Two thoughts:
The two independent drivelines don’t need to be synchronized in any way, they self-synchronize. It’s pretty much a non-issue. With electric drive, on nearly all mainline locomotives, each axle (or most of them) has it’s own electric motor. Most now have a computer-controlled anti-slip system which compensates for differences in rail surfaces, small differences in wheel diameter due to wear or age, etc. They don’t really have any problams with this.
The other item: Citroen did this as a production thing many years ago by taking a 2CV (small car, layout like some Audis with engine longitudinally driving the front wheels) and simply adding another complete driveline, with engine, trans, wheel drives, etc. In the rear. Bolt the steering cross arm to the chassis (no frame) and off they go. It was marketed as the Sahara, supposedly for the French north africa market. They made them for a few years and sold them fairly well.
One shifter and clutch pedal for both engines?
Agreed, at least for variable frequency controlled AC motors. But only because electric motors of that type are controlled by RPM, not by power output. Totally unlike an IC engine.
Ranger Jeff:
Yes, one clutch pedal and one shifter and one gas pedal. Two starter switches, though. You could run the thing on one engine for better highway mileage. There was a separate lever that would disconnect one or the other (or neither) end by disconnecting the shift link while it was in neutral. It also had two gas tanks, one under each front seat, and a battery under each rear seat. In theory you could bump start one engine from the other if it’s battery was dead. I don’t know how the wiring worked for lights, blower, etc. Each engine had a generator.
Well Citroen actually built and sold a twin engine car. A 2CV is about as far from “fine computer controlled” as you can get.
Here look: Citroën 2 CV Sahara 1958
Cadillac Eldorados used to have a 472CI V-8 attached to a transaxle version of a TH-400. People have stuffed them into the engine compartments of VWs. Given enough time and money, I could see someone putting one in front as well. The 500 engine is a bolt in replacement, so it would be plausible to have 1000 cubic inches of torquey engine in a bug. Buy a neck brace first.
Chevy engineers tried it with a Citation(?!)
ACaddy Eldorado with twin Northstar drivetrains.
An old Cooper Mini with twin engines…the Twini!
The Eldo, twini, and Citation all have transverse drivetrains.
Once in Philly about 25 years ago there was a Hot Rod show at the Civic Center and someone had a Chevelle (or maybe a Nova) with a Caddy 473 and TH400 driving the front wheels, and another TH400 with a 500 caddy motor out back.
Both auto trannys…Column shifter for the front, Console t-bar for the rear, two separate key ignitions on the dash.
Not a VW, but I remember this one from the 80’s, here in Australia…
BLOBAK 2: a Holden HQ ute with a blown 350 Chev in the tray and a 253 up front!
"For BLOBAK 2, Alan added a 253 Holden donk up front to keep the nose on the deck and to run the alternator and other accessories and converted the whole lot on LPG. Mated to the 253 was a gutted Trimatic, which supported a steel driveshaft bolted directly to the rear of the 253’s crank.
Behind the transmission was a modified tailshaft that bolted directly to the Chev’s crank snout, making the 253 the starter motor for the Chev."
No front wheel drive though! from: http://www.streetmachine.com.au/features/1408/where-are-they-now-blo-bak-2/
But back on the VW thing, has a ‘new’ beetle engine been relocated to the back yet?
There have been El Caminos (that’s the Chevy car-pickup, like a Ford Ranchero, for you kids in the audience) with an Eldo/Toro driveline mounted in the bed. One I saw was primarily a show car, so I have no idea how the driving dynamics and performance might have been, but the other (and I think there was more than one around) was a drag machine with very impressive numbers. High 7s? Mid 8s? Something around there. Of course, that only speaks to its straight-line performance. I am not sure such a beast would have even adequately safe handling.
That’s 100% false. There is precisely zero need for any kind of synchronization.
Sure, the car will drive nicer if the power distribution is approximately equal. But it’ll drive adequately if the power distribution is 80/20 or 20/80. For most drivers in most situations the difference between 60/40 and 40/60 would be imperceptible.
This. Road traction and rolling force on something the size of a car, even a small one, is more than enough to iron out small differences in driveline speeds.
Doing everything you can to make sure that the throttle response and curves are as matched as possible, so that you can whomp on the gas and have equal response at both ends, is good and (to a point) necessary. But the idea that you have to precisely “gear” the two drivelines together is wrong. You aren’t going to have one set of tires spinning with a tiny bit of slippage because the throttles aren’t precisely synched.
In a real-world design intended to be driven by completely ordinary drivers in all normal situations, a precisely engineered AWD system is not optional. But as long as the driver is skilled and aware of the potential problems, dual drivelines, separate, will work just fine. And have, in a number of test vehicles. As C&D found out, it’s all a lot simpler than it might seem when sketching on that blank piece of paper.
LSL, you keep referring to power and various terms that are absolutely correct for an airplane. I’m not sure if this is how a car actually works.
If I understand it correct, the engine cycles at a given rate. This is the speed of the crankshaft at the bottom of the engine, and is what shows up on the tachometer.
Most transmissions most of the time, the cycling crankshaft is connected via a specific ratio - a gear ratio - to a driveshaft. That is, some number of crankshaft turns are needed to turn the driveshaft once. The driveshaft goes to a differential, which somehow sends the power to one wheel or the other depending on which one is easier to turn.
So as you step on the gas, the engine and drivewheel are directly coupled. Every engine turn is connected by a ratio to a drivewheel turn by a solid block of metal gears.
