Enlighten me: Diesel emissions 101

All the VW TDI stuff lately has uncovered for me a concern which I can't seem to pin down. I have this sense that apart from VW's difficulty with NOx, there are some other things (partuclates, mostly carbon soot?) which tend to be significantly worse with diesels, and I'm not sure how bad, or how completely a "good" modern diesel copes with them.

Anybody have a "technical layman" summary of diesel emissions concerns, and how they stack up compared to modern gas vehicles?

I've got this concern that while diesels's may get more mpg, they may be intrinsically more dirty per gallon with current tech, thus making them more efficient but possibly less clean than a gas car for a given trip... But I can't seem to find a simple summary that doesn't bleed over into related topics like suggestions that a lot of diesels are operating outside their stated parameters even if it's just a matter of under differing loads in real word driving, as opposed to VW's cheat mode...

If one of my primary concerns is emissions, should I even be looking at diesels?

Ransom wrote: All the VW TDI stuff lately has uncovered for me a concern which I can't seem to pin down. I have this sense that apart from VW's difficulty with NOx, there are some other things (partuclates, mostly carbon soot?) which tend to be significantly worse with diesels, and I'm not sure how bad, or how completely a "good" modern diesel copes with them. Anybody have a "technical layman" summary of diesel emissions concerns, and how they stack up compared to modern gas vehicles? I've got this concern that while diesels's may get more mpg, they may be intrinsically more dirty per gallon with current tech, thus making them more efficient but possibly less clean than a gas car for a given trip... But I can't seem to find a simple summary that doesn't bleed over into related topics like suggestions that a lot of diesels are operating outside their stated parameters even if it's just a matter of under differing loads in real word driving, as opposed to VW's cheat mode... If one of my primary concerns is emissions, should I even be looking at diesels?

As I understand it, diesel engines have higher compression than gasoline engines and are usually running with a mixture that's lean of stoich. This results in higher temperatures during combustion, which tends to result in higher NOx emissions. You can combat this by running more EGR, but those higher combustion temperatures are a key part of why diesel engines are more fuel efficient than gasoline ones, so your mpg suffers from doing this.

On the flip side, because they run lean, diesels tend to be less likely to produce the incomplete combustion pollutants like carbon monoxide or unburned hydrocarbons. Diesels are not inherently worse than gasoline engines, they're just different.

Note that the relative impact of the different pollutants varies depending on the climate of where they are produced. NOx is particularly a problem in LA, so US standards tend to be a lot more stringent than those of Europe.

^^ this sums it up very nicely.

It is also important to note that the carbon soot that is produced is not as awful as it seems. People see the black smoke and smell the exhaust and assume its deadly and toxic. It does contain some nasty stuff, but not really that much more than a 90s gasoline car. Most of the stuff you can see coming from a diesel's exhaust is heavy particulates that fall to the ground and have very little impact on the atmosphere. One article in Diesel Power magazine tried to compare the amount of particulates (soot) that a typical diesel emits is about equivalent to the rubber that gets scrubbed of your tires. He was equating mass, not its environmental impact of course, but his assertion was that mile per mile, you are putting out as much mass in exhaust particulates as you scrub off your tires.

Of course, are we talking about a 50mpg TDI with 185mm rubber, or a dually with 6 massive tires and 15 mpg? Hard highway rubber, or off-road knobbies?

Either way, the article successfully pointed out the psychology that diesel haters miss; just because you can see or smell diesel exhaust doesn't make it bad, and just because you can't see gasoline exhaust doesn't make it good.

Newer diesels (08-up) use particulate filters in the exhaust to capture soot, then it uses fuel in a regeneration cycle to burn the soot off from time to time. So it doesn't really solve the issue of net carbon out the tailpipe, it just postpones it for periodic release. Urea injection is also used to reduce NOx. Some newer diesels also use throttles to reduce the amount of air being ingested during cruise and therefore reducing combustion temps and NOx.

It is also important to note that as of 2008, diesel emissions for light vehicles (including trucks) are more or less the same as gasoline. That is to say, of the measured exhaust gasses, diesel now has to fit in with gasoline emissions in many ways. There are, however, hundreds of compounds coming out of the tailpipe. Another thing to consider; emissions testing measures the ratio of gasses. It is not necessarily a good indication of how many tons of any particular gas will come out of the tailpipe for a given lifespan of the vehicle.

