Boost compression

[Tony D]

It most certainly does.

 

Volumetric efficiency is a ratio of the engine's airflow to the engine's displacement and rpm. The more air the engine flows' date=' the better the VE. Adding boost (positive manifold pressure) increases airflow to the engine, thus yilelding a higher VE.

 

The formula is VE= 3456 x cfm / displacement x rpm[/quote']

 

I would have to disagree!

 

You are confusing the issue here, and using a N/A VE formula which is inaplicable to a boosted operation---especially with a turbocharger.

 

VE will determine what BOOST you make, as boost is a relationship to restriction to FLOW!

 

If your VE goes up, with constant CFM to the engine, the Boost observed will DECREASE. Problem is a turbocharger is a variable displacement dynamic compressor feeding a positive displacement compressor (the engine).

 

VE refers to the efficiency of the positive displacement flow through the engine in reference to ideal cylinder filling.

 

The Turbocharger "Increases" VE by making the mass flowed through the engine increase, but the efficiency of the pumping action of the engine is not really changed at all.

 

If the VE of the engine actually increased, the turbo boost pressure shown would drop.

 

It's splitting hairs to be sure, and most people get lazy and simply use VE as that sort of generic term that relates to total mass flow through the engine, instead of the actual efficiency of the pumping action of the engine----which is actually what VE is supposed to refer.

 

VE increases come through decreasing pumping losses, and a turbo doesn't do that. It simply boosts the head pressure available for the engine to draw. in all actuality, the VE of the engine (as discussed obliquely above) remains the same----the formula should incorporate inlet pressure to the engine, and related mass flow (hidden in the constant of "3456" above, which only relates to an engine operating at atmospheric pressure). When you properly calculate it, the VE will not change appreciably when considering on boost or off-boost operation. A notable exception is the RB26DETT. When the VE is calculated on that engine off-boost, you find pumping losses are higher than when under boost. This was primarily because the inlet manifold was optimized for flow conditions under higher boost pressures, and not the lower pressures encountered while in N/A operation.

 

What you have stated is the same assumption of "Boost Compression Ratio" using two stages of compression and lumping them together as if they were a single system. That's not how it works!

[dr_hunt]

A supercharger (also known as a blower) is an air compressor used to force more air (and hence oxygen) into the combustion chamber(s) of an internal combustion engine than can be achieved with just normal atmospheric pressure. Any device which does this is a supercharger.

 

The additional mass of oxygen-containing air that is forced into the engine improves on its volumetric efficiency which allows it to burn more fuel in a given cycle - which in turn makes it produce more power. A supercharger can be powered mechanically by belt-, gear- or chain-drive from the engine's crankshaft. It can also be driven by a gas turbine powered by the exhaust gases from the engine. Turbine-driven superchargers are correctly referred to as turbo-superchargers - or more commonly as turbochargers.

 

Edit:

 

Volumetric efficiency in internal combustion engine design refers to the efficiency with which the engine can move the charge into and out of the cylinders. More correctly, volumetric efficiency is a ratio (or percentage) of what volume of fuel and air actually enters the cylinder during induction to the actual capacity of the cylinder under static conditions. Therefore, those engines that can create higher induction manifold pressures - above ambient - will have efficiencies greater than 100%. Volumetric efficiencies can be improved in a number of ways, but most notably the size of the valve openings compared to the volume of the cylinder and streamlining the ports. Engines with higher volumetric efficiency will generally be able to run at higher RPM, and thus power, settings as they will lose less power to moving air in and out of the engine.

[Clifton]

Increasing the air density doesn't increase the engines ability breathe (v/e) regardless if there is more air entering the enigne from the higher manifold pressure.

[Tony D]

As a system you are correct. As a system the efficiency increases.

 

But when you calculate the VE of the engine itself, using a proper formula and not one with a lumped-together constant making N/A assumptions, you will find the VE is unchanged.

