coyoteboy
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| posted on 10/9/10 at 10:46 AM |
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Pros and cons of twin engined setups
So if you're considering a twin-engine 4WD setup you have the choice of combining the two engines into a xfer box, then outputting to a centre
diff, or simply driving the two engines to opposite ends. My thinking is that with two 750's, one pointing forward with a flipped diff, one
pointing backward with a normal diff, make a ~200hp/140lbft 6spd sequential 4wd with little more than the added mass of the extra engine and
diff/shafts, which is a nice proposition considering the extremely low price of 750s, plus reduces the wear that you'd normally get dragging
half a ton round with a 750 (not normally a good idea).
But obviously two engines need to be fairly closely controlled or you end up with some odd torque splits. Assuming you maintain the same gears and
shift at the same times, without an Xfer box you should essentially have fairly close match of revs and torque from two identical engines, which
should give you approx 50:50 torque split, and pretty much locked due to the similarity of the engines, almost like a viscous centre diff. Two
seperate engines would have additional costs of two ECUs unless you have them hard-linked where they would act practically as one engine (plus some
harmonic problems).
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MakeEverything
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| posted on 10/9/10 at 10:50 AM |
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You would need to have a common gearbox i would imagine. The chances of synchronous gear changes would be tight, though not impossible.
Kindest Regards,
Richard.
...You can make it foolProof, but youll never make it Idiot Proof!...
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scootz
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| posted on 10/9/10 at 10:52 AM |
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I'd have thought that trying to control the two engine outputs so that they're not constantly fighting each other would be nigh on
impossible for your average 'home' builder.
I know it's been done by the likes of Z-cars, but how successfully? I've not heard of their twin-Busa Ultima since it was finished.
It's Evolution Baby!
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tomgregory2000
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| posted on 10/9/10 at 10:54 AM |
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there is a twin engine tiger somewhere
might be worth looking at that for some info
link to tiger click
[Edited on 10/9/10 by tomgregory2000]
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coozer
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| posted on 10/9/10 at 10:54 AM |
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Z cars twin engine ZX9R Westy was a party piece.
Take off up to the ton and then switch the second engine on! That was Chris's explanation to me and the two engines could be in different gears
as well.
1972 V8 Jago
1980 Z750
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mrwibble
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| posted on 10/9/10 at 11:02 AM |
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pros:
woooooooooohoooooooooooo
cons:
how the feck
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coyoteboy
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| posted on 10/9/10 at 11:04 AM |
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I'm not sure they would be fighting that much to be honest but I'm hoping we can think it through out loud here. If you have a physical
parallel link on the stick they either both go into gear or both don't. Slow take-off is the sticky point where clutches may be more or less
engaged than their neighbour.
IIRC the zcars one (that was the one on fifth gear wasn't it?) had 2 engines feeding one axle, that's not for me. Essentially if you have
2 axles seperately driven the only time you're really going to have problems is with breaks in traction at one end or the other. If that happens
you're going to end up having one engine revving out while the other bogs, which may be a terminal problem, but that's essentially the
same as an open centre diff, where one end spins out and you have to ease off. Maybe something like the megasquirt rev-rate traction control on each
engine would keep things in tow, automatically throttling back the slipping axle, but I'm nto sure how well MS handles bike-engine rates.
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rallyingden
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| posted on 10/9/10 at 11:08 AM |
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A mate of mine made a twin engine 4WD VW scirroco by grafting the front of one into the back of another. First setup was manual box at one end Auto
box at the other but then he found out that as there was so much wheel spin he could put two manual box's in with a single lever selecting
gears. Off the line it spun the wheels so much that the fact that one set spun a bit slower than the others didn't matter  .
To drive home from an event he selected neutral then disconnected the linkage to one box and drove home on one engine two wheel drive
Billiant
RD 
[Edited on 10/9/10 by rallyingden]
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jkarran
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| posted on 10/9/10 at 11:16 AM |
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They'll not fight each other, having one doing more work is the equivalent of one going slightly uphill, the other slightly downhill.
They needn't even be in the same gear or with the same final drive ratio giving you the possibility of an uneven torque split since bike engines
have such a wide rev range. The problem there is over-reving one or wasting the potential of the other. In fact, they needn't even be exactly
the same engine (though you'd get some weird variable torque distribution going on!).
In the real world the main issues will be keeping the gearboxes synchronised and avoiding an accidental overspeed when one doesn't shift up.
The beauty of an engine/axel is obviously 4wd and the ability to use a very simple, lightly built drivetrain.
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MikeR
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| posted on 10/9/10 at 11:17 AM |
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if the torque balance is such an issue - then why not get two identical engines and detune one. That way you'll always have one engine more
powerful than the other and therefore the torque balance will always be one way in favour. The amount in favour will change through the rev range -
but on a two wheel drive car the torque isn't linear.
My understand is the biggest problem is the gear change on bikes. They're a bit agricultural and its far easier to get a miss shift on a bike.
