Where does an engine stop? experiment

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Fri 2 Sep 2005 09:36
Where does an engine stop? experiment - Cliff Pope
On another recent thread concerning possible starter motor problems, it was suggested that a worn flywheel ring could be to blame. It is popularly believed that an engine always stops in the same position, thus concentrating the initial starter wear.
Someone suggested that this was always TDC.
I have experimented by marking TDC with a splash of snopake, and observing over about a dozen stops where the engine has come to rest in relation to this mark.

The results show that the engine (4-cylinder), ALWAYS stops at either 45 degrees before, or after, either TDC on cylinders 1/4 or 2/3. This is I think what might be expected. I observed the engine at the moment of switching off, and as the engine slows to a stop, it either just succeeds in plopping over a compression, or else it just fails to make it over the hill and runs back by a similar amount in the other direction.
There are thus four points around the circumference of the starter ring where wear would be concentrated.
However, with modern pre-engaged starter motors I don't think there is the kind of crash engagement that old inertia motors used to have with the flywheel teeth.

I just thought someone might be interested.
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Fri 2 Sep 2005 18:22
Where does an engine stop? experiment - pmh
From memory I cannot remember on the 4 cyl engines that I have taken apart in the past how many bolts hold the flywheel to crankshaft. If it is anything other than 4 it means that ring wear could be overcome by rotating the flwheel by 360/n degrees.

Worth thinking about if you run old cars.



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pmh (was peter)


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Fri 2 Sep 2005 18:58
Where does an engine stop? experiment - mfarrow
The problem comes pmh where you have semi-modern cars with crankshaft sensors on the flywheel (i.e. Fords). Only solution then is to rotate the ring by removing from the flywheel and refitting 45 degrees later.

A very worth while experiment Cliff. Thank you.

--------------
Mike Farrow
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Fri 2 Sep 2005 21:51
Where does an engine stop? experiment - pmh
Yes I take your point! My engine/clutch etc out experience is fortunately in the very distant past BMC A, BMC B, Imp, Old Fiesta, Morris 918cc sv, Ford 1172 sv ......... that sort of thing.

Crankshaft pos sensors werent even a dream in thos edays.

I hope that this post is not tempting providence on any of the current fleet. Clutch changing may be a bit of a struggle these days. Altho my daughter does nneed to extend her education.....





--

pmh (was peter)


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Sat 3 Sep 2005 22:04
Where does an engine stop? experiment - Cliff Pope
Cars I remember from which I have ever removed a flywheel had locating studs on the crankshaft flange,as well as bolts, specifically to stop the flywheel being refitted in the "wrong" position. Presumably to maintain original balance?

Out of habit I keep glancing at my paint mark. The 45 degree rule continues to hold true, but I do notice a slight preference for one of the 4 positions. Slightly uneven compression pressures, perhaps?
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Sun 4 Sep 2005 08:40
Where does an engine stop? experiment - jc
A lot of cars do not use dowels to locate the flywheel-tho'if the timing marks are on it you would have to but unevenly spaced flywheel bolts were also used for this.
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Tue 14 May 2013 11:04
Where does an engine stop? experiment - TeeCee

ring wear could be overcome by rotating the flwheel by 360/n degrees.

Er, surely if you had the flywheel off and were thus able to rotate it, you'd replace the knackered ring gear while the opportunity presented itself?

There's economy and there's false economy.........

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Sun 4 Sep 2005 12:35
Where does an engine stop? experiment - artful dodger {P}
I can see the logic in where the engine stops at 45 degrees as the cylinder pressure should be equalised.

However this has brought another question. My car is fitted with a 5 cylinder block, so without testing, would you expect it stop in relation to tdc on cylider 1? My guess would be 36 degrees before or ofter tdc. Does anyone else agree?
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Mon 5 Sep 2005 10:58
Where does an engine stop? experiment - John S
Cliff

There's no doubt engines stop at the same position due to the retarding affect of compression. I'm surprised at you finding four stop points though. I'd have said for a 'four' it's actually two wear points because 1/4 and 2/3 stop at the same point on the compression stroke. These two points were quite apparent last time I looked at my Minor's flywheel! This has, of course, the old inertia pinion on he starter.

