine running at a fairly high speed should be 80%. In some engines, it may drop at 50% at high speeds because the cylinders are half-filled at high speed.

#17 Relative Efficiency

Relative efficiency is the ratio of the actual cycle and the thermal capacity of the ideal cycle. It is also known as the Efficiency ratio.

#18 Mean Effective Pressure

It can be defined as the average pressure inside the cylinder of the internal combustion engine based on the resulting power output. For any type of engine, there will be two mean effective pressures. It is the Indicated mean effective pressure (pim) and brake mean effective pressure (pbm).

#19 Mean Piston Speed

The mean piston speed is the twice length of stroke and rotation speed of the crankshaft.

Mean Piston Speed = 2 x L x N


  • L = Length of stroke
  • N = Crankshaft speed in r.p.m.

#20 Specific Output Power

The specific Power output can be commonly defined as the power output per unit piston area.

Specific Power output (Ps) = Brake Power/Area of the piston

#21 Specific Fuel Consumption

It can be defined are the fuel consumption per unit time per unit power generated by the engine.

#22 Air-fuel Ratio

It is considered very important in the performance of the engine. In a spark-ignition engine, the air-fuel ratio will be the same for most operations.

Whereas in the compression engines, the fuel is entered individually with the help of a fuel injector so that if the load needs to be increased, the amount of fuel will increase directly into the cylinder.

#23 Calorific Value of the Fuel

The calorific value of the fuel can be defined as the amount of thermal energy delivered per unit quantity of fuel when it is fully burned.