internal combustion engine - heat engine driven directly by the expansion of combustion gases, rather than by an externally produced medium, such as steam. Basic versions of the internal combustion engine are: gasoline engine and gas engine (spark ignition), diesel engine (compression ignition), and gas turbine (continuous combustion). Diesel compression-ignition engines are more fuel-efficient than gasoline engines because compression ratios are higher, and because the absence of air throttling improves volumetric efficiency. Gasoline, gas (natural gas, propane), and diesel engines operate either on a four-stroke (Otto cycle) or a two-stroke cycle. Most gasoline engines are of the four-stroke type, with operation as follows: (1) intake - piston moves down the cylinder, drawing in a fuel-air mixture through the intake valve; (2) compression - all valves closed, piston moves up, compressing the fuel-air mixture, and spark ignites mixture near top of stroke; (3) power - rapid expansion of hot combustion gases drives piston down, all valves remain closed; (4) exhaust - exhaust valve opens and piston returns, forcing out spent gases. The diesel four-stroke cycle differs in that only air is admitted on the intake stroke, fuel is injected at the top of the compression stroke, and the fuel-air mixture is ignited by the heat of compression rather than by an electric spark. The four-stroke cycle engine has certain advantages over a two-stroke, including higher piston speeds, wider variation in speed and load, cooler pistons, no fuel lost through the exhaust, and lower fuel consumption. The two-stroke cycle eliminates the intake and exhaust strokes of the four-stroke cycle. As the piston ascends, it compresses the charge in the cylinder, while simultaneously drawing a new fuel-air charge in to the crankcase, which is air-tight. (In the diesel two-stroke cycle, only air is drawn in; the fuel is injected at the top of the compression stroke.) After ignition, the piston descends on the power stroke, simultaneously compressing the fresh charge in the crankcase. Toward the end of the power stroke, intake ports in the piston skirt admit a new fuel-air charge that sweeps exhaust products from the cylinder through exhaust ports; this means of flushing out exhaust gases is called "scavenging." Because the crankcase is needed to contain the intake charge, it cannot double as an oil reservoir. Therefore, lubrication is generally supplied by oil that is premixed with the fuel. An important advantage of the two-stroke-cycle engine is that it offers twice as many power strokes per cycle and, thus, greater output for the same displacement and speed. Because two-stroke engines are light in relation to their output, they are frequently used where small engines are desirable, as in chain saws, outboard motors, and lawn mowers. Many commercial, industrial, and railroad diesel engines are also of the two-stroke type. Gas turbines differ from conventional internal combustion engines in that a continuous stream of hot gases is directed at the blades of a rotor. A compressor section supplies air to a combustion chamber into which fuel is sprayed, maintaining continuous combustion. The resulting hot gases expand through the turbine unit, turning the rotor and driveshaft. See fuel injection, turbine.