In an Otto cycle, how does heat addition take place?
Reversibly at constant volume
Understanding the Otto Cycle Heat Addition Process
The Otto cycle is an idealized thermodynamic cycle that describes the functioning of a typical spark ignition piston engine. It is a useful model for understanding how these engines convert heat into work. The cycle consists of four internally reversible processes:
- Isentropic compression (adiabatic and reversible compression of the air-fuel mixture)
- Constant volume heat addition (combustion)
- Isentropic expansion (power stroke)
- Constant volume heat rejection (exhaust blowdown)
Ideal Otto Cycle: Heat Addition
In the ideal Otto cycle, heat addition occurs during the combustion phase. This process is modeled as happening instantaneously while the piston is momentarily at its highest point (Top Dead Center - TDC). Because the piston is not moving during this instant, the volume of the working fluid (air or air-fuel mixture) remains constant.
The heat addition process in the ideal Otto cycle is also assumed to be reversible. This is an idealization. In reality, combustion is a complex, irreversible process. However, for the purpose of analyzing the ideal cycle, it is assumed that the heat transfer occurs reversibly at a constant volume.
Therefore, in an ideal Otto cycle, heat is added to the system at a constant volume and is considered to be a reversible process.
Comparing Otto Cycle Processes
Let's look at the characteristics of the different processes in the ideal Otto cycle:
| Process | Description | Ideal Condition |
|---|---|---|
| 1-2 | Compression stroke | Isentropic (Reversible, Adiabatic) |
| 2-3 | Heat addition (Combustion) | Constant Volume, Reversible Heat Addition |
| 3-4 | Expansion stroke (Power stroke) | Isentropic (Reversible, Adiabatic) |
| 4-1 | Heat rejection (Exhaust blowdown) | Constant Volume, Reversible Heat Rejection |
Based on the description of the ideal Otto cycle, the heat addition process (Process 2-3) is defined as occurring reversibly at a constant volume.
Important Questions from Otto Cycle
A petrol engine theoretically operates on
In actual four-stroke petrol engine ignition takes place when the piston approaches.
Otto cycle is also known as
The efficiency of diesel cycle approaches to Otto cycle efficiency when
In an Otto cycle, the compression ratio is 9 : 1 and the pressure and temperature at the beginning of the compression process are 100 kPa and 10°C. The heat addition by combustion gives the highest temperature as 2500 K.Thermal efficiency of the cycle is:
