Engine is switched off, does it stop at the same point?
Discussion
Not sure if posting this in the correct section, and it may only be of interest to those who have several cars or a classic car etc. When an IC engine is switched off, does it for any reason stop at more or less the same point in a revolution, or will the position of the crankshaft be different each time the engine is turned off?
I ask, because when an engine is switched off, some valve will be shut, and the valve springs, at their max extension, whilst some valves will be open, with the valve springs fully compressed.
Certainly not really an issue for cars used on a regular basis, but for others which are laid up for lengthy periods, would a valve spring which has been fully compressed for weeks. months. or even years, have its power reduced (e.g valve springs can become tired over time) not to mention that the cylinder which has valves open to air, might be more prone to cylinder bore corrosion? Just a question, that I have sometimes wondered about, because it does not seem to have much effect on an engine, but I could be wrong.
I ask, because when an engine is switched off, some valve will be shut, and the valve springs, at their max extension, whilst some valves will be open, with the valve springs fully compressed.
Certainly not really an issue for cars used on a regular basis, but for others which are laid up for lengthy periods, would a valve spring which has been fully compressed for weeks. months. or even years, have its power reduced (e.g valve springs can become tired over time) not to mention that the cylinder which has valves open to air, might be more prone to cylinder bore corrosion? Just a question, that I have sometimes wondered about, because it does not seem to have much effect on an engine, but I could be wrong.
If indeed some engines do stop at a specific place, my concern would be for the valve springs in the cylinders which have open valves, where the valve spring could remained fully compressed in the same position for weeks, months (or in some cases even years) and as posted before would there be an increased chance of corrosion occurring in those cylinders where the exhaust valve is open, and therefore the cylinder is open to ambient air via the exhaust pipe?
This is a very interesting question.
Have a look on YT and you'll see plenty of videos of old motors being started up with only just a bit of prep work done - prime with oil, change the fuel, clean the plugs, crank by hand etc. The compression seems to be fine.
The valve springs in an engine are quite big and working well within their tolerance, for them to suffer fatigue they would need to be working very near to their Yield Strength - the point at which they would stretch or suffer from fatigue. If this was the case, they would be failing during normal use too, so leaving one compressed for a length of time should not affect it.
Assuming the engine is laid up in one piece, cylinder corrosion would be unlikely, as most cylinders are lined (steel / aluminium?) which resist corrosion quite well. Also, the amount of ambient air in the cylinder would be fairly small so corrosion would be unlikely. Even if there was a small bit of surface corrosion, as soon as the piston ring slides over it, it will clean if off anyway. You often seen stored vehicles with the intakes / exhausts covered however, for this reason and to stop other things - dust / creatures etc. This would be a different story if the engine block was exposed - but then you'd be rebuilding it anyway right?!
Watching this question with interest for more learned members to correct / inform me...
Have a look on YT and you'll see plenty of videos of old motors being started up with only just a bit of prep work done - prime with oil, change the fuel, clean the plugs, crank by hand etc. The compression seems to be fine.
The valve springs in an engine are quite big and working well within their tolerance, for them to suffer fatigue they would need to be working very near to their Yield Strength - the point at which they would stretch or suffer from fatigue. If this was the case, they would be failing during normal use too, so leaving one compressed for a length of time should not affect it.
Assuming the engine is laid up in one piece, cylinder corrosion would be unlikely, as most cylinders are lined (steel / aluminium?) which resist corrosion quite well. Also, the amount of ambient air in the cylinder would be fairly small so corrosion would be unlikely. Even if there was a small bit of surface corrosion, as soon as the piston ring slides over it, it will clean if off anyway. You often seen stored vehicles with the intakes / exhausts covered however, for this reason and to stop other things - dust / creatures etc. This would be a different story if the engine block was exposed - but then you'd be rebuilding it anyway right?!
Watching this question with interest for more learned members to correct / inform me...
.
Was it not the case (at least in the 'classic car' years) that flywheel teeth wear was worse at one spot (or two spots?) on the circumference because that was always where the starter dog engaged each time? In other words, the engine stopped at a set point(s) in its cycle each time.
Was it not the case (at least in the 'classic car' years) that flywheel teeth wear was worse at one spot (or two spots?) on the circumference because that was always where the starter dog engaged each time? In other words, the engine stopped at a set point(s) in its cycle each time.
Let's assume a 4 cyl engine.
As above the resistance to turning increases with compression, so (all else being equal) it would be fairly safe to assume that the engine would normally stop more frequently at a similar point on one of the 4 compression strokes.
Does a modern stop/start engine stop at a specific point?
I seriously doubt it would make an significant difference where on its cycle it stops as to how easily/quickly it would restart as (as per above) it would naturally be likely to stop on a compression cycle, which is where it would want to be to start quickly.
On top of that, to stop at a different specific place would require additional equipment & additional failure possibilities.
As above the resistance to turning increases with compression, so (all else being equal) it would be fairly safe to assume that the engine would normally stop more frequently at a similar point on one of the 4 compression strokes.
Does a modern stop/start engine stop at a specific point?
I seriously doubt it would make an significant difference where on its cycle it stops as to how easily/quickly it would restart as (as per above) it would naturally be likely to stop on a compression cycle, which is where it would want to be to start quickly.
On top of that, to stop at a different specific place would require additional equipment & additional failure possibilities.
Arnold Cunningham said:
I have read that with stop/start technology some engines now stop at a predetermined position so that they'll start again without having to complete a full revolution to sync up all the position sensors. Unsure if this is a fact, but it seems logical.
