Week 2 : Basic Calibration of Single cylinder SI-Engine

Calibration of SI Engine

Aim:

• The main objective of this project is to set up and run a model of a single cylinder 4-stroke SI Engine at 1800 rpm in GT-Suite
• To run the same model at 3600 rpm and increase the power-output by 10%

Introduction:

The Spark-ignition (SI) engine is an internal combustion engine, where the air-fuel mixture is ignited by a spark from the spark plug.

The 4-storke SI engine is an Otto cycle engine and consists of 4 strokes as shown in the figure. These comprise an intake stroke when a fuel-air mixture is drawn into the engine, a compression stroke where the mixture is compressed, a power stroke when a mixture is ignited and expands and an exhaust stroke when the combustion products are expelled from the engine. During each stroke, the crankshaft completes 180 degrees of rotation and hence the entire 4 strokes are completed in 720 degrees of crankshaft rotation. Therefore, for every cycle the crankshaft completes two revolutions whereas there is only one power stroke [1]. This is in-contrast to 2-stroke engine where a power-stroke takes place during a single rotation of the crank-shaft

There exist some important parameters which characterize the operation of internal combustion engines and they are important to an engine user for comparing performance of different reciprocating engines. The factors include

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 air flow produces incomplete combustion leading to CO emissions whereas a higher air flow leads to NOx emissions. Therefore, an air flow 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 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 setup from scratch by following the in-built tutorial on single-cylinder SI engine. The following figure shows the final structure of the model.

This model uses the WoschniGT model for heat-transfer. This model closely emulates the classical Woschni correlation without swirl. A Wiebe function is used here to impose the combustion burn rate. In this model, the fuel is injected into the inlet port and the amount of fuel injected depends on the specified Air-Fuel ratio and the air-flow rate in the inlet-runner (intrunner). The in-built indolene-combust model was used for the fuel. The timing of the fuel injection is controlled by the cylinder. The specifications of the engine used in this model is summarized in the table below

Geometric Parameter	Value
Number of cylinder	1
Bore [mm]	86
Stroke [mm]	86.07
Connecting rod length [mm]	175
Compression ratio	9.5

Results:

The model was run with an rpm of 1800 and 3600 for 10 cycles and the important operating parameters were noted.

Parameters	RPM
	1800	3600
Air Flow Rate [kg/h]	24.6	58.6
BMEP [bar]	9.5	11.1
BSFC [g/kW-h]	239.2	241.8
Max-Cylinder Pressure [bar]	48.89	58.9
Brake Power [kW]	7.1	16.7
Brake Torque [N-m]	37.7	44.3
Brake Efficiency [%]	34.4	34.1

From the above table, it can be observed that the engine produces higher power at a higher rpm.

Objective 2: Increase the power by 10% at 3600 rpm

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 [2]

`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 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 higher heating value
6. Increasing the density of the incoming air
7. Increasing the Fuel-air ratio or decreasing the air-fuel ratio i.e to increase the richness of the fuel

In this project, we investigated the effect of displacement volume, density of incoming air and the air-fuel ratio on the engine brake power.

Case 1 - Increasing the displacement volume:

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

Case 1.1 – Increasing the bore:

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

The following figure shows the case setup from GT-Suite

The results obtained are shown in the figure

It can be observed that the brake power increases with increasing the bore. It can also be observed that the volumetric efficiency also increased with increasing bore value. The bore value of 90 mm gives a 10% increase in brake power

Case 1.2 – Increasing the stroke:

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

The following figure shows the case setup from GT-Suite

The same trend can be observed here. The brake power increases with increasing stroke. To get a 10% increase in brake power, the stroke needs to be increased to 98 mm

Case 2 – Increasing the density of air:

Assuming an ideal gas, the density is given by

`rho = p/(R*T)`

The density of air is inversely proportional to its temperature. Therefore, the model was run with 3 different temperatures: 300K, 295K, 290K, while the other parameters remained constant. In practical cases, the air density will be increased with a turbo-charger or a super-charger

The following figure shows the case setup from GT-Suite

It can be observed that by decreasing the inlet temperature, the air flow rate increased and thereby the brake power also increased. However, maintaining a low temperature will not be possible at every operating conditions. Therefore, in practise the air density is usually increased using a turbo-charger or super-charger

Case 3 – Decreasing the air-fuel ratio:

In this case, the air fuel ratio was changed to 8, 10, 12, 14 and its effect on the engine power is studied. The following shows the case setup from GT-Suite

It can be observed from the above figure, that richening the fuel actually leads to decrease in the brake power. This is because at low concentrations of air, not all fuel is converted into energy and most of the fuel remains unburnt, which also increases the CO emissions as seen from the table below

The same effect can be observed by increasing the fuel inject rate while keeping the air-fuel ratio constant.

Conclusion:

In this project, the 1D model of a single cylinder 4-stroke SI engine was sucessfully developed in GT-Suite. The model was then used to study the effect of different parameters in increasing the brake power of the engine at 3600 rpm.

References:

1. Breeze, Paul. Piston engine-based power plants. Academic Press, 2017

Heywood, J. B. (2018). Internal combustion engine fundamentals. McGraw-Hill Education