At this point you risk an engine going pop a second or two later. On the tiger twin rear wheel drive car 5th gears tiff blew up one engine as he
couldn't get the hang of shifting both engines. (mates lad was the motec guy who tuned the engines and the comment i heard back was the owner
was rather 'gutted' when he saw one engine go bang)
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mikeb
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| posted on 10/9/10 at 11:18 AM |
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Sure a guy did twin engine's in a scirocco called it a durocco or something, it was two 16v lumps seperately driving each axle. Went like stink
theres some youtube footage somewhere
http://www.youtube.com/watch?v=eQREhPfA5w4
http://www.durocco.com/
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adithorp
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| posted on 10/9/10 at 11:21 AM |
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I think the main con is that it's illegal for a road car to have two engines driving two axles independantly!
adrian
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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iank
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| posted on 10/9/10 at 11:31 AM |
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quote:
Originally posted by adithorp
I think the main con is that it's illegal for a road car to have two engines driving two axles independantly!
adrian
I've also been told that's a myth based on the crash that nearly killed John Cooper.
Do you have a reference to the law in question?
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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scootz
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| posted on 10/9/10 at 11:32 AM |
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This is one of those things that gives me a headache (bit like pondering the Universe!).
If one is driving the front wheels and the other is driving the back... and they are not putting down exactly the same power at exactly the same time,
then surely one axle is always going to be turning faster than the other...
So would we get a push-me-pull-you effect and one engine being ragged by the other?
I can get my head around it when you introduce uber-fancy electrickery as we all know that this is a black-art where magical things happen, but I
can't quite fathom it otherwise!
Will follow this thread with interest...
It's Evolution Baby!
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matt_claydon
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| posted on 10/9/10 at 11:38 AM |
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quote:
Originally posted by scootz
I'd have thought that trying to control the two engine outputs so that they're not constantly fighting each other would be nigh on
impossible for your average 'home' builder.
I know it's been done by the likes of Z-cars, but how successfully? I've not heard of their twin-Busa Ultima since it was finished.
Why would they be fighting each other? If the throttle's open, they will both be pushing the car forward, if the throttle's closed, they
will both be engine braking. It doesn't matter if they aren't outputting the same amount of torque, it just adds together. The only
problem is if they are so out of sync that one is trying to accelerate and the other is trying to engine brake, but that would require a seriously bad
setup!
Simple example: Let's suppose two people are pushing a car, it doesn't matter if one is pushing harder than the other, they still help
each other out.
Engines don't try to run 'at a speed', they provide a torque based on throttle position. The speed they then run at is dictated by
what they are pushing against.
[Edited on 10/9/10 by matt_claydon]
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scootz
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| posted on 10/9/10 at 11:46 AM |
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quote:
Originally posted by matt_claydon
If the throttle's open, they will both be pushing the car forward, if the throttle's closed, they will both be engine braking.
It's the bit in the middle that gets me Matt (doesn't take much!)... you pick two engines off the assembly line and they will not have the
exact-same accelerating / decelerating properties. They will differ somehow and by varying margins.
Throw in the variables caused by a twin throttle linkage and this gap in operation will surely widen (can be a complete PITA setting up the linkage on
a pair of throttle bodies on a V engine where they are only a foot apart, never mind them being at either end of the car!).
Am I getting too concerned about the need for exactness here!?
It's Evolution Baby!
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mcerd1
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| posted on 10/9/10 at 12:03 PM |
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and two engines = twice the weight (engine wise at least)
[Edited on 10/9/2010 by mcerd1]
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adithorp
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| posted on 10/9/10 at 12:19 PM |
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quote:
Originally posted by iank
quote:
Originally posted by adithorp
I think the main con is that it's illegal for a road car to have two engines driving two axles independantly!
adrian
I've also been told that's a myth based on the crash that nearly killed John Cooper.
Do you have a reference to the law in question?
Not exactly, just hearsay.
When Scrapheap did thier program where the teams built vehicles and then drove them around the country doing various challenges along the way, most of
them had to go through SVA and the "twin engined Taxi" had to have one engine disabled for the test and whenever on the road... at least
that how I remember it.
"A witty saying proves nothing" Voltaire
http://jpsc.org.uk/forum/
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hughpinder
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| posted on 10/9/10 at 12:58 PM |
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I would have thought you would need to run both engines from the same ECU to ensure that at least the ignition timing is the same, so the engines
follow the 'same' response. I assume both clutches will be release using the same pedal, assuming you're using the bikes
gearboxes/clutches- This means at some point, even if its only for an instant one clutch will bite and take the whole load of starting the car moving,
until the second one cuts in. I would have thought this would lead to heavy load/unload cycles on the gearboxes and give early failure. On the other
hand, it may be that the first one just starts to put load on the tyres as the second starts to bite, and the boxes may then last a bit better. How
will you prevent the 'missed gear' problems experienced on bikes - if one engine changes up and the other doesnt, it could get messy.
