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

SL. No.	Characteristics	SI engine	CI engine
1	Thermodynamic cycle	OTTO cycle	Diesel and duel combustion cycle
2	Combustion	Spark ignition	Compression ignition
3	Governing (Load and speed control)	Quantity governed by throttling; Almost constant A/F ratio.	By rack, quality governing, air constant, fuel charge variable, no throttling, variable A/F ratio.
4	Compression ratio	6-11, restricted by detonation (average 7-9)	13-22 (average 15-18) Higher CR reduces knocking. Restricted by mechanical and thermal stresses.
5	Operating pressures (a) Compression pressure (b) Max. pressure (c) Operating speed	7 to 15 bar 45 to 50 bar Max. 10 bar	30 to 50 bar 60 to 70 bar Max. 20 bar
6	(a)Operating Speed (b) Piston speed	High speed (2000-6000 rpm) High 16 m/s	Low sped (4000 rpm) medium speed (400-1200), high speed (1200-3500) up to 11 m/s
7	Distribution of fuel	A/F ratio is not optimum in multicylinder engines	Excellent distribution of fuel in multicylinder engines
8	Supercharging	Limited by detonation, used only in aircraft engine	Inherently suitable, widely used. Limited by blower power and mechanical and thermal stresses.
9	Exhaust gas temperature	High, due to low thermal efficiency	Low, due to high thermal efficiency
10	Starting	Easy, low cranking effort	Difficult, high cranking effort
11	Weight per unit power	Low (0.5 to 4.5 kg/kW)	High (3.3 to 13.5 kg/kW)
12	Power per unit displacement	High (30 kW/liter)	Low (15 kW/liter)
13	Acceleration	Not so good, but compensated by acceleration pump	Good
14	Reliability	Good, normal troubles in carburetor and ignition system	Good, greater reserve of power, rated by smoke, not max. power, normal trouble in injection and generating systems
15	(a) Specific fuel consumption (b) Fuel economy @Full load @Part load	Full load low, worse at part load and idling Costly fuel, density low, a calorific value slightly higher, fewer calories per liter Medium Poor	Full load better, part load much better than SI as no throttling Cheaper fuel, density high„ calorific value slightly low, more calories per liter Good Good
16	Fuel safety. (fire hazard)	Volatile fuel, more fire hazard	Less volatile, less fire hazard
17	(a) Initial, capital cost (b) Running cost	Low High	High due to heavyweight and sturdy construction, costly construction, 1.25 - 1.5 times. Low
18	Operating life	Less	More due to sturdy construction. Rating lower than maxi-mum power
19	Maintenance cost	Minor maintenance compared to CI	Major overall maintenance required less frequently
20	Noise and vibration	Less	More idle noise is a major problem
21	Odour and smoke	Less objectionable	More objectionable
22	Two-stroke operation	Less suitable, fuel loss in scavenging	More suitable, no fuel loss in scavenging
23	Applications	Passenger cars, small mobile applications aircrafts. Two stroke engine scooters, motorcycle mopeds due to cheaper and simpler engine.	Buses, trucks, locomotives, stationary generating plants. Heavy duty equipment like tractors, earthmoving machinery; ships
24	Combustion problem	Knock in unburnt mixture.	Diesel knock caused by long delay.
25	Fuel	Petrol high octane number, high self-ignition temperature	High cetane number, low self-ignition temperature
26	Air-fuel ratio	10 to 17	18 to 100
27	Fuel supply method	Carburettor(cheap), Port fuel injection and direct injection.	Expensive; fuel pump and fuel injectors.
28	Very high power	NO	Yes

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.