how much power does a turbo make

[mechman600]

Originally Posted by jamesqf

How do airplanes harness power from turbojet engines? Which are, after all, basically scaled-up turbochargers pushing bypass air out the back end instead of stuffing it in the cylinders of an IC engine.

Simplest answer is that you don't do it mechanically at all. Turbine shaft is mated to the rotor of an alternator, which generates electricity. By switching the alternator so it only generates at higher exhaust flow, you pretty much eliminate any excess turbo lag. If you happen to have a hybrid, you can store the electricity for later use...

Compound turbos do it mechanically through a gear set. The DD16 in my link above does, as did all the WWII airplane engines.

[Occasionally6]

Originally Posted by pete c

I don't have any actual stats, but, I believe that statement is backwards. I believe that a properly sized turbo uses its waste gate only at high loads.

Isn't that what I said? Reread my post in the context of the post I was replying to.

Originally Posted by Occasionally6

Current turbochargers use a waste gate to dump excess exhaust gas around the turbocharger once the desired boost (compressor work) is achieved.

It's really interesting to play around with the variables in the MatchBot page pgfpro provided the link to.

There is one other problem here. How do you harness power from a device that is spinning at crazy rpms and is very hot? You do it with a rather complicated transmission that will cause turbo lag like you've never seen.

The bottom line is that yes, there is some wasted energy here, but harnessing it is gonna be awfully tricky if possible at all.

In addition to the turbo compounding (geared to the crankshaft) and turboprop aircraft, that problem has been solved in every gas or steam turbine power station.

If lag is an issue, there's no reason, other than conversion efficiencies, why electrical energy can't go both ways, whichever is appropriate at the time.

It will cost money to do it but it has been tried:

AutoSpeed - HyBoost

[jamesqf]

Originally Posted by mechman600

Compound turbos do it mechanically through a gear set. The DD16 in my link above does, as did all the WWII airplane engines.

Sure, it CAN be done that way, at the cost of adding weight, complexity, and wear/failure points. That might not matter much for combat aircraft - after all, the average service life of a B-17 in combat was something like 4 months.

The amusing part is the research done after the war to discover the most efficient size of turbocharger for a given piston engine. Turned out the optimum was to reduce the size of the engine to zero.

[ConnClark]

Originally Posted by ChazInMT

Yeah, it is true. There is power required to spin the turbo charger, that has to come from the engine. People always think it's just "Waste Gas", but it isn't. There is a load placed on the engine to make the turbo charger spin, if you just let the air blow into the open by not hooking it up to your engines air induction, it would really slow you down. But since it is packing in a lot of air into your engine, the net gain is that it does help, you're MPG increase is due to the turbo allowing you to operate in a better region of the BSFC chart for your engine. This is basic turbo charger stuff.

You don't get something for nothing, but the benefits outweigh the losses in this case. It is understandable for you to think as you do, but it is not accurate.

No its not true. If you pack more air into the cylinder it and supply the same amount of fuel the resultant combustion gases are cooler because of the excess air. The end result is less heat energy lost to the cooling system and more energy in the combustion gases that can be extracted by the piston. Also a turbo that is properly matched at a load point and rpm will generate more pressure in the intake manifold than in the exhaust manifold.

[pete c]

Originally Posted by Occasionally6

Isn't that what I said? Reread my post in the context of the post I was replying to.

Here is what you said.

At very light loads maybe. Current turbochargers use a waste gate to dump excess exhaust gas around the turbocharger once the desired boost (compressor work) is achieved. There's still plenty of energy available in the exhaust that is being routed around the turbine.

The way I read that, you are saying that anything above "very light loads" results in waste gate bypass.

This very well might be the case, but, I kind of doubt it. My thought was that the waste gate only came into use at high loads.

Perhaps you are right, as I make no claims to being anything remotely close to a turbo expert.

As for aircraft applications, lag is not an issue as engines are run at relatively stable rpms. Also, these engines spend a lot of time near max power. An aircraft engine really is an entirely different animal.

[ChazInMT]

C'mon people, listen to yourselves, you sound like a bunch of HHO proponents with "Cooler Gases" with "More Energy" WTH??

Bottom line, think about this, The discussion was started based on the question "How much horsepower a turbo generates at the turbine shaft?"

Who here said None? No horsepower? I'm guessing nobody. So if there is horsepower being generated by the turbine, it has to come from somewhere, and I don't think anybody is going to say it isn't coming from the engine.

Now I completely understand that if the turbocharger is using say 5HP, the net effect of it packing in more air is going to be an additional 10-20HP or something, resulting in a net gain in HP. Great. Works as advertised.

How it improves fuel efficiency? I'm not real clear on these nuances, but I don't dispute they do.

What I do dispute, is the notion that the turbocharger is running on magic fairy dust to make it propel the compressor which requires a fairly substantial amount of energy in order to raise the pressure of a large volume of air.

The energy comes from somewhere, and it is the engine. Net Gain, Yes, all day. But the turbocharger is using power to make power, and the power used is not "free".

[niky]

It's only free in the sense that it's power that isn't being used for anything.

It's likely you would only have excess boost pressure to bleed off to produce electricity at very high engine speeds. If you're getting excess at moderate or low speeds, then you have a poorly sized turbo and you're throwing a lot of energy away, in the first place. Hybrid-electric turbochargers that generate electricity only make power at high engine speeds, and use it at low engine speeds. As a means to increase efficiency, it doesn't seem very efficient for ecomodders who tend not to drive flat out.

Also, looking at the diagrams and what goes into making power-generating turbos, seems like a lot of work for recovering what is potentially a few measly horsepower from the tiny turbine you would attach to a car engine. And that's before you go through the losses in converting it back to electricity. Would make more sense to simply have the turbine properly sized, in the first place, or to use a variable-geometry unit, instead.

This kind of thing makes sense on the industrial level or with ships, which have huge turbochargers, but on cars... I don't see the benefit.

[freebeard]

That's an interesting Autospeed article.

Here's what BMW patented

They use three turbos; the electric turbo is to control lag. Most of the time it's feeding the battery/ultracapacitor.

[jamesqf]

Originally Posted by ChazInMT

So if there is horsepower being generated by the turbine, it has to come from somewhere, and I don't think anybody is going to say it isn't coming from the engine..

You think wrong. It is not coming from the engine, in the sense that it is robbing power that would otherwise go to the wheels. It is using the energy in the hot exhaust gas, which otherwise would just go right out the tailpipe.

[ConnClark]

Originally Posted by ChazInMT

So if there is horsepower being generated by the turbine, it has to come from somewhere, and I don't think anybody is going to say it isn't coming from the engine.

I'll say power is not coming from the engine.

The energy comes from somewhere, and it is the engine. Net Gain, Yes, all day. But the turbocharger is using power to make power, and the power used is not "free".

The energy comes from the fuel, not the engine. The engine and the turbo convert this energy to usable work. There is no rule in thermodynamics forbidding use of a turbo to improve the efficiency of an engine.