dual two cylinder engines

[comptiger5000]

That's a good point. I hadn't thought of the longevity issue.

[dcb]

I am not terribly concerned about noise or longevity, those can be addressed obviously in a theoretical and are not germane to Efficiency. One of the assumptions here is that "it works". The other is that it isn't a 30 year old carbureted air cooled (keep it rich) engine, but something with adequate cooling and is tuned for efficiency.

I do understand the 80mpg comment, I think it is good to have a point of reference.

So lets simplify it a bit for now,
lets say you have an aerocivic, and you can choose an engine size that will operate at bsfc peak in cruise (say 60mph? flatland)

so some basic questions:
1. what sized engine are we talking about here?
2. what peak bsfc can you get from the smaller engine?
3. how does that bsfc peak compare to the current aerocivic operating parameters @ 60?
4. If there is an advantage in continuous fuel consumption for the smaller engine, is it negated (or even beaten) by p&g/EOC of the larger engine (maintaining average 60mph)? Or lean burn or whichever is most efficient?
5. if better power management (i.e. p&g) gives the larger engine an advantage, how do you determine optimum engine size?

[ceej]

I haven't got any specific fuel consumption numbers for you. Don't know. Most motorcyclists don't need that information. It wasn't until the last year or so that manufacturers bothered to list fuel economy ratings, and that is spotty at best.

As far as size, that isn't what I was trying to explain. Motorcycle engines are "Overbuilt" to a normal operation standard ~20%. They will take 20% more load than normal riding places on them without breaking. They are also generally rated for intermittent operation at high power settings.
An automotive engine is "Overbuilt" to anywhere from 200 to 300%. It will take up to 3x the loading that would be considered normal operation. The parts are bigger and heavier to do this.

This is easiest to visualize where we have a motorcycle that operates at peak power and torque at greater than 8000 rpm. Most car engines peak somewhere around 5000 rpm. Some lower, some higher.

The weight of the reciprocating assembly is made very light to allow for these engine speeds. The crank shafts use roller bearings. Low drag, and highly susceptible to shock damage. The engine is designed to accelerate very quickly. Much more so than an automotive engine, running plain bearings, and with much heavier rods, pistons and so forth.

Theoretically the lighter engine should operate more efficiently at a given engine speed, It also means that there is less mass in the crank and flywheel to provide torque in between power pulses. A two cylinder vertical twin rotates 360° between power impulses. That's where pumping losses start to become noticeable as well. A four cylinder engine provides power impulses 180° apart. An 8 cylinder rotates 90° between power pulses. That's why they run smoother as well.
Once up to speed, operating somewhere near torque peak, your application could very likely get better economy than the 1.6L Honda engine that came out of the Civic. All things being equal as far as acceleration and load, I think it will be close.

There are very few modern motorcycle engines that are 400cc here in the states. There are 250's then we step right up to 500's.
The most common 400cc twin around here was the Honda Hawk. The operating speed of that engine is somewhere above 6000 rpm to get into any power. Peak power was somewhere above 9500. Below that, HP was considerably lower. If I remember right, Honda claimed 35HP or so from the Hawk. On the Honda engines that we dyno tested, their power claims were optimistic by about 30%. Operated well into the redline they got closer.

As an experiment, your idea has merit. It was unclear how you intended to operate the vehicle. I wouldn't want to rely on that particular configuration for a Daily Driver.

Until we get to very late model machines that are fuel injected, and utilize catalytic converters, motorcycles were much dirtier than cars. In grams per mile of particulates, a five litre v8 car was cleaner than a quarter litre four stroke street bike of just a few years ago. If raw fuel economy is the goal, you may be able to get pretty good results. As far as pollution, the civic engine is far better to the environment.

CJ

[rmay635703]

Just take the motor from a subaru 360 2cycle 2 cylinder, despite itself it is still more efficient than most 4cycle motorcycle rigs as it was tuned for FE, it puts out an amazing 25hp and may be large enough to run alone in an aerocivic if you could figure out how to get 6+ gears coupled efficiently to it of coarse in my line of thought if it only takes 15hp to go 90mph then bear the acceleration, enjoy the weight savings and just use a smaller motor by itself, even better would be a little diesel.

[ShadeTreeMech]

I wouldn't rule out making it front wheel drive because of it being too hard.

Simply use the original 5 speed tranny with the motorcyle engine, and then you will be using the existing design when it comes to driving the front wheels. And with the lesser HP torque steer is irrelevant.

You could attatch the drivechains of both engines to a common drive shaft that would then go to the transaxle. When shifting, you could use the car gears, or make it where you can use the motorcyle transmission in the cab and essentially you could have a multitude of gearing ratios. If you want to disengage one engine, use the clutch cable to dissengage one engine from the system and have a way to lock the clutch into the dissengaged position. If I'm not mistaken, the thrust bearing on a motorcyle is immersed in oil so there shouldn't be any extra wear due to the clutch being disengaged for extended periods of time.

It also takes some of the guesswork out, since you are reengineering less of the car.

[bestclimb]

single engine accept the acceleration, or forced induction on tiny motor to get acceptable acceleration and cruse at 0 vac. go all aero with light weight rag and tube construction. Could get a 2+2 in under 1000# and shaped very well.