That was a big, long way of saying this: Diesel is not quite as clean as gasoline at the tailpipe, but that doesn't tell the whole story. Diesel fuel has a much lower vapor pressure than gasoline, so it doesn't evaporate as easily. As you will notice from many fuel pumps these days, they go to great extents to recapture the fumes from fueling because it is a very significant source of raw HC. Diesel fuel doesn't have that problem as much. Diesel fuel also has a higher BTU content than gasoline, so it theoretically takes less fuel to produce the same work. Diesel engines are also significantly higher compression which means they can typically convert more BTUs per gallon into work than a gasoline engine does.

So for me (while I have yet to see hard numbers on it) it certainly seems like a wash. The research I have seen at least doesn't suggest that I should feel guilty about driving my diesels.

A huge advantage that diesel has over gas engines is that you can convert to biodiesel, which significantly reduces greenhouse gases. Plant based biodiesel reduces it by 57% and waste grease reduces greenhouse gases by 86%. However, plant based biodiesel is a poor selection (especially mass produced) due to deforestation carried out to produce it and damage natural ecosystems, or by reducing food production for fuel production, leading to food shortages in poor countries. One way to reduce it even further is to convert it to a hybrid diesel vehicle.

However, the most eco friendly cars are full electric cars, and run your house off of solar power so your car is completely run with renewable energy, along with your home. Coal is still used too much for energy production in the U.S. right now.

Historically, diesels have been high compression engines that use turbocharging and direct fuel injection to achieve decent power/fuel economy. They often struggle with NOx emissions and particulates.

That hasn't completely changed, but the gasoline engines are beginning to run into similar issues as they have adopted many technologies that diesels have used. Today's turbo/direct injected engines often run increasingly higher compression ratios. Simultaneously, diesel engines have had to reduce their high compression ratios, so the two types of engines are kind of headed toward the same common ground.

To meet increasingly strict emissions standards, diesel engine makers initially tried to make the engines run as cleanly as possible (dropping the compression ratio, lots of EGR, playing with the fuel injection). This alone wasn't enough (and hurt performance/fuel economy), so they had to add aftertreatment systems to neutralize the bad stuff that results from the combustion process. Today's aftertreatments are actually so effective, that the engines themselves can actually be run "dirtier" than the past, which results in improved fuel economy and power. The problem comes, when these aftertreatments are damaged, modified or removed and you have a "dirty" engine without any of the post processing going on.

The NOx issue with diesel isn't the high combustion temperatures- in actual application, they are lower than gas engines. The problem with NOx is that it's really, really hard to convert to N2 and CO2. If you look at the feedgas emission of diesels, they are considerably cleaner than gas engines. But the chemistry of combustion means that there is a huge amount of excess O2, which makes converting NO really, really hard. To do that, the current state of the art is to inject urea, which has enough to convert the NO into CO2 and NH4.

While it is possible to be as clean as the cleanest gas cars, it's enormously expensive- the catalyst is expensive, the urea injection is expensive, and EGR is required to lower NO even further.

As for the law, in the US, diesels always had to meet the same rules as gas engines. Where they got some leeway was the phase in of LEVII and Tier2- where you saw the diesels cert at T2B10 and LEVII- which were offset by some T2B4 or PZEV cars.

Particulates are a different thing- and since diesels have been working on that for a while- they generally don't have an issue meeting the 10mg/mi PM rule, with some well developed particulate filters. PFI gas engines don't make PM, and Direct injection engines do make it- but mostly right now, they don't have particulate filters.

As for the smell- if you can smell it, that's generally a hydrocarbon. NO does not have an odor, nor does CO. Ammonia does- but that's not being made- so what you smell are HC's, and most of the time, if you can smell, it's bad (level wise). Some gas engines do odors as well when running WOT.

One last thing- US rules vs. the EU- as I mentioned before, in the US- diesels have to meet the same standards as gas engines- relative to the vehicle classification. But the EU biases their rules for diesels- they have higher allowances for NOx and HC for diesel vs. gas- pair that with the lower tax for diesel fuel- it's pretty natural that most cars are diesels.

