tveltman

What would happen if you designed a system to constantly pressurize the manifold to a given limit? Would this enable you to run the engine at lower RPMs without lugging the engine, or would it make the sucker explode (or both)?

Thanks!

Imo000

Huh?!?!?!?!......... You mean to delete the bypas/blow off valve? Or to run it off a large compressed air tank (has been done in the begining of NHRA)? I don't understand the question.

tveltman

Well, as you may know, I have been poking around with variable geometry turbos, though I haven't even attempted to get the unit set up on a 928, as mine is burning oil at the moment. I thought of a simple way to control the boost level using a pneumatic actuator, the result being that I could control exactly the onset boost pressure (within the limit of the tightest aspect ratio of the VGT). It would thus be possible to have the thing fully spooled at idle (or any arbitrary RPM), and I was just wondering what this would do to the engine (aside from making first gear launches very tricky). It would be like driving the car with the manifold pressure at ~10 psig all the time, rather than just when you stomp on the gas and get the engine RPM up around 3500-4000 as in a more standard turbo setup. I would think it would make for instantaneous throttle response (not that the 928 particularly needs that), but I would be concerned about potential consequences of running constant boost in that fashion. The reason I am considering it in the first place is that the solution to controlling the VGT is very simple and straightforward, but could have the side effect of pressurizing the manifold at all RPMs, depending on the optimal setup. Perhaps I ought to have explained it more clearly at the outset, but I didn't really want to confuse the issue or get into a discussion of the actuator mechanism until I actually built and tested a proof-of-concept. It would, however, be nice to know if controlling it in such a fashion would be deleterious to the engine, in which case I wouldn't bother building a test piece and come up with an alternative instead. Hope it clears it up, thanks!

IcemanG17

a properly designed turbo system will make boost for a given powerband that you choose.... superchargers will make more boost lower in the rpms than a turbo..but once turbos are "spooled up" they can make boost at silly low rpms....look at modern high compression turbos that make torque tables from 1500-1800 to 5000+rpms.....the new twin turbo V8 BMW is a good example...or the 335i twin turbo using the large & small turbo to accomplish the same goal....which is interesting since the actual boost drops off well before redline in that motor....which why aftermarket tuners that hold peak boost gain so much HP and torque over the stock setup

tveltman

So essentially you are telling me that it is okay to run the thing at max boost all the time, provided that max boost doesnt blow head gaskets or generate too much heat?

danglerb

Displacement type superchargers create boost at all rpm ranges from idle up, you just have to manage the fuel and spark, and the throttle opening controls the actual pressure in the manifold. If the manifold pressure was higher the motor would make more power than needed and use more fuel etc.

For a street car I think its an advantage to have a "normal" operating condition at lower rpms without boosted pressure in the intake.

tveltman

What advantage? Simply a more well-mannered drive or is there some other benefit? I was thinking that if it produced more power at lower rpms you could end up saving fuel my driving in a higher gear, but maybe not.

76FJ55

Couple initial thoughts. I'm tired so will look back at this subject tomorrow.
One thing to keep in mind is that you will still need a throttle plate to regulate the actual manifold pressure. At low throttle settings you will still have low manifold pressure. This is necessary in a homogeneous mixture combustion engine (typical gasoline). Many Diesels (non-homogeneous mixture) run with no throttle on the air side and therefore do operate at high MAP even in low load conditions. the diesel may benefit from your design more so than a gas engine.

Roy928tt

This is one of the strangest threads I've seen on here.

The initial post was very hard to grasp, Thomas' second post added some meat to the concept, but was so hard to conceptualise it still made little sense. Everyone has added constructive and correct coments.

The most confusing aspect was to differentiate between "boost at idle" and "the capacity to make boost at idle speed." Thomas kept referring to a constantly pressurised manifold implying "boost at idle".Well that is just B.S. I'm not aware of any engine that has positive manifold pressure at idle, if one were to be built it would have to be capable of operating in a severely lean fuel/ air condition.ie a diesel.