JS
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Mon 5 Sep 2005 12:25
Where does an engine stop? experiment - Cliff Pope
No, there are definitely four possible stopping points - just before, and just after, each compression point.
However, as time passes and more results come in (this thing gets obsessive) I do notice that the BTDC points are becoming more common than the ATDC.

Presumably the higher the cylinder compressions, the more chance of a slowing engine being stopped before a compression, rather than just struggling over the top.
An engine that had very poor compression might show more even distribution between the four points. Also flywheel weight might be a factor.

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Sat 11 May 2013 11:22
Where does an engine stop? experiment - Neville 23

Surely the just-after-TDC is the just-before-TDC of the next piston. Two compression cycles for each rotation of the crank on a four cylinder. My Ferguson T20 tractor engine (same as the Standard engine fitted to the Vanguard and the TR2 and the Morgan +4) is in bits at the present and has two chewed sections on the starter ring opposite each other - one much worse than the other.

Only the cylinders with both valves closed will affect the stopping position so induction and exhaust are irrelevant. The compression stroke will drive a stopping the engine backwards and the power stroke which is no longer ignited will push the engine forwards because of the compressed air it contains. Under perfect conditions this should be equal on all cylinders and the engine should stop low down on the compression stroke of any one of the four cylinders - hence two chewed places on the starter ring.

However, as noted above ring wear and valve seat wear will make the compression efficiency different for each cylinder so the engine will have a preference to stop where the differential is greatest between the compression of two cylinders which fire next to each other in the sequence - so in a 1,3,4,2 engine 1/3, 3/4, 4/2 and 2/1. The well sealed piston will stop on the compression stroke and the leaky one on the power stroke.

Obviously, I want to use the unchewed bits on my starter ring, but none of this is any use to me because the pistons, rings, and sleaves are all new and the valves re-ground so the engine will probably stop in a different place and replacing the flywheel rotated by 90 degrees, probably won't help. Just bolt it back together andd hope as usual.

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Sat 11 May 2013 15:57
Where does an engine stop? experiment - unthrottled

Surely the just-after-TDC is the just-before-TDC of the next piston

Not if the cylinders are 180 degrees out of phase (which they should be). Two pistons simultaneously pass TDC as the other two pass BDC.

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Sat 11 May 2013 17:23
Where does an engine stop? experiment - Neville 23

Depends how far 'just-before' and 'just-after' are away from TDC (In Clff Pope's observations). If they are small (as you imply) then there must be 4 stopping points for the engine - 2 for each piston of which 4 overlap the other 4. However, if 'just-before' and 'just-after' TDC are something close to 90 degrees then there should be 2 stopping points, one for each piston and 2 overlapping the other two.

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Sat 11 May 2013 17:28
Where does an engine stop? experiment - Neville 23

On second thoughts 90 degree rotation of the flywheel should do it. Pity I did not mark the original position of the flywheel relative to the crank

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Sat 11 May 2013 17:43
Where does an engine stop? experiment - Neville 23

The perfect engine must surely stop halfway down the (unignited) power stroke of one piston and halfway up the compression stroke of the next in sequence as it is the same volume of air trapped in both. Pressure differences due to adiabatic cooling of the gas in the power cylinder and heating in the compression cylinder can surely be ignored due to the far greater heat transfer from the cylinder walls.

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Sat 11 May 2013 18:08
Where does an engine stop? experiment - unthrottled

The perfect engine must surely stop halfway down the (unignited) power stroke of one piston and halfway up the compression stroke of the next in sequence as it is the same volume of air trapped in both.

Think of things in terms of torque flux rather than work done by a gas on a piston or vice versa.

A work balance implies that a (non-firing) power stroke exactly cancels out a compression stroke. But the bulk of the compression work is done at the end of the compression cycle. This corrosponds to the end of the expansion stroke in which very little positive work is done.

