turbo vs compression thought experiment

Without getting too complicated and trying to say things that I don't quite understand....

Say you have a 8:1 CR turbo motor at 14.7 PSI. Your effective compression ratio is 16:1.

Now say you have a 12:1 CR turbo motor at 5.0 PSI. Your effective compression ratio is 16:1.

Given that a 12:1 CR motor will make more power before the turbo gets going, why is it rare to see 12:1 CR motors with a turbo, but it is common to see 8:1 CR motors pushing 20 PSI?

Why does the high compression motor make less power even though the effective compression ratio is the same?

http://www.rbracing-rsr.com/compression.htm

Taiden wrote: Why does the high compression motor make less power even though the effective compression ratio is the same? http://www.rbracing-rsr.com/compression.htm

because there is more air and fuel packed in the combustion chamber in the high boost motor.

After thinking for awhile, I agree with 2002maniac.
Taiden, think of it this way: the more air and fuel, the more power (easy enough). A 2.0L engine is only able to displace 2.0L of volume every two revolutions (disregarding volumetric efficiencies). Say, if you pack 2.0L bottle with 14.7 psi of air, it will have more air than if it were packed it 5 psi of air (all things being equal...etc etc). Same thing with a motor.

Yeah, but I thought 'effective compression ratio' was how much air was squeezed between piston and head/valves/etc.

Meaning that a motor with effective compression ratio of 16:1 would compress 16 units volume of air fuel mixture into one unit of volume space. I am assuming that a 16:1 (effective CR) motor compresses as much gas. My whole point being that the two motor comparisons would see the same amount of air... (and fuel)

I see this being true because any motor when you give it 14.7 PSI of boost, the effective compression ratio DOUBLES. This makes sense, because you are forcing it to take in TWICE the amount of air molecules (due to atmospheric pressure being about 14.7 PSI).

Yes? No?

But to compress that mixture, you have to get it into and out of the combustion chamber. Blowers do that more efficiently. A blower will increase intake pressure, which means it will move a greater mass of air per volume past your valves. You may only be compressing 16:1, but each of those 16 units is bigger. It's not the volume, it's the mass/density.

There are some airplane engines (mostly high-volume radials, I think) that use low pressure blowers just to help move fuel/air into the compression chamber.

I would imagine the airplane engines are doing that to battle pressure drop at high altitude. ???

I think I am taking the 'effective compression ratio' term too seriously. I understand how turbos work, but I assumed that two motors with identical 'effective compression ratios' would encounter a very similar amount of air given a constant displacement.

On the intake stroke the non-turbo engine can only pull in the air and fuel that will fit in the cylinder at a pressure of 14.7 psi whereas the turbo engine will have additional air+fuel coming in due to the increased manifold pressure. Beyond that, I dunno. 12:1 probably likes to detonate a lot more than 8:1 + 20psi boost.

This was bothering me all day. So here is what I did to prove to myself (if you actually want the excel file, I can email it out; I wont even be offended when you fix a mistake ).

There are a bunch of assumptions in there (ie: the autoignition temp of gasoline being 520°K)

The first case is atmospheric pressure filling a 500cc cylinder with a 25cc combustion camber to try to match the effective CR for the second case. The second case has double the atmospheric pressure (ie: 14.7 psi of boost). What is vital to the conversation is that little number of moles of air. The more moles, the more air in the cylinder. The more air will have a higher potential for making power. (Whew... )

I'm definitely open for discussion on my methods...

think of it in terms of absolute pressures. assuming 100% VE in this little comparo as well.

the low compression motor with the higher boost will have an absolute pressure of 29.4psi in the cylinder before it starts compressing the charge.

the high compression motor will have an absolute pressure of 19.7psi in the cylinder before it starts compressing.

the volume of the cylinder at BDC in these two theoretical engines is the same, so the one running 1 bar of boost will have more molecules of fuel and air in the cylinder, and thus, will make more power.

Strizzo wrote: the low compression motor with the higher boost will have an absolute pressure of 29.4psi in the cylinder before it starts compressing the charge. the high compression motor will have an absolute pressure of 19.7psi in the cylinder before it starts compressing.

Okay, but when you compress 29.4 psi into 1/8th the space, you get 235.2psi. When you compress 19.7psi into 1/12th the space, you get 236.4 psi.

