A boost is useless until the throttle is near wide open…
Rod thats not true, at least on a turbo car… I drove of all things a CX-7 the other day. Turbo charged Mazda SUV… You could 100% feel the boost coming on even on minor acceloration… In most turbo charged cars I have driven you can almost keep your foot at one spot in the throttle and the car will start pulling harder and harder with out giving it more gas as the turbo spools up.
It used to be that tubos only ‘came on’ at near wide open throttle, but now that they’re being used to increase fuel economy with a small (1.4 l) 4, and/or maintain ‘v6’ performance with a ‘4’, they’re boosting power much of the time.
Mr. Knox is correct. Supercharged on not, any time the throttle is partially closed the initial cylinder pressure at the beginning of the power stroke is less than atmospheric. Any energy consumed to pressurize air, or fuel-air mixture against the throttle plate is wasted. Adding boost at partial throttle only means the cylinder fills more for the same throttle position. It doesn’t represent an increase in performance.Opening the throttle on a non-supercharged has the same effect. Supercharging doesn’t accomplish much until the intake tract runs out of its normally aspirated capacity.
For most normally aspirated engines at high RPM even with WOT the cylinders don’t fill to atmospheric. The intake tract acts as a throttle. A partially filled cylinder has a lower final pressure at ignition. The engine’s compression ratio is designed to take this into account. Generally the compression ratio for a normally aspirated engine is set higher than would be possible given a %100 atmospheric cylinder fill. This is one reason detonation occurs at low rpm and WOT. It’s also the reason engines designed with superchargers have lower compression ratios. Controlling detonation is one of the problems that will need to be addressed if adding a supercharger to an engine not designed for one.
“Mr. Knox is correct. Supercharged on not, any time the throttle is partially closed the initial cylinder pressure at the beginning of the power stroke is less than atmospheric. Any energy consumed to pressurize air, or fuel-air mixture against the throttle plate is wasted.”
This is incorrect. The throttle plate introduces a restriction, but it does not automatically cut the pressure to below atmospheric. One could easily see partial boost conditions at the intake valve at less than full throttle.
Um…the “beginning of the powerstroke” follows the end of the compression stroke. The inside of the cylinder had better not be less than atmospheric.
Now if one were discussing the beginning of the INTAKE stroke, than yes, it could be below atmospheric unboosted…especially if the driver is downshifting to scrub speed!
Any air pump adding pressure continuously, whether it’s driven by the crankshaft or the exhaust, is going to improve power throughout the stroke in proportion to the amount of pressure it’s adding. It may not be obvious excet at WOT, because anywhere below that one can simply press the pedal a bit harder on a naturally aspirated engine to get the equivalent of what the boosted power would be. But the power curve would be higher throughou the entire RPM curve.
Note that I said “continuously”. Obviously, a turbocharger that doesn;t kick in until the throttle plate is in WOT position won’t change the power curve one bit until that happens.
The throttle controls rpms by blocking the flow of air into the cylinders, causing a vacuum. If a blower pushes air into the intake and causes a positive pressure the engine will be at wide open throttle. When the throttle is closed, or near closed, the blower is whistling in a partial vacuum. When the throttle is opened air is allowed to feed the blower but until the throttle is opened wide enough to give the blower sufficient air to produce a net positive pressure it is just eating up horsepower spinning and churning for naught.
Read my post. I didn;t say that the boost device (super or turbocharger) would bring the manifold pressure up to a level higher than below ambient when the throttle is other than wide open. What I said was two things:
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the cylinder pressure at the beginning of the POWER stroke will always be above ambient…because it’s compressed! It had better be, or someone has a missing valve…
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any boost device constantly supplying boost will improve power throughout the band, not just as WOT. All the way from idle to WOT the engine will see compression levels higher that it would in an unboosted (naturally aspirated) state. With the throttle plate closed, the wastegate (blowby valve, popoff valve, whatever) should be allowing pressure to be relieved, and a pump trying to compress air against an open valve will essentially be freewheeling, not using any substantial power at all. Granted, some amsll amount of power will be used to blow the valve open.
Okay, I guess in thinking about it some power would be wasted at idle, but the point at which the power gained by the additional pressure would be greater than the power used to operate the compressor would be not much above idle. The engine definitely would not have to be at WOT to feel the effects of increased pressure into the manifold. I’m countering the argument that an engine needs to be at WOT to feel a difference. It doesn’t.
