Comparing SI and CI engine using GT-Power and studying the effect of Main Duration phase of combustion on the engine performance

Objective :

To compare SI and CI engine, comparing factors such as BSFC, Exhaust Temperature and Air fuel ratios and observe impact of MFB50(Mass fraction burnt = 50%) on the performance of CI engine.

Spark Ignition Vs Compression Ignition (Theoretical) :

Sr. No	Parameters	SI	CI
1.	Ignition mechanism	Spark plug	Self ignition
2.	Compression ratio	Low compression ratio upto 12.0	Higher compression ratios.
3.	Flow mechanism	Tumble	Swirl
4.	Ports	Directed	Helical
5.	Fuel	Gasoline, LPG, CNG	Diesel
6.	Speed	High speed (upto 12000 rpm)	Low speeds( upto 6000 rpm)
7	Knocking	Commences after end of combustion	Commences at the start of combustion(also called detonation)
8	Flame propogation	Homogenous	Heterogenous
9	Emissions	CO, HC,NOx	CO,HC,Nox, Soot
10	Fuel burnt	Not complete, as spark plug initiates the flame and flame propagates in the cylinder.Some part of fuel is unburnt.	Complete, as fuel is self ignited, no unburnt gases remain.
11	Efficiency	Less	More
12	Application	In motorcycles, cars, light duty applications	Heavy duty applications such as, trucks, locomotives.

Spark Ignition Vs Compression Ignition (Simulation)

A single cylinder SI and CI engines have been modelled with similar specifications to compare the important aspects in the engine.

1. SI engine

Bore - 86.0 mm

Stroke - 86.05 mm

Compression Ratio - 9.5

Conc rod length - 175 mm

Air fuel ratio - 13.5

Fuel injected - 40.2 mg

Engine speed - 3600 rpm

2. CI Engine :

Bore - 86.0 mm

Stroke - 86.05 mm

Compression Ratio - 16.5

Conc rod length - 175 mm

Air fuel ratio - 25.5

Fuel injected - 39.9 mg

Engine speed - 3600 rpm

3. Simulation :

1. BSFC :

It is the amount of fuel required to produce 1kWh of power.

BSFC = mf/Pe

where:

Mf [kg/s] – fuel mass flow rate (measured on the engine dynamometer)
P_e[W] – effective (brake) engine power
BSFC [kg/J] – brake specific fuel consumption

Usually, the fuel mass flow rate is measured in [g/s], the engine power in [kW], which gives the brake specific fuel consumption in [g/kW·h]:

BSFC = mf *3600/Pe

The lower the brake specific fuel consumption, the more efficient the engine is.

BSFC=m˙f