In this type of engine a piston descends in a cylinder, drawing in a charge of fuel and air through an inlet valve after reaching the bottom of its stroke the piston rises in the cylinder with the valves closed and compresses the charge at or near the top of its stroke the charge is ignited by a spark and the resulting increase in pressure from the explosion forces the piston down again on the subsequent upstroke the exhaust valve opens and the burnt gases are pushed out of the combustion chamber.
The first working engine was the four-stroke Otto engine produced in 1876 by Nikolaus Otto (1832–91). A heat engine in which fuel is burned in combustion chambers within the engine rather than in a separate furnace (as with the steam engine).
