Engine efficiency

Engine efficiency of thermal engines is the relationship between the total energy contained in the fuel, and the amount of energy used to perform useful work. There are two classifications of thermal engines-
(1) Internal combustion (gasoline, Diesel and gas turbine, ie., Brayton cycle engines).
(2) External combustion engines (Steam piston, steam turbine, and the Stirling cycle engine).

Each of these engines has thermal efficiency characterisics that are unique to it.

Modern gasoline motors, have an average efficiency of about 25% when used to power an automobile. In other words, of the total energy of gasoline, 75% is consumed by the motor itself and dissipated in the form of heat and only 25% of energy moves the vehicle. At idle and slow speed the efficiency is much lower than average and improves considerably at open road speeds. Diesel motors are more efficient. The most efficient type, direct injection Diesels, are able to reach an efficiency of about 40% in the engine speed range of idle to about 1,800 RPM. Beyond this speed, efficiency begins to decline due to air pumping losses within the engine.

The efficiency depends on several factors, one of them is the compression ratio; most gasoline engines, have a ratio of 10:1 or 8:1 with some high performance engines reaching a ratio of 12:1. The greater the ratio the more efficient is the machine. Higher ratio engines need fuel with higher octane value.

Diesel engines have a compression ratio between 14:1 to 25:1. In this case the general rule does not apply because Diesels with compression ratios over 20:1 are indirect injection Diesels which use a prechamber to make possible high RPM operation as is required in automobiles and light trucks. The thermal and gas dynamic losses from the prechamber result in direct injection Diesels despite their lower compression ratio being more efficient. An engine has many parts that produce friction and this friction increases at high RPM. A motor is more efficient at low RPM than at high RPM. The loss of efficency as RPM rise becomes proportionately greater due to air pumping losses which increase much faster than friction losses.

Piston steam engines are very inefficient which is why there are no longer any steam locomotives in commercial use. Large output steam turbines equal or exceed the efficiency of the Diesel, which is why they are used for electric utility generating plants. The Stirling engine has the highest efficiency of any thermal engine but it is very expensive to make and is not competitive with other types for normal commercial use.

The gas turbine is most efficient at maximum power output. Efficiency declines steadily with reduced power output and is very poor in the low power range. This is one reason, among several, why the gas turbine is not used for automobiles and trucks where much of the operating cycle is at idle and low to intermediate speeds. Detroit at one time tried to make a gas turbine for an automobile and gave up. This is also why gas turbines can be used for peak power electric plants. In this application they are only run at full power where they are efficient or shut down when not needed.