Engine running in reverse?
 

My father drove a concrete mixer for 30yrs and told me that the engines sometimes started to run in reverse, to stop this event they simply turned off the engine and then restarted it. Anyone ever heard of this? Why do they do that? Jerry.:eek:

Absolutely. Some boat diesels run in reverse in order to back up the vessel. Certainly can happen in some trucks, usually 2 cycle diesels I think. You then get multiple reverse gears and only one forward gear.
It can happen if a person attempts to pop start the engine with the clutch and it turns the engine in reverse.

It happens because the injection is near TDC. If the first combustion forces the crankshaft in reverse, it can keep running that way. The injection will then be after TDC, but close enough to still run.

I don't know if it's a common problem for modern MB engines as I have not heard of an occurance, but the old engines had a flap on the intake which closed if it did start in reverse. The danger of running backwards is that your oil pump is pumping the wrong way.

An MB would not run in reverse. It would cause the chain to go slack before the injection pump and skip teeth or jam.

I think only the 2 cycle will run backwards
 

That is because the timing is very close to TDC and of course they only have exhaust valves, intake air comes in through ports, and of course it fires every time it come to TDC. On the 4 cycle engine the way the valves operate, (valve timing) would preclude a backwards running engine.

Many 4 stroke diesels will run backwards - albeit a bit roughly. I've seen it happen on older British trucks, where the timing is all gear driven, and there were no anti-backfire devices fitted.

This can happen on 4 strokes because the valve timing and injection timing is close enough to symmetic.

Running forwards;

The piston goes down on the intake stoke, pulling in fresh air from the inlet manifold
The piston goes up on compression, squeezing that fresh air (all valves closed)
Injection happens sometime shortly before TDC
The piston goes down on the power stroke (all valves closed)
The piston goes up on the exhaust stroke, pushing products of combustion out of the exhaust

Running backwards,

The piston goes down, pulling fresh air in from the exhaust system
The piston goes up, compressing this air
The injection happens sometime shortly after TDC, but the gases in there are still hot, so the fuel burns (but not quite so well)
The piston goes down
The piston comes up, pushing the products of combustion out of the inlet valve

There are a few ways to prevent this - as mentioned, one way throttle valves, and also making the cam in the injector pump asymmetric so that in the reverse direction, injection happens at a useless point in the cycle (a so-called anti-backfire cam)


The exhaust gas passing through the inlet ruins the air filter in short order!

My grandfather's Oldsmobile (gasoline engine) used to do this. If the RPM was a little high when you turned off the very warm engine, it would start "dieseling" - my grandfather's word - chugging backwards. To fix it, we had to turn the ignition back on, thus somehow allowing the thing to start going around forward again. Then we could kill it with it in gear (to lower the RPM) and it would shut down.

It wasn't going backwards during those after-runs. The engine is still turning forward. The engine is very rough because ignition is probably occurring on only a few cylinders. Fuel is still supplied by the carburetor whenever the engine is turning, and hot carbon deposits can ignite the mixture.

my yamaha golf car went fwd/bkwd depending on which way you wanted to go.

it would really move in reverse.

Not sure about running in reverse, but my Ford IDI will run with the pump a full 180 degrees out of time. Not very well, and they will smoke like a house afire, but they will run. The first one I bought had the pump installed 180 degrees out, but I drove it home.

As far as accidently running backwards heard of 2 stroke diesels taking a gasp and chugging backwards..

I had a Harley XR750 flat tracker that on several occasions at start up took a deep breath, stopped, and chugged backwards until you killed it.