(light bulb goes on)

Okay... I think I get it. Those two are basically the same pressure (236psi +/- 1psi), but the volume of air/fuel mixture in the first motor is larger, 62.5ccs versus only 41.6ccs in the second.

ReverendDexter wrote: Okay... I think I get it. Those two are basically the same pressure (236psi +/- 1psi), but the volume of air/fuel mixture in the first motor is larger, 62.5ccs versus only 41.6ccs in the second.

Right! And your energy doesn't come from volume of combustible mixture, it comes from combustible mass.

Taiden wrote: I would imagine the airplane engines are doing that to battle pressure drop at high altitude. ??

There are a number of airplanes that do that too. The particular trick I'm thinking of was frequently used with old, stupid WWII era radial engines (like Russian engineered ones). Those things had huge combustion chambers with incredibly low compression ratios (about 6:1, so that they could burn damn near anything). They'd put a supercharger on that would only push about 2-4psi. But that was enough to help fill those huge compression chambers.

As I understand, it also helped even the fuel air mixture within and between all the cylinders, what with the really crude carbs. I could be wrong on that count though.

In reply to Salanis:

Re: volume vs mass. I think that for this discussion, assuming a/f is the same in both, volume of mixture is interchangeable with combustible mass. At 12:1 a/f, 1.5L of mixture will make more energy when burned than 1L of mixture.

Not sure if you were disagreeing or not. Apologies for preaching to the choir if not.

RossD:

Unless I misunderstand your spreadsheet...

Pressure is higher in the second example, but so is your effective compression ratio.

Check this out:

3,822,218 Pa / 4,004,229 Pa = ratio of 0.9545

21 CR / 22 CR = ratio of 0.9545

EDIT: I see now. I was looking at cylinder pressure, which we already agreed would be the same. You have almost double the moles of oxygen in the second motor configuration!

Interesting thread. what bearing does the cooling capacity of the charge have on the ability of a motor to make power? Can the lower compression motor run more timing due to the lager volume of uncompressed gas cooling the combustion chamber surfaces as the fuel atomizes? Huh.

I was wondering something along the same lines as Shaun. Air temperature affects detonation. Would the smaller engine with higher boost be more susceptible to it?

yes, the turbo would have to flow more air and make more boost in the lower compression motor than the high compression motor, so if the required extra flow/boost pushed the turbo out of its efficiency range, there could very well be a lot of heat/detonation issues along with the associated power loss.

Strizzo wrote: yes, the turbo would have to flow more air and make more boost in the lower compression motor than the high compression motor, so if the required extra flow/boost pushed the turbo out of its efficiency range, there could very well be a lot of heat/detonation issues along with the associated power loss.

So would the increased gas and fuel flow of the lower compression motor yield more cooling? It would seem so, but then it would be making a bigger bang as well, so it would need more cooling. coupled dependencies galore, this stuff gets complicated quick.

Strizzo wrote: Re: volume vs mass. I think that for this discussion, assuming a/f is the same in both, volume of mixture is interchangeable with combustible mass. At 12:1 a/f, 1.5L of mixture will make more energy when burned than 1L of mixture. Not sure if you were disagreeing or not. Apologies for preaching to the choir if not.

I think we are disagreeing.

I'm saying that volume and compression remain the same between both engines, but that the engine running higher pressure will actually move a greater mass on intake. Higher Pressure within the same Volume, means more mass. D=M/V. If V remains constant, and you increase D, you must also increase M.

1L of mixture under 1bar of pressure will have a certain mass. 1L of mixture under 2bar of pressure will have double that mass.

In terms of boost pressures, you'd probably get a bit less than 150% of the mass, since at 0 boost, your mixture is technically at 1bar of pressure. Plus the downstroke of the piston creates a vacuum that draws in mixture at a bit more pressure.

That might also be why boosted engines like lower compression. They're not just seeing greater pressures, they're also compressing more mass.

I think we are agreeing actually. What you described is the way I see it in my mind, maybe the way I described it is lacking clarity. Or we are discussing the same concept using different variables to quantify it. Or I'm so wrong that I can't even begin to fathom how wrong I am.

Boosted engines like lower compression so that there is less chance of detonation. It's about heat management in the combustion chamber.

When you compress a gas, there is heat generated. If you compress "more" gas, you get more heat. Boosted engines have a triple whammy - the compressor adds heat, there is heat added when the piston compresses the gas, and more heat is generated because the gas is already under pressure.