Four-stroke Spark Motor – Otto Cycle

Motor quatro tempos ciclo Otto - Four-stroke engine Otto cycle - Motor de cuatro tiempos ciclo de Otto
Interactive animation of an internal combustion engine with spark the spark Otto cycle

Interactive animation demonstrating the relationships between the various systems of an internal combustion engine with spark ignition that develops the Otto cycle and plots in a diagram Pressure x Volume idealized approximations of the curves of the theoretical Otto cycle, allowing us to study the system more easily .

This type of engine uses as fuel: gasoline; alcohol and CNG – vehicular natural gas.


– It is possible to stop the animation, or to take it to a certain point, drag and drop the pointer of the diagram that marks the four times of the cycle, this action will show more information and supporting texts;

– Continue scrolling down the page and go to a synoptic table to understand the aspects of the Otto Spark cycle.

Description of internal combustion four-stroke engine Otto cycle

Theoretical description of the four strokes of an engine associated with Otto cycle


Name Description of the processes Valve





With the piston at TDC – upper dead point – the valve control opens the intake valve, the inertia of the crankshaft moves the rod that moves the piston on the way to the PMI – lower dead point – When moving, the piston reduces the pressure inside the cylinder and the created vacuum sucks the air-fuel mixture with virtually no change in pressure.

The cylinder volume is filled with fuel-air mixture to

approximately one atmosphere pressure.





Otto cycle Compression


With the piston at BDC the inlet valve closes and the crankshaft, which continues to turn, pushes the piston on the way to the TDC by compressing the air-fuel mixture in the combustion chamber, without any thermal energy exchange with the environment.

Some degrees of crankshaft rotation, before the piston reaches TDC, the air-fuel mixture is ignited by the spark created by the spark plug. The chemical process of combustion of the fuel-air mixture requires some time, constant, therefore it happens at the end of the second half and prospers when the engine speed increases -RPM.




Only providing mechanical energy



The combustion is fast and intense, an explosion, and instantaneously increases the pressure and temperature inside the combustion chamber; In an analogy, giving hitting a hammer to the piston’s head; pushing it to the PMI, forcing the crankshaft to turn, while providing power, at the only engine driving time of the cycle



Providing heat

(Isochoric) Late

Otto cycle

With the piston approaching the PMI the exhaust valve begins to open, the gases resulting from the combustion, which are at higher pressure and temperature than atmospheric, quickly escape to the exhaust manifold and the pressure within the cylinder drops to 1 Atmosphere, in fact, is what we hear in a well-regulated engine.

The third time is the only one that provides mechanical power to the motor cycle.

Start of opening




With the piston in the PMI and the exhaust valve open, the piston, driven by the inertia of the crankshaft, resumes its path towards the PMS by expelling the resulting gases from the cylinder with practically no pressure change.

Shortly before the cylinder reaches the PMS the intake valve begins to open, the difference in temperature and density between the intake and exhaust gases, and the inertia of the gases, carry the remnant of the burnt gases to the exhaust manifold and the exhaust valve closes in preparation for the restart of the four-stroke cycle


Start of opening

Top closure

Eduardo Stefanelli

Engenheiro por profissão, professor por vocação