So that brings us back to "the capacity to make boost at idle speed" as Brian correctly points out a positive displacement supercharger will do this, note: a centrifugal supercharger will not, I expect it could be possible with modern turbocharger technology to create boost at low revs, but simply due to the fact a turbo is driven by the exhaust gasses a positive displacement supercharger will have a faster response time.

As to making more power lower in the rev range, to use a higher gear and reduce fuel consumption. The combustion process is a chemical reaction that creates power which is transmitted via various means to move a motor vehicle. As long as night follows day, it will take, X number of calories, to move Y pounds, Z distance, in A lenght of time. You can fudge around and improve the current situation some, but the science never changes.The most economical car is the lightest most aerodynamic one. The most powerfull car is the one that turns the most fuel into drive.

missive ended.

Cheers Roy

danglerb

The purpose of idle rpm is that it is the lowest practical fuel consumption for no load, or low load operation of the engine.

Somebody way smarter than me would need to give the "real" answer, but I think it is unlikely that the same rpm would be the most efficient at both minimum power levels and much much higher average power levels. This could be the intent of the engine designer, or dictated by the physics, but as I understand it the peak higher power efficiency is something like 5x the idle rpm for most gas engines.

The range of power practical from fixed rpm and variation of manifold pressure is not very large, about double power is possible from one bar of boost, triple from two bars, with more boost than three bars I think engines start needing extensive internal improvement and control becomes much more complex.

tveltman

First, sorry for the long post, lots of ground to cover...

I guess perhaps I was confused, but my impression of the situation was that when the turbo spools to the RPM where the compressor would churn out 10 psi at the manifold, then the compressor will pressurize the manifold to 10 psi, regardless of the engine RPM necessary to achieve this. Inintially, it would be like feeding a very small turbo with all 8 cylinders, so it would spool very quickly and put out maximum boost. When the engine speed is increased, more exhaust flows through the turbo, and the variable geometry vanes must expand to permit additional flow while still maintaining this level of boost. The control mechanism I was considering balances a static force against the manifold pressure, so as the manifold pressure increases, the vanes in the turbo would open up more, allowing for more exhaust flow. The increased size of the aperture in the turbo would keep the backpressure from building up, son once the turbo spools once, it is constantly running at its final intended speed. From the way I understand it (which I admit is perhaps incomplete), this means that you will be pressurizing the manifold at whatever RPM you set the turbo to spool at.

As to fuel economy, the chemical reaction is exactly my point. Fuel + air = power + waste heat. The MAF is going to detect the amount of air being forced into the engine and adjust the fuel flow to the stoichiometric level. Since, for a given RPM, you have more air flowing in with the turbo, you will have more fuel flowing in to meet it, and so wherever the engine is run under boost, you will be producing more power per RPM. You only need a limited amount of horsepower to overcome drag at a given speed, and for most highway speeds this is far under what the engine produces. Increasing the boost at low RPM would therefore provide more power at that RPM, meaning that you could run the engine at a lower RPM than a non-boosted unit and extract the same power. If what danglerb says is correct about the peak efficiency being some value over idle, what that really means is that at some RPM, you are extracting the most work per unit fuel. By increasing the fuel flow at lower RPM, you can effectively shift that original efficiency point down. Although the engine will still run most efficiently at the 5x idle RPM, it will be putting out more power at that RPM. If you look at a plot of efficiency vs power, you will see (i think) that the boosted engine produces consistently more power per unit fuel, and that is the key point. You are exploiting the fact that the engine favors a certain fuel consumption per revolution to be efficient, so when you run the engine boosted to say 1 bar, you are putting in (approximately) twice the fuel per revolution, so the effective efficiency of the boosted engine should be close to that of NA engine at twice that RPM. If you compare the two engines at the same RPM (boosted and non boosted), you will find that the boosted efficiency will be higher than the NA efficiency.

