I was talking to mueller a while ago about building turbine engines out of turbo chargers, and he said that that was basically what an aircraft APU (auxillary power unit) does.

Which gets me to wondering. I think it would be really cool to build an electric 914 that is powered by a turbine generator running on CNG.

Is this viable? With Otmar's setup this would be an interesting way to get 1000 ft lbs, but still maintain range. Plus you ought to be able to get access to Californias HOV lanes (either by running CNG or as a hybrid.

What kind of power output can you get from a small turbine generator?

I see a few locomotives here and there that use turbine genereators vs. diesel. They look/sound cool pulling a grade with a high-piched whine and a 6ft blue flame shooting straight up.

You're not the first to think about this. But... I think on a car it would end up being much less efficient than direct drive from a piston engine.

Were you thinking about using a turbo from a car, or using an APU? If you were thinking about an APU you're going to fall out of your chair when you see the prices. About 15 years ago I worked for a big flight school and saw an APU from a Gulfstream II go for $100k.

-Ben M.

Ben,

I believe Fiid is thinking that the drive wheels would be powered by the electric motors (as are the locomotives)....

I think the hybrid cars are pretty cool (engineering wise, design wise most are fugly )

With the generator powerplant, you'd want to make sure the "donor" motor is effecient....diesel powered hybrids have been made by the automanufactures, but the high cost of the diesel powerplant coupled with the higher cost of the electronics make it a very expensive project which the consumers more than likely would not buy.

A 4WD 914 is possible if run as a hybrid, gas to turn the rear wheels, electric for the fronts

Wouldn't the noise be a serious issue for a street vehicle? All the turbo-fabbed turbines (admittedly 2) and APUs I've seen you needed ear protection up close to them.

Ocassionaly you can get APUs for a much better price military surplus (still ain't exactly cheap). BTW our local surplus store has been sitting on a turbine from an F-100 for several years now, any takers? Who doesn't need 20k+ lbs of thrust? ... in a several ton, 20' long package...

Hehe, I've thought about this The problem seems to be a combination of eficiency of energy conversion, and weight.

Turbines aren't that heavy, but the generators often are.. even if you get a light one, you will still need a battery pack because turbines don't like varying loads. So you basically have an electric car (already pretty heavy unless you have VERY expensive batteries) and then you add a turbine, a generator, all the charging/regulating electronics, and a fuel tank of some sort. In the end you basically have a locomotive

The energy conversion problem applies to all series hybrids. You convert the chemical energy in the fuel to mechanical energy (turbine) then to electrical energy (generator) then to chemical (battery) then to electrical (battery) then to mechanical (motor). In the end you only get a small percentage of the fuel energy as mechanical energy. That is why all the commercial hybrids are parallel hybrids (both the engine and electric motor are mechanically coupled to the wheels)

As for power consumption, remember that at cruise, the car is only consuming about 10hp-15hp to maintain 60-65mph. So, you don't need a lot of turbine power to keep the batteries topped up while cruising. Theoretically, one could get by with a pretty small engine to power a generator just to extend range. A 250cc bike engine should be quite adequate. Perhaps you could run that on CNG just to make it all more complicated.

I'd also wonder about noise with a turbine. I have no idea how a turbine reacts to exhaust backpressure from a muffling system. I know Volvo make a turbine/electric concept car a few years ago, but I have no idea if they actually made it run.

I just happened to be thinking about the efficiency issue with this kind of setup the other day. A turbine gets about 80% efficiency (fuel energy converted into mechanical energy). A four-stroke Otto engine has about 30% efficiency BEFORE a gearbox is involved. Turbine to generator to electric power to electric motor to mechanical drive I think cascades to about 20% efficiency, which is roughly the same as an Otto engine and gearbox. However, you can run on batteries alone in town and under acceleration, so no need for an engine of better than 20-30hp just to keep the batteries charged. Main downside is limited acceleration time before you have to wait for more charge.

Every turbine generator I've seen had at least 10:1 gearing between the turbine and the generator. There's got to be some loss there.. Also, are all turbines that efficient? I was under the impression that most were quite inefficient, with only the most modern and sophisticated ones up there. Here's a neat idea.. a turbine parallel hybrid. You could gear the turbine down to drive the wheels at cruising speed. The electric motor would accelerate the car up to cruising speed, and the the turbine would take over. At cruising speed and during breaking, you would recharge the batteries.

fiid,

If you wait a short time, you'll have an abundance of Hydrogen fuel cells at your disposal - with those you can build all the 914 4WD cars you want.

Its certainly on my list of things to do.

Hm.. I started looking up some of those numbers, and it looks like a single stage gas turbine is only 40-50% efficient, not 80% as I stated.

I'd certainly not try to have the turbine directly drive the car at any time, as it would really complicate the driveline. As stated earlier, turbines like to run at one fixed speed, and have terrible throttle response characteristics. One big advantage of building a mostly electric hybrid is the driveline is fairly simple. No gearbox required. No clutch required. You'd still need a diff. I'd try to save the weight and ONLY run a diff directly off the electric motor. Or run two electric motors, one for each wheel, and forget the diff altogether (hey, controllable limited slip!). The gas engine is only there to recharge the batteries, so it can be located anywhere, in any orientation, and requires little to no gearing, other than what the generator would need (turbine, yes. Otto engine, no).

Perhaps step one would be build an all-electric car. Only add the gas engine later.

When talking about the efficency of the conversions from one form to another from the turbine to the wheels, the electricity from he generator would go directly to the motor except when the motor is not drawing any amps. The batteries would be a buffer, only get the leftover energy that would then be released when the generator could not meet the full draw of the motor.

So, the turbine would only need to be powerful enough to maintain cruzzzzzing speed, and the battery pack would only need to be large enough to aid in acceleration.

There are some 24v bats that are the same size as the 12v bats used now. So six bats could give you the performance (short term though it may be ) of a 144v system. If your batteries did get too low you could just stop for a moment and let the generator top them up.

one problem with your idea of a small high volt pack is the discharge/charge rate of the batterys, you would need either high end nimh or nicads. lead acid batterys like around a 10 to one rate either way except cranking batterys. so simply put you would need batterys that would cost 5 grand. which is way modern hybribs use a capicitor pack for regenerative breaking

Scott, the 24v bats I was speaking of are deep cycle. nad would be equal to about 1/8 th of the usual size pack used for full electric conversions. On their own that would give them about a 10 mile range without any recharge.

If you're going to use a turbine to genereate the elec, why not rely on some of the new current control technology going into wind turbines? Since the rpm of the shaft changes with the force of the wind, they've developed electronic controllers that maintain steady output current. Not sure about the cost though.

Just a thought!