The efficiency of internal combustion engines depends on several factors, the most important of which is the expansion ratio.
The greater the expansion ratio, the more efficient the engine, in principle, and higher compression / expansion -ratio conventional engines in principle need gasoline with higher octane value, though this simplistic analysis is complicated by the difference between actual and geometric compression ratios.
In fact, lower-octane fuels, typically rated by cetane number, are preferable in these applications because they are more easily ignited under compression.
The thermal and gas dynamic losses from the prechamber result in direct injection diesels (despite their lower compression / expansion ratio) being more efficient.
Along with friction forces, an operating engine has pumping losses, which is the work required to move air into and out of the cylinders.
An excessively rich fuel to air ratio will increase unburnt hydrocarbon pollutants from the engine.
As combustion temperature tends to increase with leaner fuel air mixtures, unburnt hydrocarbon pollutants must be balanced against higher levels of pollutants such as nitrogen oxides (NOx), which are created at higher combustion temperatures.
With direct injection this effect is not as dramatic but it can cool down the combustion chamber enough to reduce certain pollutants such as nitrogen oxides (NOx), while raising others such as partially decomposed hydrocarbons.
The air-fuel mix is drawn into an engine because the downward motion of the pistons induces a partial vacuum.
At high speeds, efficiency in both types of engine is reduced by pumping and mechanical frictional losses, and the shorter period within which combustion has to take place.
[7] Approximately half of this rejected heat is carried away by the exhaust gases, and half passes through the cylinder walls or cylinder head into the engine cooling system, and is passed to the atmosphere via the cooling system radiator.
[8] Some of the work generated is also lost as friction, noise, air turbulence, and work used to turn engine equipment and appliances such as water and oil pumps and the electrical generator, leaving only about 20-40% of the energy released by the fuel consumed available to move the vehicle.
[citation needed] Mixtures with slightly less fuel, called lean burn are more efficient.
Modern turbo-diesel engines use electronically controlled common-rail fuel injection to increase efficiency.
General Motors at one time manufactured a bus powered by a gas turbine, but due to rise of crude oil prices in the 1970s this concept was abandoned.
[15] Steam engine efficiency improved as the operating principles were discovered, which led to the development of the science of thermodynamics.
Comparisons of efficiency and power of the early steam engines is difficult for several reasons: 1) there was no standard weight for a bushel of coal, which could be anywhere from 82 to 96 pounds (37 to 44 kg).
[15] The first piston steam engine, developed by Thomas Newcomen around 1710, was slightly over one half percent (0.5%) efficient.
[17] Higher-pressured engines were developed by Oliver Evans and Richard Trevithick, working independently.
These engines were not very efficient but had high power-to-weight ratio, allowing them to be used for powering locomotives and boats.
The valves were quick acting, which reduced the amount of throttling of the steam and resulted in faster response.
The variable cut-off was responsible for a major portion of the efficiency increase of the Corliss engine.
The higher speed minimized the amount of condensation in the cylinder, resulting in increased efficiency.
Using statistics collected during the early 1940s, the Santa Fe Railroad measured the efficiency of their fleet of steam locomotives in comparison with the FT units that they were just putting into service in significant numbers.
They determined that the cost of a ton of oil fuel used in steam engines was $5.04 and yielded 20.37 train miles system wide on average.
Steam power stations operating at the critical point have efficiencies in the low 40% range.
Turbines produce direct rotary motion and are far more compact and weigh far less than reciprocating engines and can be controlled to within a very constant speed.
For this reason, despite their high power to weight ratio, steam turbines have been primarily used in applications where they can be run at a constant speed.