Motoring Discussion > 4-cyl Engine always stops in 1 of 2 positions Miscellaneous
Thread Author: Skoda Replies: 11
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New Mon 24 Jan 11 22:30 
 4-cyl Engine always stops in 1 of 2 positions - Skoda
I never knew this but it interested me - a 4 cylinder engine always comes to rest in one of 2 positions, 180 degrees apart if looking at the flywheel. A 6 cyl stops in one of 3 positions.

It means the starter ring on the flywheel always wears out more prominently in 2 locations 180 degrees apart, caused by the starter motor meshing in the same locations every time the car's started.

You learn something new every day.
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New Mon 24 Jan 11 22:34 
 4-cyl Engine always stops in 1 of 2 positions - Lygonos
And a 5 cyl ?
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New Mon 24 Jan 11 22:35 
 4-cyl Engine always stops in 1 of 2 positions - Skoda
Shhh, they don't exist :-)
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New Mon 24 Jan 11 22:46 
 4-cyl Engine always stops in 1 of 2 positions - bathtub tom
>>a 4 cylinder engine always comes to rest in one of 2 positions

Not always, but usually.

Many years ago,I was in a car when the driver turned on the ignition and it started, spontaneously. We could only assume the engine was in such a position, with compressed petrol fuel vapour in a cylinder that was just past TDC and the act of turning on the ignition caused a weak spark that was enough to ignite it.

IIRC it was possible to do this with some large old RR engines intentionally.
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New Mon 24 Jan 11 22:58 
 4-cyl Engine always stops in 1 of 2 positions - Bigtee
In the days of bodging we turned the fly wheel around so where it's worn teeth are unbolt it and turn it re bolt it back on a new starter supplied and it ran again.
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New Mon 24 Jan 11 22:59 
 4-cyl Engine always stops in 1 of 2 positions - Ted

You could do this with vehicles that had a manual advance/reterd.
I recall we often started a neiughbour's 20HP Rolls this way.

Although I do recall a very different way of getting it to move.
It was a Thrupp & Maberly Landaulette, big wheels. On a wedding ( it was specifically a wedding car ), the regular driver had to start the car on the handle. He had left it in gear and the car lurched forwrd, knocking him over. He disappeared under it as it set off down the road slowly. He appeared from the back, white coat covered in oil, ran after it and jumped in. No harm except for pride dented.

Ted
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New Tue 25 Jan 11 08:24 
 4-cyl Engine always stops in 1 of 2 positions - Cliff Pope
I did an experiement a few years ago to answer this question. I marked TDC on the pulley with snopake, and each time I switched off I noted where the mark was.

The engine never stopped at TDC. That is the point of maximum compression, so it will always spring one way or the other away from TDC.

There were in fact 4 different positions - 45 degrees after and before TDC on nos. 1 and 4.
But just after was more common, presumably because of the bias imparted by the momemtum of the crankshaft.

With modern pre-engaged starters there is no wear on the ring anyway, because the starter pinion engages before the motor turns. On old ones they wound themselves into engagement with a crash of teeth.
Last edited by: Cliff Pope on Tue 25 Jan 11 at 08:24
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New Tue 25 Jan 11 11:31 
 4-cyl Engine always stops in 1 of 2 positions - bathtub tom
>>There were in fact 4 different positions - 45 degrees after and before TDC on nos. 1 and 4.

Isn't TDC the same on 1 and 4?
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New Tue 25 Jan 11 12:49 
 4-cyl Engine always stops in 1 of 2 positions - Cliff Pope
Yes, but only one of them is on the compression stroke.
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New Tue 25 Jan 11 12:52 
 4-cyl Engine always stops in 1 of 2 positions - RichardW
So the crank will be in the same place, but the cam won't be, so Skoda's thread should have been titled "4-cyl engine always stops in one of 2 crank positions." There are 4 engine positions - just as each cylinder is coming up to compression
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New Tue 25 Jan 11 13:24 
 4-cyl Engine always stops in 1 of 2 positions - Skoda
Another day another tid bit. I think i can polish off my application to appear on QI now :-)
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New Tue 25 Jan 11 14:19 
 4-cyl Engine always stops in 1 of 2 positions - bathtub tom
>>Yes, but only one of them is on the compression stroke.

The OP said: It means the starter ring on the flywheel always wears out more prominently in 2 locations 180 degrees apart.

The flywheel doesn't differentiate between which cylinder's on the compression stroke.
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