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Consider the following statement with reference to combustion and performance in a four-stroke petrol engine: 1. The auto-ignition temperature of petrol as a fuel is higher than that of diesel oil as a fuel. 2. The highest compression ratio of petrol engines is constrained by the possibility of detonation. 3. A petrol engine is basically less suitable for supercharging than a diesel engine. Which of the above statements are correct?

Consider the following statement with reference to combustion and performance in a four-stroke petrol engine: 1. The auto-ignition temperature of petrol as a fuel is higher than that of diesel oil as a fuel. 2. The highest compression ratio of petrol engines is constrained by the possibility of detonation. 3. A petrol engine is basically less suitable for supercharging than a diesel engine. Which of the above statements are correct? Correct Answer 1, 2 and 3

Explanation:

Auto-ignition temperature, it is the temperature at which the substance will ignite without the presence of a source of ignition. The auto-ignition temperature of petrol is higher than that of diesel as a fuel.

Higher compression ratio will lead to a higher temperature of the charge in the petrol engines, this will increase the detonation tendency. The compression ratio in the petrol engine is limited to 10 as it leads to detonation in petrol engine whereas in diesel engine the compression ratio is limited to 20 approximately.

Superchargers are used to increase the mass of air circulated, the pressure after supercharging is 1.5 to 2 times the entry pressure, thus the temperature of the charge entering the engine cylinder is much higher, therefore, it will lead to detonation in the petrol engines, therefore superchargers are less suitable for petrol engines. The increased pressure and temperature of the charge at the end of compression reduce ignition delay in CI engine, thereby the combustion becomes smooth, and the tendency for knocking is avoided. 

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Factors affecting knocking in SI engine:

i) Compression ratio:- Higher compression ratio will result in the higher temperature of the charge, the tendency to knock will increase.

ii) Inlet air temperature:- The effect of inlet air temp is the same as the compression ratio. High inlet temp of the air-fuel mixture will increase knocking.

iii) Advancing the spark:- Advancing the spark will result in more compression during sparking. The increase in temperature during the sparking will be higher. Hence tendency to knock will increase and so by retarding the spark, the tendency to knock will decrease.

iv) Coolant water temperature:- By increasing coolant water temperature, less heat will be carried from the engine which will increase the engine temperature. So the knocking will increase.

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