In the following animation you can see the four strokes of an Otto engine. Most of modern mixture-compressing engines work after this plan.
Otto engine: Mode of operation
1. intake stroke: The air-fuel mixture is sucked in the cylinder by the piston sliding downward.
2. compression stroke: The piston sliding now upward compresses the mixture using work of the crankshaft.
3. power stroke: In the upper dead-center, the mixture is ignited. The pressure of the burning mixture pushs the piston back into the cylinder: Work is performed.
4. ejection stroke: The burned exhaust is ejected by the rising piston through a second valve.
After the publication of "Ottos new engine" by the gas engine factory "Deutz" in Germany and as "Otto silent" in England, problems appeared that probably emerge with each ingenious invention. Because of the relatively simple mode of operation, many factories began to copy the engine. Nikolaus Otto got problems with patents, and when it became public that already Beau de Rochas had invented a plan for a four-stroke engine earlier, Otto couldn't hold his patent. An incomparable competition began, the otto engine was developed very quickly therefore, better accessories (carburetor, ignition mechanism, valves, pistons) were produced, the cylinder capacity, engine speed and the horsepower performances were increased dramatically. Otto experienced a 100 HP Otto engine before he died (His first four-stroke engine had a power of approx. 3 HP)
Source: Helmut Hütten, "Motoren", Motorbuchverlag Stuttgart, Cover
Fuel question
The question of the fuel interested not only the engineers before Otto, but himself, too. After a strong occupation with the combustion process for his first four stroke engine, he was now able to burn the gas he used in a regulated way. 1883, Karl Benz and Gottlieb Daimler, who are still very famous because of their cars, built an engine, where gasoline is induced into the induction air through a surface carburetor. For the first time people succeeded in using liquid fuels. A large step was done toward the automobile with this, because liquid fuel needs less space than gaseous and can be transported more easily.
sv, ohs, ohv
The sv-engine
In the sv-engine the camshaft is stored laterally, the valves likewise. sv stands for side valve. This type of engine is considered as outdated and is practically not used any more. The combustion chamber is not compact since the valves are attached laterally, and therefore it is quite difficult to manage a regular burn. Of course the second valve is not missing, but it can't to be seen behind the other one.
Animation: sv-engine
The ohv engine
Like the sv-engine, the ohv engine (over head valve) has a lateral camshaft, which can be constructed more easily than overhead camshafts (see ohc). These "head-controlled" engines permit more compact combustion chambers. The disadvantage of this design are the long levers which are acting very slowly and develop strong mass forces.
Animation: ohv-engine
The ohc-engine
The most modern, most efficient, but alst the most complicated construction uses the ohc engine (overhead camshaft). The camshaft is situated directly over the valves. Sometimes even two seperated camshafts are used to control the valves individually. Since there is a considerable distance from the camshaft to the crankshaft, the connection between those has been a problem for a long time. Meanwhile there exist solutions with cogwheel-, chain-, "kingshaft"- or belt-transmission. (mode of operation see Otto engine animation above).
