Yes, they are among one of the four items identified in improving fuel economy: Aluminum frames (carbon fiber is better but cost prohibitive at this time), diesel, streamlining (read shrink), and the other that I am currently blanking on.

a. Diesel engines get 33% better fuel economy. [1] b. Have a lower total cost of ownership. [2] c. They are better than petrol gasoline on three out of five emission items, the one that is noticeable to the naked eye, particulates, is what causes concern. [3]The caveat is that petrol gas emits particulates at a smaller micron level, unnoticeable to the naked eye, and may actually be of greater concern to the general public.

1. http://www.carsdirect.com/car-buying/diesel-fuel-vs-unleaded... 2. http://www.dieselforum.org/files/dmfile/20130311_CD_UMTRITCO... 3. https://uk.answers.yahoo.com/question/index?qid=200609260829...

There's one small issue with diesel engines. Diesel exhaust fumes cause cancer: http://scienceblog.cancerresearchuk.org/2012/06/14/diesel-fu...

Take a look at the list of Category 1 carcinogens, at https://en.wikipedia.org/wiki/List_of_IARC_Group_1_carcinoge... .

Also on that list are 'Alcoholic beverages', 'Salted fish (Chinese-style)', and 'Wood dust'.

The article mentions that particulate is in a classification that is known to cause cancer, however, the article admits that "IARC isn’t in the business of telling us how potent something is in causing cancer – only whether it does so or not."

The amount needs to be identified, because it could be similar to the radiation levels in bananas... There is some, but it may not be enough to be deadly.

I mentioned this mostly as a reply to:

[3]The caveat is that petrol gas emits particulates at a smaller micron level, unnoticeable to the naked eye, and may actually be of greater concern to the general public.

The diesel exhaust fumes are proven to be worse than the petrol engines in at least one respect - health.

Maybe if every truck on the road and a lot of the cars ran on bananas and not on diesel, then I would be concerned about the radiation levels in bananas.

Moreover in the place where I live people are allowed to remove their car DPFs which makes things even worse.

They're also easier to stop/start, esp on long downhill stretches.

Does all of this include Hybrid engines or just more traditional gasoline engines?

The thing to remember is that hybrids and electrics tend to moderately trade off emissions for an increase in toxic waste from production. Neodymium for motors, tantalum for capacitors, lithium for batteries - all of those are bad news bears for the environment. Even stuff like copper isn't particularly awesome to produce. All other things being equal (mileage, emissions, etc), you should prefer the vehicle with a simple aluminum frame and engine over the high-tech wundercar because the impact of producing and disposing of it is lower.

To the actual question - yeah, diesels running on biodiesel actually produce less GHG emissions than even hybrids. Ultra Low Sulfur Diesel is pretty close but just a bit above hybrids. The National Academy of Sciences published a study back in 2010 on the topic. The report itself is paywalled but there's a copy on the Arkansas state website. I snapped an image of the chart for ease of viewing.

Note that ULSD was only introduced in the US in 2010 (2006 for Cali). The Euros have had it for quite a bit longer. So when you're reading studies, if it predates 2010 and it doesn't specifically say ULSD or Biodiesel then you should disregard it, because the new fuels are vastly cleaner and the playing field has shifted dramatically as a result. Biodiesel is often notated as something like B100, B85, or BD20 denoting the percentage of biodiesel in a mixture with fossil diesel. Pure B100 has a fairly high gel point and can cause problems starting in cold areas, so these mixtures include a fraction of ULSD to lower the gel point.

http://imgur.com/FEOo1zB

http://1.usa.gov/1gO9jPL

"Despite lower carbon dioxide emissions, diesel cars may promote more global warming than gasoline cars."

http://news.stanford.edu/pr/02/jacobsonJGR1023.html

http://www.sciencedirect.com/science/article/pii/S0301421507... (available without cost from http://earthjustice.org/sites/default/files/black-carbon/bou... )

