Author Topic: Volkswagen diesel scandal  (Read 1132 times)
sol
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Volkswagen diesel scandal « on: February 24, 2016, 09:23:03 PM » Author: sol
I had planned on getting a new VW Jetta this coming summer, but, plans may have changed because of the diesel scandal. Has anyone heard anything regarding the eventuality that VW will resume the sale of diesel cars ? The most recent I could find is this article from 4wheelsnews on Feb 4, 2016.

I learned to drive on a 1986 Jetta diesel and would like to now get a modern one.
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Medved
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Re: Volkswagen diesel scandal « Reply #1 on: February 25, 2016, 02:28:09 AM » Author: Medved
It is not that unlikely the VW brand will go out of the North American market due to the scandal...

The problem is, it is not possible with a small diesel to meet the official emission limits and still maintain the economy and reasonable reliability.
Nor in the US, nor in Europe. The chemistry and physic s laws just do not obey the "environmental laws". The main problem is with the fuel bein just liquid droplets at the time of combustion, which either produce soot (then the need for DPF and it's frequent cleaning, so extra fuel needed for that and aggressive use of EGR, so a lot of soot enteroing the piston-cylinder area, acting there as an abrazing sand) or an excess of oxygen and more time to burn through (hence utilizing all the fuel, so the high efficiency regime), so generating some NOx. To get rid of that down to the limits, you need an SCR and the AdBlue or such liquid. By the way the same problems starts to apply for the new efficient gasoline engines as well - they inject the fuel directly into the cylinders amd it has not enough time to evaporate and mix with the air prior ignition. But it allows greater compression ratio (the still evaporating fuel cools down the compressed mix, so allows more compression heat to be absorbed), yielding higher thermal efficiency.

All car makers go around this by changing the operation mode to a less emitting (so more wearing and/or fuel consumption) mode under situations, which are close or equal to the test procedures. Some do that less selectively (e.g. only with cold engine, as NEDC is focusing on that, etc), others are more selective to identify the test procedure, so they may offer better fuel and maintenance economy and at the same time "meet the limits on the test".

All this is perfectly legal in Europe, because the European laws (and technical rules) require the cars to meet the limits "in the NEDC test cycle only". And all cars, include VW, do meet that.
But in the US the laws just state "meet the emission limits during operation" and the test procedure is just offered as an example how to prove that. And that means not only the test cycle, but just anywhere. So practically all the "tricks" above becoming illegal in the US, because they are all aimed to fool the test measurement.
The error VW did and the only difference towards other makers designing for an European market was, they used European design for the US market without realizing what is still perfectly legal in Europe, is illegal in the US.

So to make the diesels fulfilling the limits, the systems would have to be rather complex, so very questionable reliability and cost.
« Last Edit: February 25, 2016, 07:41:15 AM by Medved » Logged

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RyanF40T12
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Re: Volkswagen diesel scandal « Reply #2 on: February 25, 2016, 08:24:40 PM » Author: RyanF40T12
Buying new, or used? 
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Aveoguy22
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Re: Volkswagen diesel scandal « Reply #3 on: February 25, 2016, 08:57:06 PM » Author: Aveoguy22
 this is a little off topic but these new GDI (gasoline direct injected) cars are as bad as the diesels in the way of soot production.  the fuel spray hits the walls of the cylinder and cool off (not as vaporized) and end up not burning as cleanly.  all the newer cars equipped with this always have very sooty tailpipes.  my 04 chevy aveo has the old MFI (multiport fuel injection) system where the fuel is sprayed against the intake valve instead and you could eat off the end of the exhaust, well not really but you know what i mean.  the GDI cars also are having problems with the backs of the intake valves are getting excessively sooty from the EGR soot and the oily vapor from the PCV mixed in that now they offer a service to clean the intake valves.  this nver happened with carb'ed cars or with the MFI system because the valves were always getting a bath of gas.  if it aint broke, dont fix it i say.  go back to MFI instead.
« Last Edit: February 25, 2016, 08:59:47 PM by Aveoguy22 » Logged
Medved
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Re: Volkswagen diesel scandal « Reply #4 on: February 26, 2016, 01:21:55 AM » Author: Medved

New: Only gasoline. Even the overcharged direct injection (TSI by VW) work quite well.

Used: Could be a diesel, but the old, simple one. The complex designs from last few years are not going to last.

The soot:
Generally the soot tends to be produced at high rpm, where the droplets have not enough time to completely burn through before the expansion in the cylinder cools the gas down and so stops the flame.
The gasoline direct injestion allows higher compression ratios, so allow the use of overcharging to reduce the mass and really boost the efficiency and lowr the mass. At moderate power levels (steady cruise, smooth, not aggressive acceleration,...) using lower part of the rpm range really means the highest efficiency of these engines. And with smooth driving style this is the regime you really operate the engine at most of the time. At the same time it has quite some power reserve for some emergency situations (although there the efficiency and emission become way worse - so you should not use that regime for normal driving).

But it is not only the direct injection, but even on classical multipoint injection the aggressive use of the EGR instead of throttle valve to regulate the power (it maintains higher efficiency at that power level, due to less flow restrictions) leads to more soot production, mainly when you push the accelerator pedal quickly from partial load to full power: The EGR has way slower response, but the controller still want to get as much power as possible (it "reads" such driver action as an emergency acceleration), yielding to temporarily too rich mixture, yielding some cloud of soot in the exhaust.
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RyanF40T12
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Re: Volkswagen diesel scandal « Reply #5 on: February 29, 2016, 01:00:15 AM » Author: RyanF40T12
Came across this just today, great read if you have a few min.

http://www.enginebuildermag.com/2015/01/direct-injection-engines-carbon-deposits/

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Medved
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Re: Volkswagen diesel scandal « Reply #6 on: February 29, 2016, 12:03:39 PM » Author: Medved
The maintenance is another story.
By the way the crank case ventilation can not be blamed so easily: Using the intake suction to make negative pressure in the crank case to prevent the oil from leaking when the engine is running is quite common practice for decades - I do not know any 4 stroke car gasoline engine not using this trick since designs originating in 50's (and older design I've not seen at all, but I think that trick is most likely even way older)...
However it is true, with the injectors upstream the intake valves the fuel washing keeps it clean.
But I know about many diesel engines using this trick as well (and there the intake valve is not cleaned by anything) and still I'm not aware about any major problems in that area (the now older, "pre-dieselgate" VW TDI's,...). The only drawbacks with these I've seen was the tendency to runaway (on the oil as fuel) if some sealing broke, but that issue was greatly supressed with the introduction of an electronically regulated turbocharger (it may even stop the turbo even with an excess of exhaust) and practically eliminated by an EGR valve (displacing the oxygen by the exhaust; of course, if some "tuning" moron haven't disabled it)...
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