Cost/Benefit of Multi Engine Vehicles

[BretCahill]

Reciprocating engines for motor vehicles still cost > $10/kW so it will be awhile before fuel costs make multi-engine vehicles economical, assuming it will ever happen.

DOE or DOT has some regulations ready to go to require semi rigs to have APUs (aux. power units) because tractors waste billions in fuel idling. All DOT needs to do is find some people willing to enforce the regs., i. e., show up at truck stops at 2 am to bang on the cab doors of violators.

While sitting in traffic near Malfunction Junction I thought it might be good to have a small "trolling" motor in every vehicle to save gas. A lawnmower engine will move most vehicles fast enough for Capital Beltway and LA traffic.

With hybrids it is possible to swap out engines. You could have a big one for towing the boat on weekends and a smaller one for commuting.


Bret Cahill

 

[motorsportsdesign]

When I was 18, I had two Olds Toronados. I removed the drivetrian from one and installed it in the rear of the other to make a twin engine 4wd car.

The car had several unpleasant characteristics:

The worst was that at certain speeds there was some sort of resonance between the two engines that was shockingly loud.

The second was that the anti dive didn't work out too well in the rear when braking. That was later corrected.

Driving with only one engine running was suprisingly sluggish.

The fuel consumption was severe.

The heavy spring on the accelerator pedal to manage both engines got old quickly in traffic.

All that said, the four wheel burnouts were pricless for an 18 year old.



Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[tbuelna]

BretCahill:

Here's a company trying to put a two cylinder gasoline APU in the back of SUVs:

http://www.cmcpower.com/html/electricity/42volt.asp

Regards,
Terry

 

[BretCahill]

If we had cheap efficient small gas turbines it would be nice to use a small (10- 15kW) GT engine in a series hybrid to get 60 - 80 mpg _most of the time_. A larger (150 kW) GT engine would also fit under the hood for direct parallel power -- they want $5,000 for a hybrid generator -- when it was needed.

The larger engine would only spin at one speed and would require something like a torque converter or other fluid coupling transmission. The energy wasted getting up to speed wouldn't be such a big deal on the interstate but it might be a problem in traffic.

Large (50 MW) aircraft GT engines typically have 70 times more power/weight than vehicle reciprocating engines. Even if the smaller GT engines had only 3 - 5 times more power / weight of reciprocating it would make sense to stuff two or more under the hood.

Assuming, of course, they were cheap enough to make good economic sense.


Bret Cahill

 

[GregLocock]

If you want a good weight/efficency/cost equation then why would you walk past a diesel engine?

I think your prices are whack, OEM gasoline engines run about $10/kW, for cheap ones. I doubt a diesel would be thrice that. The cost/weight tradeoff for cars is completely different than for aircraft.






Cheers

Greg Locock

 

[motorsportsdesign]

I was listening to a radio show where they computed the added cost of the hybrid cars compatred to the value of the fuel they saved. IIRC the car had to be driven 100s of thousnads of miles to break even.

Jonathan T. Schmidt

http://www.schmidtmotorworks.com

 

[drwebb]

Fuel efficiency may be a strong suit of turbines (as long as they're at a constant speed, anyway) but the emissions would kill it for widespread deployment, and aftertreatment usually costs fuel efficiency.

 

[BretCahill]

GE has a 100 + ton industrial gas turbine that gets 60% efficiency -- the highest for any engine or power plant -- but that really hasn't been the case for smaller GT engines where diesel has been better. The Abrams tank has been criticized because its 1500 kW gas turbine is so inefficient it cannot carry enough fuel to go very far.

Nevertheless, the GE example may encourage efficiency increases in smaller gas turbine engines, lower economies of scale and lower Reynolds numbers not withstanding.

Wankels may be compact enough to stuff two under the hood.

The problem is, if the smaller engine is going to be prime prime mover as in a series hybrid, then you really want it to be the clean and efficient engine, which works against small engines.


Bret Cahill

 

[tbuelna]

BretCahill:

As far as PRACTICAL prime movers are concerned, consider the following:

CI recip piston engines are the best choice efficiency-wise.

Recip piston engines are the best choice cost-wise.

Turbine engines are the best choice power-to-weight wise.

However, emissions-wise, turbine engines can be pretty good:

http://www.catalyticaenergy.com/xonon/how_it_works.html

http://www.capstoneturbine.com/applications/emissions.asp

Regards,
Terry

 

[EdDanzer]

We are working on a digitally controlled linear engine so only the amount of power needed for driver demands is provided this could be from 1 to 10 or more.
I have been looking for a plot of power delivered to the wheels for acceleration and available for regenerative braking of a car or truck during a city driving cycle to estimate the possible savings. In the last 2 years I have not found such information.
My estimate is it is possible to reduce fuel use by 50% is some cases.

 

[GregLocock]

Ed if you look at my website

http://www.geocities.com/greglocock

in the gallery it has a zipped spreadsheet FTP75.xls which on sheet2 has the speed time profile for FTP75, the Federal urban driving cycle . You can use that for the acceleration part of your estimate.

Braking is much more difficult. Your best bet would be to talk to someone with an electric car and instrument their battery.

Here's some approximate stats

almost all braking is at 0.35g or less
average braking is 0.2g

If you contact Peter, he has electric car data for real world driving,

http://people.unisa.edu.au/Peter.Pudney

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[GregLocock]

Also, Howard Wilson from GM, who was closely involved with the Impact program, estimated that the range benefit from regen was of the order of 15-20% for their heavy, aerodynamic, electric car. He did not specify a drive cycle. It was designed very much as an LA car if that helps.

The real world experience that GM got from running the Impact program would probably make them the best source of information like that, sadly the rest of us are unlikely to see it.


Incidentally you would probably be able to start generating your own data if you have an OBDII scanner, using vehicle speed to work out the decel rate, and "brakes on" to activate it.




Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[EdDanzer]

Greg,
I took a snap shot of the data and up to about 20 MPH the vehicle only puts about 2hp to the ground during acceleration. (For those how have not seen the data the engine is listed at 22075 W, or 75,320 BTU’s at this snap shot) If that is converted to BTU’s that is only 5091 BTU/hr, or .037 gallons per hour at 100% efficiency, or 540 mpg on diesel. This does not seem possible.
This is why I would like better data.
My preferred method would be to measure torque and rpm at the drive wheels.

Thanks, Ed Danzer