 

What you have is a "total compression ratio" across the two compressors of 14:1 in my original posted example.

 

In reality, the individual parts are still operating individually and should be looked at as such.

 

Which was the point of the "Boost Compression" calculator fallacy.

 

Same goes for VE calculators using that kind of formula.

 

The VE (Pumping efficiency) of the engine doesn't change. it still has the same mechanical compression ratio.

 

The VE (Pumping Efficiency) of the total system of turbocompressor and engine does increase.

 

Which is what I'm trying to get across. Like "Boost Compression Ratio" is inaccurately applied, so is saying "VE of the Engine increases" which it does not. It's still pumping with the same efficency. That it flows more through it is a function of the supercharger raising that inlet head, it's not pumping that pressure any more efficiently than it would N/A. It's a VE increase for the system, not the engine.

[Tony D]

I think the point missing is that turbochargers use energy in the exhaust gas to pressurize the intake charge, hence zero effect on pumping loss of the engine itself. Roots blowers or other belt driven is a different kettle of fish.

 

Actually, the argument can be made that turbos are not "free" as the exhaust restriction imposed by many costs power.

 

That the payback of the non-linear response of a dynamic compressor outpaces the linear requirements of a positive displacement compressor is the only reason there is a net gain.

 

The belt driven items have a easily calculatable loss, figuring out the loss from exhaust restriction is much more difficult.

 

F1 engines have (had) very low backpressure, but weren't really efficient but near design point.

[Tony D]

Er...

"We Digress!"

 

lol

[dr_hunt]

Er, what he said.

 

Right the turbo is a hard to figure component. Basically the exhaust restriction is the only loss but it has to be calculated somehow. But don't you agree at least in part that the turbo engines operate at a higher VE than those calculated for N/A??

[TimZ]

As a system you are correct. As a system the efficiency increases.

 

But when you calculate the VE of the engine itself' date=' using a proper formula and not one with a lumped-together constant making N/A assumptions, you will find the VE is unchanged.

 

What you have is a "total compression ratio" across the two compressors of 14:1 in my original posted example.

 

In reality, the individual parts are still operating individually and should be looked at as such.

 

Which was the point of the "Boost Compression" calculator fallacy.

 

Same goes for VE calculators using that kind of formula.

 

The VE (Pumping efficiency) of the [i']engine[/i] doesn't change. it still has the same mechanical compression ratio.

 

The VE (Pumping Efficiency) of the total system of turbocompressor and engine does increase.

 

Which is what I'm trying to get across. Like "Boost Compression Ratio" is inaccurately applied, so is saying "VE of the Engine increases" which it does not. It's still pumping with the same efficency. That it flows more through it is a function of the supercharger raising that inlet head, it's not pumping that pressure any more efficiently than it would N/A. It's a VE increase for the system, not the engine.

 

Tony - I actually started to make that exact same post regarding VE of the engine not changing while under boost, and I completely agree with you. Unfortunately, when I looked around the 'net for supporting material, all I found was the same definition that Dr. Hunt posted - actually, that exact definition is posted word for word on about 20 different websites - not sure who posted it first, but there sure is a lot of plagerism going on out there.

 

I decided to let it go, since it wasn't germaine to the conversation. But now that you mention it...

 

It all depends on where you want to draw your system boundaries. The Wikipedia (et al) definition draws it around the entire engine/supercharger. However, I've never been convinced that this is a useful thing to do. You need to consider how you might use the measurement of VE. For instance, if you are trying to create a fuel map for your EFI system, it makes much more sense to think of the VE of the engine seperately from the turbo. The VE of the engine itself is pretty well understood, and is repeatable (i.e., at X manifold pressure and Y RPM, the VE is Z - correct for temperature, and I know how much fuel to put in). If you try to lump the turbo/intercooler into this, it becomes very messy very quickly, to the point that it is no longer useable.

