JNTU B.Tech III THERMAL ENGINEERING I Previous Question Paper (March - 2021 - R16)

Time loss factors account for the deviation between the ideal combustion process and the actual combustion process in an internal combustion engine. In an ideal cycle, combustion is assumed to be instantaneous at top dead center (TDC), whereas in an actual cycle, combustion takes a finite amount of time to propagate through the cylinder charge.

The effect of time losses is that the peak pressure and temperature are reached after the piston has already moved past TDC, resulting in a reduction of the expansion stroke length and a decrease in the work output. Efficiency is reduced because the pressure rise does not occur at the optimal volume, leading to lower peak pressures and higher exhaust temperatures, which signifies that more energy is being rejected rather than converted into work.

The actual cycle efficiency is significantly lower than the air-standard cycle efficiency due to several fundamental differences between ideal assumptions and real-world engine operation:

1. Dissociation: At high temperatures, combustion products like CO2 and H2O dissociate into CO, O2, and H2, an endothermic process that limits the maximum temperature and pressure reached.
2. Variable Specific Heats: Air-standard cycles assume constant specific heats, whereas in reality, specific heats increase with temperature, reducing the peak pressure and temperature during combustion.
3. Heat Losses: Actual engines experience significant heat transfer from the hot gases to the cylinder walls, cooling fins, and coolant, whereas the air-standard cycle assumes an adiabatic process.
4. Pumping Losses: Real engines require work to intake the air-fuel mixture and exhaust the burned gases, which is not accounted for in the theoretical cycle.
5. Incomplete Combustion: Due to non-homogeneous mixtures and limited time, combustion is rarely 100% complete, leading to chemical energy losses.
6. Friction and Mechanical Losses: Mechanical components such as pistons, rings, and bearings create friction, reducing the brake thermal efficiency compared to the indicated efficiency.