SI Engine Case Study

Select any SI engine model, get engine specifications, assume suitable data wherever possible, and run for at least Max Power and Max Torque condition.

1. Compare the results and list down important parameters

2.Post-process and take snaps for the following data

• PV and log PV diag
• in-cylinder pressure and burn rate
• valve mass flow and pumping loop

To complete the challenge a nineteen years old engine has been chosen (all the specs are taken from this website: https://www.carfolio.com/specifications/models/car/?car=77044)

The engine chosen was mounted on the Mercedes-Benz C180 (W203) from 2001 to 2005. It's a SI, naturally aspirated, straight 4 cylinders, 4-stroke, SOHC engine, with four valves per cylinder, water-cooled, characterized by a fuel system called "HFM fuel injection" that stands for "hot film engine-management, sequential multiport fuel injection".

Main geometrical specs are presented below:

Bore: 89,9 mm

Stroke: 78,7 mm

Bore-Stroke ratio: 1,14, so it's an over-square engine

Total displacement: 1,998 L (1998 cc)

Single displacement: 0,499 L (499 cc)

Compression ratio: 10,6

BMEP: 11,6 bar

Maximum power: 95 kW at 5500 rpm

Maximum torque: 185 Nm at 3500 rpm

Every simulation was based on this GTP example:

Let's start showing all the modifications done on the model:

The "connecting rod length" was chosen as twice the stroke of the engine.

Then in "case setup" the cases were launched: one at the maximum torque (3500 rpm) and the other one at maximum power (5500 rpm):

It's very hard to find detailed information about engines, for this reason, the simulation of these cases was based only on the parameters shown, obtaining the following values:

the first column was for the "case 1", run at 3500 rpm, so at the maximum torque. The official value and the one obtained by simulation are quite close, in fact:

• real maximum torque: 185 Nm;
• simulated maximum torque: 175 Nm.

The second column was for the "case 2", run at 5500 rpm, so at the maximum power. Even in this case the official and the simulated values are quite close:

• real maximum power: 95 kW;
• simulated maximum power: 108,1 kW.

Another important value that can be compared is the BMEP:

the real value is referred to the highest value of BMEP in the engine, corresponding generally at the point of maximum torque, as visible in the following equation:

"BMEP=(2 pi ε M)/(V)"

where:

M: is the torque [Nm]

V: is the total displacement of the engine

So, the highest value of torque corresponds to the highest value of bmep.

As visible, the real value and the one obtained by simulation are quite near:

• real value: 11,6 bar
• simulated value: 11,01 bar

Because of the lack of more detailed information about the engine, the simulations were run with all the other parameters given by the tutorial model of GTP, such as valves timing:

3500 rpm 5500 rpm

Following there is a comparison between different important values, for both cases:

• in-cylinder temperature

• in-cylinder pressure

3500 rpm 5500 rpm

plotting the cases on the same diagram is visible how the maximum pressure is reached at 5500 rpm:

• heat transfer rate (total)

• Burn rate

• PV diagram

at 3500 rpm at 5500 rpm

differences are highly visible plotting both cases in the same diagram:

• logPV diagram

both cases are plotted on the same diagram. Is visible how increasing the speed of the engine, there's an increase of the losses, figured as the lower area, that's bigger in the case of 5500 rpm than in the 3500 rpm case.

• Valve mass flow

what this diagram represents it's very important: it describes the airflow rate that enters into the cylinder, for every crank angle. Obviously, being a naturally aspirated engine, the air that enters inside the cylinder is a function of the depression generated by the movement of the piston from the TDC to the BDC, and so it depends on the rpm of the engine. After the BDC, some degrees before the IVC, there's a negative mass flow rate: this is caused by a reflux of the air inside the intake runner, due to the movement of the piston to the TDC. Notice that IVO and IVC reported in the diagram are referred at the case run at 3500 rpm.

• Pumping loop

3500 rpm 5500 rpm

These diagrams permit to show:

the pressure inside the cylinder;

valve lift and exhaust lift;

intake pressure;

exhaust pressure.

Very important is what happens around the TDC: both intake and exhaust valves are open, more air can enter inside the cylinder and can push the exhaust gases through exhaust valves. So, this effect increases the volumetric efficiency of the engine. The best situation would be a depression wave of exhaust gases around the TDC, at the closing of the exhaust valve, increasing the emptying of the cylinder by exhausted gases.