Week-4 : Basic Calibration of Single cylinder CI-Engine

OBJECTIVE:

The aim of this project is to simulate a single cylinder CI engine in GT Power and

1) Conduct a comparison study between SI and CI engines on various parameters

2) Observe the impact of MFB 50 on performance

Case Setup:

SI Engine:

CI Engine

1. Geometry differences:- CI engine (Left image) has a large bore and stroke i.e. a larger cylinder than SI engines(Right image). This is due to the fact that it has high Compression ratios hence swept and clearance volume is higher and it has to have a larger cylinder size.

2. Compression ratio (CR):- From figure 1 we can see that the compression ratio of SI engine is low and max can be taken up to 9:5 this limitation is to avoid knocking in SI engines. Whereas for CI engines CR can go up to 16:5. Hence, the CR of CI engines is on the high side compared to Si engines.

Fig.1 CI engine (left) vs SI engine (right)

Results:

The performance, as well as the emissions of the engine at 3600 RPM, is listed below. The table on the right shows the performance of the SI Engine whereas the left one shows CI Engine.

From the above table the following performance points are noted:

• The airflow rate is higher in CI engines. This is because CI engines are usually operated under lean conditions with a minimum A/F ratio of 19.
• The BSFC of diesel engines is lower compared to SI. This means, to achieve the same power, lower fuel is consumed by CI engines.
• The exhaust temperature is higher in the SI engine because of a richer mixture of air and fuel when compared to CI, which gets combusted in the SI cylinder.

EFFECT OF MFB-50 ON THE PERFORMANCE OF CI ENGINES:

The position where 50% of fuel mass burned over an engine cycle is reached (MFB50) provides very important information about the effectiveness of combustion, burn rate, and speed of combustion that is taking place.

To find out the effect of MFB 50, a CI engine case was run with increasing order of MFB50 values. Following are the 3 cases. The image of the case setup is also shown below:

• MFB50 in 30 CA
• MFB50 in 35 CA
• MFB50 in 40 CA

PERFORMANCE COMPARISON:

It can be observed from the table above that, as the MFB50 values increase, the performance decreases. Essentially MFB 50 means, the amount of time in CA for the combustion to reach 50% of the burnt stage. As we increase MFB50, it means more time is to be taken to reach 50% burn point. This implies a reduction in the burn rate of the fuel. As the intensity of combustion decreases, there will be a decrease in performance which is very evident from the below plots that as the main duration is increased the maximum values of pressure, temperature, heat transfer rate in the cylinder is decreased.

Pressure V/s Crank angle

Temperature V/S crank angle

Conclusion:

Difference between SI & CI studied using GT.