Aircraft Engine Performance
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.
Some background
For some understanding how engines create power we need to go back to basic physics once again...
Force, Work and Power
To get an object to change its speed or direction we need to act on that object with a force. The formula is: F = m x a, where m is mass and a is acceleration. Force is expressed in Newtons or kgm/s2.
Work is done when a force on a object causes it to move. So force over a distance is work. In a formula: W = f x d, where f is force and d is distance. Work is expressed in Newton x meters, Nm. It is the same as Joule and used to be called ft/lb.
Power is simply the rate of doing work. Thus the amount of work in a certain time period. Formula: P = W / t, where W is work and t is time. Power is expressed in watts (W) and equivalent of 1 Joule per second (J/s).
Horsepower, Energy 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.
Energy is the capacity to do work and expressed in Joules. One kg of gasoline or JET contains about 44 MJ, but as JET is denser it has more energy per liter. Hence the longer range a diesel engine powered aircraft has compared to an avgas model with the same fuel tanks.
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 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.
Measuring engine power
To determine the power an engine can deliver it is installed on a test bank and the torque is measured at certain RPMs. It is known as brake horsepower, BHP. The formula used to calculate the BHP is BHP = ((2 PI RPM) / 60) x Torque. This is the power at the propeller shaft. The engine itself produces more somewhat power as it must overcome its internal friction. Which is measured by connecting an electric motor to the engine (at operating temps) and measuring the power required to rotate the engine. Engine oil, design of the engine and accessories all determine the power lost to friction.
The rated power is normally expressed in a form like: 160 BHP at 2700 RPM. Rated altitude is the altitude where rated power is developed with full throttle, which will be at MSL for a normally aspirated engine and at a higher altitude if the engine is super- or turbocharged.
Engine efficiency
Efficiency is rate of energy in the fuel used and useful work done by the engine. So Brake Thermal Efficiency = Brake Power (joules/sec) / Fuel Consumption (joules/sec). Or put differently: Brake Specific Fuel Consumption = Fuel Flow / Brake Power in kg/hr per watt, or lb/hr per BHP. AVgas (or gasoline) engines have efficiencies of about 20 - 30% where as diesels can run from 30% for light weight four stroke diesels and up to 50% for large two stroke marine diesels.
Volumetric efficiency
Another type of efficiency important to engines is volumetric efficiency. This relates to the volume of fuel/air the engine breathes at intake stroke. A normal aspirated engine pumps the mixture in by opening the intake valve and the downward motion of the piston. The design of the intake manifold and enertia of the mixture results in less mixture taken in than there is room for when the piston is at bottom dead center.
The highest volumetric efficiency is obtained with a full open throttle and low RPM. And ofcourse using a super- or turbocharger will help too.
To conclude: for an engine to to achieve the best BSFC it must be run at high MAP, low RPM, be at full throttle height, mixture leaned and carb heat set to cold.
About horsepower and Torque
As a diesel engine generally has more torque than a gasoline engine of the same horsepower rating. I have included an explanation of both and the results of torque on a propeller.
Horsepower versus Torque
Simply defined, 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.
The diesel engine is better suited for producing torque than producing power. The force driving the engine's 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 (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 turbo-prop 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.
