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The Otto cycle can be seen in Figure 2 (real Otto Cycle) and Figure 3 (ideal Otto Cycle). The changes in these will create heat, and use this heat to move the vehicle or machine (hence why it's a type of heat engine). The pressure volume diagram (PV diagram) that models the changes the fuel-air mixture undergoes in pressure and volume in a four stroke engine is called the Otto cycle. A higher ratio will allow a larger fuel-air mixture to enter, causing a higher pressure, leading to a hotter chamber, which increases thermal efficiency. This ratio is the difference between the minimum and maximum volume in the engine chamber (seen as TDC and BDC on figure 2). One way thermodynamic efficiency can improve in engines is through a higher compression ratio. However in general, gasoline engines convert 20% of the fuel ( chemical energy) to mechanical energy-in which only 15% will be used to move the wheels (the rest is lost to friction and other mechanical elements). The thermal efficiency of these gasoline engines will vary depending on the model and design of the vehicle. The remaining exhaust gas is pushed out by the piston as it moves back upwards. Exhaust stroke: As the piston reaches the bottom, the exhaust valve opens.Power Stroke: As the fuel reaches the end of it’s combustion, the heat released from combusting hydrocarbons increases the pressure which causes the gas to push down on the piston and create the power output.At the end of this stroke, a spark plug provides the compressed fuel with the activation energy required to begin combustion. Compression stroke: The intake valve is closed, and the piston moves up the chamber to the top.Intake stroke: The piston moves downward to the bottom, this increases the volume to allow a fuel-air mixture to enter the chamber.There is an animation to the right (Figure 1) of a four-stroke engine and further explanation of the process below. A four stroke engine delivers one power stroke for every two cycles of the piston (or four piston strokes).
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The four-stroke engine is the most common types of internal combustion engines and is used in various automobiles (that specifically use gasoline as fuel) like cars, trucks, and some motorbikes (many motorbikes use a two stroke engine).
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1:fuel injection, 2:ignition, 3:expansion(work is done), 4:exhaust.