Compound turbo engines - realistic or not?

[Rainbow Man]

I just stumbled upon this concept on the interwebs after doing a random search on google images.

Frankly I had no idea it was possible to hook the exhaust up from a piston engine and use it to help drive the crankshaft. I always thought the gas volume exiting the cylinders to be much too small to be able to extract any kind of useful kinetic energy from the process (beyond that of driving a turbocharger I mean).

Anyway, the crank cool running this website seems to have MUCH enthusiasm for the concept!

So is it feasible, or is the guy just running a debate with voices in his own head? I didn't do much research before posting here. I know there's some smart people at this place so I took the easy way out heh.

Peace.

 

[Basic]

I don't know much about it, but I thought it was rather common on big trucks.

 

[MPI]

Frankly I had no idea it was possible to hook the exhaust up from a piston engine and use it to help drive the crankshaft. I always thought the gas volume exiting the cylinders to be much too small to be able to extract any kind of useful kinetic energy from the process (beyond that of driving a turbocharger I mean).

Well, there are a few things to be said. The first is that the bulk of the wasted energy goes out the exhaust. As petrol engines are only 20-25% efficient at most, there's a LOT of horsepowers going out with the exhaust gasses. Gas turbines operate on this fact.

Also, don't underestimate the power needed for the compressor stage in a turbocharger! If you drive a compressor mechanically (i.e. a supercharger) we're talking power draw from 20-30hp up to 100-150hp+!

So is it feasible, or is the guy just running a debate with voices in his own head? I didn't do much research before posting here. I know there's some smart people at this place so I took the easy way out heh.

Peace.

Sure it's feasible, the question is for what purposes.

Combined cycle engines are pretty common already, like COGAS and the like. It's a Combined gas and steam turbine, with the exhaust gasses coming from the gas turbine heating water to drive a steam turbine. But the scale of the thing just doesn't make it all that suitable for personal use... It's used in electric power plants and warships.

As basic said, some Scania and Volvo heavy trucks use Turbo-compund engines as described on the website, but I don't think it would be all that suited to be scaled down for cars for instance. There would be some extra weight, but the gearing for the turbine would also add a lot of rotational inertia I would think. For big stonking heavy duty engines or engines that doesn't change much in acceleration (airplanes, boats) neither is much of a problem, but for a car it is.

 

[_xxx_]

It's being done for ages already. Its use is pretty limited though, the guy is a bit overly enthusiastic. He obviously never heard of LEV, ULEV, SULEV or even Euro5.

 

[Rainbow Man]

So what are those things?

Vehicles? Engines? Thingamabobs screwed onto an engine?
Peace.

 

[Mintmaster]

Anyway, the crank cool running this website seems to have MUCH enthusiasm for the concept!

Remember the scope of his enthusiasm. He's talking about aircraft engines, which have two attributes that make the idea useful.

One is that the load and RPM is constant. The turbine and the gearing to the shaft can be designed for this specific operating point. Trying to use this on automobiles is tough. Trucks spending a lot of time on highways would benefit, but that's about it.

The other is that it's much more useful on a rotary engine (which has flaws but has the high reliability needed in aviation), as it doesn't have exhaust valves. This allows the exhaust energy to get out of the combustion chamber without dissipating.

The talk of Mazda engines in the LeMans racer is about how the amount of turbocharging possible suggests that the exhaust energy is less impeded.

Well, there are a few things to be said. The first is that the bulk of the wasted energy goes out the exhaust. As petrol engines are only 20-25% efficient at most, there's a LOT of horsepowers going out with the exhaust gasses. Gas turbines operate on this fact.

Gas turbines don't have most of the usable energy sucked out by pistons, though.

After ignition, an ICE uses (mostly) adiabatic expansion to extract work from the initially hot, high pressure cylinder contents. Even though there's a lot of energy in the exhaust, it's hard to get much more useful work out of it.

There's a much lower temperature difference with the environment compared to what a gas turbine or ICE piston deals with. Moreover, any back-pressure would be felt by the pistons. An engine consuming a certain amount of air and gas needs to push the combustion products out of the engine. Think of this as a fan. Now if you put a turbine after a fan and feed the energy from the former into the latter, you're not going to make it more efficient.

