Eco stop/start.

[geraldrobins]

Surely the main stress for a timing chain is opening the valves which are heavily sprung, so its under load whenever the engine is running?

 

[rf065]

Surely the main stress for a timing chain is opening the valves which are heavily sprung, so its under load whenever the engine is running?

The main stress is starting, as in moving the entire engine internals from a static position to running while overcoming compression.
Once running, the camshafts open the valves and leverage comes into play.

 

[geraldrobins]

The main stress is starting, as in moving the entire engine internals from a static position to running while overcoming compression.
Once running, the camshafts open the valves and leverage comes into play.

I'm not an expert but I'm not convinced. The starter motor overcomes the compression and the timing chain is under continual stress opening valves?

 

[190]

The only mechanism for additional timing chain wear I can think of is if there is a hydraulic chain tensioner actuated by oil pressure it will take a finite time to tension the chain after every start. Unless of course that problem was engineered out when stop start was introduced. This can be heard at the first start of the morning at least mine can with the window wound down but it only lasts for a second or so.

A test would be, can any timing chain noise be heard when start stop is functioning. Mechanical noise is generally not a good thing and could be an indicator of wear taking place.

 

[st13phil]

Why is the torque that’s required to turn the camshaft at (say) 50rpm more than that required to turn it at (say) 3000rpm. Clue: it isn't. And I honestly don’t think the additional torque required to accelerate the camshaft from zero to 500rpm at startup is significant either.

 

[st13phil]

The only mechanism for additional timing chain wear I can think of is if there is a hydraulic chain tensioner actuated by oil pressure it will take a finite time to tension the chain after every start. Unless of course that problem was engineered out when stop start was introduced.

This is one of the reasons why engines with stop / start have auxiliary (or main) electric oil pumps that maintain pressure when the engine is stopped.

 

[st13phil]

The main stress is starting, as in moving the entire engine internals from a static position to running while overcoming compression.

I think there's a common misconception here. The camshaft is driven at half-speed from the crankshaft, so as far as the camshaft drive is concerned, it doesn't care whether the crankshaft is being turned by a starter motor or by combustion above the pistons. The load on the camshaft drive is caused by the cam lobes lifting the valves against (primarily) the spring tension that keeps them closed, i.e. it's nothing to do with cylinder compression.

Typically, engine wear at start-up is caused by lack of lubrication in bearing journals etc. allowing metal-to-metal contact until the oil film builds up. Unsurprisingly, manufacturers go to great efforts to ensure that engines with automatic start-stop maintain oil pressure / oil feed to components while the engine is temporarily stopped so as to eliminate this issue. A re-start from an auto stop-start stop cycle is therefore a different proposition to a start-up from an ignition off condition.

In short, I am totally unconvinced that auto stop-start is responsible for cam-drive failures.

 

[Tonygw]

Can you please explain the wear mechanism?

I've seen the “engine braking wears out he timing chain / belt” statement a few times, but I’m yet to hear a cogent argument as to why.

Why does stop / start cause the timing chain to stretch (wear)?

I dont have an argument as to the reason sorry, perhaps others could comment but apparently this is the main cause of chain wear on this engine. Its a single row chain not the double rows used on some MB engines so its probably not ideal for stop start. I guess the continued starting, during the initial cranking of the engine puts a little extra stress on the chain each time and causes it to wear prematurely. Its certainly a theory I hear a lot. Mine did make an awful rattle at start up and it was found to be the chian had stretched in a couple of places along its length. At 91K miles a chain shouldnt really wear out that quickly and I know the previous owner did a lot of stop start journeys in it. so I guess that was the issue. The MB indy I use said it was quite early for this to happen, they service a taxi with the same engine and it had done 450K on the same chain.
The rest of the car and engine are in tip top shape though so I was just unlucky.

 

[st13phil]

Its certainly a theory I hear a lot.

Thanks for responding with honesty. I hear the "theory" of stop-start causing premature failures too, but I've never heard a plausible engineering explanation as to why, so I remain highly sceptical.

Mine did make an awful rattle at start up and it was found to be the chian had stretched in a couple of places along its length.

