Our Long-Term MPG Results May Shed Light on Future Emissions Recall Expectations - 2013 Volkswagen P
Edmunds.com
Member, Administrator, Moderator Posts: 10,316
Our Long-Term MPG Results May Shed Light on Future Emissions Recall Expectations - 2013 Volkswagen Passat TDI Long-Term Road Test
Our long-term MPG tests of a 2013 Volkswagen Passat TDI diesel and its gasoline counterpart may shed light on how any future recall fix may affect performance.
0
Comments
Most manufacturers today tend to make compromises between fuel economy and drivability in order to get good EPA numbers while still keeping customers from revolting over the way their car feels and responds in normal driving (and there are usually still complaints). If what has been said about the 'test' ECU program is true, using it basically would result in greatly reduced drivability, so not a choice, Volkswagen may have to do some compromising in the reprogrammed 'fix' code to still meet emissions requirements while still compromising fuel economy a bit to meet owners expectations, reducing real-world fuel economy.
I would expect that if the current EPA numbers are based on the cheat ECU program, then retesting with a 'fix' will lower the EPA ratings. The car might still get better than EPA ratings, in line with most diesels today, but only in comparison to the new ratings, and probably not the old.
Maybe a comparison with the Chevy Cruze Turbo Diesel isn't so far off the mark after all..
To lower NOx on a diesel they could have done a few different things. They could have run the engine lean and relied on the NOx trap to capture the oxides of nitrogen but not allow the engine to go back to a brief period running very rich and catalyze it correct, but I'm not sure if the trap/catalyst VW used has enough capacity to last for the ~9 minutes of the test without ever going back to rich operation.
They could also artificially richen the mixture which means there is more fuel compared to the amount of air in the cylinder, which means less nitrogen from the air that can combine with oxygen into NOx particles during combustion. The richer mixture also helps cool the combustion chamber which also lowers NOx. The down side is that the CO, CO2 and hydrocarbon would normally spike higher when the engine is run richer. These and other diesel engine do use a diesel oxidation catalyst (similar to the catalyst in a gasoline powered engine) that can process some of these molecules.
NOx and particulates are usually the most difficult parts to clean/neutralize in diesel exhaust emission, but the particulate traps work really well to capture and later burn fine particulate matter. The NOx is still a problem because to get better economy and efficiency the diesel engine needs to run lean which increases NOx produced. The NOx trap/catalyst by itself was able to meet older diesel exhaust emission standards but the most recent tier/bin standards are much tougher. That's why the selective catalyst reduction (SCR) systems in newer diesel vehicles usually have to include urea injection (i.e. diesel exhaust fluid (DEF)) to increase the efficiency of the NOx catalyst.
It's possible VW richened the mixture to reduce NOx and relied on the diesel oxidation catalyst to handle the CO, hydrocarbon, and oxides of sulfur just for the test, but if they ran the engines like that in the real-world it would excessively load the oxidation catalyst and might damage it over time. Another problem with running rich is that when more fuel is burned it increases carbon dioxide (CO2). The oxidation catalyst also breaks down carbon monoxide (CO) into water and CO2 making this slightly less likely they significantly richened the mixture.
There's a few other ways they could have possibly tailored the emission results when the test vehicle is on a chassis dyno and not under the normal operating loads normally experienced out on the street (e.g. no wind resistance, etc.). If significantly less engine power was required to spin the giant roller drum on the dyno than is required to move the car, they could have altered the fuel injection timing to produce a cleaner combustion process at the expense of power. The engines in question don't have variable valve timing, but that's another way a manufacturer could tweak the combustion process.
Thinking about it from a technical standpoint, a more likely scenario is the calibration (software tuning) engineers partially activated the throttle plate and opened the exhaust gas recirculation (EGR) valve while testing. Normally diesel engines don't have a throttle body/plate and are inherently lean burning, instead rely on the amount of fuel injected to produce power. Having no throttle to restricting air flow along with the higher energy density of diesel fuel is what allows them to be more efficient compared to a gas engine.
Many new diesel engines do have a throttle body (plate) in the intake tract that can be closed under certain engine loads to allow the engine to more easily draw in recirculated exhaust gas form EGR system (upstream in the intake manifold from the throttle plate) instead of outside air. Creating a slight vacuum with the throttle plate allows the cooled exhaust gasses to be pulled into the intake tract and since they've already been burned once they don't contain as much oxygen. That means less power will be created but there's also less oxygen molecules to form NOx.
If VW activated and closed the throttle plate during the test it would reduce NOx while also keeping the other emission elements in check. They probably couldn't run like that all the time because it would have a significant impact on the amount of power the engine could produce and might also impact economy from the greater intake restriction.
That's just my speculation of some of the ways they could have gotten the car to pass testing on a chassis emission dyno and then run differently out on the open road. Until the code is examined or reverse engineered from data logging and testing, we probably won't have an answer as to exactly how and when they got the 2.0L TDI engines to pass. And until we know what they did it's going to be difficult to figure out what might need to be changed going forward to get the vehicles to comply and the modifications required.
I'm sure VW has already been formulating a plan, possibly including costly hardware changes, but until a technical paper comes out we may have to play the waiting game.
How this mess plays out in the coming months will be interesting to observe. The categories are technical solutions, financial reimbursement for owners, dealerships and various governments. The last category is how to manage outliers.
There will be a technical solution to get the emissions back in line, which will involve software and perhaps hardware changes. The impact on fuel consumption and engine longevity may be factors.
Owners will expect to be reimbursed. The questions are what to be reimbursed for and by how much? Reimbursement for historical usage is not likely. Reimbursement for potential loss of future engine longevity or diminished future value is most likely. Expect a lot of discussion on this legal aspect.
Dealerships will want to be compensated. Again, the challenge will be by how much.
Governments will want to be compensated for two reasons. One as a form punishment. The second may due to impact on health care costs due to increased pollution. This latter point is hard to measure, unlike GM's faulty ignition casualty numbers.
How will outliers be managed? There will always be people who do not want to have their car repaired, as they do not want to have their fuel consumption and potentially engine longevity compromised. Should these people be forced to comply? Can outliers be caught?