So an engine will always stop near the end of a compression stroke-even though the forces on the pistons are not balanced.

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Sat 11 May 2013 18:39
Where does an engine stop? experiment - Neville 23

Fair enough, so an engine with insufficient angular momentum to pass the next compression TDC bounces back and stops - where? Of course, that will be critically affected by the valve timing, just a crack of exhaust valve opening at the end of the unignited power stroke will make the compression stroke piston run all the way back near BDC (or just-after-TDC for the power stroke piston in Cliff's observations). But if no valve opens maybe friction will cause the engine to stop just before TDC of the power stroke piston for long enough for the gasses to leak round the rings. That would explain Cliff's observations and irregularities in the compression efficiency of the cylinders might favour stopping on some cylinders rather than others.

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Sat 11 May 2013 19:42
Where does an engine stop? experiment - unthrottled

TDC bounces back and stops - where?

At very slow angular speeds, hydrodynamic lubrication is lost and the friction in the bearings and piston assembly will increase markedly. I think this should be more than sufficient to hold the piston assembly stationary while the pressurised cylinders bleed down.

Of course, that will be critically affected by the valve timing, just a crack of exhaust valve opening at the end of the unignited power stroke will make the compression stroke piston run all the way back near BDC

At the end of the unignited power stroke the ressure in the cylinder will be roughly atmospheric so opening the exhaust valve will not introduce any additional irreversibility to the system!

That would explain Cliff's observations and irregularities in the compression efficiency of the cylinders might favour stopping on some cylinders rather than others.

Possibly. I'm not sure what effect variations in cylinder sealing efficiency would have on where the piston assembly stops.

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Sat 11 May 2013 22:19
Where does an engine stop? experiment - Neville 23

So basicly friction wins at just before TDC. Many thanks for your help with this.

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Sat 11 May 2013 23:22
Where does an engine stop? experiment - unthrottled

Yup.

The question isn't as trivial as it might seem. Interest in knowing what position the engine stops has resurfaced with Stop-start systems because a warm engine can (sometimes) be restarted by firing a cylinder that is at the start of the expansion stroke.

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Sun 12 May 2013 15:19
Where does an engine stop? experiment - Neville 23

An engine recently stopped with a cylinder near the beginning of the expansion phase might be made to re-start reliably if a bit of petrol were injected followed by a spark. Maybe that's the direction you're heading in. Do it on a cold engine and you could get rid of a whole heap of heavy junk. Its a bit reminiscent of the probably apocraphal stories of old Rolls Royces starting when the ignition was turned on.

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Sun 12 May 2013 15:59
Where does an engine stop? experiment - galileo

An engine recently stopped with a cylinder near the beginning of the expansion phase might be made to re-start reliably if a bit of petrol were injected followed by a spark. Maybe that's the direction you're heading in. Do it on a cold engine and you could get rid of a whole heap of heavy junk. Its a bit reminiscent of the probably apocraphal stories of old Rolls Royces starting when the ignition was turned on.

I think the trick on old R-Rs was to manually advance the spark, using the lever on the steering column; if there was mixture in the cylinder this fired it.

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Sun 12 May 2013 19:36
Where does an engine stop? experiment - unthrottled

It was a trick used with any 4 cylinder (or more) engine with a manual timing lever. You also had to remember to retard the spark timing when hand cranking to avoid nasty kickbacks...!

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Mon 13 May 2013 00:36
Where does an engine stop? experiment - bathtub tom

Ever kicked over an old 500cc single cylinder motorbike?

You only forget to retard the timing once!

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Mon 13 May 2013 09:50
Where does an engine stop? experiment - Neville 23

I can see that advancing the ignition through the firing point might start the engine, but surely simply turning on the ignition would cause a current change in the coil if the points were closed / worn which might give a spark in the piston at the start of its power stroke if the rotor arm were close enough to the lead. I guess the attachment of these stories to R-R is an early exmple of viral advertising - our tolerances are so good that............

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