Engine revs without foot on accelarator

[BigKev2]

Xman - Good post my man. Its good to get a bit of tech detail. I think you are on the right track.

The Miller Cycle has been around for decades but not used much. With cars now having forced induction via turbos its possible to make use of the efficiency gains offered by Mr Millers cycle. 

The Miller Cycle is basically the same as the Otto cycle ie valves, pistons, spark plugs etc. The difference is that the compression and expansion strokes are different. Not physically of course but effectively. The compression stroke is shortened by leaving the inlet valve open for a while as the piston rises on the compression stroke. You would think that's daft as the fresh charge would return to where it came from but not so. The clever bit is the inlet manifold is under pressure from the Turbo so reverse flow is limited. So we get a reduction in compression losses without a loss in the power stroke. 

I am getting 53MPG long term which I reckon is remarkable for my 1.5TSI petrol. MPG like that is well into diesel territory.

The only way to beat the kangaroo is to over rev the engine on pullaway - great when you want to nip out quick at a roundabout, but crap at all other times - clutch wear ? 

[AMC150]

1 hour ago, xman said:

 

 

At idle, the 1.5tsi is configured in full Miller mode for max economy, that is maximum inlet/exhaust overlap via the cam adjusters. However very little take off torque is available because at idle there is no turbo boost (which is a requirement for correct/true Miller operation). When you try to pull away, the ecu detects lifting the clutch and raises the idle speed asap to both spin up the single scroll turbo and perhaps back off the cam adjusters to reduce valve overlap. Naturally this takes time to become effective. Some engines are slower to respond than others. So if the driver releases his clutch too quickly, the power/torque cannot build quickly enough and the revs dip, killing the turbo.

 

Also note that because of the large inlet/exhaust overlap that Miller uses, any drop in rpm causes a much larger drop in power/torque around idle speeds than conventional cycles.

 

The natural consequence is a tendency for kangarooing in a manual car because of the drivers natural reaction and subsequent lurches affect the drivers clutch control.

All sounds incredibly technical and plausible. The only bit I have difficulty with is the paragraph immediately above re driver reaction. My left foot is perfectly still at the point when the revs suddenly die and then recover. 

I couldn’t possibly comment on the rest of your post which is beyond my basic suck, squeeze, bang, blow knowledge. Thanks for the info. Very interesting! 

[AMC150]

19 minutes ago, BigKev2 said:

Xman - Good post my man. Its good to get a bit of tech detail. I think you are on the right track.

The Miller Cycle has been around for decades but not used much. With cars now having forced induction via turbos its possible to make use of the efficiency gains offered by Mr Millers cycle. 

The Miller Cycle is basically the same as the Otto cycle ie valves, pistons, spark plugs etc. The difference is that the compression and expansion strokes are different. Not physically of course but effectively. The compression stroke is shortened by leaving the inlet valve open for a while as the piston rises on the compression stroke. You would think that's daft as the fresh charge would return to where it came from but not so. The clever bit is the inlet manifold is under pressure from the Turbo so reverse flow is limited. So we get a reduction in compression losses without a loss in the power stroke. 

I am getting 53MPG long term which I reckon is remarkable for my 1.5TSI petrol. MPG like that is well into diesel territory.

The only way to beat the kangaroo is to over rev the engine on pullaway - great when you want to nip out quick at a roundabout, but crap at all other times - clutch wear ? 

Thank you. At last some plausible possible answers. Please tell Skoda!!!!

Totally agree with your final paragraph.

[SoupDragon]

Xman and BK-2, thanks for the input. I think you have found out what is the reason for the jumpy start. I suspected that the ECU was very slow to detect the throttle position but I can understand that it is the turbo lag that is the reason for the slow reaction from the engine. Your theory also makes sense of the earlier run-away revs. problem, that it was intended to get the turbo spinning when moving very slowly. So would you agree that a smoother start could be achieved by just waiting a short time, about a second, between pressing the throttle and lifting the clutch for a gentle pull away? I shall try for myself soon. I have already found that a heavier right foot gives a smooth start when you need to pull away quickly but you don't always want to do that.

[nidza]

These newer engines are Otto and Miller (actually Atkinson, when they are supercharged, as they are). When you need full engine capacities it is in Otto regime, but when you coast economically you have Atkinson. ECU will adjust camshaft (0-60 degrees) and intake valve accordingly. This cannot be a reason of jumpy start, but may be what @SoupDragonwrites. Something between throttle sensor and ECU. 

