Why not run an "undersized" turbine?

[bryson]

I feel like I may be overlooking something completely obvious, but I'm trying to figure out what the downfall of having a small turbine side is. My first thought is that it will create too much backpressure in the exhaust, but with a very large wastegate (or two), couldn't backpressure be controlled? The turbine works based on the pressure difference from the turbine entry to exit, but I don't think that a smaller turbine requires a larger pressure differential. If anything, I would think it would be smaller. With an adequately sized wastegate, couldn't you theoretically release enough exhaust to keep the exhaust pressure as low with a small turbine as you could with a large one?

What am I missing? Is the mass of the compressor wheel so much that sizing the turbine differently wouldn't help significantly? Would balancing become such a large issue that this isn't a possibility? I'm guessing that I'm overlooking something simple, or else there would be more people with T25/T88 hybrids and incredibly large wastegates. It seems that the best approach to choosing a turbocharger would be to size the compressor accordingly, then to use the smallest turbine possible and the largest wastegate.

Where is the error in my train of thought? Although I would really like to think that I'm not mistaken, i'm pretty sure I must be because nobody seems to do this.

[cactus bastard]

I'm not so sure I follow you? The ability of a pressure difference to generate a force is dependant on the surface area it's able to act on. So a tiny turbine would have a hard time generating enough force to spin the much larger compressor. The pressure acting on the turbine would rise, but not enough to compensate, and it would become extremely innefficient.
The way I see it anyways. I'm not a turbo guy, and I also could have missed your point completely.

[bryson]

That makes sense -- for some reason I never considered that it may take more force to spin a wheel with smaller surface area (assuming the mass is the same).

[The Dark Side of Will]

The turbine has to power the compressor. The more air you want the compressor to move at higher pressure, the more power it will take. To get a lot of power from a small turbine (small mass flow), you have to have a HUGE pressure ratio across it in order for it to have energy inflow that it can extract enough energy to run the compressor.

[bryson]

Yeah, I don't know why I didn't think of that. I guess the exhaust has to drive the turbine with the same amount of work that I want to get from the compressor. I can increase velocity by going to a smaller housing, but I don't think that I could increase it enough to make up for the lower mass flow rate in order to manage the same work across both the turbine and compressor. I guess that pressure required would shoot through the roof as well, and there really isn't much else that I could use to make up for it. A greater temperature differential isn't easily attainable.

I've seen some good sequential turbo setups out there -- all of this is way in the future, so I'm not too worried about it now.

[Shaun41178(2)]

I am guessing you feel you have too much lag?

[Kohburn]

the answer tot he original question is ---- you can, its just less efficient ( and a few other side effects depending on sizing)

[The Dark Side of Will]

Yeah, I don't know why I didn't think of that. I guess the exhaust has to drive the turbine with the same amount of work that I want to get from the compressor. I can increase velocity by going to a smaller housing, but I don't think that I could increase it enough to make up for the lower mass flow rate in order to manage the same work across both the turbine and compressor. I guess that pressure required would shoot through the roof as well, and there really isn't much else that I could use to make up for it. A greater temperature differential isn't easily attainable.

I've seen some good sequential turbo setups out there -- all of this is way in the future, so I'm not too worried about it now.

You can actually figure out how much power you need to get from the turbine.

Figure out your mass flow and boost level, then use the formula for power required to compress air (available at www.google.com ), divide by compressor efficiency, then divide by turbine efficiency (not sure exactly, but can probably use compressor efficiency) and you have how much exhaust energy you'll need.

That's why backpressure is higher than boost. You need to get significantly more energy from the exhaust than you need to compress the air on the inlet side. If the turbine & compressor efficiencies are both .75, then you'd need to pull 78%(!) more energy from the exhaust than you put into the intake air.

[bryson]

78%..wow! It really makes turbochargers seem inefficient. That's the other thing I've wondered about (kind of off topic) -- how can a turbocharger make SO much more power than an equivalent naturally aspirated engine? The whole thought behind turbocharging is that there is a greater difference in pressure across the intake valve, filling the cylinder more. But, wouldn't the increase in exhaust pressure cause the cylinder pressure to be higher? Especially if the increase in exhaust pressure is so much greater than that of the intake, how does more air get into the cylinder? I understand how the engine actually produces power (we dealt with dual cycle engines a little while ago in thermo), but I feel like I don't have a really strong grip as to how a turbocharger works as well as it does. It makes supercharging seem like a more attractive alternative.

[The Dark Side of Will]

Keep in mind that the 78% figure can come from both temperature and pressure. It's not that turbos are incredibly efficient... it's just that other methods of forced induction are WORSE.

Complete scavenging and exhaust/intake interactions do not work well on turbo engines because of the large pressure difference. Turbo engines like minimal overlap or they suffer from exhaust reversion into the intake tract during the overlap period. Supercharged engines suffer from blow-through with excessive overlap.

IOW, there's very little interaction between the end of an exhaust stroke and the beginning of the next intake stroke on a well tuned turbo engine.

[cactus bastard]

But, wouldn't the increase in exhaust pressure cause the cylinder pressure to be higher? Especially if the increase in exhaust pressure is so much greater than that of the intake, how does more air get into the cylinder?

Don't forget that on the exhaust stroke the rising cylinder forces most of the remaining exhaust out of the combustion chamber.

[Weponhead]

its forcing more air into the cylinder because the air in the intake manifold down to the cylinders is compressed, thus the PSI rating you run at, its compressing all the air in the intake mani to say 15 psi making the air more dense, more oxygen even be it in a smaller space = better combustion when combined with more fuel = more power... am i right guys? ..

[p8ntman442]

this thread makes me feel smarter.

[Kohburn]

this thread makes me feel smarter.

your brain-


your brain on RFT

[cactus bastard]

This one's even better than staying at a holiday inn
http://www.ebaumsworld.com/2006/08/peopleidiot.html