# Question about Otto Cycle: Why Does Compression Increase Temperature?

• kamranonline
In summary, during an isentropic compression in the Otto cycle, the temperature of a gas increases due to the change in internal energy. This is because the change in internal energy is proportional to the temperature and a positive external work (compression) increases the internal energy.
kamranonline
I have a general question about otto cycle. If we heat a gas with constant volume its temperature rise according to PV=mRT. However during Isentropic compression(delta Q and delta S is zero), the volume is becoming less and pressure is increasing but i don't know why the temperature should increase during this process. According to the above equation the temperature can increase or decrease but i don't know why it always increases..

The internal energy of an ideal gas is proportional to its temperature. The change of internal energy in a process is

$$\Delta U = Q + W$$

where Q is the heat gained from the surroundings and W is the work of the external forces.
When Q =0, a positive external work (compression) will increase the internal energy, so the temperature will increase, too.

ehild

Thanks alot... failed to think from internal energy's perspective.

## 1. Why is compression necessary in the Otto Cycle?

The Otto Cycle is a thermodynamic cycle used in internal combustion engines. Compression allows for the air-fuel mixture to be compressed into a smaller volume, increasing its temperature and pressure. This results in a more efficient and powerful combustion process.

## 2. How does compression increase temperature in the Otto Cycle?

When the air-fuel mixture is compressed, its molecules are forced closer together, resulting in an increase in temperature. This is known as the adiabatic compression process, where no heat is exchanged with the surroundings.

## 3. What happens to the temperature during the compression stroke in the Otto Cycle?

The temperature of the air-fuel mixture increases during the compression stroke due to the decrease in volume. This is in accordance with the ideal gas law, where an increase in pressure and decrease in volume leads to an increase in temperature.

## 4. Does increasing compression ratio always lead to a higher temperature in the Otto Cycle?

In theory, yes. However, in practical engines, there are limitations to the compression ratio that can be achieved without causing engine knocking or pre-ignition. In these cases, increasing the compression ratio may not necessarily lead to a higher temperature.

## 5. Is there a limit to how much temperature can be achieved by compression in the Otto Cycle?

Yes, there is a limit known as the autoignition temperature. This is the temperature at which the air-fuel mixture will spontaneously ignite without the need for a spark. Beyond this temperature, the compression stroke can lead to engine knocking and damage.

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