Also, don't underestimate the power needed for the compressor stage in a turbocharger! If you drive a compressor mechanically (i.e. a supercharger) we're talking power draw from 20-30hp up to 100-150hp+!

For a supercharger, yeah, but remember that with a turbo you get most of that power back through higher pressure at the turbine stage. It basically just compresses everything in the engine with friction and heat transfer (esp. with an intercooler) being the only source of energy loss.

It's countercurrent exchange, much like a heat exchanger. A gas/liquid can flow in and out of a black box at a low temperature whilst being held at a much higher or lower temperature inside the box, but you don't need any heaters or refridgerators expending energy to do it (except to make up for losses).

 

[_xxx_]

So what are those things?

Vehicles? Engines? Thingamabobs screwed onto an engine?
Peace.

The exhaust level regulations/laws. To say it bluntly, if you can't get below the limits, no business for you.

It's not the technical side of things slowing all that down, but the mandatory minimum exhaust levels. You could technically build an insanely powerful or economic car, but it would produce much more not-so-healthy exhaust.

 

[Rainbow Man]

Maybe I'm not too bright but why would hooking up a turbine to the exhaust to harness power increase pollution levels?

I mean... Turbochargers have been around on standard cars since what, the 70s? Ansd they're not banned now either due to pollution. Only difference - that I can see - is they drive an air compressor instead of the crankshaft..
Peace.

 

[_xxx_]

I can't really explain that correctly either, but it does change the pressures in the exhaust section, which affects many other things. Exhaust is already being re-used or partly recycled if you so wish - for turbos or for feeding it back into the combustion chamber.

 

[Frank]

Most likely, turbo chargers are more efficient, simpler and cheaper than compound turbos.

 

[Squeak]

For a turbine driving a generator in an electric car this could a good idea though.

 

[_xxx_]

For a turbine driving a generator in an electric car this could a good idea though.

Nope, too weak.

And if it's an electric car, where should it come from?

 

[Squeak]

Nope, too weak.

And if it's an electric car, where should it come from?

I was thinking more a long the lines of those interim fuel cell electric cars, where a small fixed speed engine drives a generator, instead of using heavy expensive current tech batteries.

 

[Mintmaster]

Most likely, turbo chargers are more efficient, simpler and cheaper than compound turbos.

Turbo chargers rarely improve thermal efficiency (although they don't hurt it like superchargers do). They basically make a smaller engine equivalent to a larger one with a couple drawbacks (turbo lag, weird power curve, expense, etc).

The purpose of a turbo compound engine is to improve efficiency. It's especially useful for Wankel rotary engines, which is what the original link is talking about. That engine is generally less efficient to begin with, so we're not talking about anything revolutionary, but it is useful for light aircraft engines.

 

[Rainbow Man]

Are wankels common in aircraft?

Not even Mazda bothers with wankels in cars these days.
Peace.

 

[BlizzardOne]

Not even Mazda bothers with wankels in cars these days.

RX-8

 

[Rainbow Man]

But surely that was years ago?

From what I've been told despite its mechanical simplicity a wankel is overall inferior to a piston engine in a car.. So why would they still be developing the tech?

Pewace.

 

[Altivec]

But surely that was years ago?

From what I've been told despite its mechanical simplicity a wankel is overall inferior to a piston engine in a car.. So why would they still be developing the tech?

Pewace.

You have to be kidding.

They have the Renisis rotary... http://www.mazdausa.com/MusaWeb/displayPage.action?pageParameter=modelsMain&vehicleCode=RX8

How hard is it to google?

 

[Rainbow Man]

So what's the point of having discussion fora? We can all just google, right?

Come on. Don't be like that.

 

[_xxx_]

I was thinking more a long the lines of those interim fuel cell electric cars, where a small fixed speed engine drives a generator, instead of using heavy expensive current tech batteries.

I haven't seen that yet, but I can't imagine it would be efficient enough. But that's just the gut feeling. Would be an interesting project, that's for sure.