karmavore

Why are they only efficient at one RPM range or another and not blowing equal pressure through out the power band?

Its seems that if a SC reaches its maximum output at, say, 3000RPM, and is then making 6psi, it should continue to push 6psi all the way to redline? If so, why do people say SCs are only good for bottom end power (or top end if so geared) but not at both?

Luke.

PatrickB

Here's my guess based on what I know about how SCs work:

If a supercharger reaches it's maximum output at 3000 rpm, that means that it is compressing (by X psi) as much air per second as it can at 3000 rpm. However, the engine continues to draw more and more air per second, and the SC doesn't have any further capacity. Because the SC is driven by an engine belt, the rate at which it can compress air is limited by the power available from this belt, which is high proportional to the amount of air needed at low revs, but insufficient as the engine revs higher and needs more and more compressed air..

A turbocharger, in contrast, is driven by exhaust gases. This means that when the engine isn't revving much, it is not giving much assist, leading to the well-known turbo-lag phenomenon. However, as the engine spins faster and faster, there are more exhaust gases to use to power the TC to compress the incoming air, potentially allowing the TC to remain useful at higher RPMs.

Of course, this is all just a wild-*** guess based on my sketchy knowledge of how SCs and TCs work, and 2 minutes of think time on the way back from the soda machine. I could be terribly, horribly wrong.

-Patrick

zoom44

sounded good though

PatrickB

When you've been in school as long as I have (I finish my PhD in comp sci. in 2 months), you learn to bullshit well based on minimal info. Once in a while, you're even right.

karmavore

Nice try...

you may be right but it doesn't make sense. What difference does it make how the turbine is powered? Obviously it does, but I can't figure it out...

Thanks,

Luke.

zoom44

yeah i don't know diddley, but i was wondering why it doesn't keep compressing more as the engine spped increases increases since it is driven by a belt? and how much power is lost to driving it?

karmavore

There's a good artice in the August 2001 issue of SCC. I'll let you know what it says.

zoom44

hey wait i think iget it. it just can't keep up with the air demand above a certain rpm right?
so why not supercharge a rotary? since it makes it's power up higher in its rev range it seems the supercharger would give it a kick off the start.

Immi

edit
-misunderstood you, sorry!

karmavore

I think he meant two more months, douchebag.

Luke.

karmavore

http://www.sportcompactcarweb.com/tech/

Lots of GREAT info here. Just read 'em more than once. I've learned a lot.

Luke.

Immi

I figured.

so relax lucas - no need to call people names

PatrickB

karma - how the compressor is powered could make a lot of difference. A supercharger has a more-or-less fixed amount of power available to use to compress air. Say at 3000 rpm the SC can compress air going through it to +6 psi, but that uses all of the available power from the fan belt for the SC. Now, say we rev up to 6000 rpm - the engine needs roughly twice as much air now, because the chambers are filling and detonating twice as fast. That means that if the SC had no excess capacity now, it can't compress all of the air needed for the engine to 6psi. It has to compress it less. How much less I don't know, probably a lot less, since it has twice the volume of air to compress now, and that air is already at 32psi.

A turbocharger, on the other hand, gets its power from the exhaust gases. Say it was giving a 6psi boost at 3000 rpm, and that was using all of the available power from the 3000 rpm exhaust gases to do this. Now, when the engine gets to 6000 rpm, the turbo has to compress twice as much air. But now there's twice as much exhaust gas leaving the engine, so the turbo should have roughly twice as much power available to use to compress air. In theory it should be able to use this power scale up to faster rpms than the SC.

Now, there are a lot of assumptions here - I'm assuming that air volume used and exhausted increase perfectly linearly with RPMs, as does power extracted from the exhaust by the turbo. It's unlikely that these are true, meaning that the turbo becomes somewhat less efficient as the engine revs higher, but it *should* be able to extract more power for better compression than the SC. I'm assuming that the SC can't get significantly more engine power at higher RPMS. I'm assuming that the turbo *can* extract the extra power available from exhaust gases at higher RPMs. I think these are all true, to a first approximation. If I'm wrong, please say so.

