An engine is a device for converting stored energy into mechanical energy, capable of doing useful work. Most engines in use today are heat engines, or combustion engines. which convert heat into work, though the efficiency of this process, being governed according to the second law of thermodynamics is often very low. Heat engines are commonly classified according to the fuel they use (as in gasoline engine), by whether they burn their fuel internally or externally, or by their mode of action (whether they are reciprocating, rotary, or reactive). They are distinguished from electric motors which, in providing their power, do not directly alter the chemical or physical composition of a substance.

 

The rules governing the way in which heat can be converted into work were discovered through experience by early Engineers. Later these rules were formulated as the science of thermodynamics. By using them it is possible to calculate how much power an engine will produce, what fraction of the heat will be turned into work, and what might be done to improve its performance.

 

The first engines invented during the 18th century, burned coal to produce steam in a boiler. The steam was then used to drive the engine. An engine of this sort, in which the source of heat is outside the engine, is known as an external combustion engine. An example of this type is the steam turbine is shown in Figure 1.

 

In a steam engine, as used in steam locomotives, heat from burning fuel (coal or oil) boils water in a boiler. The pressure of the steam produced then forces a piston back and forth i a cylinder.

 

By far the most successful form of engine, however, is the internal combustion engine. In an engine of this sort fuel is burned and the expansion of gases produced is employed to drive a piston back and forth inside a cylinder and is transmitted via a crankshaft to drive the wheels.

 

Early gas engines

The internal combustion engine was invented in the second half of the nineteenth century. The first successful design was produced by a German engineer, Nicklaus August Otto (1832–1891), who used coal gas as his fuel. Otto's gas engine used the four-stroke cycle first proposed in 1862 by Alphonse Beau de Rochas – and so called because only one stroke in four produces power. The other three are taken up in drawing in the fuel, compressing it and finally, after it is burned, exhausting it.

 

Since only one stroke in four produces power the output of a single-cylinder four-stroke engine is very uneven and requires a heavy flywheel to smooth it out. Most practical four-stroke engines have more than one cylinder – usually four or six, but often as many as sixteen. Increasing the number of cylinders produces smoother power because each cylinder is arranged to produce its power in turn.

 

The internal combustion engine using the four-stroke cycle is the engine found in almost all cars and trucks and many motor cycles. Such engines will run on gas but are normally fuelled with a liquid fuel, using petrol (gasoline). The fuel is tuned into a vapor , mixed with air in the carburettor, and ignited inside the cylinders of the engine by the sparking plugs which are timed to produce a spark at exactly the right moment in the four-stroke cycle.

 

The diesel engine

Many internal combustion engines, including most of those used in modern commercial vehicles, employ a heavier petroleum fuel called diesel oil. This type of engine is named after the German inventor Rudolf Diesel (1858–1913). Instead of a carburettor to mix the fuel with air, diesel engine has a fuel injector which pumps a measured quantity of fuel into the cylinder at precisely the right moment. And instead of sparking plugs diesel engines depend on the compression of the gas in the cylinder to achieve ignition. As the pressure of a gas increases so does its temperature. In a diesel engine the fuel ignites spontaneously at the end of the compression stroke. For this reason a diesel engine is more correctly called a compression-engine engine.

 

Converting heat into work

Internal combustion engines do not convert heat into work efficiently. A typical car engine converts no more than a quarter of the energy contained in the fuel into useful work and even a highly developed engine is little better than 35 percent efficient. The explanation of this disappointing performance lies with the laws of thermodynamics.

 

The first law says that you cannot get more energy out of an engine as work than you put into as heat. The second law goes further, It shows that the amount of work produced by an engine is always less than the amount of heat supplied. In other words the efficiency of any type of engine is always less than 100 percent.

 

The efficiency of a heat engine depends on the difference in temperature between the heat used to drive it and the heat rejected as waste. The higher the temperature of the engine's working gas and the lower the temperature of the waste heat the more efficient the engine. Complete 100 percent efficiency can be achieved only if the waste heat is at absolute zero (–273°C). In practice, of course, this cannot be and thus the efficiency of a heat engine cannot be 100 percent. Under ideal circumstances a heat engine operating at 1,000°C (1,832°F) – possible for an internal combustion engine – and rejecting waste heat at normal exhaust temperatures (say 150°C (302°F)), has a theoretical efficiency of 67 percent.

 

The difficulties of designing a perfect engine mean that all actual engines fall well short of the theoretical efficiency. Friction, vibration, and the energy consumed by the engine itself in driving camshafts, fans, and so on, and losses in the transmission reduces the efficiency to much lower levels.