Why is Otto cycle called constant volume cycle?

Question

I really don't know why they name it "constant volume". In a reciprocating engine, the piston moves up and down. The volume of the air always changes, so how do they call it "constant"?

Thankyou guys, I think I understand why they call that constant volume cycle. Constant volume happens at combustion process and exhaust process. In a very moment, the pressure and temperature change very quickly without volume change. The reason why they call it constant volume also because there is another cycle called constant pressure cycle; it is Brayton cycle for gas turbine engine. The constant pressure also happens at combustion and exhaust.
Hope you guys understand my English and my “kind” of knowledge. Thanks again for helping me.

Answer 1

The volume is not constant throughout the process. The Otto cycle is a COMBINATION of adiabatic processes (the expansion and contraction) and constant-volume processes where the temperature and pressure change so fast the cylinder essentially doesn't move while it's happening. This occurs at two points: at the end of the (adiabatic) compression cycle, the fuel/air mixture detonates which drives the temperature and pressure up almost instantaneously (hence constant volume), and the opening of the exhaust valve decompresses the cylinder so fast it's essentially instantaneous too. So the whole cycle is NOT constant volume, it's partly adiabatic and partly constant-pressure as well.

Answer 2

Otto Cycle

Answer 3

It's called a constant volume cycle because, to a first approximation, the heat energy is added at top dead center (TDC), when the volume is not changing. Engine cycles are studied by graphing pressure on the y-axis versus volume on the x-axis. This is called a P-V diagram. In a constant-volume cycle, you see the pressure increase dramatically at the minimum volume - see the graphic in the link. Compare this to a constant-pressure process, in which heat is added *after* TDC, resulting in a P-V diagram with a flat top. Diesel engines are modeled as a 'limited pressure' cycle, in which heat addition at constant volume is followed by some heat addition at constant pressure. You won't see a pure constant-pressure cycle in an internal combustion piston engine.

Answer 4

Please read the following:
*Stages of Otto Cycles:
Otto Cycles have four stages:
expansion, cooling, compression, and combustion.

*Expansion:

In the Otto cycle, fuel is burned to heat compressed air and the hot gas expands forcing the piston to travel up in the cylinder. It is in this phase that the cycle contributes its useful work, rotating the automobile's crankshaft. We make the ideal assumption that this stage in an ideal Otto cycle is isentropic.
Piston: moving from bottom dead center to top dead center.

*Cooling:

Next, the expanded air is cooled down to ambient conditions. In an actual automobile engine, this corresponds to exhausting the air from the engine to the environment and replacing it with fresh air. Since this happens when the piston is at the top dead center position in the cycle and is not moving, we say this process is isochoric (no change in volume).
Piston: at top dead center.

*Compression:

In preparation for adding heat to the air, we next compress it by moving the piston down the cylinder. It is in this part of the cycle that we contribute work to the air. In the ideal Otto cycle, this compression is considered to be isentropic.
It is at this stage that we set the volumetric compression ratio, r which is the ratio of the volume of the working fluid before the compression process to its volume after. It will turn out that the efficiency of the Otto cycle (assuming that air is an ideal gas) can be described entirely in terms of this ratio.

Piston: moving from top dead center to bottom dead center.

*Combustion:
Next, heat is added to the air by fuel combustion when the piston is at it bottom dead center position. Combustion is not initiated until a spark (from a spark plug, for instance) is generated in the cylinder. Because the piston is essentially immobile during this part of the cycle, we say that the heat addition is isochoric, like the cooling process.
Piston: at bottom dead center.