Technical Stop Start Story

[UFI]

Maxi stated that technology had moved on so didn't see that S/S was going to be much of an issue to a modern turbo.

You stated that technology hadn't changed since the 80s....

The example given were of how technology had moved on since the 80s including VGTs but also materials, design, tolerances and electronic control.

Look see!

View attachment 147376

The improvements in materials, tolerances and design means that bearings are far less likely to fail on a modern turbo, than they are on a unit from the 80s, this is the sort of thing I suspect Maxi was getting at, VGTs was just an example of how turbo technology had moved on, where you were claiming it hadn't.

In the 80s turbos were fitted to only special cars where as now with the huge growth in demand for turbo diesel cars and manufacturers pushing down the co2 outputs of all models, virtually all cars on the forecourts today come with at least one turbo engined option. As a result of course you will see higher numbers of turbos failing, from all manufactures

Someone's feeling pedantic. Perhaps your definition of the word 'much' is different to mine. My dad would have argued that cars haven't changed 'much' in the last 100 years.

Switching off a hot turbo still kills it, so the advances are still irrelevant to the discussion. Turbos were never intended to race from one light do the next with the engine switching off in between, we simply don't know what happens when they are asked to do so. Of course turbo failures are more common as more turbos are fitted, but the point if they're STILL failing without stop/start. What happens when you add S/S stresses to the mix?

 

[306maxi]

Yes, but do small turbo's like the one in the twinair actually get properly hot during typical use? Yes, if you are pushing on it'll get hot, but how often does this happen on the typical car and how often does the drover come to a stop soon after?

 

[Most Easterly Pandas]

Yes, but do small turbo's like the one in the twinair actually get properly hot during typical use? Yes, if you are pushing on it'll get hot, but how often does this happen on the typical car and how often does the drover come to a stop soon after?

But define pushing it, the turbo is designed to be used even in sedate driving conditions, its not necessarily to do with heat, more turbine RPM.

 

[306maxi]

Yes, but in typical conditions your foot is not going to be flat to the floor pushing the turbo at high rpm for long periods of time and causing a buildup of heat into the turbine housing.

 

[Sweetsixteen]

They also don't account for the fact that you can always kill the engine on any car if you know you're going to be stopped for awhile.

The driver will always know better than the computer how long the car is likely to remain stopped. A manually activated switch on the steering wheel would, to my mind, be a useful addition, together with a (persistent) menu option for setting S/S to Off/.

Picked up on this, and heartily agree. I hate nanny cars, that assume they know better than you. Where possible, I knock out the auto-lights and auto dip features, the auto rain wipers, etc - and many of the other increasingly auto features - including stop/start, I would rather be the judge of when to cut my engine.
By watching the road and traffic, including junctions, traffic lights and other hold ups, I choose whether it's worth cutting the motor at each stop or not. Better for the battery, the motor-reliant accessories, and the environment.

 

[UFI]

Yes, but do small turbo's like the one in the twinair actually get properly hot during typical use? Yes, if you are pushing on it'll get hot, but how often does this happen on the typical car and how often does the drover come to a stop soon after?

Small turbos are actually where the problem lies. Look at the size of a turbo diesel intercooler even on a small capacity engine - it's huge. These engines are designed for a high turbo duty cycle and so need a high thermal capacity intercooler, much more so than the occasional boost event that happens on more performance oriented machines.

My 174hp (stock) TDi intercooler has about triple the thermal capacity of my 220hp Skyline. Worryingly, the TA intercooler is sized more along the lines of the Skyline even though the boost map looks very much like a TDi.

I see a lot of people racing between lights every day, with 875cc just keeping up with traffic gives that baby snail quite a work out.

 

[306maxi]

But the intercooler cools down the charge, not the turbo.....

 

[jrkitching]

Look at the size of a turbo diesel intercooler even on a small capacity engine - it's huge.

UFI, be careful not to confuse what's happening in the intercooler with what's happening to the exhaust turbine.

The intercooler has to be massive to cool the incoming charge, but it's not running at the sort of temperatures (perhaps 80C max) which are going to compromise lubrication.

The exhaust turbine, OTOH, can get very hot indeed (450C typically and perhaps significantly more in a worst case scenario) - it's what's happening in the parts of the system that are directly exposed to the exhaust gases, and the heat transferred from those parts to the bearings, that's going to cook the oil.

Remember also that modern cars need to run very close to peak EGT for economy reasons. It can get pretty warm in there.

