Reverse Engine Rotation??
Discussion
All,
People talk about damaging the engine during a spin due to "reverse engine rotation", I assume this occurs when the car travels backwards in a forwards gear as the car comes to rest.
How does this damage the engine?, logic would suggest that the engine, which is basically an air pump, would just cycle exhaust gases back up into the intake manifold during reverse rotation or am I missing something?
Thanks,
Geo.
People talk about damaging the engine during a spin due to "reverse engine rotation", I assume this occurs when the car travels backwards in a forwards gear as the car comes to rest.
How does this damage the engine?, logic would suggest that the engine, which is basically an air pump, would just cycle exhaust gases back up into the intake manifold during reverse rotation or am I missing something?
Thanks,
Geo.
Hi 
In a race car..its called hydraulicing...
Yes you are missing something
The wrong valves are open and the normal result is a very big bang !!!
Most engines lose the inlet system at best and I have seen many race cars throw a piston or two out of the side of the block and sometimes the crank takes a trip out of the sump....
Not a good thing to do....although its unusual to manage it just by spinning etc....
Race cars have a nasty habit of doing it on start up...
All the best Brett
In a race car..its called hydraulicing...
Yes you are missing something
The wrong valves are open and the normal result is a very big bang !!!
Most engines lose the inlet system at best and I have seen many race cars throw a piston or two out of the side of the block and sometimes the crank takes a trip out of the sump....
Not a good thing to do....although its unusual to manage it just by spinning etc....
Race cars have a nasty habit of doing it on start up...
All the best Brett
"In a spin, both feet in"
Once past the point of no return, lock up the brakes and declutch. Engine will be idling ready for a quick restart once you come to rest, provided you didn't find something solid to park against in the meantime.
In my 20 yeats of racing I have not seen any engines damaged in the manner described though.
Once past the point of no return, lock up the brakes and declutch. Engine will be idling ready for a quick restart once you come to rest, provided you didn't find something solid to park against in the meantime.
In my 20 yeats of racing I have not seen any engines damaged in the manner described though.
I've had my share of spins, even without declutching and on the dry (and the wet and...), and I don't think you can ever get the engine to operate in reverse in a spin. The wheels are sliding by definition and they will not have the time to catch. I'd imagine the clutch might take a hit when you spend too much time going backwards but not the engine. This is just my perception. Never happened to me.
Kay
Kay
Engines with timing belts particularly don't like having the belt "pull" applied in the wrong direction but the same problem can happen with the timing chain. The tensioner works on the "slack" (a relative term) portion of the belt/chain; the "tight" portion is usually a straight run betwen the cams and the crank when the engine's rotating in the right direction. If the engine rotates in the wrong direction slack appears in the wrong area and the valve timing's off. Not an issue so far as running because 4 stroke engines won't run in reverse but if the engine's tolerances are very close valves and pistons can collide.
And I think bike engine cars can have clutch damage as well from trying to drive the engine the wrong way. When in a spin both feet in. When in doubt both feet out.
And I think bike engine cars can have clutch damage as well from trying to drive the engine the wrong way. When in a spin both feet in. When in doubt both feet out.
On the inlet stroke of a 4 stroke engine, it's the inlet valves that are open and the plug fires just before top dead centre ( of the piston). So if the engine is in reverse it's going to be firing just after top dead centre. If you're running a lot of advance on the ignition the timing difference is marginal as is the compression. So basicaly the engine is going to run backwards; Not as efficently as it does forwards and it isn't designed to run backwards so I'll take on board anything to do with cam chains etc but I can't see why you would think that it would exhaust through the inlet valves or pistons would contact valves, con rods through the block, crankshaft failures, or any other catastrophic events would occur at the first instance. I can understand a high compression engine dieseling on the exhaust stroke if there's unburnt fuel in the cylinder head and that this would be a bad thing. Anyway didn't seem to do my engine any harm and the instructor wasn't overly concerned - it was his idea to put the car in 1st and see what happened next - but what would he know.
Edited by tony.t on Wednesday 15th August 23:36
Edited by tony.t on Wednesday 15th August 23:37
Think it really depends on the engine, but i cannot see a 4 stroke working in reverse because of the valve sequence, as someone mentioned in race engines, they are designed very light and the pistons i can see being very upset with lots of unburnt fuel in a very high compression situation (hence the hydraulicing)
Low compression road engine with very little valve over lap, would not expect too much of a problem.
Low compression road engine with very little valve over lap, would not expect too much of a problem.
Certainly not my experience of a 4 stroke going backwards, least wise not a road going car. Valve timing and ECU control over fuelling and ignition will make this very unlikely.
IMHO you'll most likely bend a valve or two, if hurried backwards whilst there's some valve lift going on, otherwise a others have said it'll bugger the cam chain/belt variety dependant. because the tensioners do'nt like working in reverse.
I have had two strokes fire up in reverse, an old RD250E to be exact, frightened the life out of me, and I actually rode it backwards in 1st with feet firmly planted on the deck, bloody hilarious and scary in one go...
Kevin.
IMHO you'll most likely bend a valve or two, if hurried backwards whilst there's some valve lift going on, otherwise a others have said it'll bugger the cam chain/belt variety dependant. because the tensioners do'nt like working in reverse.
I have had two strokes fire up in reverse, an old RD250E to be exact, frightened the life out of me, and I actually rode it backwards in 1st with feet firmly planted on the deck, bloody hilarious and scary in one go...
Kevin.
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