 

2.0 TSI Gen-3 in RS is also Atkinson when needed.

 

What is confusing me, why this is not already fixed and part of some recall, but so many jumpy frustrating cars are on the streets?! 

 

 

[BigKev2]

Hi Soup. As you suggest inlet manifold pressure is a function of turbo spool speed which in turn is a function of exhaust gas flow . Give what you say a try, as  there will always be some lag as the input feeds through the system. Just remember to leave the house a bit earlier 

[xman]

30 minutes ago, nidza said:

ECU will adjust camshaft (0-60 degrees) and intake valve accordingly. This cannot be a reason of jumpy start, but may be what @SoupDragonwrites. Something between throttle sensor and ECU. 

 

2.0 TSI Gen-3 in RS is also Atkinson when needed.

 

What is confusing me, why this is not already fixed and part of some recall, but so many jumpy frustrating cars are on the streets?! 

 

 

 

That's part of the problem, I'm speculating that it takes probably hundreds of milliseconds to precisely adjust the camshaft angles by a large amount as it is moved hydraulically by engine oil pressure and solenoid valves. And at low rpm oil flow is low.

 

Miller is similar to Atkinson but Miller has a supercharger (replaced and compromised by a turbo in the 1.5tsi) , Atkinson does not and so Miller can go further with valve timing over a wider range.  The new 2.0tsi is ”Budack cycle” which is another variant which VW have trademaked and probably patented. The new 2.0tsi will only be offered with DSG AFAIK, so maybe this problem won't be evident.

[SoupDragon]

I have noticed another effect: if the car has been on over-run for a bit with the throttle closed, and I suddenly open the throttle, there is always a momentary hesitation and a slight noticeable nose down if the car is in second gear. It is not enough to cause concern but again it reminds me of driving an old car with a worn out carburettor. Could this be the same effect, i.e, the valve timing needs to adjust from one of these special cycles to normal mode?

[xman]

Could be

[nidza]

19 hours ago, xman said:

 

That's part of the problem, I'm speculating that it takes probably hundreds of milliseconds to precisely adjust the camshaft angles by a large amount as it is moved hydraulically by engine oil pressure and solenoid valves. And at low rpm oil flow is low.

 

Miller is similar to Atkinson but Miller has a supercharger (replaced and compromised by a turbo in the 1.5tsi) , Atkinson does not and so Miller can go further with valve timing over a wider range.  The new 2.0tsi is ”Budack cycle” which is another variant which VW have trademaked and probably patented. The new 2.0tsi will only be offered with DSG AFAIK, so maybe this problem won't be evident.

 

You are right, I have mixed up Miller and Atkinson, writing straight from my mind.

 

18 hours ago, SoupDragon said:

I have noticed another effect: if the car has been on over-run for a bit with the throttle closed, and I suddenly open the throttle, there is always a momentary hesitation and a slight noticeable nose down if the car is in second gear. It is not enough to cause concern but again it reminds me of driving an old car with a worn out carburettor. Could this be the same effect, i.e, the valve timing needs to adjust from one of these special cycles to normal mode?

 

Whatever is the reason, I think ECU must be aware and adjust everything properly. It is rather strange so many cars on the streets are affected, but it wasn't detected in the testing phase. Must have been some late change before serial production. It should be fixable via software update.

 

[BACUPIAN]

In reading this topic on the "ill running " problems on the 1.5 tsi engine, beginning to think if I should cancel the order on [build wk 48] Octavia, is a fix it in the Skoda pipe line or is likely to be frequent visits to the dealer ? The 1.4 tsi was NOT an on offer due to me wanting "factory options"...........keep the 1.2 tsi Rapid ????Manual gear box 

Edited 28 August, 2018 by BACUPIAN
omitted manual gear box

[Gerrycan]

I cannot remember if this has already been covered but does the addition of  a pedalbox address the issues for the 1.5tsi?

[xman]

General consensus is it only affects manual cars and not those with DSG. And then not every car, perhaps not many.

 

I'm wondering if dealers are blocking feedback to the factory. Ordering a manual 1.5tsi in any model seems a bit of a gamble at the moment.

