Jawaharlal Nehru Technological University Hyderabad B.Tech III Year IC ENGINES AND GAS TURBINES Previous Question Paper (January/February - 2021 - R16)

The ideal cycle for a Compression Ignition (CI) engine is the Diesel cycle (or Dual cycle), which assumes reversible processes and ideal working fluids. In contrast, the actual cycle deviates significantly due to thermodynamic, mechanical, and chemical irreversibilities.

Differences between Ideal and Actual Cycles:

• Working Fluid: The ideal cycle assumes air as the working fluid with constant specific heats. The actual cycle uses a mixture of air and fuel, and specific heats vary significantly with temperature.
• Combustion Process: The ideal cycle assumes instantaneous heat addition at constant pressure (or constant volume). The actual cycle involves a finite duration of combustion, leading to pressure losses.
• Compression and Expansion: The ideal cycle assumes adiabatic (isentropic) processes. The actual cycle involves heat transfer across cylinder walls, making the processes polytropic.
• Valve Timing: The ideal cycle assumes instantaneous valve opening and closing at TDC/BDC. The actual cycle employs valve lead and lag to account for gas inertia and flow resistance.

Losses in the Actual Cycle:

• Pumping Losses: These occur during the intake and exhaust strokes due to pressure drops across valves and friction in the manifolds. The net work done during the pumping loop is negative, reducing the total indicated work.
• Time Loss (Combustion Loss): Since combustion is not instantaneous, the peak pressure is not reached at TDC. If combustion starts too early, work is lost during compression; if too late, expansion work is reduced.
• Heat Loss: Significant heat is transferred from the hot gases to the cylinder walls, piston crown, and cylinder head, reducing the internal energy available for expansion.
• Dissociation: At high combustion temperatures, CO2 and H2O dissociate into CO, O2, and H2, absorbing heat and preventing complete combustion at peak temperatures.
• Exhaust Blowdown Loss: The exhaust valve opens before BDC to allow gases to escape. This results in a loss of potential work that could have been extracted during the final part of the expansion stroke.
• Friction Losses: Mechanical friction between piston rings, bearings, and auxiliary components (oil pump, water pump) reduces the brake power output compared to the indicated power.

A carburetor is designed to supply the correct air-fuel (A/F) ratio to an internal combustion engine under varying operating conditions. This is achieved through specific auxiliary circuits integrated into the carburetor body:

• Idling Condition: During idling, the throttle valve is nearly closed, resulting in low airflow through the venturi and insufficient pressure drop to draw fuel from the main jet. To compensate, an idling circuit is used. It consists of an idle jet and an idle port located just below the throttle valve. The high vacuum existing below the throttle valve draws fuel through the idle jet, which is then mixed with air from an idle air bleed. This provides a rich mixture (approximately 10:1 to 12:1) necessary to overcome the high internal friction and maintain smooth engine operation at low speeds.
• Normal Operating (Cruising) Condition: As the throttle opens, the main venturi becomes active. The air velocity increases, creating a pressure drop that draws fuel from the main discharge nozzle. To maintain efficiency and economy, the engine requires a leaner mixture (approximately 16:1 to 17:1). This is achieved via an air-bleed system, where air is introduced into the fuel nozzle before it enters the venturi. This air-bleed breaks the surface tension of the fuel, improves atomization, and compensates for the tendency of the mixture to become richer as airflow increases.
• Acceleration Condition: Rapid opening of the throttle causes a sudden increase in airflow, but fuel, being denser and having higher inertia, lags behind, leading to a temporary lean mixture that can cause engine stalling or 'hesitation'. To prevent this, an accelerating pump is employed. This is a mechanical plunger or diaphragm pump linked to the throttle linkage. When the throttle is opened quickly, the pump forces an extra 'shot' of fuel directly into the venturi, providing the extra rich mixture (10:1 to 12:1) required for rapid acceleration.
• Full Power (Cold Start/High Load) Condition: For maximum power or cold starting, an economizer circuit or a choke valve is used. The choke valve restricts air intake to create a very rich mixture for starting, while the economizer valve opens at wide-open throttle to allow additional fuel flow, ensuring the engine produces maximum power output.