Week 2 : Basic Calibration of Single cylinder SI-Engine

Aim:

• To run the single cylinder SI engine setup at 1800 rpm.
• List down parameters such as air flow rate, BSFC, BMEP, and In-cylinder pressure.
• To run the same setup at 3600 rpm and increase the output by 10%

Introduction:

The working cycle of both spark-ignition and compression-ignition engines may be either two-stroke or four-stroke. A four-stroke spark-ignition engine is an otto cycle engine. It consists of the following four strokes: suction or intake stroke, compression stroke, expansion or power stroke, and exhaust stroke. Each stroke consists of a 180-degree rotation of crankshaft rotation and hence a four-stroke cycle is completed through 720 degrees of crank rotation. Thus for one complete cycle, there is only one power stroke while the crankshaft turns by two or more revolutions.

Important parameters which characterize the operation of internal combustion engines

Air-flow rate:

It is the amount of air that is entering the engine and it is important because it decides how the fuel combustion takes place. A low airflow produces incomplete combustion leading to CO emissions whereas a higher airflow leads to NOx emissions. Therefore, an airflow close to stoichiometry is desired.

Break Mean Effective Pressure:

It is the ratio of gross work done per cycle to the volume displaced per cycle. It provides a valuable measure of the engine performance which is independent of the engine displacement.

Brake Specific Fuel Consumption:

It is the amount of fuel consumed per unit of power produced in a cycle. Low values of BFSC are desirable

In-cylinder pressure:

It is the maximum pressure that is developed inside the engine during a single cycle. It provides a measure of the work that can be extracted from the engine

Model Setup:

The model is set up from scratch by following the in-built tutorial on a single-cylinder SI engine. The model is imported from tutorials > modelling applications > engine_performance > 1cylSi-final.gtm

Results:

• At 1800 RPM

Air flow rate: 24.6 kg/h

Break mean effective pressure: 9.5 bar

Brake specific fuel consumption: 239.2 g/kw-h

In-cylinder pressure: 48.89 bar

• At 3600 RPM

Air flow rate: 58.6 kg/h

Break mean effective pressure: 11.1 bar

Brake-specific fuel consumption: 241.8 g/kw-h

In-cylinder pressure: 58.9 bar

2. Increase the power output at 3600 rpm by 10% 

At 3600 rpm, the engine with the current configurations and operating conditions produces 16.7 kW. A 10% increase will result in 18.37 kW.

For a four-stroke engine, the relationship between power and other operating parameters is given by the following equation:

`P = (eta_f * eta_v * N*V_d*Q_(hv)*rho_a*(F/A))/2`

where, 

P = engine power

`eta_f` = fuel conversion efficiency

`eta_v` = volumetric efficiency

N = rotation speed of the crankshaft

`V_d` = displacement volume

`Q_(hv)` = heating value of the fuel

`rho_a` = density of incoming air

`(F/A)` = Fuel-air ratio

From the above equation, it can be observed that the power of an engine can be increased by:

1. Increasing the volumetric efficiency
2. Increasing the fuel conversion efficiency
3. Increasing the rpm
4. Increasing the displacement volume
5. Using fuel with a higher heating value
6. Increasing the density of the incoming air
7. Increasing the Fuel-air ratio or decreasing the air-fuel ratio

Case 1: Increasing the displacement volume

The displacement volume can be increased by either increasing the bore, stroke, or both.

• Increasing the bore:

In this case, the model was run with 4 different bore values: 86mm, 88mm, 90mm, and 92mm. Here the stroke was maintained constant at 86.07mm.

• Increasing the stroke:

In this case, the model was run with 4 different stroke values: 86mm, 88mm, 90mm, and 92mm. Here the bore was maintained constant at 86  mm

Case 2: Increasing the density of air

The model was run with 3 different temperatures: 300K, 295K, and 290K, while the other parameters remained constant.  

Case 3 – Decreasing the air-fuel ratio

In this case, the air-fuel ratio was changed to 8, 10, 12, and 14.

Conclusion:

• By increasing the cylinder volume by changing the bore and stroke more burning of air and fuel can take place.
• By increasing the connecting rod length, better torque can be obtained.
• By increasing the intake pressure better combustion can be obtained, But increasing the intake air pressure requires a turbocharger or supercharger which increases the cost of an engine.

The above steps increase the power output of the engine by 10%.