Assignment 4
Ans 1. Difference between SI and CI engine are as follows:
1.Burn profile
SI combustion model: In Burn profile is modelled with position of CA50 and duration of burn (10 to 90%). These are the main parameter for plotting burn rate profile.
CI combustion model: In burn profile modeling, we need to input ignition delay (because in diesel engine ignition delay is considerably high), premix and tail fraction indicates amount of fuel burn in respective burn period. Duration of premix, main and tail duration (in cranf angle deg)are also entered.
2. CA 50 position
SI engine: It is near TDC firing. Generally vary from 8 to 12 deg.
CI engine: CA 50 position is on greater than that for SI engine Generally vary from 15 to 22 deg.
3. NOx output
Comparing both tutorial (1cylDI-final ans 1cylSI-final) it is observed that for 3600rpm, NOx emission is more in CI engine than SI engine.
4. Air fuel ratio:
As shown in above table A/F ratio for SI is 14.51, whereas for CI it is 20.78.
Ans. 2
BSFC in case of CI engine has improved as compared to SI engine by 1.5%. This increases the thermal effciency of CI engine as compared to SI engine.
(SI engine)
(CI engine)
Exhaust temperature in case of CI is comparatively high because of higher compression ratio of engine.
A/F ratio: For diesel engine air fuel is 20, whereas for SI it is about 14. A/F ratio of CI engine is higher than SI engine.
Ans 3.
For SI engine: Here CA50 position is varied by varing anchor angle.
For case 1, 2 and 3 it is observed that when CA 50 position is moved away from TDC firing, the BSFC detoriates percentage of energy in exhaust has increased.
For CI engine:
In this model CA 50 position is varied by change of injection timing.
It is observed that with retarding of injection timing the power output/ effciency has considerably reduced. This is because when engine is advance in injection timing the position of crank angle w.r.t peak pressure move towards TDC firing, this increases pressure rise rate and thus in cylinder pressure also increases. This leads to increase in torque output with same quantity of fuel input.