Understanding Heat Rejection in an IC Engine Cycle

Internal Combustion (IC) engines operate on thermodynamic cycles where fuel is burned inside the engine cylinder, producing high-temperature, high-pressure gases that push a piston to generate work.

A complete engine cycle involves several processes: intake, compression, combustion (heat addition), expansion (power stroke), and exhaust (heat rejection).

Heat Rejection Process

Heat rejection is the process where the engine expels waste heat to the surroundings, typically through the exhaust gases and cooling system. This process brings the working fluid (gases) back to a state where a new cycle can begin.

Common IC Engine Cycles

Two primary theoretical cycles are used to model IC engines:

• Otto Cycle: Ideal cycle for spark-ignition (petrol) engines. Heat addition and heat rejection occur at constant volume.
• Diesel Cycle: Ideal cycle for compression-ignition (diesel) engines. Heat addition occurs at constant pressure, and heat rejection occurs at constant volume.

The question asks about heat rejection in "a" IC engine cycle, implying a general or typical cycle. Both the ideal Otto and Diesel cycles feature heat rejection at constant volume.

Why Constant Volume Heat Rejection?

In the ideal Otto cycle, the combustion happens instantaneously at Top Dead Center (TDC), which is considered a constant volume process. Similarly, at the end of the expansion stroke and just before the exhaust valve opens, the volume is at its maximum (Bottom Dead Center or BDC). When the exhaust valve opens, the pressure inside the cylinder drops rapidly as gases escape, while the piston is momentarily at BDC (constant volume). This rapid pressure drop at constant volume is where the heat rejection to the surroundings is considered to occur in the ideal model.

In the ideal Diesel cycle, although heat addition is at constant pressure, the heat rejection process at the end of the cycle (at BDC) is also modeled as occurring at constant volume, similar to the Otto cycle.

Conclusion

Based on the theoretical Otto and Diesel cycles, which are standard models for IC engines, the heat rejection process is represented as occurring at constant volume.

Therefore, in a typical IC engine cycle, the heat is rejected at constant volume.