Oh, and for the record, yes, I meant two more months until I get my PhD. I though that was perfectly clear from what I said.

Immi

simple misunderstanding - my fault

PatrickB

No problem. Mistakes happen. I make plenty of them.

For the record, It's taken me 8 years after my BS (from Mississippi State University) to finish Piling it Higher and Deeper. That's a little longish, mostly because my first advisor left the University of Arizona after I had been here not-quite 4 years, and I had to find a new advisor (who has also since left.) Still, I've managed it, and enjoyed myself in the process.

I take a new job as CS faculty at the University of New Mexico starting this spring. To get this post back on topic - Do you think I'm looking to get a new car once my salary jumps from grad-student level to professor level? You bet. The money becomes available early this next spring. Any guesses what car is at the top of my list?

One concern - does anyone have any experience how well rotaries perform at altitude? (Albuquerque is at abiut 5500 ft. above sea level)

-Patrick

zoom44

karmavore thanks for the article, but really watch the name calling dude. lets keep it freindly folks!
so back to my other question. why not supercharge a rotary to give it a kick in the rear from the start since the already have the power up higher?

karmavore

I'll call 'em as I see 'em.

Did you see the one design the employs a rotor? Now wouldn't that be cool: a rotory supercharged rotary!!

Luke.

Immi

what does that mean?

I apologized.

MyT13B

Most turbochargers have a waste gate to prevent too much air pressure.

A small turbo will wind up quickly where a large turbo will be delayed due to its internal mass and often causes a lag.

My turbo is an Aireasearch with a 2" outlet. I had to modify the wastgate spring to limit the boost to around 6 psi above normal. I am also using the stock compression ratio 9.2:1. I also use 90 octane pump gas.

I believe a belt driven supercharger would work ok but would add enough drag to the engine to where the end result would be worth less than what you get using a turbo.

fuz

Supercharger is always less efficient, as it saps power directly off the engine. The only good thing about it is that you can get a linear power delivery that's predictable, unlike a turbo which can spool up at any part of the RPM line.

It's like NA tuning, but with much better results. (Which is why I'm sure it's not included in Gran Turismo 3 for PS2) There is no lag (depending on they type of charger though), and not sudden power rush, which also makes for a smoother ride.

The downside is that you can't turn off a supercharger so it eats up that much more gas. A turbo only runs part time so is a fair amount more efficient, unless you're really lead footed--in that case neither will be better for the MPG. Unless you have an expensive magnetic coupler to switch it on and off, the charger is continually leeching off the engine. Have to remember that forced induction is just another way of adding liters/cubic inches.

Sputnik

Okay, let me try to actually answer this man's questions.

First, with an SC or turbo, the power gains come from stuffing as much air as possible in the engine. The amount of boost pressure that it's running at doesn't mean anything, it's the amount of air you can get into the engine. Basic physics will tell you that a cubic foot of air at 15 psi and 80 degrees is much more dense than a cubic foot of air at 15 psi and 150 degrees. When we talk about a compressor's "efficiency", we're talking about how much air it can push versus how much it heats that air. This is important because you can cram 50 psi into an engine, but if it's so hot that the air density is too thin, then you didn't gain anything. Not only that, you want as cool of an intake air temperature as possible. The higher the temperature, the less power an engine will produce. And when you are talking about a SC or turbo, if you put too much heat into the engine, it will not only kill performance, it will self-destruct.

1: Why are they only efficient at one RPM range?
Whether it is a turbo or SC, and whether the SC is a roots or a centrifugal type of compressor, the compressor has a rpm range where it is most "efficient" (pumping the most amount of air with the least amount of heat). The problem with an SC is that it is directly connected to the crankshaft, so it's rpm range is dictated by what rpms the engine is turning. Depending on the setup, a Centrifugal SC could be geared so that it spins ten times as much as the engine. So on an engine that ranges from 1k at idle to 6k at redline, that SC will spin from 10k to 60k. So, depending on where the engine rpm is at, that SC could easily be outside of it's "sweet-spot", where it will either not be able to produce much boost, or be over-heating the air.

