This insight was going to be my last ditch effort to convince people to stop getting hung up on the mistake of thinking that the various engines would necessarily be producing specific RPMs and any difference would be causing massive tire slippage. Congrats for recognizing it.
Consider a single engine car with a V-8 engine. The eight cylinders are not identical. One is strongest, one is weakest, the other 6 somewhere in the middle. They’re each pulling unequally, but they’re all yoked together. And it works. It even works if one or two cylinders quit because, say, the spark plug wire falls off.
I don’t disagree with any of this, except that the linkage in a dual driveshaft transmission is a set of metal gears. In the stated example, the linkage between the engines is back tires->ground->front tires. I was concerned that tire slippage would mean the car might drive ok but you’d burn through tires much more rapidly than normal.
There’s a certain amount of truth in that, but as Rick pointed out, tires slip all the time on pretty much all cars, and it is a factor in long-term tire life. Differentials do a lot to accommodate different rotational speeds for short distances (through turns and on uneven terrain) but tires do still slip and scrub a bit even on a Prius being toodled to the grocery store.
When you are talking about high-performance cars, as are most of what this thread has referenced, you’re going to get a lot of slippage even on non-driven tires, so some measurable percentage of ongoing slip between drive tires and the road is expected, and will shorten tire life, but all of that is in the “who cares” category for this kind of driving.
If Ford was to come out with a twin-engine car, for whatever unimaginable reason, you bet they’d use every kind of fluid coupling, differential, computer-assisted trick to reduce slippage and differential tire speeds… but that’s a specific, highly engineered solution to an optimum end, not a one-off racing or show vehicle.
No idea, but (completely off-topic) I have to mention that I knew a guy (slightly) once who put an air-cooled VW engine into a BMW /2 motorcycle frame.
Anyone who’s gone to a custom car show could wear this on a T-shirt.
My fave memory: “The Hustler.” Imagine a top-drawer-show-worthy custom car, front half something Chip Foose might have turned out, rear half… a pool table. All details done in similar motif - 8-ball shifter, pool cue parking brake handle, all turned up to 11.
Beautiful work. Thousands of hours. Why, I have no idea.
My favorite racing series, the 24 Hours of LeMons has seen a few dual engined cars. I think the most successful has been the MRolla, which is the back half of an MR2 welded to the front half of an AE92 Corolla: LeMons Good/Bad Idea of the Week: Twin-Engined Toyota MR2+Corolla!
The major problem they had in the first few races was that the rear engine has an automatic and the front engine has a stick so hearing when to shift was hard and the rear transmission has a penchant for randomly downshifting at bad times. Overall seemed to work pretty well, though!
I’m a little late to the party, but just in case anyone is still having trouble getting this.
Can we agree the car is moving at 60 mph?
If you agree that the car is moving at 60 mph, this is where you have gone astray. Assume the front wheels are carrying the car at 60 mph. It requires no contribution from the rear engine to drive the car at 60 mph. So, the rear wheels are also being driven at 60 mph. This would be true even in a car that sends no power to the rear wheels (like every front wheel drive car in the world). The rear wheels receive no power directly but they turn because the front wheels are pulling the car body, and the car body pulls the wheels, and the rear wheels are dragged on the ground. There is more friction between the rear tires on the ground than is necessary to overcome the friction in the wheel bearings, etc. so the real wheels turn at the same speed as the front wheels.
If you push or tow a manual transmission car in a gear with the engine switched off, the engine starts to spin at the same speed it would as though the engine were on. This is how you bump start a manual transmission car. This tells me that the driven wheels’ friction against the road is sufficient to drive the engine through the transmission even when the engine isn’t powered.
The difference with the twin engine car described here is that the rear wheels are attached to another engine or transmission. In effect, if the the rear engine weren’t powered, the front engine would continuously bump-start the rear engine.
Well, if the rear engine is off, yes. But if you turn the rear engine on, it will begin to provide power. If the car is travelling at 60 mph, then the rear engine is being driven at 60mph. This will translate to some engine speed in whatever gear the car is in. The question is, if you turn the engine on, will the engine generate some power to help the front engine overcome the friction of dragging the rear engine around? If the answer is yes, then the rear engine is helping to propel the car to some degree. It will be able to go faster with the rear engine running.
Why wouldn’t the rear engine be able to generate power? Perhaps at 60 mph in the gear you are in, then engine is spinning so fast that it exceeds its redline and the connecting rods break. In that case, the rear engine won’t create any power to contribute to moving the car. You can avoid this by having the rear engine geared sufficiently to produce some power at the speeds you plan to travel.
As long as both the front and the rear engines are geared to produce power over the car’s entire speed range, you could have very different engines providing the power at each end and neither engine would particularly care about the other.
It’s called an epicyclic transmission. Multi engine helicopters have been using them for years, and if I was going to build a twin engine car it’s the way I would go. Linky. or here.
I’ll point out, relevant or not, that Navy landing craft of late WWll and Korean vintage were powered by two pair of Detroit 6-71 diesel engines. Each pair of engines was connected to a gearbox with two inputs and one propeller shaft output. four engines, two gearboxes, two props.
I salvaged and refurbished a number of these craft.
A drag racer named Tommy Ivo had a 4 engine dragster. He hooked each side together so he had a V-16 on each side. One side powered the front wheels, the other the rears. Lots of smoke from the tires. I’m not too great on the linking but there are a ton of pics on google, several show the car without the body.
Imagine that the rear engine were there, but not running. As it were dragged along, it would turn, with only a loss caused by the driveline friction and fighting the compression of the engine. Like a stalled car in gear- the whole engine doesn’t seize up, it continues to turn albeit with a great drag on the car’s forward momentum. Now, if that rear engine is running, as its revs come up, it would initially be reducing the drag and eventually be adding to the total output.
Don’t know if it has been mentioned but there is a Russian military vehicle that has a separate engine for each of it’s IIRC 6 wheels. Done here obviously for redundancy and off-road in mind however these things also have to be able to run on roads too.
Normally, in a bug, you have a rear-engine layout; air cooled flat four in the back, the drive wheels, the gearbox, and finally the stickshift up at the driver.
If you mirror this arraingement up front, the front wheels will be spinning THE WRONG WAY…unless you flip the differential/gearbox upside down, or rig the drivetrain to run the other way :dubious:
I would think in terms of making the drivetrain fit in the front of the car, what you’d do is have the transaxle in front (and maybe the engine sitting on the driver’s lap,) so it’d still be pointing the same direction as the rear-mounted one.
Although I do recall hearing stories of people screwing up transaxle rebuilds on old VW’s and ending up with 4 reverse gears, so it may not be all that difficult to do if you did want to mount it the other direction.