Week-4 : Basic Calibration of Single cylinder CI-Engine

Basic Calibration of Single Cylinder CI Engine

Aim:

To compare the differences in developing a single-cylinder SI and CI engine model in GT-Suite and determine the influence of the MFB parameter on the CI engine performance.

Introduction:

The Spark Ignition (SI) and the Compression Ignition (CI) engines were developed to convert the chemical energy to mechanical work. However, they employ different operation principles to achieve their target. Some of the key differences between the two engines are summarized in the table below [1]:

Model Development:

GT-Suite was used to develop a 1D model of both engines. The following figures show the completed model of the engines:

SI

CI

The following table summarizes the major differences in their setup

The CI engine uses a turbocharger, but it was not modelled. Hence a higher inlet pressure was specified in the env-inlet to include the presence of a turbocharger.

The SI engine has a longer inlet pipe to prevent the reverse flow of the fuel from the cylinder into the inlet runner. The reverse flow will increase the air-fuel ratio and reduce its performance. This is not a problem in the CI engine because the fuel is injected directly into the cylinder during the power-stoke.

Comparison of specific parameters:

Both the models were simulated at 3600 rpm, and the important parameters are compared

The CI engine operates at a higher compression ratio, and hence the brake power and the efficiency are higher than the SI Engine. This high compression ratio also leads to the lower BSFC. At the same time, the CI engine often operates in the lean region leading to higher NOx emissions.

Influence of the MFB50 on the CI Engine performance:

Mass Fraction Burned (MFB) in each engine cycle is a normalized quantity that describes the probes of the release of chemical energy as a function of crank angle. It varies from 0 to 1 and shows how the in-cylinder combustion progresses with crank angle.

In the CI engine model, the parameter “main duration” in the combustion object is changed to vary the MFB. The main duration is the duration in crank angles where controlled combustion is taking place.

MFB50 is used to denote the crank-angle where 50% of the fuel is converted to energy. This is an important parameter in comparing the combustion control and duration of different fuels.

Three simulations were performed by changing the main duration from 20 to 50, and the corresponding MFB50 values are shown in the table below

When the MFB50 was increased, the following changes were observed:

1. The Brake power and torque decreased
2. The BFSC increased
3. The Brake efficiency decreased
4. The Air-Fuel ratio remained the same
5. The Exhaust Temperature increased

The Mass Fraction Burned (MFB) for the three cases were plotted as shown in the figure below.

As observed from the above figure, the in-cylinder pressure also decreased with the increase in MFB50, leading to lower power output.

Conclusion:

In this project, the SI and CI engine characteristics were compared and summarized, including their modelling strategies. In addition, the influence of the MFB50 on the CI performance was evaluated by modifying its combustion model parameters. An increase in the MFB50 signifies that the fuel burns slowly, and hence the power output was less.

References:

1. Spark Ignition (SI) Engine and Compression Ignition (CI) Engine: A Comparison - Bright Hub Engineering
2. DMJ (lukasiewicz.gov.pl)