Aircraft Engine Performance, II
In a piston engine chemical energy is converted into heat energy and then into mechanical energy 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. The power of an engine is a way of measuring the rate at which it is doing work.
On this page we dive into how power is developed and show you some action into engine efficiency, it will involve some basic physics as we used to learn at school.
Engine specification is usually given in power (kW and HP) and sometimes is torque is given. With the explanation of Force, Work and Power ofn the previous page we expand that knowledge a little.
About horsepower and Torque
There seem to be a bit of confusion about what torque and horsepower exactly is so I have included an explanation of both and the results of torque on a propeller.
Horsepower and Torque
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 the SI system its equal to 746 Joules/s or 746 W.
Torque is defined as a force which produces (or tends to do so) a rotating force on a object. This can be the piston rotating the crankshaft connected to the propeller. Torque is also measured in Nm (Newton meter) and is the same as Work. Engine torque will rotate the propeller and the propeller, while creating thrust (lift and drag) will resists that engine torque with propeller torque (drag). If engine and propeller torque are in balance the RPM is constant.
Horsepower versus Torque
As stated above, power is the rate at which the torque 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 from SAE, 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 fast a car, boat or airplane accelerates up to a required speed.
Diesels and torque
The diesel engine is better suited for producing torque than producing power. 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. Diesel fuel contains more BTU energy per volume than gasoline.
The extra heat available allows for a larger percentage of expansion than gasoline does. This greater 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), increasing engine life.
Torque and Propellers
For aircraft useage, higher torque means that you can turn a larger propeller with wide turboprop like blades which produces higher thrust at lower RPMs with less noise than smaller propellers that turn at higher RPMs.
The larger propellers can give faster take-off and better climb performance as well as better cruise. Propellers are more efficient at lower RPM levels of 1600 - 2000.
