Haflinger engine failure after running backwards

[101RRS]

Not my engine thankfully but a contact has had engine failure after it fired up and ran backwards.

The Haflinger engine is a normal 4 stroke engine with carbs and distributor - the only odd bits is that it is air cooled and a flat twin.

This guy connected the power leads to the starter the wrong way around so when started it turned the wrong way and fired up backwards and soon self destructed. One piston impacted the exhaust valve putting a hole in the piston - the valve did not break but as it was pushed sideways the valve guide was damaged, I noticed the valve spring was broken and the pushrod was bent. I am told there was no other damage. All to be expected when a valve impacts with the piston though a broken spring is not expected - did it break in the incident or is it the result of the incident.

Now the lobe profile on the camshaft is the same in opening the valve and closing the valve so there is no reason why the camshaft could not open and close the valve as per normal if going in reverse.

Now the owner is adamant the failure is because the engine ran backwards - I am not so sure because if it fired up why isn't there similar damage in the other cylinder (it is totally undamaged). Also I turned my engine backwards for over 50 revolutions by hand and there was no issue.

I suspect that there could have been a sticky valve allowing the impact to happen but more likely (as there is a broken valve spring) the spring broke and was no able to fully close the valve (maybe sticky as well) and as the engine is a "hemi" when the piston hits the valve instead of simply pushing it closed it is hit on the side and into the piston.

The guy has ordered new pistons, valves and a pushrod but did not realise he had broken valves so he needs to get them as well.

So what do the engine gurus think? If the lobe profile on the camshaft is suitable - should an engine be able to run backwards if accidently fired up?

Thanks

Garry

Broken Spring.jpg

[PhilipA]

You do not talk about how the timing chain is tensioned or its length, but it is conceivable that if the timing chain was old the timing backwards could be quite different to the timing forward allowing a valve to hit the piston.
Regards PhilipA

[101RRS]

Its a basic design so no timing chain -the cam shaft lives in the bottom end and is geared to the crankshaft then pushrods to the rockers on top of the heads which operate the valves - all these are not adjustable with the exception of the tappets.

Garry

[Graeme]

Turning backwards would be expected to slacken the timing chain if the tensioner was spring or oil pressure operated as the pull would be against the tensioner rather than against the camshaft. A fixed tensioner would maintain normal valve timing.

[justinc]

I'm suspicious of the ability of the starter to even engage the ring gear in opposite rotation. Normally a starter has a one way clutch / Sprag on the drive pinion to help it disengage after the engine kicks. Do these starters operate like any other?

[101RRS]

Hi Justin - there is no flywheel gear or throw out gear etc on the starter - the starter/generator is a combined dynastarter connected to the engine via a twin V belt to the crankshaft pulley. Power is applied to the starter which turns the engine via the belts and when it is running the starter being driven by the engine is then the generator.

The dynastarter is between the fan and the carb. It drives the fan and the pulley on start turning the crankshaft and when running the crank drives the fan and the generator. All DC so can go the wrong way if connected incorrectly.

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[JDNSW]

I wonder if running backwards loosened a nut on either the camshaft or crankshaft and resulted in a sheared key followed by mistiming?

[Blknight.aus]

not buying it...

sticking valve meeting piston I'm buying.

being a carbed engine it should be incapable of running backwards.

Being rolled over backwards maybe but not running.

[justinc]

Hi Justin - there is no flywheel gear or throw out gear etc on the starter - the starter/generator is a combined dynastarter connected to the engine via a twin V belt to the crankshaft pulley. Power is applied to the starter which turns the engine via the belts and when it is running the starter being driven by the engine is then the generator.

The dynastarter is between the fan and the carb. It drives the fan and the pulley on start turning the crankshaft and when running the crank drives the fan and the generator. All DC so can go the wrong way if connected incorrectly.

Hochzeit_1.jpgHochzeit_2.jpg

Wow, thanks Gary what a great set up, learn something every day!!😊

[101RRS]

being a carbed engine it should be incapable of running backwards.

As Pauline would say - please explain

Suck, squeeze, bang and blow still happens - I can see how the crank can go the wrong way and spark occurs when it should but I am not sure about the valves - valves will be closed at the right time but I have a suspicion that because the cam is turning the wrong way then exhaust valve might be open when it should be closed and likewise the inlet open when it should be closed - so not fuel/air mix gets into the engine but I am am having trouble to conceptualising this.

We know that an engine can fire backwards but can it actually run - without understanding the action of the inlet and exhaust valves I am not sure. However some ships I have been on actually fired their engines in reverse to go astern.

When I had the tappet covers off my engine - I should have checked what was happening with the valves when I manually rotated it in reverse.

Cheers

Garry