 

On the boost compression thing, I would still take issue where you mentioned that in your example the "total compression ratio is 14:1" - it isn't. Don't give in so easily.

 

For the purposes of this conversation, "compression ratio" refers to the compression ratio of the thermodynamic cycle (in this case, the Otto cycle). The compression resulting from the turbo/superchager is not part of this cycle, and can only influence it's initial conditions (i.e., the starting temperature), not the cycle itself. This is fundamentally why the "boost compression" idea doesn't work.

 

Actually, now that I'm thinking about it, it's also a pretty good argument for keeping it out of the VE calculations, too.

[TimZ]

actually, that exact[/b'] definition is posted word for word on about 20 different websites - not sure who posted it first, but there sure is a lot of plagerism going on out there.

 

Before I make anybody mad, I did not intend for this to imply that Dr. Hunt was plagerizing - it was clear to me that he included that as a reference.

 

What I did mean to imply was that seeing this same definition in so many different places kind of made me wonder about it's validity - did anybody see Steven Colbert editing Wikipedia to help out the elephant population?

[zguy36]

I believe that I got into a rant a while back about the definition of VE. I agree that the turbocharger should be left out of the equation and that the system boundaries should include inlet pressure as the pressure AFTER the turbo. This then means that VE doesn't increase with a turbo.

 

The reason that I am posting is to point out the fact that the VE does in fact DECREASE with the installation of the turbo. We cannot completely remove the turbo from the equation because it is such a huge restriction on the exhaust. This restriction makes it much harder for the engine to breathe. Hence the reason two engines make different power at the same boost left with different sized turbos.

 

A supercharger does not affect the VE value over NA since it is belt driven and does not impose a restriction on the flow through the engine. This can be a bit confusing though, since a small supercharger doesn't flow enough air for high rpm. This does NOT decrease VE though, since the equation is only taking the ratio of air drawn in vs. what could be drawn in at the given pressures.

[dr_hunt]

Yeah, it was a reference, yes I did plagarize since I didn't quote my reference. Sorry! It better be verbatim since I did the control c, control v thingy!

 

I'm speaking clearly from an overall mass balance point of view with my block diagram just looking at the engine itself. One would have to look at the turbo itself as well and calculate the HP it takes to compress a given volume of air to a certain pressure with a certain temp increase, exhaust temps befor and after turbo too, which is relatively easy.

 

I agree with you in part, but I disagree with you in part that the turbo is such a huge restriction in the exhaust as to negate any increase in VE. But there is no free lunch, I know that much. So I want you to ponder this. This is an actual engine on a chassis dyno and the car has run a best of 7.97 at 178mph, which I witnessed in person and the dyno too. HP calculates with that MPH to be over 1400rwhp!

 

Stang, 3200 lb, 347 cubic inches, single turbo, water/air, race gas, maxed the chassis dyno at 1200rwhp at 5000 rpm. What's the VE on that engine at 25lb of boost, 7000 rpm to achieve that HP level? My calculations show that it's got to be 100% to get that HP. So, clearly a turbo increases the VE IMO.

 

Edit: VE depends on combustion efficiency, I think that's the missing link since you are pressurizing the intake and adding more fuel/air mass and simultaneously increasing the compression which should increase efficiency IMO.

Perhaps you have something else that maybe we could compare numbers on.

[dr_hunt]

Quote from the sticky on turbocharging;

 

"Many will argue that nothing is free and you need pressure to spin the turbine, and this must take pumping losses." (This is your argument) "They are wrong because the turbo is not getting anything for free at all, it is just making the engine more efficient. It is true, there are pumping losses, but on the other hand there are pumping gains as well. If the exhaust back pressure is lower than the intake, the intake pressure makes more force on the intake stroke to help push the piston down. At the same time, another piston is on it's exhaust stroke. So, the intake pressure is more than cancelling out the exhaust pressure. Not FREE, JUST MORE EFFICIENT."

 

So, turbocharging increases efficiency, thats all Im saying.