2: Why doesn't it produce the same boost throughout the rpm range? The amount of boost that an SC generates depends on how fast it is spinning. And, since the SC speed is directly connected with engine speed, the amount of boost it develops depends on how fast the engine is running. For example, a roots SC could develop 3 psi at 1500 rpm, 6 psi at 3k, 12 at 6k, etc. etc. This is a very general assumption, and there are many other variables, but that is the basic idea.

3: Why do people say SCs are only good for bottom end power (or top end if so geared) but not at both? That is mostly because there are two common types of SCs, the roots type, and the centrifugal type.

The roots type is the one that looks like a couple of paddlewheels next to each other. It is better for "bottom end", and the type of SC that you will see in most factory setups (one common one is GM's 3800SC that is installed in Grand Prixs, Bonnevilles, Regals, etc.). The faster a roots SC spins, the less efficient it becomes. So, with my example above, the SC could be pumping 6 psi of boost at 3k, which may equate to pumping 40% more air into the engine. That same roots SC pumping 12 psi of boost at 6k might be heating the air up so much that the density is much lower. So, instead of pumping 80% more air at 12 psi, it might only be pumping 60% more air at 12 psi. By the time you take the higher intake temps into consideration, you might only be getting as much of a power benefit out of it at 12 psi as you do at 6 psi, and you are running a higher risk of detonation at 12 psi.

The centrifugal type is the one that looks like a jet engine. The turbo has the same compressor, actually. In both cases, a turbo and centrifugal SC both have to spin pretty quickly for it to start producing any kind of boost, but once it gets going, it is more efficient than the roots ever thought of being. So, this is the type of SC that works best at the "top end". But, like the roots, if you go too far, you will create problems. In the case of the centrifugal SC, the difference is not like 6 psi at 3k, and 12 psi at 6k. The numbers for the centrifugal SC would be more like 1/4 psi at 1500 rpm, 2 psi at 3000, 6 psi at 4500, and 12 psi at 6000.

But it basically boils down to the fact that an SC is limited by what the engine rpms are set at.

This is where a turbo really benefits over an SC. The turbo gets it's power from the exhaust gasses spinning the turbo. Once the turbo gets up to it's efficiency range, it can open up a valve (called a "wastegate") which will cause some of the exhaust gasses to bypass the turbo. So, regardless of what rpms the engine is runnng, the turbo will always be at it's efficiency speed by simply the amount of exhaust that is driving the turbo (once it gets up to speed, that is). There are many variables, but a turbo could be able to produce 12 psi of boost anywhere between 3000 and 6000 engine rpm.

I realize that this "book" probably brought up more questions than answers, but I hope it gives the uninitiated some basics.

---jps

fuz

Playing the game of "confuse the newbie" is always fun. :D

SmokingClutch

Actually you can turn off a supercharger - a clutched pulley will do just that.

The supercharged AW11 Toyota MR2 has just that - there's a little light in the instrument panel that tells you when the supercharger is engaged - I believe it disengages at light throttle.

wakeech

ya, centrifugal clutches are used in thing like some types of karts (to disengage the engine near idle, obviously so as not to stall it 'cause they're direct drive), and the same could be used in this application for light-boost SCs.

Sputnik

That's not quite accurate. The '88/'89 MR2 does have a roots SC, but it does not have a clutched pulley.

What the supercharged MR2 does have is a bypass valve that opens at idle, cruise, and other low load situations (because the SC becomes a restriction at that point), and there wasn't an indicator light for that either (at least not from the factory). Odds are, the light you saw was an indicator for the TVIS system that some owners set up in their car.

Other SC setups, factory and aftermarket, have a similar bypass valve (it's mostly internal to the SC itself), the GM 3800SC has it, and the Jackson racing SC for the Miata has it.

---jps