"Jacobson (2002, 2005) suggested that, despite their lower fuel efficiency, gasoline cars were better for climate than BC-emitting diesel cars. However, Jacobson’s analysis relied on radiative forcing (RF) and climate equilibrium calculations, which is artificial and possibly misleading because any policy or technology will only be implemented for a finite period of time"

For about the last ten years, most of the diesel fuel in the US has been ULSD (Ultra-Low Sulfur), and in 2007 the EPA tightened emissions standards for new vehicles. If the VW vehicles made after 2009 met those emission goals, they would be much cleaner than the smoke-belching cars and trucks you may be thinking of.

http://www3.epa.gov/cleandiesel/technologies/index.htm

I think part of the problem in the US is "off-road diesel" is taxed far less and therefore much cheaper encouraging people to use it on road, and it is terrible. By the way does anyone know of a good way to test for diesel particulates? I swear Boston commuter rails are leaking diesel fumes into the passenger cabins.

Here, in the US, untaxed diesel is dyed red. The authorities typically don't bother checking passenger vehicles but commercial trucks are routinely checked. If the fuel is darker than some arbitrary limit, there are severe fines imposed.

Is "off-road" diesel actually formulated any differently? My understanding is that it's taxed differently because it's only legal to use it in machines that aren't road-legal (farm & construction equipment, for instance), since much of fuel taxes goes toward road maintenance. I thought that the only real different is the presence/absence of a dye used to prove that you're using the correct type.

As I understand it, it's legal to use it in other vehicles but only if those vehicles are used off of the public roadways. For example, if you have a large site and must move material internally to that site, you can use untaxed diesel while the vehicles are moving material on site.

Apparently they used to have higher sulfur but as of 12/2010 are all supposed to be Ultra Low Sulfur, although diesels will run on heating oil.

I have heard from someone who worked with a diesel fuel distributor that most of the "off-road" diesel in the US is exactly the same as over-the-road diesel and that the only difference is the tax paid and the colorant added.

Build a small box with a piece of an unused HEPA filter for a vacuum cleaner, sandwiched between two small computer case fans in push/pull config, add some batteries and a on/off switch. Take that on the train and leave it whirring. Store it in a ziploc bag that you open after getting on the train and close before leaving again.

You then have to send that sample off to a lab, since DYI-ing an elementary carbon analysis looks bloody complicated and expensive.

The device for measuring particulate concentration is called an aethalometer: https://en.wikipedia.org/wiki/Aethalometer

> Are diesel vehicles actually more environmentally friendly?

Yes (legal) diesel passenger vehicles and light trucks sold in the US since [edit: 2007] are significantly cleaner in all respects than their gasoline equivalents, although that difference goes away soon as the gas regulations catch up.

It's very unlikely you could detect whether the vehicle in front of you is gas or diesel just by looking at it or smelling the exhaust.

That said, there are a lot of pre-2005s out there, showing smoke and smelling bad. Also the heavy trucks are just now getting the emissions requirements that lighter vehicles have had for the last 10 years.

I have a 2012 heavy-duty diesel pickup. Except for the logo on it and maybe the sound of the engine, you'd never know it was diesel.

The sound of the engine is usually the giveaway to me. Clatterclatterclatter... I sometimes have to roll up my window when I am at a stoplight next to a heavy-duty diesel.

Even the sound of the newer engines is reduced. Common-rail fuel injection reduces injector noise, which is most of that rattle you hear. The emissions equipment muffles the exhaust a bit. Even valve noise is reduced because they need to control things to finer tolerances.

But a big diesel will always sound a bit beefy I think. The smaller 4-cylinders shouldn't be discernable from gas at all.

My 2.0L turbo diesel engine (on non-VW variety) sounds very different than its 2.0L gas sibling. And I definitely like it :)

Our now-maligned cheater TDI really doesn't sound any different than other 4 cylinder engines of its size and power. The exhaust does smell different, I prefer it to gasoline exhaust (don't _like_ it, but find it less offensive).

more a result of the co2 craze. then they discovered diesel contains helluvalot of particulate compared to standard fuels (ops!) and to reduce it you have to burn at higher temperatures (and now you have NOX) at which point it was too late, at least in Europe, where diesel is usually 10% cheaper to buy and overall 20% cheaper to run, so demand is high enough to keep it alive.

