Comparison of Diesel Cycle and Otto in Four-Stroke Motor

Comparação entre ciclo Diesel e Otto

Interactive comparison which shows the relationship between the various systems of an internal combustion engine with fuel injection ignition which develops the Diesel cycle and the relationship between systems of an internal combustion engine with spark ignition that develops the Otto cycle and plots synchronously in two diagrams Pressure x Volume approximations of the idealized theoretical curves of theoretical Diesel cycle and Otto cycle, allowing us to compare the cycles with greater ease.


– Keep rolling the page and go to a summary table to understand the comparative aspects of the Diesel cycle and Otto cycle.

Comparison of the curves in the diagram Pressure Volume of Diesel and Otto cycles

Description and Comparison of Theoretical Curves in the PV Diagram of Diesel and Otto Cycles

Curves Comparison
Diesel Cycle Otto Cycle
The compression ratio is too high, part of the mechanical energy of the piston is transferred to the air by increasing its temperature enough to ignite the injected fuel. Compression of the air-fuel mixture without thermal energy exchange with the environment. Compression
The burning of the fuel is slow, the piston moves to expand the volume increases at constant pressure by supplying torque. A spark initiates combustion, explosion increases the pressure and the temperature, pushes the cylinder providing power. Expansion
End of combustion, reduces pressure without exchange of thermal energy, the increase in volume. Opening of the exhaust valve from the combustion gases escape and the pressure within the cylinder retracts the atmosphere. Thermal energy transfer
Providing Heat
Opening of the exhaust valve, the gases escape and the pressure within the cylinder retracts the atmosphere.
The work performed by this cycle is greater than in the Otto cycle, the area of the graph of the interior is bigger, and the torque is also bigger, which enables i.e. the speed of fuel combustion, slow, not that it be injected. The fuel injection is to prevent it from burning before the critical moment (pre-ignition). The rapid burning of the fuel provides the main feature of engines operating under this cycle, the power and fast speed variation. Fuel is flammable at room temperature the engine operates at much lower compression ratios to Diesel cycle, allowing lighter and less complex engines.
Eduardo Stefanelli

Engenheiro por profissão, professor por vocação