theoretical: what's a blower got that high compression ain't got?

[mikedc]

I understand a lot of the basics of forced induction but this kind of stumped me.


If the whole point of forced induction is to squeeze the A/F mixture harder before blowing it, then why not just use compression to do the job and forget the mechanical air pumps?

Of course the limitation on high compression becomes the octane rating of the gas . . . but that's also a limitation when you do super/turbocharging. What does S/Tcharging have that makes it better?

[GetMore]

Forced induction squeezes MORE air into the cylinders, it doesn't just increase the pressure in the cylinder.

[gkring]

and more compression plus more boost equals even more horsepower, again assuming you have the correct fuel to keep from igniting until it is time.

[mikedc]

Thanks for the replies, but I still don't get it.



What about this:

If an OEM wants to squeeze some more power out of an existing motor, they might put a blower onto it. If they do this they will probably only boost it a few PSI over atmospheric, so as not to outrun the capabilities of pump gas.

Now, in this scenario, why not just raise the compression instead of going to all the trouble of using that blower?

Isn't the ultimate limit on the usable cylinder pressure gonna be the gasoline's octane rating either way? Regardless of how that XXX pounds of pressure was achieved, mechanical air pump or piston compression?


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[TonyL]

Because raising the combustion pressure lowers the overall volume of air molecules in the chamber. Boost stuffs MORE in. A LOT more. More molecules, tighter packed. Simply increasing compression doesn't create more molecules in the chamber, the vacuum stays the same, so only pulls in enough to fill the volume of the chamber. See?



The fuel curve is changed in a supercharged car to deal with this "boost" of air. And more power is created.

/simplest i can make it.

[DeltaT]

Plus all the good setups use an intercooler that strips out much of the heat added by the compressor or blower, keeping temps down and allowing pump gas. Much of detonation revolves around the temps seen in the chamber, not the octane of the gas.

Jim

[WS6]

Tony is correct. It is not a pressure issue it is the volume of air in the cylinders that matters. Your engine is an air pump. It is burning the oxygen in the air. It merely consumes the fuel. Dumping in more fuel doesn't make more power. Dumping in more air does. Having the appropriate amount of fuel ensures that the metal parts in the engine are not consumed by the burning oxygen. That's why going too lean destroys the metal pistons and metal spark plugs.

N2O works because when it burns it creates more oxygen in the cylinder because again, it is the oxygen in the air that is creating power. Spraying N2O is closer to boosting an engine instead of raising compression.

The simple answer to your question of what a blower has that high compression doesn't is more air(oxygen).

[shmoov69]

Higher compression still relies on volumetric effency of the engine (since I can't spell, VE!). Boost throws VE out the window by cramming in more air/fuel than what can naturally be "sucked" in.
That is the easy way it was explained to me years ago.

[mikedc]

AAAHHH!

Okay, got it.

I was thinking too much about the pressure alone creating power, and forgetting that the raw "displacement" of the motor was being artificially increased by the blower method too. Makes sense to me now.

[WS6]

AAAHHH!

Okay, got it.

I was thinking too much about the pressure alone creating power, and forgetting that the raw "displacement" of the motor was being artificially increased by the blower method too. Makes sense to me now.

Not necessarily. Roots blowers are positive displacement blowers and I think screw blowers are too. A centrifugal blower(I believe Prochargers are called this) are not positive displacement blowers. A turbo is not positive displacement either. The difference can be easily felt when driving/riding in a car that has the different types of blowers on them. A Procharger is like a turbo, it must have RPMs in order to produce boost. A roots blower does not. It produces full boost from the get go. However, they all do the same thing and that's cram more air into the cylinders than can be achieved naturally. That's how they make the engine produce more power.