Assignment 4

Objective : To simulate a CI engine and observe the changes in its performance for different MFB 50 timings.

1.Compare SI vs CI and list down differences

1. The basic difference between an SI engine and a CI Engine is of the thermodynamic cycle. The SI engine works on an Otto cycle which a constant volume combustion process. The CI engine works on a Diesel cycle which a constant pressure combustion process.
2. The compression ratio is very high in case of CI engines (15-20) as compared to SI engines which have low compression ratios (8-12).
3. The size of a CI engine is quite bigger and heavier than that of a SI engine due to high cylinder pressures.
4. The ignition process is different in CI than SI engines. In CI engines, the fuel is atomized into small droplets and gets self ignited dut to high in-cylinder temperatures and pressures.
5. The thermal efficiency of a CI engine is higher than SI engine.

2.Comments on following parameters

• BSFC

CI Engine SI Engine

It can be seen that the BSFC value of a CI engine at the same rpm is lower than SI engine. A lower BSFC value indicates that the engine is efficient and requires less amount of fuel to produce 1 kW-h of power.

• Exhaust Temperature

CI Engine SI Engine

The exhaust gas temperature of CI engine is lower than SI engine. The reason for that being the in-cylinder temperatures of a CI engine are quite higher than SI engines and hence during the expansion stroke the exhaust gases reject more heat compared to SI engines.

• A/F ratios

CI Engine SI Engine

The A/F ratio of a CI engine is higher than SI engine as SI engines normally burn stoichiometric mixture. The diesel engines usually burn lean with A/F ratio greater than 14.5.

3.Change MFB 50 and observe impact on performance

The case is run at 3600 rpm for different MFB 50 timings.

As the MFB 50 timings are increased the Brake Power produced decreases. The reason being most of the heat release happens during the expansion stroke and hence less power is extracted during the power stroke.

Higher peak pressures are observed in case 1 when the MFB 50 combustion happens closer to the TDC, the reason being that the burning starts closer to TDC thus generating higher peak pressure.

High temperature is observed in case 1 when the MFB 50 combustion happens closer to the TDC, the reason being that the burning starts closer to TDC thus generating higher pressure and temperature in low volumes due to compression.

The Burned fuel fraction in case of case 1 is higher because more mass is burnt at a higher burn rate closer to TDC in very small volume as compared to the other cases where the burning event expands into the expansion stroke.