All questions of IC Engine for Mechanical Engineering Exam

To find the clearance volume of each cylinder, we need to understand the concept of compression ratio and displacement.

Compression Ratio:
The compression ratio is the ratio of the total volume of the cylinder when the piston is at the bottom dead center (BDC) to the total volume when the piston is at the top dead center (TDC). It is an important parameter that affects the performance and efficiency of an engine.

Displacement:
The displacement of an engine is the total volume swept by all the pistons in all the cylinders during one complete cycle. It is usually measured in cubic centimeters (cc) or liters.

Given Data:
Number of cylinders (n) = 3
Total displacement (D) = 770 cc
Compression ratio (CR) = 8.7

Calculations:
The displacement of each cylinder can be calculated by dividing the total displacement by the number of cylinders:
Displacement per cylinder = Total displacement / Number of cylinders
Displacement per cylinder = 770 cc / 3
Displacement per cylinder = 256.67 cc

The clearance volume (CV) can be calculated using the formula:
CV = (Displacement per cylinder) / (Compression ratio - 1)

Substituting the given values into the formula:
CV = 256.67 cc / (8.7 - 1)
CV = 256.67 cc / 7.7
CV ≈ 33.33 cc

Therefore, the clearance volume of each cylinder is approximately 33.33 cc.

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.

Exhaust gas recirculation (EGR) is a technique used in internal combustion engines to reduce nitrogen oxide (NOx) emissions. It works by recirculating a portion of the engine's exhaust gas back into the intake manifold, where it mixes with the fresh air-fuel mixture before entering the combustion chamber.

Decreasing thermal efficiency:
- One disadvantage of EGR is that it can decrease the thermal efficiency of the engine. This is because when the exhaust gas is recirculated, it displaces a portion of the fresh air-fuel mixture in the combustion chamber, leading to a lower combustion efficiency.
- The presence of exhaust gas reduces the oxygen concentration in the combustion chamber, which affects the combustion process and can result in incomplete fuel combustion.
- Incomplete combustion leads to the production of less useful work and increases the fuel consumption, thereby reducing the thermal efficiency of the engine.

Increasing HC emission:
- Another disadvantage of EGR is that it can increase the emission of hydrocarbon (HC) pollutants. HC emissions are a result of incomplete combustion, and as mentioned earlier, the presence of exhaust gas in the combustion chamber can lead to incomplete combustion.
- The recirculated exhaust gas contains unburned hydrocarbons, which can further contribute to the HC emissions.

Both a and b:
- Therefore, the correct answer is option 'c' - EGR has the disadvantages of decreasing thermal efficiency and increasing HC emissions.
- The decrease in thermal efficiency and increase in HC emissions are two significant drawbacks of using EGR in internal combustion engines.
- These disadvantages need to be carefully considered when implementing EGR systems, and engineers should strive to find a balance between reducing NOx emissions and maintaining optimal engine performance and efficiency.

Increasing aldehydes:
- The option 'd' - increasing aldehydes - is not a correct answer because EGR does not have any direct influence on the production of aldehydes.
- Aldehydes are organic compounds that can be formed during the combustion process, but their formation is not specifically attributed to the use of EGR. Other factors such as fuel composition, combustion conditions, and catalyst efficiency can contribute to the production of aldehydes.

In conclusion, while exhaust gas recirculation has the advantages of reducing NOx emissions, it also has the disadvantages of decreasing thermal efficiency and increasing HC emissions. These drawbacks should be taken into consideration when implementing EGR systems in internal combustion engines.

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.

To determine the number of cycles completed per second for a four-stroke diesel engine running at 6000 rpm, we need to understand the concept of engine cycles and the relationship between engine speed and cycle frequency.

Engine Cycles:
A four-stroke engine goes through four different strokes to complete one cycle. These strokes are:

1. Intake Stroke: The intake valve opens, and the piston moves downward, drawing in air and fuel mixture into the cylinder.
2. Compression Stroke: The intake and exhaust valves are closed, and the piston moves upward, compressing the air-fuel mixture.
3. Power Stroke: At the top of the compression stroke, the fuel is ignited by a spark plug. The expanding gases push the piston downward, generating power.
4. Exhaust Stroke: The exhaust valve opens, and the piston moves upward, pushing the burnt gases out of the cylinder.

Cycle Frequency:
The cycle frequency is the number of cycles completed per second. It is directly related to the engine speed, which is measured in revolutions per minute (rpm). The formula to calculate the cycle frequency is:

Cycle Frequency = Engine Speed (rpm) / 2

Explanation:
In the given scenario, the engine speed is 6000 rpm. By using the formula, we can calculate the cycle frequency as follows:

Cycle Frequency = 6000 rpm / 2
Cycle Frequency = 3000 cycles per minute

Since the question asks for the number of cycles completed per second, we need to convert the cycle frequency from minutes to seconds:

Cycles per second = (3000 cycles per minute) / (60 seconds per minute)
Cycles per second = 50 cycles per second

Therefore, the correct answer is option A) 50. The four-stroke diesel engine running at 6000 rpm completes 50 cycles per second.