When a chain "stretches" (in common parlance) it hasn't actually stretched, rather the individual link pivots have worn. For a chain that is properly specified regarding power transmission characteristics that is not the subject of a manufacturing fault, this generally points to a failure of lubrication. This could be a result of insufficient lubricant delivery due to an oilway being blocked, inappropriate oil being used, contamination (diesel engines are poor for this), or simply running the oil for too long.

The rest of the car and engine are in tip top shape though so I was just unlucky.

Hopefully now it's been replaced you can enjoy the car without further premature failures

 

[190]

This is one of the reasons why engines with stop / start have auxiliary (or main) electric oil pumps that maintain pressure when the engine is stopped.

I'm happy to hear they have engineered out the problem of oil pressure. I might be forgiven for being sceptical as the problem arises in the first place because of the move to prioritise emissions/fuel consumption over engine durability.

The other big improvement in stop/start will come from the adoption of belt driven starter/alternators which will reduce wear on the starting mechanism and make the whole operation smoother.

 

[wongl]

Thanks for responding with honesty. I hear the "theory" of stop-start causing premature failures too, but I've never heard a plausible engineering explanation as to why, so I remain highly sceptical.When a chain "stretches" (in common parlance) it hasn't actually stretched, rather the individual link pivots have worn. For a chain that is properly specified regarding power transmission characteristics that is not the subject of a manufacturing fault, this generally points to a failure of lubrication. This could be a result of insufficient lubricant delivery due to an oilway being blocked, inappropriate oil being used, contamination (diesel engines are poor for this), or simply running the oil for too long.Hopefully now it's been replaced you can enjoy the car without further premature failures

My only experience of chain stretch on a Mercedes Benz is with the old 190E. At circa 40,000 miles there was an awful chain clatter at start up and sometimes at idle. I paid for the chain to be replaced at an MB dealership only for the clatter to return circa 10,000 miles later. Luckily for me, MB UK took care of the second replacement including the replacement of a modified (improved) oil/lubrication system. Following that replacement I didn't experience any more chain stretch issues right up till when I sold the car at circa 130,000 miles

This is before start/stop, and most of my journeys were motorway miles cruising at circa speed limits. So the theory that one of the root causes for the chain stretching due to improper lubrication does support my personal experience and those of MB.

 

[Tonygw]

Well I can only comment on my experience but my car does have a comprehensive merc dealer service history at the proper intervals so its has been lubricated properly according to the Merc recommendations.

I first heard the chain rattle at start up a few months before I got it repaired and it got gradually worse. At first I wasnt sure what it was.

The MB indy took off the cam cover and rotated the engine manually using the crank and found at certain parts of the chain there was to much play / slack in the chain. He could lift it from the cam sprokets, another small rotation and it would be within tolerance and tight. So he concluded the chain was stretched slightly. what ever this actually means I'm not sure, stretched links or chain pins loose due to wear.. who knows.

Anyway. he replaced it and the tensioner and its all quiet now.
I do know the tensioners on these are spring loaded but also use oil pressure to keep tension on when the engine is running.
It was replaced using genuine merc parts.
So I guess that was the problem, as to what actually caused it... I have no way to prove, all I know is the car was used for stop start journeys at one point in its life. And this seems to be the talking point if nothing else

 

[HB]

So we’re having a few interior rattling issues with new Conti in the family.
I asked Bentley if the technician could could come to the office and I would take him out for a drive so we could both pinpoint where the noises were coming from - we did this and it’s going in next week.

Now,it was the workshop foreman who came out who has been with them 12 years and has worked for Porsche, Mercedes etc and he’s hugely knowledgable. First thing he did was disable the stop start system before he drove off. I asked him why and his reply was simply, “this system is there to tick a box for the manufacturer to comply with emission rules. Most customers have no idea the long term damage caused to the engine switching on and off regularly. It gets fixed under warranty if you’re lucky or as we’ve seen on the Bentleys, huge sums of money to repair/replace parts. Fuel pumps, hot oil draining out a hot spinning turbo, starter motors etc”

His advice , if you enjoy your car and don’t want the engine suffering, disable it every time you get in it.

 

[400ixl]

TonygwYou have most likely just been unlucky with an early failing part. Tensioners have always had a variable life across pretty much all brands be it on belt or chain driven cars, just like chains themselves. Sometimes these things do just happen.