If you were to attach a big cable to the car while it’s driving down the road and force it to stop without reducing power, the weak link of the system will fail - the engine and transmission and driveshaft keep turning, but the drive wheel will start slipping, leaving a cloud of burnt rubber.
I think this is what will happen in the 2 engine case. The engines won’t quite be the same speed. The drivewheels won’t quite turn at the same speed. And so one drivewheel or the other will slip some to make up the difference, leading to excessive tire wear.
Tires do slip. They slip all the time. Not in the dramatic fashion you mention but in small unnoticed ways. Step on the gas? The drive wheels slip a bit. Steer the wheels? The tires slip a bit. Step on the brakes? The tires slip a bit. In fact ABS is designed to limit the amount of slip to no more than about 10-12% as this is where maximum brake force is.
So the car won’t care if the front engine is at 3,250 RPM and the rear is at 3,253 RPM the tires will slip a tiny bit and it’s all good.
The fact that a number of twin engine cars have been built should tell you that it is possible.
Nope (ETA: @Habeed). The “slippage” is here instead:
Assuming identical engines, transmissions, and tires, you’re right that the two engines will end up turning at the same RPM. It’ll be a different RPM for different speeds, but at, say, 30mph in 3rd gear while going straight ahead, both engines will be turning exactly the same 1950 rpm.
Where folks are confused is thinking they have to have the exact same throttle setting to achieve that. Or that some computer needs to monitor torque to ensure the engines are pulling equally. The “slippage” is entirely in how hard each engine is working to turn that 1950 RPM. One may be bustin’ ass while the other is loafing. That’s fine.
Consider this thought experiment. Take an ordinary 2WD car. One with a single engine, transmission and two drive wheels. Drive it in, say, top gear at 60mph on level ground. Now without changing the throttle setting = without the driver moving his/her foot even a smidgen, have the car encounter an up hill. What happens?
The car slows down. The exact same throttle setting produces a slower speed. Why? Because the effort required to move the car increased due to the hill. Is anything slipping while going uphill? Nope. The car’s speed and the engine’s RPM decrease in sync with each other. The only change is the relationship between throttle angle and engine RPM.
Now start down the other side of the hill, again without moving the gas pedal at all. What happens? The car speeds up. Because the effort required to move the car decreased due to the hill. Is anything slipping while going downhill? Nope. The car’s speed and the engine’s RPM increase in sync with each other. The only change is the relationship between throttle angle and engine RPM.
Now consider a car with two completely separate engines and drivelines going the same 60mph on the same level ground. And let’s assume by some magic you’ve got the two gas pedals set exactly matched, so each engine is shouldering exactly 50.000000000000% of the burden of pushing the car. They’re both pushing equally hard and turning the exact same RPM with exactly the same relationship between throttle angle and engine RPM.
Now stomp on just one of the two gas pedals. Don’t change anything on the other one. What happens now?
The stomped-on engine will generate more power. And will start to pull harder But it will feel some drag because its partner isn’t putting out the same increased power. IOW, the stomped-on engine will “think” it’s starting up a hill. From it’s POV, the symptoms are the same as a hill.
Meantime the other unchanged engine will notice that suddenly it’s gotten easier to push. It’ll “think” it just started down a hill.
The car’s speed will settle at whatever is appropriate for the total output of the two engines. One will think it’s pushing up a hill while the other will think it’s coasting down a hill. They’ll both be right in what they’re feeling, but wrong about why. The car is still on the flats. the two engines are just unequally sharing the load of pushing it against the total drag forces.
I think you’re correct, LSL. The additional piece you’re adding to the model is that you’re pointing out there’s feedback the other direction, from the wheels to the engine. Work an engine harder and it’s RPM drops at the same throttle setting, since the amount of air permitted into the engine is the same so the total power is the same.
Of course, if RPM drops, power might also drop, dropping RPM further…not sure why engines don’t have this kind of negative feedback. Or do they…
Anyways, I’m not an automotive engineer, just my thought was that the power differential between 2 engines would be expressed by the tires slipping a lot. Maybe this isn’t a big deal.
Anyone that played the gran turismo series will be familiar with pikes peak escudo. A real world twin engine 1000 hp monster that won the pikes peak rally in 1995. So not only is it possible you can win international rallies with such a design Twin-Engined & Terrifying: A Monster Suzuki - Speedhunters
LSLGuy is right. Conventional single engine cars already encounter the same push/pull loads. When engine braking, towing (or under differing load weights) and driving up hills. A two-engine model might just encounter two opposite cases at once.
I guess a two-engine car might be more tricky on snow, if the two engines were throttled very differently, but if they were on the same pedal, that wouldn’t happen.
Isn’t there a way to tie 2 engines to a single transmission with some mechanical wizardry? I would assume that’s what the Suzuki guys did.
I think there are multi-engine technologies in aircraft, trains and the like that use what are essentially multiple torque converters to couple two crankshafts to one “transmission.” The problem then becomes the old one of trying to couple all that power to the ground, a problem I am intimately familiar with at only the 500HP level. 
A few years back, a crazy Brit built a… well, car is the wrong word. It was an aircraft V-12 displacing 36 or so liters with a body shell wrapped around it, and a cockpit for insane people on the rear. He used two rear drive axles and four huge tires to try and get useful work out of the 3-4,000HP up front. The engineering of such dual drive is the mirror image of what we’re talking about here, but otherwise quite similar.