What is Otto Cycle?

The name Otto is not picked from any physics book nor it is any phenomenon. Otto was the last name of Engineer Nikolaus Otto who created the very first working four-stroke engine. Four-stroke engines with spark plugs are also called as Otto engines.
Otto cycle is basically the thermodynamic cycle on which a four-stroke engine works. It is usually represented with a Pressure-Volume diagram shown below.

The above diagram explains the changes in pressure and volume which take place while the combustion chamber is at work. All the four-strokes, intake, compression, combustion (power) and exhaust stroke contribute to the reciprocating motion of pistons in some way.

Let’s understand these four-strokes according to Otto cycle (refer figure above):
1. Intake Stroke – During the intake stroke, the pistons go down and take inside air-fuel mixture. While doing this, there is no change in pressure as the change in volume keeps pressure constant (Isobaric Process is 0-1).

2. Compression Stroke – In this stroke, pistons move from Bottom Dead Centre(BDC) to Top Dead Centre(TDC). During this time, no change in heat is there (Adiabatic Process is 1-2) but a change in pressure and volume is found.

3. Combustion Stroke – This is the Power Stroke which gives power to the engine. At the initial stage of this stroke, fuel is combusted because of increasing heat at constant volume (Isochoric Process is 2-3). But later, as we know, increased heat and constant volume will increase pressure, so, the pressure pushes the piston down which creates the power for engine and during this stage, since, volume and pressure are changing, so heat remains constant (Adiabatic Process is 3-4).

4. Exhaust Stroke – At the initial stage of this stroke, when the piston is at BDC, at constant volume, air loses heat, so, change in heat takes place at constant volume (Isochoric Process is 4-1). After that, the unburnt fuel goes out from valves as pistons come up keeping the pressure constant.