dual two cylinder engines
 

imagine, you have a small car, i.e. civic sized.

and you have a couple, say 400cc motorcycle sized engines under the hood, complete with transmissions and clutches/etc. And they are tuned for efficiency (feedback/fuel injected/etc).

Each engine drives one wheel. They are only connected through the ground.

You can put the engines side by side in back and make it rwd if you like, whatever. Ladder bars and dual chain drive might be easier to visualize anyway, assume you have the drivetrain sorted (and cooling and exhaust and shifting and charging and everything). Assume the layout works for purposes of this discussion, and you can live with the acceleration both engines provide.

One engine is sized just so that it is very close to bsfc peak on your normal hiway excursion, or wherever you cruise.

The other engine is the same size.

You can engage/disengage/restart them independently.

You can (should) use both when accelerating/climbing, and use one when cruising, and none when coasting of course.

Bonus, if one fails you can probably limp home on the other one, even if it means removing a master link with your pocket knife.

If you went all aerocivic on the body and optimized the engine size and gearing for best bsfc cruise, what kind of city/hiway mpg do you think would be possible?

Has this been attempted?

dcb -

I don't know. Educate me on how the steering would work if only one rear wheel was moving the car (I just don't know).

CarloSW2

works just fine, My mom has a one wheel drive trike, no problemo. Maybe an argument for rear wheel drive though, I could see it being an issue in fwd now that you mention it, unless you got fancy with the articulation.

Special measures would need to be taken so that the engine that only sees intermittent use doesn't cool down i.e. it's always in "cold start" mode.

No MPG speculation?

You might want it to cool down if you cruised a long way, then switch engines on the return trip, or share coolant around town or whatever makes sense. You might not need as much radiator in one engine mode if you have the mass of the other engine there.

More Performance than economy... Ok all Perfomance but same concept could be used

http://www.youtube.com/watch?v=yofZIcTkCiw

Yah cadillac had a two engine monstrosity too :)

Ok, so Frank and Carlos hint at some good points, lets make the following additional assumptions for simplicity:

It is rear wheel drive (i.e. ladder bars and chain/sprocket) so you don't have to worry 'bout torque steering or complicated steering geometry and drivetrains.

It is liquid cooled with sufficient valving so you can keep either non-running engine warm with waste heat from the other.

You shouldn't need a differential.

I would guess somewhere between the Aerocivic (80mpg) and the VW 1 Liter (235mpg). Split the difference and decrement the results some for what I assume to be a much larger (than the 1 Liter) vehicle. Maybe 130mpg? Just so you know, the Aerocivic uses barely any power when cruising... less than 15hp at 75mph if you don't include any parasitic power (headlights and such). If you only had a need for 30hp, you might get by with a much smaller than 400cc engine... what does a 250cc engine put out?

well, the question is what hp does it put out at bsfc peak. My 250 is rated at 25hp peak, so maybe 1/3 of that at bsfc peak, don't know fer sure of course.

And yes velocity is important, but I am just thinking of a course like the epa test circuit for now for mpg guesses, with a hypermiler at the wheel.

You also don't want to stress an engine with a 100% duty cycle i.e. if cruise requirement is 15 HP you want, say, a 20 HP engine.

Believe me I've thought about dual engine set-ups before.