supercharged vs normally asperated engines 2

[jriordan]

If the compression ratio(CR) of an engine is set as high as is reasonable for the fuel used, how can a supercharger (or turbosupercharger) not cause detonation when it increase the incoming charges pressure? Or, to ask the question another way, isn't supercharging the same as increasing the CR.

 

[sreid]

You will get much better and more elaborate answers than this but

1) At higher RPM the cylinders are not completly filling due to air flow dynamics. The blower increases the filling pressure to compensate for this.

2) Blown engines usually have a reduced compression ratio to compensate for the increased cylinder filling pressure. The engine can still make more HP because there is more air/fuel in the cylinders.

 

[patprimmer]

I agree with sreid, but would like to expand a little.

1) Positive displacement blowers like Roots blowers can increase air supply to the engine, depending on blower displacement, engine displacement and blower drive ratio.

2) Turbos tend to work better at higher engine speeds, but this also depends on a few variables.

3) centrifugal blowers fall somewhat between these.

The objective of supercharging is not to increase compression, but to increase the amount of oxygen in the cylinder, so it can burn more fuel to make more power. The increase in effective compression is a co-incidental effect of this.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[ejit]

The combustion cycle is not continuous, due to the characteristics of the fuel, rod ratio, compression ratio etc. There may be a point in the engine's rotation where the combination of cylinder volume, pressure etc. causes the combustion pressure to be momentarily higher than the anti-knock quality of the fuel can manage.
This "spike" in cylinder pressure during combustion occurs later in the crank's rotation with supercharged motors, and has a lower maximum value for the same average pressure, and is therefore less knock-sensitive.

 

[patprimmer]

To more precisely answer the original question, if the engine is on the edge of detonation, then you do nothing else but increase manifold pressure, without changing temperature (yes I know it's near on impossible to actually do), you will get detonation.

Once the inlet valve is closed on the compression stroke, the chamber does not know how the gas got there, it just compresses what there is, which results in a pressure and temperature increase, Once this pressure/temperature increase exceeds a certain value for a given fuel, autoignition (detonation) will occur and cause knock.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SBBlue]

Predetonation is not caused by the pressure being too high.

It's caused by the temperature of the compressed air/fuel mixture being too high at the height of compression.

Now it is true that the more the compression ratio is increased the higher the temperature of the compressed air will be. But its possible to have high pressures normally seen with a high compression ratio without having the high temperatures.

Let me give an example.

Let's suppose you have a naturally aspirated gas engine with a 10:1 compression ratio. If you start with ambient air at 80 deg F and one atmosphere of pressure, after a 10:1 compression the pressure will be about 24 atms (360 psi) and the temperature will be 854 deg F.

Now let's use a supercharger with a 2:1 compression ratio. The temperature of the gas coming from the supercharger will be 198 deg F. With this gas you can do a 6.1:1 compression to get back to 24 atms of pressure, again at a temperature of 854 deg F.

But you can also use an aftercooler, and cool the temperature of the gas coming from the supercharger down to, say, 120 deg F. To get to 24 atms of pressure from this temperature and pressure, you only need to do a 4.4:1 compression, and the resulting temperature of the gas will be. . . .707 deg F.

So with aftercooling you get a much lower temperature at full compression -- and maintain the same amount of absolute pressure.

That's why aftercoolers are such nifty things!!!