 

Note that no 1.5tsi models are as yet WLTP certified so there are no new builds being delivered at the moment. Just existing dealer stock.

Edited 28 August, 2018 by xman

[BACUPIAN]

Seems a bit of gamble if  you are "lucky" or NOT ? Odds are stacked against you if its a MANUAL gear box then ? 

[xman]

Personally I would wait until I see enough positive posts from people who had their problems properly fixed and understand whats involved in achieving that.

But if you have placed an order already, it may be too late. Maybe email your Skoda importer and ask if there is any acknowledgement of this issue and fix in the pipeline? Not that I expect them to be open and honest though....

[SoupDragon]

Does anyone know if there is a VW/Skoda fix for this? 1.5 TSI ACT engine hesitation on pull-away with manual gearbox. I am not aware of any VW/Skoda statement or even recognition of this problem.

[Ootohere]

Ask Skoda UK Customer Services, they might know a number for someone at Skoda CZ or VW Germany, and maybe someone will answer the phone 

or an email.  They can not all be deaf, dumb, blind and flat earthers.

http://skoda.co.uk/about-us/contact-us 

[Ootohere]

New WLTP Approved 1.5 TSI vehicles will have a GPF so different engine management / software so a whole new experience.

Time will tell what if any difference that makes when those cars are available to be driven.

[AMC150]

47 minutes ago, Offski said:

Ask Skoda UK Customer Services, they might know a number for someone at Skoda CZ or VW Germany, and maybe someone will answer the phone 

or an email.  They can not all be deaf, dumb, blind and flat earthers.

http://skoda.co.uk/about-us/contact-us 

I tried contacting them. Waste of time. Referred me back to my dealer

[Ootohere]

^^^ Exactly, nobody knows anything or says anything,  but Skoda UK got a new CEO / Brand Director early 2018 in the form of Rod McLeod, so he heads this crowd of hopelessness.

He will appear at some point saying what a successful year Skoda UK has had, and then off he will be to some other post with VW Group or move on to some other job, just as the last Brand Directors have.  Next to useless for anything other than cutting down on what Skoda UK pays out on Warranty Work, 

but not on customer complaints.

[gm73]

My 1.6TDi does this. At first I thought there was something wrong with the car. It’s a bit of a pain though and makes stop start driving uncomfortable. 

[SoupDragon]

Since last Monday, I have been driving with these comments in mind. I mean I now wait about half a second between pressing the pedal and lifting the clutch. I can now pull away without any nose diving. However, the smooth getaway is achieved by replicating the original over-revving complaint with 'wetware', that's my brain, rather than the original software. At least it's predictable and controllable and I know what is going on but I am concerned about additional clutch wear. It seems to me that a software fix for this problem needs the ECU to anticipate when the driver wants to change from idle to power at least half a second before the driver. That is difficult. On the plus side, 50+ mpg is excellent and  important these days of rising fuel cost. The complete lack of engine braking on overrun is very noticeable and I guess this is also to do with the engine mode used when the gas pedal is released.

[DaveLees]

I've been using my Octavia in sport mode for the last couple of weeks and it certainly does seem to lessen the nose dive problem to the point that I don't notice it now. The sport mode hasn't affected my fuel consumption and I prefer the slightly heavier steering.

[vegit8]

I have now done almost 5000 miles in our 1.5Tsi since April

 

I have had the overrevving on clutch lift fixed by the dealer via an ECU update.

I have fitted a DTUK Pedal box, and love the improved throttle response.

 

Like many others I struggle to get a smooth pull away when moving slowly in a queue of traffic.

Pulling away and accelerating through the gears is less of a problem.

Is just that thing when you want to change from stationary to moving but not hit the car in front that is also creeping along, just as you get going you come to an almost standstill. My passenger does not like it one bit!.

Now that the holiday season is over I will be going back to the dealer and getting them to send out a 'technician' with me and then get him to do the same after I have proved the point.

I wonder if its worth starting a poll on here to find out how many 1.5Tsi owners have the same problem!

[AMC150]

I don’t have the problem with the engine revving itself, quite the contrary. If I allow the revs to drop below 2000 when moving off, the engine dies before picking up again causing the nose to dive. Put me down for a yes re the poll. The more who complain the better (1.5tsi manual Karoq)