[TimZ]

So' date=' turbocharging increases efficiency, thats all Im saying.[/quote']

 

No argument there - I think the only thing we are saying is that maybe VE isn't the best place to express this.

[zguy36]

The loss of VE from the exhaust restriction is not due to pumping loss, but loss from leftover exhaust gasses remaining in the combustion chamber. If there 30psi in the exhaust manifold, then you get respectivly more exhaust gas in the combustion chamber. Decrease the leftover gas, you can bring more gas into the cylinder for the next charge. Exhaust gas left in the chamber is bad for making power, hence the reason small restrictive turbos don't make huge numbers. Let the exhaust breathe more with a bigger turbo, horsepower numbers increase along with VE.

[Tony D]

The loss of VE from the exhaust restriction is not due to pumping loss, but loss from leftover exhaust gasses remaining in the combustion chamber. If there 30psi in the exhaust manifold, then you get respectivly more exhaust gas in the combustion chamber. Decrease the leftover gas, you can bring more gas into the cylinder for the next charge. Exhaust gas left in the chamber is bad for making power, hence the reason small restrictive turbos don't make huge numbers. Let the exhaust breathe more with a bigger turbo, horsepower numbers increase along with VE.[/i']

 

Reversion is a pumping inefficiency.

 

But if your turbine is sized correctly, there will be no more pressure in the intake manifold than in the exhaust manifold pre-turbine. That means it's all back to pumping efficiency to determine VE.

 

But your last statement is exactly what was said earlier: decrease the pumping losses, and you increase VE. Putting a bigger, less restrictive turbine wheel on the exhaust decreases pumping losses. Air in and air out.

[Clifton]

Reversion is a pumping inefficiency.

 

But if your turbine is sized correctly' date=' there will be no more pressure in the intake manifold than in the exhaust manifold pre-turbine. That means it's all back to pumping efficiency to determine VE.

 

But your last statement is exactly what was said earlier: decrease the pumping losses, and you increase VE. Putting a bigger, less restrictive turbine wheel on the exhaust decreases pumping losses. Air in and air out.[/quote']

 

Having measured my turbine pressure with 2 different P trim wheels I would have to say this is not realistic on a turbo setup that will be driven on the street. For a race car that will be in the rev range need for an extremely large a/r, sure.

[dr_hunt]

You still have to consider the pumping gains from pressurized intake charge ,which is greater than the pumping loss, since the intake pressure exceeds the exhaust pressure.

[Clifton]

since the intake pressure exceeds the exhaust pressure.

 

Not on street driven turbo engines.

[Tony D]

You still have to consider the pumping gains from pressurized intake charge ,which is greater than the pumping loss, since the intake pressure exceeds the exhaust pressure.[/u']

 

This is most definately not the case! On an undersized turbine perhaps. But for a perfect example I refer you to JeffP's last setup where his exhaust gast pressure was 23psi at the turbine inlet when his intake manifold was at 23psi!

 

This turbo came on violently at 3000rpms to full boost, not one of the laggy an controlable progressive gradual increasers with a 4500 boost threshold.

 

Everyone said his turbine was "too small" and "restrictive" but actual testing on his engine proved the theorists wrong in that case.

 

I would argue his setup at that point was ideal for what it was, even though he has gone to a larger A/R turbine now, with a more progressive boost building.

 

I guess having a 450hp differential in about 500 rpm can be a bit of a control problem in a corner!

 

Usually exhaust pressure is "higher" than intake pressure. On racing engines like F1 (as mentioned earlier) they could have far more intake than exhaust pressure, but then again the engine was optimized to have a VE under boost and was optimized as such. The VE of the engine without boost (like an RB26) is shot to hell when they aren't running turbo bost through their optimized ports and inlets.

[Clifton]

Even though he posted it I find that extremely hard to believe, especially with a .63 hotside. I've seen other GT series stuff with higher turbine pressure too.