Ibiza Cupra TDI turbos: why are they blowing?

[Avispon]

Hi There

I'm from what I think of as a sister site in South Africa, and I am concerned by the number of Ibiza 1.9 TDI Cupra turbos that are blowing at our 1800 metre altitudes. Seem not to be so common at the coast. Some say it's caused by boost leaks and the resulting overcompensation of the turbo. VW in South Africa has no answer for us.

Any ideas would be really helpful.

 

[Gooner_Mike]

Higher altitude and the thinner air and lower atmospheric air pressure, so worse combustion maybe...

These might help... http://answers.yahoo.com/question/index?qid=20090409081140AApKHEf and http://forums.macresource.com/read.php?1,567376,567443

 

[Muttley]

Um.

The whole point of a turbocharger, the reason it was developed in the first place, it that it compensates for the ambient pressure drop at altitude by blowing a bit harder, using energy that was otherwise going to waste in the exhaust: as the outside air pressure decreases, the pressure difference between exhaust and ambient increases, giving more power to drive the turbocharger. The desire to get aero engines to work at higher altitudes was a significant driver of turbocharger development.

And on top of that, we are talking about diesel engines here, that do not have issues with air/fuel ratios in the same way that petrol engines do. Diesels always swallow a full charge of air at every stroke, and at idle inject a tiny amount of fuel into the cylinder, which still burns ( and very lean, too, very efficient ).

Apparantly these failures are associated with a specific turbo, the VNT17a, at least that's what a brief google tells me.

I would hazard a guess that the turbo is not up to working at high boost levels for long periods. The SA veldt is higher than most european roads, where you would also only experience the altitude for a brief period while going over a pass.

 

[basssound]

Um.

The whole point of a turbocharger, the reason it was developed in the first place, it that it compensates for the ambient pressure drop at altitude by blowing a bit harder, using energy that was otherwise going to waste in the exhaust: as the outside air pressure decreases, the pressure difference between exhaust and ambient increases, giving more power to drive the turbocharger. The desire to get aero engines to work at higher altitudes was a significant driver of turbocharger development.

And on top of that, we are talking about diesel engines here, that do not have issues with air/fuel ratios in the same way that petrol engines do. Diesels always swallow a full charge of air at every stroke, and at idle inject a tiny amount of fuel into the cylinder, which still burns ( and very lean, too, very efficient ).

Apparantly these failures are associated with a specific turbo, the VNT17a, at least that's what a brief google tells me.

I would hazard a guess that the turbo is not up to working at high boost levels for long periods. The SA veldt is higher than most european roads, where you would also only experience the altitude for a brief period while going over a pass.

I'll correct you on this, modern diesel engines work on volumetric efficiency for burning the fuel correctly
Yes the diesel engine will still work perfectly fine in the thinner atmosphere it will be alot more smokey and produce less power because the engine
has to compensate for a smaller volume of air entering the chamber, regardless of the chamber vacuum when the inlet valve opens.
The turbo's are having to work over time in the high altitudes to try and cram as much as possible in the chambers.
Also a diesel engine isn't very eficient when on tickover as the amount of boost is 0
so the engine is sucking in air at atmospheric pressure, the engine is most effiecient on run down, say coasting up to a junction or at a low load driving situation, constant 60mph on the motorway.

 

[Muttley]

basssound wrote

I'll correct you on this, modern diesel engines work on volumetric efficiency for burning the fuel correctly

I'm sorry, you'll have to explain that.

As I understand it, volumetric efficiency is a measure of how good the engine is at filling the cylinder with air, compared to the actual geometric volume of the cylinder. Turbocharged engines have volumetric efficiency > 100% when on boost.

This can be viewed as increasing the compression ratio, which in turn improves the thermal efficiency of the engine, directly.

But diesels don't depend on volumetric efficiency for burning the fuel, correctly or otherwise. The fuel burns because it is injected as an extremely fine spray into a combustion chamber full of hot air, well above the ignition temperature. So even a small amount of fuel burns, completely (due to the abundance of oxygen present) producing a power pulse as the combustion products are hotter, and want to occupy more volume, than the liquid diesel fuel that was injected.

Varying the amount of fuel injected controls the amount of combustion product and heat generated, and so the amount of the instantaneous pressure rise. The power pulse.

Yes the diesel engine will still work perfectly fine in the thinner atmosphere it will be alot more smokey and produce less power because the engine
has to compensate for a smaller volume of air entering the chamber, regardless of the chamber vacuum when the inlet valve opens.

I've read this three times, once upside down, and I still don't understand what you're getting at.

The original question is about turbocharged diesel engines at high altitude. As I've already said, the turbocharger will compensate for the lower ambient pressure by stuffing more air up the inlet manifold, so the engine will see exactly the same mass of air as it would at sea level. No extra smoke, no loss of power, exactly the same mass of air.

What you describe would be true for a naturally aspirated diesel.

Which would not suffer from turbocharger failure.

The turbo's are having to work over time in the high altitudes to try and cram as much as possible in the chambers.

I think I said this earlier?

Not "as much as possible", the expected mass of air according to the ECU map.

Also a diesel engine isn't very eficient when on tickover as the amount of boost is 0
so the engine is sucking in air at atmospheric pressure, the engine is most effiecient on run down, say coasting up to a junction or at a low load driving situation, constant 60mph on the motorway.

Diesels are the most efficient internal or external combustion engines, even at idle. Turbocharging does improve on that efficiency.

On the overrun the engine efficiency is zero, no fuel is being injected and the engine is being run as an air compressor, absorbing energy and decelerating the car.