Relationship of boost, torque, and cylinder pressure 2

[obanion]

Hope you guys can help settle a engineering argument.

Let's make the example very static to keep things simple.

Case 1:
3.0L motor, 6000RPM
25psi boost pressure
20psi exhaust back pressure
Making say 800ft/lbs of torque

Case 2:
3.0L motor, 6000RPM
45psi boost pressure (compounded turbos)
40psi exhaust backpressure
SAME intake temp as case1, due to good intercooling
Due to different cam, valves, head porting, etc, still only makes the same 800ft/lbs of torque

Disclaimer: Please keep to question below. Let's not get pulled off on a tangent of the how and why of the turbo setup or intercooling. Just assume for the sake of this case, that what I posted above for case1 and case2 are correct.

Now, would the peak cylinder pressures be significantly higher in case 2 as case 1? I don't think so. Here's my reasoning.

All that matters is what's IN THE CYLINDER when the intake valves close. The pressure in the manifold matters not, it's just what makes it into the cylinder. Case 1 has superior VE, and is able to pack X amount of air:fuel into the cylinder well. Case 2 has lower VE, but through higher manifold pressure, is able to pack the same X amount of air:fuel into the cylinder.

Now the only difference I can see on the composition in the cylinder is leftover exhaust content. Case1 has 2.36BAR absolute pressure (or close) exhaust left in the cylinder at the end of the exhaust stroke. Case2 has 3.72BAR absolute pressure. So there will be about 57% more leftover exhaust gas. Since exhaust gas is neutral to combustion (no oxygen), I don't see this as making much of a difference. Some, but VERY minor.

Anyone concur with my conclusion?

 

[patprimmer]

To make the same 800 ft lbs net torque output, you will need to generate slightly higher cylinder pressures so as to overcome the extra pressure over the piston during the exhaust stroke. You have to pay for the extra work to get the extra boost to overcome the low VE of the head, as you have correctly pointed out by upping the boost and exhaust back pressure.

I expect that it will be a relatively small increase, but I have no data to support that opinion. It is based on intuitive assessment.

I agree with your assessment of the residual exhaust gas if things are kept static.

Do any of the motor specialist wish to comment on the overall significance of pumping losses

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.

 

[obanion]

A quick sim on a 25psi vs 45psi through Engine Analyzer Pro shows about a additional 15 pumping HP required. Not much.

 

[SBBlue]

For all practical purposes, the scenario you present is not realistic -- for a gas engine.

Here's why. For all practical purposes, a gas (spark ignition) engine will undergo stoichiometric combustion. The air/fuel ratio for stoichometric combustion is 14.55:1. That means for every 14.55 pounds of air, one pound of gasoline will burnt.

Now consider your engine. A 3 liter normal aspirated engine running at 6000 rpm will pump 9000 liters/min a minute through the engine. This is about 25 pounds of air a minute. An engine with 25 pounds of boost will pump about 65 pounds of air a minute; an engine with 45 pounds of boost will pump about 100 pounds of air a minute.

With stoichometric combustion the 45 pound boost engine will be using about 50% more fuel than the 25 pound boost engine. An engine that is using 50% more fuel will develop more torque at equal rpm.

 

[Azmio]

Obanion,

Interesting question, my initial guess is that the IMEP for both cases are different. However, the two cases generate roughly the same BMEP at 6000rpm. So I suspect that the FMEP for both the cases are different.

For engine with higher boost pressure, the engine FMEP is higher because the engine requires more energy to compress more air inside the cylinder. The energy requires to reject more combustion gases and by product after combustion is also higher. The higher backpressure from the turbine will make it worse. This makes the piston side load higher. Moreover, the piston ring rubbing against the bore is also generating higher friction. In addition to that, the load to the crank bearing, conrod bearing and piston assembly are also higher. All these will definitely increase the FMEP

Now you may not agree with me but I think case 2 has higher peak pressure, because the higher boost pressure and higher fuel used to get around 8-10:1 A/F ratio will generate higher peak pressure.

Now one thing that you can do is to check the heat load rejected to the coolant, oil and exhaust gas. I am quite sure that case 2 will have higher overall heat rejection load. Otherwise, with all the differences in opinion among us will get this forum longer than what it is supposed to be.

May I know what engine is this? Both are with 6 cylinders? Does case 2 have lower compresssion ratio and retarded ignition timing?

AO

 

[patprimmer]

Azmio and SBBlue

I took this to be a hypothetical case.

The original question strongly implies this by stating:- Disclaimer: Please keep to question below. Let's not get pulled off on a tangent of the how and why of the turbo setup or inter cooling. Just assume for the sake of this case, that what I posted above for Case 1 and Case 2 are correct.

What has air fuel ratio got to do with it. The type of fuel is not even mentioned, nor is the method of metering it.


To do this realistically, you would need to have case 2 have maybe only a few mm of inlet valve lift or maybe less than 180 deg duration.

I was reaching for the red flag, as I was starting to wonder if obanion is a student having us help him cheat with his homework, but he has started several other very hypothetical threads that are to far from reality to even be hypothetical cases for "homework" assignments, I guess he is just someone with some knowledge and a vivid imagination.