Here is a graph illustrating what I am talking about (note that this is obviously not for a 928, so the results would certainly be different, but I am using it to make a point)

Notice the circled region is where the engine is most efficient (about 1750 RPM in this case). Imagine you were to run the engine boosted to double the original manifold pressure. From the engine's perspective, the point at which it would be consuming the same amount of fuel per revolution would be half of the RPM of the NA engine, or about 875 RPM, so it would be like taking that whole line and shifting it down by about 25 g/kWh. The maximum efficiency of the *boosted* engine would still be at 1750, but comparatively the boosted engine is more efficient over the entire range than the NA engine. Note that the horsepower line would increase with increased boost, and the reduction in g/kWh would be offset by increased kW/revolution, resulting in very little change in fuel consumption, but producing significantly more power.

It has long been known that turbos boost engine power with little to no impact on fuel economy, and this is why they are chosen in fuel-efficient applications. The above is the only mechanism I can come up with that explains this phenomenon, because all things being equal, the engine would burn MORE fuel per RPM and although it would generate more power, most of that power would be completely wasted at ordinary speeds, and so the net result ought to be a significantly LESS efficient engine. Experimentally this is not what we see, so there must be some other factor at work. I am a chemist, not an engineer or an engine builder, but this forms a self-consistent picture in my head and makes sense, so I would be very interested to hear from actual experts in the field, as I am rapidly swimming into uncharted waters here.


EDIT: I found a graph showing (mostly) what I am getting at:

This is from an engineering paper discussing the effect of boost and fuel composition on engine efficiency. For any given composition (substitution ratio), the SFC decreases with increased boost. Note the tremendous difference between 1.3 bar and 3.4 bar in the first graph. The gain here is huge, compared to the gain from 3.4 to 9 bar. The engine is approaching the theoretical limit for efficiency beyond which thermodynamics says it can't get any more efficient. Hopefully this makes my above argument slightly more lucid.

Mike Murphy

True. But you aren't saving much fuel at WOT, you're burning it like mad. However, your point is that an engine is generally more efficient at WOT than at idle, and this is true. It's because the ratio of work performed over heat generated is better.

So if you wanted your car to achieve the highest MPG, you would run at WOT to get the car up to 40mph, then shut off the engine and let the car coast down to 5mph, pop the clutch and resume the process. It's not that running full boost with a turbo is what does the trick, because you can do the same 100mpg with a naturally aspirated engine.

The other way to run full boost at idle is with a diesel engine. This is one reason why diesel's have better economy at low or part idle. But you can't lean-burn a gasoline engine, so that doesn't work in your case.

Imo000

Thomas,

Unless the throttle plate is wide open or near being wide open, the manifold (not talking about the boost/intake pipe) will always be under vacuum. Please familiarize yourself how a conventional (like the 928) internal combustion engine works and then propose and idea that is physically possible.

tveltman

Well, fair enough, perhaps it wouldn't be more fuel efficient, I can live with that. Mainly I wanted to be sure I wasn't going to blow up my engine by running it in this manner. Mechanical control is far simpler than designing a building a computerized electronic control system

tveltman

I understand what you are getting at here, but the manifold is under vacuum relative to the source of air. If the air source is not the atmosphere, but is instead the boost pipe from the turbo, then the relative vacuum as compared to the true atmospheric pressure must be less. Some of this confusion is a result of my own poor explanation. The butterfly valve creates a pressure differential, resulting in a vacuum on one side and pressure on the other. My point was more that because of the increased pressure on the inlet side, the manifold would be under higher absolute pressure for every operating condition than one without any forced induction. It was an error on my part to assume that the manifold pressure always equals the boost pipe pressure, however hopefully with this correction, the issue becomes more clear. In point of fact, I don't really care what the exact pressure/vacuum inside the manifold is, I just want to not explode my car. If you are certain that pressurizing the manifold to the max boost pressure is impossible under all circumstances but WOT, then perfect, my problem is solved and I can use this control mechanism without fear of damaging the engine.