All questions of IC Engine for Mechanical Engineering Exam

Understanding Mean Effective Pressure (MEP)
Mean Effective Pressure (MEP) is a crucial parameter in internal combustion engines, representing the average pressure in the combustion chamber throughout the power stroke. It directly influences the engine's performance and efficiency.
Impact of Air-Fuel Ratio on MEP
The air-fuel ratio (AFR) affects the combustion process within an engine, impacting the MEP significantly. Here’s how different ratios play a role:

• Higher than Stoichiometric: When the AFR is higher than the stoichiometric value (the ideal ratio for complete combustion), there is an excess of air. This allows for more complete combustion, resulting in higher thermal efficiency and consequently higher MEP. The engine can extract more energy from the fuel, leading to improved performance.
• Lower than Stoichiometric: A lower AFR results in a rich mixture (more fuel than air). While this can increase power output temporarily, it leads to incomplete combustion, producing excess hydrocarbons and carbon monoxide. This inefficiency lowers the MEP due to wasted fuel and energy.
• Equal to Stoichiometric: At the stoichiometric ratio, combustion is optimal, but the energy extraction potential is not maximized compared to a richer mixture. Thus, while performance is decent, it does not reach the peak levels achievable with a higher AFR.

Conclusion
In summary, MEP is maximized at higher than stoichiometric air-fuel ratios due to improved combustion efficiency, resulting in better engine performance and energy extraction. Balancing the AFR is vital for optimizing engine operation and efficiency.

To find the compression ratio of the ideal Otto cycle for maximum work output, we need to consider the heat capacity ratio (γ) of the gas and the given minimum and maximum temperatures.

Given:
Heat capacity ratio (γ) = 2
Minimum temperature (T1) = 200 K
Maximum temperature (T3) = 1800 K

The compression ratio (r) is given by the equation:

r = (V1/V2) = (T3/T2)^(1/(γ-1))

where V1 and V2 are the initial and final volumes of the gas, and T2 is the temperature at the end of the adiabatic compression process.

To find the compression ratio, we need to determine the value of T2. To do this, we can use the equation for adiabatic process:

T1/T2 = (V2/V1)^(γ-1)

Rearranging the equation, we get:

T2 = T1 * (V2/V1)^(γ-1)

Now we can substitute the given values to find T2:

T2 = 200 K * (V2/V1)^(2-1)

Next, we substitute the values of T1, T2, and T3 into the equation for the compression ratio:

r = (1800 K / T2)^(1/(γ-1))

Substituting the value of T2, we have:

r = (1800 K / (200 K * (V2/V1)))^(1/(γ-1))

Simplifying further:

r = (9 / (V2/V1))^(1/(γ-1))

Since the work output of the Otto cycle is maximum when the compression ratio is maximum, we need to find the maximum value of r.

As the heat capacity ratio (γ) is given as 2, the maximum value of r is obtained when the ratio of V2/V1 is minimum. In other words, V2 should be as small as possible compared to V1.

Therefore, the compression ratio for maximum work output is when V2 = V1/3, which gives:

r = (9 / (1/3))^(1/(2-1)) = 3

Hence, the correct answer is option C) 3.

An octane number is a measure of the knocking tendency of gasoline fuels in spark ignition engines. The ability of a fuel to resist auto-ignition during compression and prior to the spark ignition gives it a high octane number. Two octane tests can be performed for gasoline.

Adiabatic flame temperature refers to the temperature reached during a combustion reaction when no heat is exchanged with the surroundings. It is a critical parameter in the design and optimization of combustion systems. Several factors can influence the adiabatic flame temperature, including pressure regulation, reactant masses, the amount of excess air supplied, and reactor volume.

Pressure Regulation:
- Pressure regulation refers to controlling the pressure at which the combustion reaction takes place.
- Changing the pressure can affect the adiabatic flame temperature by altering the equilibrium composition of the reaction products.
- Higher pressures can favor the formation of more stable products, leading to a higher adiabatic flame temperature. Conversely, lower pressures can result in lower temperatures.

Changing the Reactant Masses:
- The masses of reactants can influence the adiabatic flame temperature by changing the stoichiometry of the reaction.
- The stoichiometry determines the amount of heat released during combustion.
- Increasing the mass of one reactant while keeping the other constant can result in more heat release, leading to a higher adiabatic flame temperature.

The Amount of Excess Air Supplied:
- Excess air refers to the amount of air supplied to the combustion process beyond the stoichiometric requirement.
- The amount of excess air affects the availability of oxygen for combustion.
- Increasing the amount of excess air can result in more complete combustion and higher adiabatic flame temperatures.
- Conversely, reducing the excess air can lead to incomplete combustion and lower temperatures.

Changing the Reactor Volume:
- The reactor volume refers to the space in which the combustion reaction occurs.
- Changing the reactor volume can influence the residence time of reactants and products.
- A larger reactor volume can allow for more complete combustion and higher adiabatic flame temperatures.
- Conversely, a smaller reactor volume can result in shorter residence times and incomplete combustion, leading to lower temperatures.

Conclusion:
Among the given options, the amount of excess air supplied (option C) is the most significant factor controlling the adiabatic flame temperature. By adjusting the amount of excess air, the availability of oxygen for combustion is regulated, resulting in variations in the completeness of combustion and consequently affecting the adiabatic flame temperature. The other options, such as pressure regulation, changing reactant masses, and reactor volume, can also have an impact on the adiabatic flame temperature but to a lesser extent compared to the amount of excess air supplied.

Scavenging in Two-Stroke Engines

Scavenging is a process that is relevant in a two-stroke engine. A two-stroke engine is a type of internal combustion engine that completes one power cycle with two strokes of the piston, compared to a four-stroke engine that requires four strokes of the piston to complete one power cycle. Scavenging is an important process in a two-stroke engine as it ensures efficient fuel-air mixing and removal of exhaust gases from the cylinder.

Scavenging Process
The scavenging process in a two-stroke engine involves the following steps:

1. Exhaust Stroke: During the first stroke, known as the exhaust stroke, the piston moves upwards, compressing the fuel-air mixture in the combustion chamber. As the piston reaches the top of its stroke, the exhaust port opens, allowing the exhaust gases to escape.

2. Scavenging Stroke: In the second stroke, known as the scavenging stroke, the piston moves downwards. As it descends, it uncovers the intake port, allowing fresh fuel-air mixture to enter the combustion chamber. This process is aided by the presence of a scavenging air blower or a scavenging air pump, which helps in forcing the fresh mixture into the cylinder.

3. Scavenging Air Flow: The scavenging air flow is crucial for effective scavenging. The direction and velocity of the scavenging air flow determine the efficiency of the process. The incoming fresh mixture should effectively displace the exhaust gases and ensure thorough mixing for a complete combustion process.

4. Controlled Port Timing: The timing of the exhaust and intake ports is critical for optimal scavenging. The opening and closing of the ports should be precisely controlled to ensure efficient flow of exhaust gases out of the cylinder and the entry of fresh mixture without any overlap.

5. Efficient Combustion: The scavenging process plays a significant role in achieving efficient combustion in a two-stroke engine. Proper scavenging ensures that the combustion chamber is filled with a fresh fuel-air mixture, leading to improved power output and reduced emissions.

Conclusion
In conclusion, scavenging is a crucial process in a two-stroke engine. It involves the removal of exhaust gases and the intake of fresh fuel-air mixture. The scavenging process ensures efficient combustion, leading to improved engine performance and reduced emissions.