Assignment 4

The Comparison between C.I and S.I engines

• Petrol engines are lighter than diesel engines.
• Diesel engines have higher torque.
• Diesel is cheaper than petrol.
• Diesel engines have better fuel efficiency.
• Diesel engines operate at a high compression ratio in a range of 18 to 23:1 and petrol engines operate at a low compression ratio in a range of 10 to 14:1.
• Diesel engines have higher torque than petrol engines.
• Petrol engines use high octane rating fuel and diesel engines use high cetane rating fuel.
• Petrol engines use a spark plug to ignite fuel but diesel engines do not.
• The diesel engines can be more easily turbocharged.
• Diesel engines are noisier than petrol engines.
• The petrol engine works on the otto cycle and diesel engine works on the diesel cycle.
• The diesel engine is more expensive than a petrol engine.

BSFC: BSFC is Brake Specific Fuel Consumption. It is an indicator of how much BHP, brake horsepower, the engine is ultimately delivering per unit mass or volume of fuel consumed. In petrol engines, the BSFC increases with the increase in load but in Diesel engines, the BSFC decreases with the increase in load because, At higher loads, the turbulence and in-cylinder temperature inside the combustion chamber will be high which helps in the atomization and proper mixing of fuel resulting in higher combustion efficiency. Also, the engine will be running with a richer air-fuel ratio at lower loads compared to the full load conditions.

Exhaust Temperature: In general the exhaust gas temperature of a diesel engine would be in the range of 800-1000K. The exhaust gas temperature should be above 800K for the oxidation of HC and carbon particles for controlling the emissions but these higher temperatures can cause the damage to the after-treatment process and it can reduce the durability of the engine also so these temperatures should be limited according to the model of the engine. These high exhaust gas temperatures are suitable for installing a Turbocharger compressor and it is more suitable to the diesel engine rather than petrol engines.

A/F ratios: The diesel engine always runs on a lean mixture because of its properties. The stoichiometric air-fuel ratio of the diesel engine is 14.5:1 but it always works higher than that varying from 20 to 32:1 depending on the model of the engine. The variations of Air-Fuel ratios in a diesel engine is more when compared to the petrol engine. It also plays an important role in the formation of emissions if the engine runs on a rich mixture there is a high chance of the formation of soot particles when it runs on a lean mixture the formation of NOx will be higher.

The simulation was done on the simple natural aspirated diesel engine by varying combustion duration in the engine.

This is the cylinder geomentry specifications with a compression ratio of 16.5.

This is the case set up with varying combustion duration of 35,32 and38 crank angle duration.

Here we can see that the brake power is increased in for shorter combustion duration and brake specific fuel consumption is also low for case2.

This is the comparison of pressure plots for all three cases.

This is the comparison of temperature plots for all three cases.

In the pressure plot, we can see that case 2 gives high pressure this is the main reason for the production of high brake power and on seeing the temperature plot we can notice that the temperature produced in the in-cylinders are also low when compared to other two cases those leads to the low formation of NOx. From the comparison, we can say that case 2 the simulation with shorter duration yields good results.