turbine engine + CVT

[BobbyT]

First post for me here (but have read hundreds)

This idea has been rolling around in my head for several years now. I know that about 30-40 years ago, Chrysler (and perhaps others?) was experimenting with turbine engines in cars. I remember a Speed Channel (when it was Speedvision) epdisode enterviewing a guy who owned one of these. It idled at 20,000 rpm, redlined at around 40,000, had something like 125 hp and 425 lb-ft. (Depending on which model you look at, hp tended to be 100-140, and torque 375, 425, etc).

The obvious problem (beside gearing it way way down) is the fact that a turbine engine is best suited for constant use under load, rather than on-off acceleration. So you either have to make a big turbine, use a lotta gas the 90% of the time you don't need power, and have the performance, or put in a more efficient turbine, but have no power when you need to pass / pull away from a stop / hill climb.

Seems to me the solutions is turbine + CVT...the infinite gearing would allow you to stay at peak torque RPM nonstop (great mileage), and the super-wide gearing would let you multiply that torque into power when you needed it.

What do you guys think? I wonder why, with all the expensive hybrids being built, none of the manufacturers have tried this recently.

Northwestern University Engineer, class of 2007

 

[GregLocock]

Part load performance of a turbine is shockingly inefficient. A car needs say 60 hp to accelerate, using a CVT. When it is cruising at 60 or 70 mph it will only need 10-15 hp - a turbine large enough to give reasonable acceleration will just be idling when cruising.

Even the most efficient point for a turbine is nothing to write home about efficiency-wise - do a search and I bet you can't find any small pure turbine installations that are better than 240 g/kWh.

Typical performance from Capstone - 28% efficiency for a unit that weighs 3/4 ton and produces 80 hp.

Toyota Prius engine would weigh less than half that, is more efficient, and more powerful.



Cheers

Greg Locock

 

[GregLocock]

Sorry, ignore the weights, that is a complete generator set, the turbine itself is only a small proportion of that.

Their 30 kW turbine with integral generator is 102 kg.

They make no claims about part throttle efficiency that I can find.

Cheers

Greg Locock

 

[BobbyT]

I'm aware of turbine engine inefficiencies under low load, but I understood them to be very efficient under high load.

The Chrysler cars, after a couple years of tinkering, went from 13 or 14 mpg up to 18 and 20 mpg. Figure the cars weighed 3600-3800 lbs, since they were big sedans. No CVTs, obviously.

So, if you built a CVT-trannied car, you wouldn't need 425 lb-ft. Let's make the car around 3000 lbs. Maybe 2800 (so 10-20% less). Also, it's not going to be a wall aerodynamically, as the original cars were. Scale down that turbine from 425 to, say, 250-300 lb-ft, use the gearing to make up for low power (dunno enough about turbines to guess how much fuel that'd save...would it be proportional to going from 425 to 250-300 lb-ft piston engine?).

Then take into consideration 30-40 years of turbine engine developments, better fuel, better overall automotive engineering (drivetrain power losses, etc)...performance will be better than the original cars easily. And we've gotta have the mileage from 20 into the mid-30s.

Or was I too liberal with my estimates?

Northwestern University Engineer, class of 2007

 

[GregLocock]

Well, find a map of specific fuel consumption for a gas turbine and I'll work out its urban cycle economy, and its highway economy, if you like.

I'll need a 3d map showing sfc vs speed vs torque, or a 2d line showing demand power and optimum sfc for that power.

Frankly, gas turbines only make sense if you can do something useful with the waste heat, and even then, you'd be better off with a diesel if you are primarily interested in power rather than heat, and you need (as I have tried to emphasise) good sfc at low power outputs.





Cheers

Greg Locock

 

[GraviMan]

Interesting thread guys! Rover cars played around with turbines in the 60's. Not suprisingly the less than 20mpg economy lead to these never being implemented into production. Check out google for the Rover Heritage Centre.

The mind boggles as to why the engineers never put two and two together, by implementing a high pressure stage at the turbine core. The best solution would be a 3-cylinder 2-stroke engine, but any piston arrangement would work. Imagine if high pressure ratio turbo supplemented systems had had the benefit of 40 years of developement! We wouldn't be arguing about CO2 emissions now.

Rover also invented the Torrotrack type CVT. Why they came up with the Metro is anyones guess...

Mart

 

[Docengineheat]

Problems with turbine engines:

1. The bigger the engine, the better the efficiency. Small engines are much less efficient.

2. Very sluggish power response. Pilots who learn to fly in piston aircraft are amazed when they transition to turbine aircraft and see how much time it takes a turbine engine to respond to a power adjustment -- on the order of ten seconds or so.

3. Very narrow power range. At 80% power a turbine is essentially idleing.

4. Takes a long time to start (five-ten minutes, at least in aircraft) -- unless you have some special Israeli type instant response starting device.

5. To be as efficient as a diesel, you need some type of waste heat energy retrieval system. In power plants a Rankine "bottoming cycle" is typically used to increase efficiency from 35-40% to 50-55%. This would significantly add to complexity of the system.

It is interesting to note that any engine that has a turbocharger is essentially a type of turbine engine.

 

[dgallup]

Mack Trucks had a turbine engined truck prototype running, I saw it at an SAE meeting around 1980. It used an exhaust heat regeneration system but was still nowhere near as efficient as a diesel. At least with a truck the throttle response and duty cycle are more appropriate than for a car. Mack developed it with AiResearch and somebody else (German) because they were afraid that the EPA was going to legislate diesels out of existance. Based on the huge fines the EPA has recently been handing out to truck companies lately they may have been right!

Also, there is a turbine powered motorcycle for sale right now, Jay Leno has one. It too is totally impractical but way cool.