Assignment 4 - Tutorial No 1- CI final

INTRODUCTION:

A model of a compression ignition engine running at 1800 rpm is given below:

• Air is allowed to enter through the inlet and moves along the runners to meet at the throttle and then allowed to enter the cylinder.
• Fuel is injected directly into the cylinder for combustion to happen and exhaust gases are allowed to leave the outlet.

DIFFERENCE BETWEEN CI AND SI ENGINES:

• CI engines require compression of fuel at higher ratios for fuel to be ignited whereas for SI engines a spark is made use for igniting the mixtures.
• CI engines use diesel as fuel whereas SI engines use gasoline as fuel.
• Compression ratios are high (15-20) in CI engines and compression ratios are low for SI engines(9)
• Catalytic converters are used for treatment of exhaust in SI engines whereas an Aftertreatment System of DOC,DPF,SCR are used in CI engines.
• Knocking which happens at the end of combustion is a disadvantage for SI engines meanwhile Detonation which happens at the start of combustion is a disadvantage for CI engines.

CI ENGINE PARAMETERS:

• We can see from the table above that the value of Air Fuel Ratio is on a higher note because we want more air to enter the engine and it is very essential to keep the mixture lean.
• The BSFC value is low when compared with that of an SI engine running at the same engine speeds which clearly indicates High fuel efficiency in CI engines.

• The value of NOx emissions are high because there is more air entering the cylinder and if suppose more fuel is injected we get more soot generated. So air fuel ratios need to be varied accordingly.

• When the cylinder is compressed to a higher pressures we can see that the temperatures inside the cylinder have risen to very high values shown in the graph above. At the exhaust cycel the temperatures are around 1000K which is on the higher side and remains throughout exhaust phase until fresh air is allowed to enter the intake after TDC phase.

CHANGE THE VALUE FOR MFB50 AND OBSERVE IMPACT ON PERFORMANCE:

The case is described below where we change only the value for main burn duration given in the table below:

When running at 3600rpm engine speed the results are obtained below as shown:

• When we allow burning to happen soon indicated in case 1 for compression ignition engines the engine achieves better break power, torque.
• Also when it comes to BSFC we see that having minimum burn duration in case 1 has consumed less fuel than other cases.

The graphs also show similiar characteristics:

CONCLUSION:

Thus the basic calibration of CI engines is understood in this exercise. And having a minimum ignition delay is suitable as they have a drastic impact on performance as well as emissions.