There are thousands of taxis that are on stop start all day everyday doing hundreds of thousands of miles. There is no rumour mill or otherwise of these falling foul of stretched chains. Spoken to a few indies who maintain taxi fleets and not one of them had any concerns about stop start causing any issues.

As for that story from a Bentley foreman, what a load of generic BS. Yes there are failures with mechanical parts all the time. Turbo's are far more likely to fail if driven hard and then just shut off. However most cars have sensors now and keep the oil flowing to them if shut off and over temperature. So stop start will make absolutely zero difference.

Totally understand that some people don't like like the idea and switch it off and I'm glad there is a way for them to do so.

 

[190]

My only experience of chain stretch on a Mercedes Benz is with the old 190E. At circa 40,000 miles there was an awful chain clatter at start up and sometimes at idle. I paid for the chain to be replaced at an MB dealership only for the clatter to return circa 10,000 miles later. Luckily for me, MB UK took care of the second replacement including the replacement of a modified (improved) oil/lubrication system. Following that replacement I didn't experience any more chain stretch issues right up till when I sold the car at circa 130,000 miles

Was yours an early 190E. They had a single row timing chain which was known to be a problem. It was replaced with a duplex chain somewhere around 1988/89.

 

[markjay]

I can see the logic of why repeatedly pulling the chain from static position into motion can cause it to stretch over time (regardless of who or what does the pulling).

Said that, I also acknowledge that my view of this isn't very scientific... it's more of a 'bloke in the pub' opinion

So I may be wrong.

 

[rf065]

Was yours an early 190E. They had a single row timing chain which was known to be a problem. It was replaced with a duplex chain somewhere around 1988/89.

And regardless of the lessons learned back then, they started fitting single chains again, and too make it as awkward as they possibly can, they fitted them to the rear of the engine and not the front where you had easy access.

 

[rf065]

I can see the logic of why repeatedly pulling the chain from static position into motion can cause it to stretch over time (regardless of who or what does the pulling).

Said that, I also acknowledge that my view of this isn't very scientific... it's more of a 'bloke in the pub' opinion

So I may be wrong.

No your not wrong, easiest way to understand it to liken it to moving a static object.
Imagine your car has broke down and you need to move it to a safer position, everyone knows how hard it is to start pushing it and to even get it moving. But once on the move it feels a lot easier to push. Thats the effect of overcoming inertia that your cam chain suffers every engine start.

 

[HB]

TonygwYou have most likely just been unlucky with an early failing part. Tensioners have always had a variable life across pretty much all brands be it on belt or chain driven cars, just like chains themselves. Sometimes these things do just happen.

There are thousands of taxis that are on stop start all day everyday doing hundreds of thousands of miles. There is no rumour mill or otherwise of these falling foul of stretched chains. Spoken to a few indies who maintain taxi fleets and not one of them had any concerns about stop start causing any issues.

As for that story from a Bentley foreman, what a load of generic BS. Yes there are failures with mechanical parts all the time. Turbo's are far more likely to fail if driven hard and then just shut off. However most cars have sensors now and keep the oil flowing to them if shut off and over temperature. So stop start will make absolutely zero difference.

Totally understand that some people don't like like the idea and switch it off and I'm glad there is a way for them to do so.

I think you’re being a bit naive if you think stop start doesn’t damage the engine in anyway ....

 

[190]

It's already been stated that timing chain stretch is misleading in that it's not literally that the metal links in the chain get stretched. The chains tensile strength is many times in excess of the loads imposed on it so that it will not stretch either at start up or under running loads. The chain does get longer but this is due to wear in the pins and rollers.

So if people think that stop/start is causing problems with the chain, the question to ask is what mechanism would cause the chain to wear prematurely under stop/start conditions.

I've found some conflicting reports of what causes chain wear. At high revs the chains rolling elements are rotating faster which all things being equal causes increased wear. On the other hand the average tension on a timing chain decreases with revs. This is probably because the tension is changing rapidly as each valve is operated and the inertia of the whole system smooths that out at higher revs. That would also suggest a chain on a 6 cylinder engine would run smoother than a 4 cylinder.