Importance of Otto cycle which no one wants you to know!


This ideal air-standard cycle is named an Otto cycle, named after one in every of the first developers of this sort of engine.


The intake stroke of the Otto cycle starts with the piston at TDC and could be a constant-pressure process at an inlet pressure of 1 atmosphere (process 6-1).

This is an honest approximation to the inlet process of a true engine at WOT, which can actually be at a pressure slightly but atmospheric because of pressure losses within the inlet air flow.

The temperature of the air during the inlet stroke is increased because the air passes through the new manifold.

The temperature at point 1 will generally get on the order of 25° to 35°C hotter than the encircling air temperature.

The second stroke of the cycle is that the compression stroke, which within the Otto cycle is an isentropic compression from BDC to TDC (process 1-2).

This is an honest approximation to compression during a real engine, aside from the very beginning and also the very end of the stroke.

In a real engine, the start of the stroke is littered with the valve not being fully closed until slightly after BDC.

The end of compression is littered with the firing of the sparking plug before TDC. Not only is there a rise in pressure during the compression stroke, but the temperature within the cylinder is increased substantially thanks to compressive heating.

This replaces the combustion process of the real engine cycle, which occurs at near constant-volume conditions. in a very real engine combustion is started slightly bTDC, reaches its maximum speed near TDC, and is terminated a bit aTDC.

During combustion or heat input, an outsized amount of energy is added to the air within the cylinder.

This increase in temperature during a closed constant-volume process ends up in an outsized pressure rise also.

Thus, peak cycle pressure is additionally reached at point 3.

The very high and enthalpy values within the system at TDC generate the facility stroke (or expansion stroke) which follows combustion (process 3-4).

The power stroke of the important engine cycle is approximated with an isentropic process within the Otto cycle.

This is an honest approximation, subject to the identical arguments because the compression stroke on being frictionless and adiabatic.

In a real engine, the start of the ability stroke is stricken by the last a part of the combustion process.

The end of the ability stroke is stricken by the valve being opened before BDC.

During the ability stroke, values of both the temperature and pressure within the cylinder decrease as volume increases from TDC to BDC.

Near the tip of the facility stroke of a true engine cycle, the valve is opened and therefore the cylinder experiences exhaust blowdown.

A large amount of exhaust gas is expelled from the cylinder, reducing the pressure to it of the manifold.

The valve is opened bBDC to permit for the finite time of blowdown to occur.

Blowdown in a very real engine is therefore almost, but roughly, constant volume. an outsized quantity of enthalpy is anxious with the exhaust gases, limiting the thermal efficiency of the engine.

Enthalpy loss during this process is replaced with heat rejection within the engine analysis.

Pressure within the cylinder at the tip of exhaust blowdown has been reduced to about one atmosphere, and also the temperature has been substantially reduced by expansion cooling.

The last stroke of the four-stroke cycle now occurs because the piston travels from BDC to TDC.

Process 5-6 is that the exhaust stroke that happens at a relentless pressure of 1-atmosphere thanks to the open valve.

This is an honest approximation to the real exhaust stroke, which occurs at a pressure slightly on top of the encompassing pressure thanks to the tiny pressure drop across the valve and within the system.

At the tip of the exhaust stroke the engine has experienced two revolutions, the piston is again at TDC, the valve closes, the valve opens, and a replacement cycle begins.

When analyzing an Otto cycle, it’s more convenient to figure with specific properties by dividing by the mass within the cylinder.

It is not uncommon to seek out the Otto cycle shown with processes 6-1 and 5-6 left off the figure.

The reasoning to justify this can be that these two processes cancel one another thermodynamically and don’t seem to be needed in analyzing the cycle.

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