I'd love to see it done though.
Regards
Hugh
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coyoteboy
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| posted on 10/9/10 at 01:12 PM |
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quote:
I think the main con is that it's illegal for a road car to have two engines driving two axles independantly!
That's something that needs to be investigated further, definitely.
As stated above I don't think the lack of sync/variation in torque is a major problem in any situation except those where you have over-revving
or mis-shifts, over-revving can be sorted by the ECU, as can an engine rev differential (something like the MS can take an input from two shaft
encoders, compare the output and tame the rev-happy engine as it slips).
Im not sure how you'd have a mis-shift, it's something I don't get without trying it I guess, but it's either home or
it's not and if it's not you get off the throttle pretty damn fast don't you? I've done it on my 3s-gte but I suppose its a
lot slower to hit the limiter
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mark chandler
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| posted on 10/9/10 at 02:21 PM |
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I cannot see an issue with not having matched engines, 4wd cars do not bias torque 50:50 front to rear, landrovers for instance have an open diff in
the middle so 100% can go to the front or rear with the diff lock out.
Clever jap cars change the bias as you drive, the only problem is making sure you do not over rev one of the engines.
Auto one box manual the other makes the gears very easy, altough car balance will be all over the place on hard bends if it shifts.
I believe the Zcars one had two gear shifts for the bikes engines next t0 each other like fiddle brakes so you could push or pull together, if one
missed a cog you could then just poke that stick, watch the rev counters to confirm they are in sync.
Regards Mark
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jkarran
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| posted on 10/9/10 at 02:32 PM |
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Getting the perfect adjustment so they both shift up together is going to be the problem. With a single engine and a sequential box you clearly feel
the clunk as it shifts. If you get the clunk from one but the other doesn't quite make it up and over into the next gear (sloppy maladjusted
link, insufficient pull on the shifter, worn barrel... whatever) then you have a problem. You may hear the difference, you may not. If you've
just pulled 1st to 2nd on one box but not the other at max rpm you now have one engine pulling strongly in 2nd rapidly overspeeding the other which is
stuck (making no power on the now ineffective rev limiter) in 1st.
Since it'd be reasonably straightforward to electronically interlock the engines so they'll only run when safe to do so it looks like the
legal issues might be the most intractable problem.
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coyoteboy
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| posted on 10/9/10 at 02:37 PM |
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Yep, it did have a twin-stick like boats with two engines. And certainly although my 4x4 does have a 50:50 torque split, most do not, I was just
concerned with some negative handling effects of having one vastly out of whack, however this is effectively a viscous coupling as I say - it allows
some torque bias but not much (only that caused by the slight variation in engine outputs, which will be in single-figure percentages).
Time to ring VOSA I think. It might be twice the weight, but even so mcerd - twice the weight is still only the same weight as a pinto or maybe zetec
powered car but with more power and 4wd. Sure it's not a 1000hp/ton, but I suspect its handling and grip would more than compensate. It would be
nice to have twin torsens to go with it. That could be arranged.
I'm still thinking through the missed gear scenario. If the selector travel is controlled, in fact limited, by the entry of the gear into its
location (as I've had it described, I've never done it on anything but a 600cc on an engine dyno) then you press and wait for it to drop
in. Now if two are linked together a few things may occur:
1) Press and hold, one will mesh, the other meshes slightly later, stick moves into place - all good.
2) Press and hold, one meshes, the second has fallen past the point that it'll mesh so never meshes, neither engine achieves a gear.
If the lever motion is controlled by the action of the gears meshing, it's an all-or-nothing affair, or rather both or none unless your
adjustment is out of whack. I'm not sure how tight that tolerance is. This could make it a pain to drive, but it's fairly easy to cut
spark during the shift to aid meshing and ease the load on the box. But I'm guessing on the details of this due to lack of experience with
them.
[Edited on 10/9/10 by coyoteboy]
[Edited on 10/9/10 by coyoteboy]
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RazMan
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| posted on 10/9/10 at 02:47 PM |
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Maybe one (or would it be two) of the electric shifter kits would minimise the missed gear changes?
I ecu controlling two engines sounds difficult but maybe MS would be a better idea.
Cheers,
Raz
When thinking outside the box doesn't work any more, it's time to build a new box
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NigeEss
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| posted on 10/9/10 at 02:49 PM |
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quote:
Originally posted by tomgregory2000
there is a twin engine tiger somewhere
might be worth looking at that for some info
link to tiger click
Sure is, it's in my mates garage just up the road from here. Not the one pictured
on the link above but the Z100WR that was indeed blown up by Mr Needell.
It runs two ZX12R engines facing opposite ways each driving a different axle.
There's two gear levers next to each other which can be locked together
but generally are not just in case one doesn't shift right.
Two Motec ecu's control things with the ABS rings used as a traction control
system.
BTW it passed SVA and is road legal although currently not T&T'd.
[Edited on 10/9/10 by NigeEss]
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