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Common Terminology of Engines.


It is the influence which tends to change the motion or direction of a body at rest or in motion. A simple explanation is pushing or pulling.

From the above, applying a force would either:

·      Start moving a body from rest or bring a moving body to rest.

·      Increase or decrease the speed of a moving body.

·      Change the direction of motion of a moving body.

Force is measured in newtons (N).



is the use of energy to overcome resistance. The amount of work done is from moving an applied force through a distance. The unit of measurement of doing work is the joule.

The force is measured in newtons (N) and the distance is measured in metres (m). From the formula Work = Force x Distance, work would be in newton metres (Nm). To prevent confusion between 'work' and 'torque', the unit given to the formula for work is the joule (j).

One newton metre = one joule.


is when a force tends to cause a movement about a point. Torque is also called a turning or twisting effort. Torque = Force x Distance. Torque is the force exerted, but notmoved, over a distance.

Force is measured in newtons (N) and distance is measured in metres (m). Torque is therefore measured in newtonmetres (Nm).

As an example, the force on the piston of an engine exerts a turning moment on the crankshaft.



is the amount of work done or energy expanded in a given time. Also expressed as the capacity to do work. Watt (W) is the unit measurement of power. A watt is the power used when energy is expended or work done at the rate of one joule per second.

Power = Force x Distance

              Time in seconds

As force is in newtons (N), distance in metres (m), and time in seconds (s), the answer will be in newton metres per second or joules per second. (1 newton metre = 1 joule). However, as one joule per second = one watt, the final answer will be in watts.

Power of an engine is measured in kilowatts (kW) rather than watts (W). 1000 W = 1 kW.

Thermal efficiency

Thermal efficiency is the ratio of work done at the flywheel to the amount of energy contained in the fuel. Thermal efficiency is expressed as a percentage.

Calorific value

Fuel contains a specific amount of heat energy or heat value which is released when the fuel is burnt. This is the calorific value of the fuel. It is measured in joules per kilogram of fuel.

Volumetric efficiency

is the ratio between the  swept volume of a cylinder and the actual volume of air drawn in during the induction stroke. The efficiency varies considerably, depending on the design and operating conditions but especially with engine speed. A turbo charged engine will have a higher volumetric efficiency (in excess of 100%) than that of a normally aspirated engine (less than 100%). Swept volume is the volume in the cylinder between TDC and BDC of the piston.


also called swirl, is the circular movement of the air as it enters the combustion chamber. The swirling motion or turbulence is encouraged by design considerations as it enhances flame propagation and is especially important at light engine loads. It is a desirable characteristic in the flow of air into the cylinder. In most engines, a rapidly swirling motion is deliberately induced and the violent movement helps ensure even mixing of the fuel and air. It also speeds up the combustion process once the fuel has ignited.


is the term used for eliminating the burned exhaust gases from a cylinder. The incoming air removes, or scavenges, as much of the burnt gases as possible. Valve overlap assists in the scavenging process.

Compression ratio

is the ratio between the volume of the air before and after it has been subject to compression. A compression ratio of 12:1 means that during the pistons travel from the lowest to the highest point in the cylinder, the air has been compressed to one-twelfth its original volume. A diesel engine needs a high compression ratio to get sufficient heat in the compressed air to ignite the fuel.


  ratio   = piston displacement + clearance volume

                           clearance volume

Valve overlap

is the period which both the inlet valve and exhaust valve are open at the same time. The inlet valve opens before top dead centre (TDC), say at 10° and the exhaust valve closes after TDC, say at 35°. The opening of the inlet valve overlaps the closing of the exhaust valve. The overlap in this case would be 35° .

The purpose of valve overlap is to ensure that are exhaust gases are discharged from the cylinder and the cylinder receives a fresh charge of air to make it more efficient when combustion next takes place. It also has a cooling effect.

Valve rotators

are devices which cause a valve to rotate each time it opens. It can be fitted to either end of the valve spring. Its purpose is to ensure even wear and prevent exhaust valves from burn out.


is the angle that the valve remains in the fully open position. The profile of the lobe of the cam causes the valve to open until the lobe flattens out. The valve stays in this fully open position which is the angle of dwell until the other side of the lobe is reached when the valve starts to close.


Cam lift

is the distance from the peak of the lobe of a cam to its axis minus the distance from the back of the cam to its axis. Another description would be the distance the valve opens plus the valve lash or tappet clearance measurement.


Cam profile


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