Even cruising at 70 mph, the throttle is restricting the air flow into the engine and a negative pressure (vacuum) will be in the intake manifold… Vacuum, not positive pressure. Only when the throttle is opened CONSIDERABLY, i.e., well beyond the throttle opening required to drive normally, will a positive pressure accumulate in the intake.
Possibly this adds more understanding than I seem able to.
http://www.mustangdepot.com/onlinecatalog/engine/paxton-info.htm
And this
And
At cruise you’re right. Just saying you don’t need WOT to have higher pressure at the inlet valve with a supercharger, compared to an engine without a supercharger. It’s not on/off like that. The wider the throttle opening, the more boost supplied to the engine
I don’t think we’re really drawing different conclusions, just comparing different things.
There’s no question that when cruising manifold pressure will be lower than ambient. I’m essentially saying that a boosted engine will take less throttle to maintain that cruising speed, because the boost will effect the power curve such that less throttle will be necessary to achieve the same amount feeding into the cylinders. I’m essentially concluding from that that the added power of the boost is felt under all conditions, not only at WOT.
I also stated that cylinder pressure even at reduced throttle position will be higher than ambient at the beginning of the power stroke. Even on a low compression ratio of 8:1, unboosted, that would be true.
SMB- good catch. I was thinking INTAKE my fingers typed POWER. My bad.
Perhaps I’m confusing effect with benefit. As an example: If you have a normally aspirated engine that has an intake manifold pressure of .5 atmosphere at 1/2 throttle the same engine with supercharging might have the same manifold pressure at 1/3 throttle. The engine will be producing the same torque. There is no benefit from the supercharger under that condition. As long as the engine is throttled to a point where the intact tract has enough capacity to fill the cylinders to the degree you want a supercharger is just going along for the ride.
For Texases’ Mustang example:
Yes, if you hold a constant throttle position, as the boost increases upstream of the throttle, the pressure downstream will also increase, more torque will be produced and the car will accelerate. If the boost remains constant, as the RPMS go up the manifold pressure will begin to drop. The engine will stabilize at a higher RPM. My point: If the new manifold pressure can be achieved at that RPM with normal aspiration you still haven’t derived much benefit from your turbo. If the boost continues to increase with the same throttle position you will either close the throttle or the engine will reach its mechanical limit.
I’m not trying to make a case against supercharging; only pointing out that its benefits only pick up where normal aspiration drops off.
Observing the boost gauge on a gasoline engine seems to give a good indication of how and when boost is introduced to overcome the engine’s declining intake volume as rpms increase under load. And a turbo charger will give an engine a greater net increase in horsepower than a belt driven supercharger but at the loss of throttle response at low rpm.
MT, beautifully said. I kind of figured that must have been a typo based on the rest of your post, but I falt obliged to answer to it anyway. Yeah, looked at from the perspective of a cruising speed there’s rally no benefit. It’s gettng to the cruising speed where the benefit is. And passing on the highway. And merging from the entrance ramps.
Rod, in general I agree, but manufacturers now are designing the engines with turbochargers included in the overall design. designing the turbos into the exhaust manifold and keeping them spooled up constantly, combined with new vane and bearing designs, and with intake manifold designs that enable instant boost straight to the cylinders, should provide almost instant response. I expect we’ll see this all designed together as a system soon. The old way of tacking turbochargers onto systems designed for onatural aspiration and living with the tradeoffs will disappear as CAFE requirements continue to get stricter. Manufacturers have the resources to do some really integrated design work.
If properly designed for reliability and longevity engines with turbo chargers could offer much greater fuel mileage but the driving public would need to become accustomed to driving with their foot a little deeper into the throttle and turning up the sound system to cover the turbo. A well engineered turbo charged engine has a much flatter torque curve and a greatly improved horse power curve compared to the same engine that is naturally aspirated. For now, supercharged engines are expensive toys for high performance gear heads in the US. Europe has some excellent turbo diesels that might be marketed here. I would enjoy trying out a VW 3 cylinder turbo diesel that is advertised at 78 mpg.
They’ll get the sound down I’m sure. Even the thing that always bothered me about turbos, the high rates of premature failure due to heat and high RPMs, is pretty much changing. The problems are being designed out and the technology to deal with the conditions (synthetic oil) is becoming commonplace. I would not be surprised if a new type of unit that operates at low RPMs was developed.