Ignition switches - safety

[Murphyv310]

Not quite true - you can also stop them with a carbon dioxide fire extinguisher aimed down the air intake. Or, simply stall them.

Yes CO2 extinguishers do work, allegedly stalling can be hit or miss, if the runaway when on the move it can be halted so long as you are in the highest gear and hit the brakes as hard as possible.
If stopped and in top gear lifting the clutch sharply doesn't always work as it can still slip and burn out.
The best methods are blocking off the air intake or CO2, beware though if that engine throws a Con Rod and your in its way you could be in a coffin along with the engine, its a very dangerous situation.

[Amie]

Discussion with a friend on the runaway engine subject. He put forward another even more bizarre scenario that he says happened to him. He owns a fordson dexta tractor (3 cylinder diesel) one day he says he almost stalled the engine and it started to run backwards! He says that the injector pump works with the engine running either way, but there is no control of the air entering the exhaust, though the engine stop (cable) fortunately does still work, didn't seriously damage the engine it just needed a new head gasket which failed between the oil and water galleries.

Thanks,
Amie

[Kalee20]

Yes I've seen a 4-stroke diesel running backwards too. Doesn't run very well, because injection timing is slightly the wrong side of top dead centre, but run it will, sucking air through exhaust and blowing exhaust via air intake

Petrol 2-strokes are often capable of running backwards, but not 4-strokes, because there's no carburettor on the exhaust side, to introduce fuel...

Surprised it damaged the head gasket though. I can see the valves not liking it, with the inlet valve having to cope with hot exhaust, and the oil pump running backwards isn't going to send oil around the engine, but would be interested to know why the gasket failed.

[Murphyv310]

Hi.
A diesel running backward will be very rough. Yes there will be no oil pressure and would seize very quickly. The timing will be retarded as rightly said will inject after TDC, this is inefficient and increases the heat which may have blown the gasket. The main issue for me would be oil starvation. Running any longer than 20 or 30 seconds would most likely ruin the big end and main bearings.

[Richard_FM]

One of my friends used to have a 125cc MZ two stroke motorbike which he could start with the engine running backwards. Sometimes he would be caught out by this!

Some bubble cars could achieve this thorough dynastart, so they didn't need a reverse gear.

[BusyBee]

One of my friends used to have a 125cc MZ two stroke motorbike which he could start with the engine running backwards.  Sometimes he would be caught out by this!

Some bubble cars could achieve this thorough dynastart, so they didn't need a reverse gear.

I had some other friends, back in my teens, who had a BSA Bantam that occasionally started in reverse rotation. With a simple two stroke the only thing that would bias rotation is the ignition timing. It is usually an error in this which results in those sort of problems. Obviously operation of the starter (kick or motor) would usually determine correct rotation.

Tracy

[Amie]

...

Surprised it damaged the head gasket though. I can see the valves not liking it, with the inlet valve having to cope with hot exhaust, and the oil pump running backwards isn't going to send oil around the engine, but would be interested to know why the gasket failed.

Can't say why it damaged the head gasket or if it damaged the head gasket, it didn't run for many seconds like that, it didn't overheat, and restarted normally, but had blown oil out and it had oil in the radiator and water in the sump. May just have been a weak spot?

He did say that it tried to rev when he pushed in the clutch, I don't know if that engine is regulated by a butterfly in the air intake, (would have no regulation in reverse), or by the control of the pump it wasn't sucking oil because it shut off on the engine stop...
(It's a Perkins engine, the tractor was made in 1960)

Thanks,
Amie.

[Murphyv310]

NASP diesels wouldn't normally have inlet manifold (air only) regulation as a diesel needs copious amounts of air to be compressed on the compression stroke to attain the high temperature needed to combust the fuel thats injected a couple of degrees before TDC. If the fuel is injected too early you get the classic diesel knock and lower output. If the fuel is late or retarded it lowers efficiency massively and combustion temperatures rise, running in reverse will make injection late and massively increase combustion temperatures, you'd never know the engine would have high combustion temperature as the thermal conduction to the iron head and block would be slow, the head gasket though is the weak link and likely to fail quickly. I don't see though how oil would have entered the sump as there would be no oil pressure as the pump would be running the wrong way. I'd surmise the gasket was likely failing anyway and there was already leakage into the sump. I can certainly accept water would get into the sump though.
Fortunately later mechanical diesel pumps don't pump in reverse as far as I know.
I dismantled and repaired a pump in my VW T4 works van many years ago and it was clear it could only operate in the specified direction, as it contained a lift pump as well as the High Pressure section, in reverse the lift pump will not lift.

[Amie]

Thanks Trevor!

Yep. One fault probably highlighted the other, revving may have been a function of the timing. I'm usually quite observant but cant remember anything other than the stop linkage on the pump. The accelerator lever wouldn't be connected by a cable.... I'll have to look next time I'm near it, every day a school day! There is a speed regulator that my friend was explaining to me so the accelerator linkage may not be connected directly, it is possible that the revving was a function of the regulator not working in reverse too?

Poor thing survived OK though.

Amie.

[Murphyv310]

The speed regulator will be done with centrifugal weights as the revs rise the pump output will be reduced. A very simple system using a control rod that is spring loaded to stop hunting. Still used on Yanmar, Honda and other single or twin cylinder engines used for many applications, E.G, generators, pressure washers, pumps etc. Did many in my last job.