Modern engines will know which engine is about to start the compression stroke and thus spark that cylinder first perhaps.Boosted LS1 said:
Arnold Cunningham said:
I have read that with stop/start technology some engines now stop at a predetermined position so that they'll start again without having to complete a full revolution to sync up all the position sensors. Unsure if this is a fact, but it seems logical.
Modern engines will know which engine* is about to start the compression stroke and thus spark that cylinder first perhaps.
An engine will stop, whereever it stops. Simple as that.
And no, there would be no way to stop the engine in a specific position for stop/start or whatever. Nor would it make any difference anyway...hence it would be pointless trying
And yes, if it's an engine with solid lifters ( ie not hydraulic of any kind ) then yes some valves may be open, some springs may be compressed. Nothing can be done about that unless you have a single cylinder engine.
And no, there would be no way to stop the engine in a specific position for stop/start or whatever. Nor would it make any difference anyway...hence it would be pointless trying
And yes, if it's an engine with solid lifters ( ie not hydraulic of any kind ) then yes some valves may be open, some springs may be compressed. Nothing can be done about that unless you have a single cylinder engine.
Some interesting replies. I asked the question because on watching some of the car restoration programs, it seemed that where engines that had not run for a long time were being stripped for a rebuild it seemed that some cylinders / pistons moved OK, but others were seized in the bores, which of course could be down to lubrication problems, but possibly also down to corrosion in some of the cylinders. Having stripped some aero engines, mainly continentals, and a gypsy major, where we did find corrosion in the bores, despite them being air cooled, I have often wondered where/how the moisture got into the cylinders to allow the corrosion to take place.
If the engine tended to stop in certain positions that would account for uneven ring gear wear, but even if the stopping position was completely random there would be more ring gear wear where the engine resistance was greater. Variations in wear don't prove that the engine was stopping there more often.
Penelope Stopit said:
Peugeot XUD 1.9 Diesel Flywheel from under my bed
I have taken a good look at the flywheel and the teeth that the starter pinion has been hitting show much more damage through 2 x 30 degree spots that are 180 degrees opposite to one another, all the other teeth do show a little wear
2 areas on a 360deg cycle 180 deg apart on a 4 cyl engine.....I have taken a good look at the flywheel and the teeth that the starter pinion has been hitting show much more damage through 2 x 30 degree spots that are 180 degrees opposite to one another, all the other teeth do show a little wear
I wonder what could account for that ?
stevieturbo said:
An engine will stop, whereever it stops. Simple as that.
And no, there would be no way to stop the engine in a specific position for stop/start or whatever. Nor would it make any difference anyway...hence it would be pointless trying
And yes, if it's an engine with solid lifters ( ie not hydraulic of any kind ) then yes some valves may be open, some springs may be compressed. Nothing can be done about that unless you have a single cylinder engine.
He's Probably thinking of this http://www.mazda.com/en/innovation/technology/env/...And no, there would be no way to stop the engine in a specific position for stop/start or whatever. Nor would it make any difference anyway...hence it would be pointless trying
And yes, if it's an engine with solid lifters ( ie not hydraulic of any kind ) then yes some valves may be open, some springs may be compressed. Nothing can be done about that unless you have a single cylinder engine.
I'm not convinced that an engine will stop completely randomly. If you turn one over by hand, it's "lumpy" and has natural points of low resistance and high resistance. Watching one stop, you can often see it come to rest at one of these low points. I'd expect that there would be a couple of points (or 4?) where it stops most commonly.
Munter said:
Boosted LS1 said:
Arnold Cunningham said:
I have read that with stop/start technology some engines now stop at a predetermined position so that they'll start again without having to complete a full revolution to sync up all the position sensors. Unsure if this is a fact, but it seems logical.
Modern engines will know which engine* is about to start the compression stroke and thus spark that cylinder first perhaps.CrutyRammers said:
He's Probably thinking of this http://www.mazda.com/en/innovation/technology/env/...
I'm not convinced that an engine will stop completely randomly. If you turn one over by hand, it's "lumpy" and has natural points of low resistance and high resistance. Watching one stop, you can often see it come to rest at one of these low points. I'd expect that there would be a couple of points (or 4?) where it stops most commonly.
It may be more prone to stop on a compression stroke...but it is still random.I'm not convinced that an engine will stop completely randomly. If you turn one over by hand, it's "lumpy" and has natural points of low resistance and high resistance. Watching one stop, you can often see it come to rest at one of these low points. I'd expect that there would be a couple of points (or 4?) where it stops most commonly.
CrutyRammers said:
He's Probably thinking of this http://www.mazda.com/en/innovation/technology/env/...
I'm not convinced that an engine will stop completely randomly. If you turn one over by hand, it's "lumpy" and has natural points of low resistance and high resistance. Watching one stop, you can often see it come to rest at one of these low points. I'd expect that there would be a couple of points (or 4?) where it stops most commonly.
Interesting, but makes no mention of them deliberately stopping at the desired range..just that it needs to be there for that to work.I'm not convinced that an engine will stop completely randomly. If you turn one over by hand, it's "lumpy" and has natural points of low resistance and high resistance. Watching one stop, you can often see it come to rest at one of these low points. I'd expect that there would be a couple of points (or 4?) where it stops most commonly.
Because how would they force an engine to stop at a specific point ?
That said, the target area does seem quite a wide range and on a 4cylinder, it's very likely a piston would end up there.
stevieturbo said:
how would they force an engine to stop at a specific point ?
The ECU has enough information to make a stab at it, but it would be very hard to get any accuracy and I don't see why you would bother. Who cares if the engine has to crank an extra quarter revolution before it fires?Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff