Thermodynamic Cycle Analysis

The goal of this project is to compare an engine's performance based on the plunger stroke and cylinder diameter for different rotation regimes. The pair of values for plunger stroke and cylinder diameter have been chosen to kept a total engine displacement of 575 cc.

Mean Effective Pressure and Engine Power

The real engine efficiency is not based on the displacement relationship or horse power but the mean effective pressure in the combustion chamber which is the one that pushes the plunger. This is the Brake Mean Effective Pressure (BMEP). The next two graphs show the BMEP and power for different plunger strokes and rotation regimes.

All curves show a maximum of mean pressure which is when the performance is optimal. For low rotation regimes, except for low plunger strokes, the behavior is similar, although it can be noticed that for higher plunger strokes the effiency is better. For high rotation regimes the situation is the other way around, lower strokes have the highest mean effective pressure.

Something similar happens for the engine's power , where the 62mm stroke gives up to double power than the engine with 100mm stroke at 8000 rpm. This way, the lower the stroke, the more capacity from the engine to give power at high rotation regimes.

Intake Valve Closing

Choosing the right intake closing will directly affect the engine's performance for different rotation regimes. If it's too small, the amount of fuel introduced will not be enough and the whole cycle will result as non efficient. On the other hand a big intake valve closing will make the fuel mixture to come backwards through the valve during the plunger displacement which is not something to look for.

Keeping the plunger stroke to 98 mm, the next graphs show how the mean effective pressure varies as well as the engine power when the intake valve closing angle changes from 15º to 50º.

It can be noticed that for low rotation regimes, the lerger the intake valve closing, the smaller the mean effective pressure. This is due to the pressure that the plunger will make when it increases will be lower. On the other hand when the engine works at high rotation engines and the intake valve closing is again too small, there's not a correct fuel admission and the performance is undesirable.

For the power behavior the performance is similar to the mean effective pressure with the difference that for low rotation regimes, the power variation is practically zero.

Exhaust Opening Advancement

If the exhaust valve opens at the TDC, there is the risk of the high pressure gases damage the engine since the valve would not be completely open. To avoid this scenario, the exhaust valve opening is advanced when the plunger reaches the TDC, so the exhaust of the gases gets started already since the pressure is higher inside the cylinder.

With the chosen value for the intake valve closing of 30º and the plunger stroke 98 mm, the graphs show the total cycle's work for different exhaust opening advancements and rotations regimes.

P-V for 2000 rpm

P-V for 3000 rpm

P-V for 4000 rpm

Result Comparison

For the above cases, it can be noticed that the cycle's peak is at a larger pressure as the valve opening advancement decreases. The results can be clarified from the data extracted from the software which gives the total work performed by the engine and it's the area under the curve.

It has been chosen as optimal angel for valve opening advancement 45º and also can be extracted from the data that as the rotation regime increases, a larger angle will be the optimal.

Compression Relationship

The usual power given by the engine has been calculated using the expression for mean effective pressure for a tipical rotation regime.

For 3000 rpm the power for compression relationships of 8, 10, 12 are 94, 99, 100 CV respectively. These values make sense with the measured results for a family van taken during class.

Ignition Advance Curve

When advancing the ignition process, danger of detonation in the engine is higher and needs to be prevented. A maximum margin for detonation danger coefficient has been settled in 0.9. When the rotation regime is at the point that the maximum mean effective pressure hasn't been reached but the canger of detonation was maximum, these results were considered for the analysis.

As it can be seen from the figure the lower the compression relationship, the earliest the optimal ignition advance angle is reached.

Conclusions

As it can be concluded from the project, combustion engines are very complex systems and it have been only analysed a few parameters invovled in the whole process.

The election of a family van was made, with high plunger stroke. Although we considered the tradeoff between power and efficiency, some parameters weren't considered that could have reached a total power of 100 CV.

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