Comparing SI and CI engines and study of effect of MFB50 on engine performance.using GT Suite

Compare the below SI vs CI tutorials and list down differences

Both the cases are ran at 1800 RPM

The SI engine tutorial was run at 1800 RPM. Hence in order to compare the important parameters, the CI engine tutorial is also run at 1800 RPM as shown below

In-cylinder pressure:

Cylinder Pressure is the pressure in the engine cylinder during the 4 strokes of engine operation (intake, compression, combustion and expansion, and exhaust). Pressure during expansion is the most important because that is the cylinder pressure pushing on the piston to produce power.

in cylinder pressure vs crank angle degree plot:

For SI engine

For CI engine

The Diesel engines always generate high cylinder pressure due to the high compression ratio.

Airflow rate:

The airflow rate is the mass of the air entering into the engine cylinder during the intake process. The airflow rate of our engine is shown in the tabular column below:

For SI engine:                                                        For CI engine

A/F ratio of SI engine: Stoichimoteric mixture(14.5 )
A/F ratio of CI engine: Lean mixture(19.18)

Since CI engine operates lean the airflow rate is high (82.8 kg/h) for CI engine when compared to SI engine.

Mean Effective Pressures(MEP):

It is an average pressure acting on the piston during the complete thermodynamic cycle in a Reciprocating internal combustion engine.

The different Mean Effective Pressures of our engines are listed below and the values are tabulated.

• IMEP – Indicated Mean Effective Pressure
• BMEP –Brake Mean Effective Pressure
• FMEP – Friction Mean Effective Pressure

For SI engine:

For CI engine:

It can be clearly seen from the above two tables that Ci engines are high power engines thus generating more BMEP and torque

Brake Specific Fuel Consumption(BFSC):

Brake-specific fuel consumption (BSFC) is a measure of the fuel efficiency of any prime mover that burns fuel and produces rotational or shaft power. It is typically used for comparing the efficiency of internal combustion engines with a shaft output. It is the rate of fuel consumption divided by the power produced.

For SI engine:                                                        For CI engine

Since CI engines are leanly burned less fuel is consumed for combustion thus resulting in low BSFC(221.3 g/kW-hr)  when compared with the SI engine (244.2 g/kW-hr).

                                                         SI Engine                           CI engine

Exhaust Temperature:

Exhaust temperature is the temperature transferred to the engine's exhaust components by the burnt gases escaping from the engine.

Diesel engine exhaust gases vary with speed and load. High loads and high speeds result in the highest temperatures. Generally, temperatures of 500 to 700 °C (932 to 1,293 °F) are produced in the exhaust gases from diesel-cycle engines at 100 percent load, to 200 to 300 °C (392 to 572 °F) with no load.

Below contour map shows the exhaust system temperatures in degree celsius

A/F ratios

Air–fuel ratio (AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process.

In petrol engines, the air-fuel ratio varies from 8 to 15. At slow speeds, rich mixtures will be formed around 7 to 9 AFR. in the economy range, the stoichiometric air-fuel ratio will be around 15 which is the highest for forming a lean mixture. That's why we get more mileage in the economy range. Again if we increase speed, the AFR ranges about 9–12 for running the engine at high speed.

In diesel engines, due to direct injection, only air will be entering the cylinder in the first stroke. So we always get leaner mixtures compared to the SI engine. Generally, it will be around 14–15 for rich mixtures and goes up to 70 for lean mixtures. This depends on our throttle input.

For SI engine:                                                        For CI engine

A/F ratio of SI engine: Stoichimoteric mixture(14.5 )
A/F ratio of CI engine: Lean mixture(19.18)

Change MFB 50 and observe the impact on performance:

Combustion control has always been a key factor in modern Internal Combustion Engines management systems, being even more crucial nowadays to improve drivability and reduce pollutant emissions. The position where 50% of fuel mass burned over an engine cycle is reached (MFB50) provides very important information about the effectiveness of combustion, such as the kind of combustion that is taking place.

Three cases were run for different percentages of the main duration in the combustion.

Case 1: MFB50 = 30 CAD

Case 2: MFB50 = 35 CAD

Case 3: MFB50 = 40 CAD

Case 1:

Case 2:

Case 3:

From the above tables following conclusions are drawn:

1. Lower the MFB50 higher is the power and torque.
2. Lower MFB50 results in low exhaust temperatures.
3. High cylinder pressures can be obtained at lower MFB's.
4. Low BSFC can be seen at low MFBso.