Thanks, guys! This helps!

That last paragraph I think may be the root of some of the murky thoughts I had surrounding the notion that current diesels were some variant of "relatively dirty per usage." Which appears to be at the very least not applicable to me here in the U.S.

I'm still not sure how to think about the PM issue... If it's just saved up and either released in bursts (I must be missing something, I don't see how that helps) or is just trapped in a disposable PM (that also sounds... odd), that all seems unhelpful. But I also don't know how bad said soot is.

I think we've opted not to buy a diesel, but with this thread in hand I'm feeling considerably better, and that fact is really just down to being off VW and not ready to spend over $30k for a used BMW...

I have read that one other relevant difference between US and European emissions standards is that the US cares about parts-per-million in the exhaust, whereas Europe cares about total quantity of pollutants per mile.

I've got a 2010 Dodge diesel truck with all the stock emissions controls on it. It's unusual in that it doesn't use urea injection. The exhaust smell is actually pretty similar to that of a car running E85 - a little bit sweet and not unpleasant at all. Just an aside, this is not the same level of science as the other posts in this thread

Of course, the first thing most truck owners do is pull out the DPF and EGR for better longevity, more power and better economy.

I don't know about you guys but I spend 40-60,000 miles a year in a van for work and every time I get behind a truck with urea injection I get sick to my stomach and either have to pass it or back way off. That urea injection sucks ass. Try passing anything in a 4cyl transit connect with a ton of tools in the back...

...and this is why I like having an '03 TDI.

Ransom wrote: Thanks, guys! This helps! That last paragraph I think may be the root of some of the murky thoughts I had surrounding the notion that current diesels were some variant of "relatively dirty per usage." Which appears to be at the very least not applicable to me here in the U.S. I'm still not sure how to think about the PM issue... If it's just saved up and either released in bursts (I *must* be missing something, I don't see how that helps) or is just trapped in a disposable PM (that also sounds... odd), that all seems unhelpful. But I also don't know how bad said soot is. I think we've opted not to buy a diesel, but with this thread in hand I'm feeling considerably better, and that fact is really just down to being off VW and not ready to spend over $30k for a used BMW...

The relatively dirty thing is kind of true during the phase in of Tier2- as the diesels were at the top of the allowance. But it's not hugely dirty.

The PM is trapped, and then burnt- most of them are carbon- but there is some soot in it that does build up over time. These are the trace elements in the fuel and the lubricating oil.

codrus wrote: I have read that one other relevant difference between US and European emissions standards is that the US cares about parts-per-million in the exhaust, whereas Europe cares about total quantity of pollutants per mile.

Where did you read that? It's not correct- the US is regulated in g/mi. Perhaps is still the idea that the sniffer tests represent the certification laws- which they do not.

The US rules are lower in gas emissions, and the cycle(s) are far more realistic. For that matter, the US there are 5 cycles that were recorded from a real car, the EU has one really odd cycle that isn't real at all.

Ransom wrote: I'm still not sure how to think about the PM issue... If it's just saved up and either released in bursts (I *must* be missing something, I don't see how that helps) or is just trapped in a disposable PM (that also sounds... odd), that all seems unhelpful. But I also don't know how bad said soot is.

Soot is partially burned fuel. The particulate trap collects soot, and periodically, the engine will "regenerate" the trap - make the exhaust really hot and burn the soot out of the trap. Hot oxygen + soot = gone.

Well, not entirely gone. There's a measure of ash left behind. Eventually the soot trap (DPF is the usual acronym) will be full of ash and it will need to be replaced. I don't know if anyone is making one that can be cleaned out. I fully expect a market to develop for remanufactured DPFs. In practical terms, the DPF shouldn't need replaced until after the car's expected life is over. I don't know what goals the OEMs build to, but I'd imagine that it's on the order of catalyst life in cars. Maybe 100k, maybe 300k, probably the car will be wrecked/rusted out/scrapped for other reasons long before it is a problem.

In reply to Knurled:

Realistically, the only real option is to get rid of the source of the ash. Which means less oil consumption and even cleaner fuel.

edit: You typed ash and I read soot. Nevermind.