Even if it is a long way into left field, it is thought provoking



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.

 

[patdaly]

Pat, I will step up to the plate for Dereck. I have known him for several years through a Chevrolet 9C1 Police car list, and can assure you, he is real, and genuinely wants to understand some pretty esoteric subjects, sp please, dont red flag him.

Thanks, Pat

 

[obanion]

The application is real. I'm building it now. Compounded turbos, Toyota 2JZ (3L, I6) motor, hopefully liquid propane electronic port injection, paired with injectors spraying a
50/50 methanol/water mix (MW50). Ratio will be between 2:1 to 1:1 propane:MW50 ratio. MW50 will only be activated under boost.

There are 2JZ motors out there making over 1000RWHP in the ~35psi range. They are of the better breathing variety, with mega headwork, cams, and of course a big turbo.

The argument is hypothetical, in that I oversimplified some of the variables by keeping them static, so as to settle a particular discussion I'm having elsewhere.

Azmio explained it very well. Just what I was looking for. However, I'm looking for a number. The argument started out as something like "is 1000HP at 45psi going to be SIGNIFICANTLY harder on the rods and head bolts than 1000HP at 25psi?" Let's assume we are also talking about the same RPM for peak HP.

No one, including myself, has suggested it won't be harder to some extent. The argument in question comes down to matter of degree. I think it would take at most 5% extra peak pressure for the extra work of the high boost, my opponent thinks more like 20%. Basically, I think he feels that 45psi will almost certainly grenade the motor, even if there isn't knock.

 

[SBBlue]

Pat asked:

"What has air fuel ratio got to do with it. The type of fuel is not even mentioned, nor is the method of metering it."

If we're talking about a spark ignition engine -- and I think we are, since it appears to be for racing -- then the air/fuel mixture going into the cylinder must be in a stoichometric range -- about 14.55 pounds of air for every pound of fuel (assuming gasoline -- it will be different for other fuels).

If you have two engines of equal displacement, running at the same RPM -- then the air flow in the engines will be the same (assuming the same volumetric efficiency, valve timing, etc.) It really doesn't make any sense to adjust the intake valve timing so the valve isn't open as long if you're increasing boost pressure. (Unless you want to do some Miller Cycle stuff).

So, the engine described that has 25 pounds of boost will pump about 65 pounds of air a minute. The engine that's working at 45 psi will pump about 100 pounds of air a minute.

Since the amount of fuel is proportional to the amount of air flow (for all practical purposes), increasing the air flow by 50% will result in the fuel flow increasing by 50%. An increase in fuel flow of 50% will result in an increase in power by about 50% (roughly).

Therefore, it's unlikely that if both engines are running at the same rpm and torque (and therefore developing the same amount of horsepower) that one will be using 50% more gas. If it did it wouldn't make any sense to run 45 psi of boost when 25 psi will give you the same power output.

 

[Azmio]

SBBlue,

We did a lot of NA production engines, I havent seen any engine running at 14.5-14.7:1 A/F ratio at full power. At the rated speed, not all the fuel injected can be evaporated and 'participate' in the combustion. Some of the fuel got stuck in the port and intake port. Some of the fuel is being absorbed to the bore surface that is wetted with oil. Therefore, 12:1 is more realistic for NA application.

For turbocharged, I have heard about the engine tuner injecting as much as 7-8:1 A/F ratio. They need the extra fuel to cool off the valves, spark plug and the sharp edges. Otherwise, preignition and later knock or knock and later preignition are likely to happen.

You also mentioned about there isnt any need to adjust the intake timing. My team on the other hand will optimize the intake timing everytime the boost pressure is increased from say x bar in the base engine.

Injecting the extra 50% of fuel to the cylinder to adjust it to the increased air mass will not necessarily increase the power by 50% at the crank. There are a lot of factors involve that will make the increse non linear. You can check the relation of the increase in boost pressure for the F1 engine in the 80s.

 

[patprimmer]

Patdaly

Thanks for putting me straight, but I had no intention of red flagging once I decided it was not a students homework. My initial reaction was that as it sounded very theoretical and hypothetical, it had the hallmarks of a homework question, and I am sure you all agree with this site's very strict rules about not helping students cheat.

It was obvious obanion was not a student once I looked up his history, so the flag was most definitely put out of consideration.

All the talk about AF ratio is sound in fact, SBBlue's points being theoretical accurate and Azmio's, points being accurate re actual practise, but they miss the point of the original question.

As I understand it the question is how much extra gross power would need to be generated to produce the same net output, and what extra strain does this put on the internals of the engine.

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.

 

[obanion]

"As I understand it the question is how much extra gross power would need to be generated to produce the same net output, and what extra strain does this put on the internals of the engine."

Correct!

 

[patprimmer]

obanion

I have answered as far as my knowledge can take me. I expect the answer you require involves some complex thermodynamic calculations that are beyond my mathematical abilities.

I have seen SBBlue post some excellent calculations along these lines in other threads, and I hope he comes aboard and helps out on this one.

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.