 

[306maxi]

But do modern oils actually get cooked? A casual google would say that modern oils don't coke up and crack like oils of yesteryear.....

 

[jrkitching]

But do modern oils actually get cooked? A casual google would say that modern oils don't coke up and crack like oils of yesteryear.....

...and indeed they don't. If you ran a modern turbo on 1950's oils it likely wouldn't last long at all.

But no amount of technology can overcome basic chemistry. If you apply enough heat to a hydrocarbon, even a well designed synthetic one, eventually it'll break down & carbonize.

 

[306maxi]

Just going to chuck something out there..... Anyone here watched Le Mans recently? Everytime a car pits for fuel or needs work doing on it they have to shut the engine down, they NEVER sit there for a whole minute letting the turbo cool down and turbo failures are fairly uncommon.

 

[306maxi]

...and indeed they don't. If you ran a modern turbo on 1950's oils it likely wouldn't last long at all.

But no amount of technology can overcome basic chemistry. If you apply enough heat to a hydrocarbon, even a well designed synthetic one, eventually it'll break down & carbonize.

But at what temperature does that occur and how often if ever is that likely to happen in a car like a twinair? How many years has the Abarth been out and how many people have been on here with blown turbochargers? What is the most common turbo related failure on the 1.3 multijet as fitted to the Panda?

 

[jrkitching]

But at what temperature does that occur and how often if ever is that likely to happen in a car like a twinair? How many years has the Abarth been out and how many people have been on here with blown turbochargers? What is the most common turbo related failure on the 1.3 multijet as fitted to the Panda?

It's the combination of S/S + turbo that concerns me; many high performance cars do not have S/S, or if they do, they disable it when not running in Eco mode (IIRC RUI test drove a Lambo recently that worked like this).

At the end of the day, it's down to the person buying the car to decide how much risk they want to take.

I think most reasonable folk would conclude that a TA, once out of warranty, is a greater potential liability from the perspective of unscheduled engine repair than a 1.2.

Whether that's significant, who knows? Time will tell.

 

[306maxi]

But the non-eco mode could be seen as a "sod off stop/start!" Mode could it not?

IMHO bringing a Lambo into this is illogical

 

[UFI]

The size of the IC is dependent on the turbo's intended duty cycle (all else like engine capacity and redline being equal). It may not cool the exhaust turbine, but it's still a valid indicator of how hard a turbo is designed to work. I can see where my TA intercooler sizing remake is a bit confused, it just struck me as I was typing that it seems rather on the small side.

Worth noting that a throttle plate less engine like a TA can potentially keep it's turbo spooling on overrun, there's no butterfly shut off.

 

[jrkitching]

IMHO bringing a Lambo into this is illogical

Anyone here watched Le Mans recently? Everytime a car pits for fuel or needs work doing on it they have to shut the engine down, they NEVER sit there for a whole minute letting the turbo cool down and turbo failures are fairly uncommon.

Less illogical perhaps than bringing a Le Mans racecar into this .

 

[306maxi]

Less illogical perhaps than bringing a Le Mans racecar into this .

Perhaps so, but this is a race car that shuts off its engine less than 30 seconds after being on the track and doesn't die

 

[306maxi]

The size of the IC is dependent on the turbo's intended duty cycle (all else like engine capacity and redline being equal). It may not cool the exhaust turbine, but it's still a valid indicator of how hard a turbo is designed to work. I can see where my TA intercooler sizing remake is a bit confused, it just struck me as I was typing that it seems rather on the small side.

Worth noting that a throttle plate less engine like a TA can potentially keep it's turbo spooling on overrun, there's no butterfly shut off.

Not really.

 

[Mick F]

It's the combination of S/S + turbo that concerns me;

Why didn't you tell me this when I was trying to get my S/S to work?


Mick.

 

[UFI]

Just going to chuck something out there..... Anyone here watched Le Mans recently? Everytime a car pits for fuel or needs work doing on it they have to shut the engine down, they NEVER sit there for a whole minute letting the turbo cool down and turbo failures are fairly uncommon.

This is something I've considered but is probably more irrelevant than than JR's Lambo.

How much do those turbos cost $50,000? What does the oil cost $200/litre? How long did they drive on electric power before hand? How much life do these hot stops take out of the turbos? How long do they get to coast in the pit lane? How many stops are made under safety car?

No one is saying a few hot stops will instantly vaporise your bearings, but do it often enough and it's got to take it's toll. I agree oil is the key and we just don't know yet.