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Lister Petter Diesel Engine

Engine Performance, II

In a piston engine chemical energy is converted into heat energy by combustion and then into mechanical energy by the pistons to rotate the crankshaft. The heat produced by the combustion process causes an increase in cylinder pressure and this forces the piston down and finally rotating the propeller.

On this page we explain the difference between power and torque with an example. This subject will involve some basic physics as we used to learn at school.

Engine specification is usually given in power (kW or even HP) and sometimes is torque is given, when we're lucky and the engine manufacturer is willing to provide the data in the specification sheets.
With the explanation of Force, Work and Power of the previous page we expand that knowledge a little.

Engine horsepower and Torque

There seem to be a bit of confusion about what torque and horsepower exactly are, so I have included an explanation of both and what the results of having more or less torque has on a propeller.


Horsepower is used to indicate how much power an engine can develop. One HP is equal to an average horse (!) pulling 550 lb per foot per second. In our modern SI system it is equal to 746 Joules/s or 746 W.


Torque is defined as a force which produces a rotating force on a object (or tends to do so). In our case this can be the piston rotating the crankshaft connected to the propeller. Torque is also measured in Nm (Newton meter) and this is the same as Work. Engine torque will rotate the propeller and the propeller, while creating thrust (propeller lift) will resists that engine torque with propeller torque (propeller drag). If engine and propeller torque are in balance the RPM is constant (Newton's first law).


If moving a box of 25 kg over 1 meter (25 kgm) takes you one hour and your copilot does it in 30 minutes. Who has more power? You both have the same torque (25 kgm) and have done the same amount of work. But your copilot has more power as he does it in half the time. Thus he/she has twice the power.

Horsepower vs Torque

As stated on the previous page, power is the rate at which torque (work) is produced (pound-feet per second or Newton meters per second). If two objects produce the same torque, the faster rotating object has more power.

According to experts of the SAE (Society of Automotive Engineers): one horsepower (1 HP) equals 550 ft-lb. per second or 33,000 ft-lb. per minute. Another familiar formula is the one which states RPM x Torque / 5252 = horsepower.
Torque is the measurement of the strength of the rotational movement and determines how a car, boat or airplane accelerates up to a required speed. The time this takes is determined by the amount of power.

Diesels and Torque

The diesel engine is better suited for producing torque than power at lower RPMs, its slower burning fuel is the reason. The force driving the engine pistons downward is the expansion of the air/fuel mixture within the cylinders. A greater expansion means more force on the piston. Also, diesel fuel contains more BTU energy per volume than gasoline.

More expansion

The extra heat available allows for a larger percentage of expansion than gasoline does. This increased expansion produces more force to push the piston downward (thus more torque).

Diesel fuel does not burn as rapidly as the more volatile gasoline. The relatively slower expansion limits the maximum piston speed and this results in a lower maximum engine speed (RPM), this wil increase engine life.

Aircraft Propeller

Torque and Propellers

For aircraft useage, having an engine with higher torque rating means that you can turn a larger propeller with turboprop like wide blades producing higher thrust at lower RPMs resulting in less noise than those smaller propellers that rotate at higher RPMs. You will climb much better!

These larger propellers are able to produce shorter takeoff distances, better climb performance as well as better cruise in terms of fuel consumption (lower RPM). Propellers are also more efficient at lower RPM levels of around 1600 - 2000.

Written by EAI.

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