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

Exhaust blow-down loss occurs during the initial phase of the exhaust process when the exhaust valve opens before the piston reaches Bottom Dead Center (BDC). Due to the high pressure of the combustion gases remaining in the cylinder, a significant portion of the gas is expelled rapidly through the valve. This sudden expansion represents a loss of potential work that could have been extracted during the expansion stroke if the pressure had been utilized more effectively. The kinetic energy of the gas leaving the cylinder is largely dissipated as turbulence and heat in the exhaust system.

The loss due to the gas exchange process refers to the pumping work required to clear the burnt gases and introduce a fresh charge into the cylinder. This process involves pressure drops across the intake and exhaust valves, which are necessary to induce flow. The work done by the piston to push out the exhaust gases against the exhaust manifold pressure, combined with the work required to overcome the pressure drop during intake, results in a net negative work loop on the indicator diagram. These losses are influenced by valve timing, valve lift, and the flow resistance of the intake and exhaust ports.

In fuel-air cycle analysis, the fuel-air ratio significantly deviates from the ideal air-standard cycle due to variable specific heats and dissociation. The effects are as follows:

1. Efficiency: Maximum thermal efficiency typically occurs at a slightly lean mixture. As the mixture becomes richer, efficiency drops due to incomplete combustion and the dissociation of CO2 and H2O, which absorbs energy and prevents it from contributing to the pressure rise.
2. Maximum Power: Maximum power is generally achieved with a slightly rich mixture (fuel-rich). This provides sufficient fuel to consume all available oxygen and accounts for non-uniform distribution, ensuring the highest energy release per cycle.
3. Temperature and Pressure: Peak cycle temperatures and pressures are highest near the stoichiometric ratio. As the mixture becomes richer, the dissociation effect becomes more pronounced, which limits the maximum temperature and pressure reached during combustion. Conversely, very lean mixtures result in lower temperatures and pressures due to the dilution effect of excess air, which acts as a heat sink.