They emit less CO2 per kilometer but much more particulates (this is why they have a filter), which is worse health wise.

"Despite lower carbon dioxide emissions, diesel cars may promote more global warming than gasoline cars.

Laws that favor the use of diesel, rather than gasoline, engines in cars may actually encourage global warming, according to a new study. Although diesel cars obtain 25 to 35 percent better mileage and emit less carbon dioxide than similar gasoline cars, they can emit 25 to 400 times more mass of particulate black carbon and associated organic matter ("soot") per kilometer [mile]. The warming due to soot may more than offset the cooling due to reduced carbon dioxide emissions over several decades, according to Mark Z. Jacobson, associate professor of civil and environmental engineering at Stanford University."

http://news.stanford.edu/pr/02/jacobsonJGR1023.html

That release is from 2002, which predates the introduction of Ultra-Low Sulfur Diesel in the US (Cali switched in '06, the rest of the US switched in '10). This drops the allowable particulate concentration from 500 ppm to 15 ppm, which as you would expect greatly reduces soot as well as emissions of all kind.

The diesel we were running back then was garbage. It really had more in common with something like heating oil than the diesel the rest of the world uses, and is the reason why the US has a perception of diesel engines being dirty. It also created huge problems for porting small diesels over from Europe - they run at a much higher compression, but at those pressures the particulates in the fuel become abrasive and eat away the fuel injectors. Now we can use the same engine designs they are. It's win-win for everyone (except the oil companies I guess).

Also, biodiesel reduces soot and other emissions even further. The only downside is that it has a higher gel point than fossil diesel. In southern states you can get away with running straight B100, in more northerly states you need to mix in 15% fossil diesel to get the gel point back down (B85 fuel).

> Cali switched in '06, the rest of the US switched in '10

The EPA mandated ULSD for all vehicle model years 2007 forward. By 2006 essentially all over-the-road diesel sold in the US was ULSD. Otherwise all those catalytic converters would have been toast.

All new vehicles, but the fleet doesn't turn over instantly. You could still buy the old No.1 or No.2 Diesel for your old engine for a couple years before it was phased out entirely. You would have been stupid to do so, of course, because the new stuff was the same price, was cleaner, and probably ran better too. As time wore on, it did get increasingly hard to find - my local gas stations all converted right around 2007 and immediately stopped carrying the old stuff. But if you were running a fleet that fueled up at your company depot or something, you may well have been buying No.1 or No.2.

It's not an ironclad rule, just a guideline of where you should apply a little extra scrutiny. There were studies of what ULSD could do that predate even 2006. But until 2010 I would look carefully at methodology, anything that just says "diesel" without elaborating may be looking at ULSD, No.1/No.2, or perhaps the vehicle fleet (which consisted of a mixture of both).

The 500ppm to 15 ppm is Sulfur not particulates and Europe is now trying to move away from diesel because of their air pollution problems. Paris wants to ban them altogether by 2020. Biodiesel does not have the same problems.

http://www.telegraph.co.uk/news/earth/environment/11878435/E...

The sulfur content of the diesel is the primary driver of particulate emissions.

> The impact of diesel fuel sulfur reduction on directly-emitted and secondary atmospheric particulate is evaluated by experiment and analysis. Experimental evidence shows that fuel sulfur conversion rate to particulate sulfate is linear down to zero sulfur level.

> Analysis shows that sulfur-derived particulate accounts for the vast majority of atmospheric particulate from diesel engines. Consequently, fuel sulfur reduction would have a far greater impact in reducing atmospheric particulate burden than any other diesel engine particulate control strategy.

http://papers.sae.org/872139/

Sorry, this one's paywalled, but it's a finding that has been repeatedly reproduced in the literature.

I completely agree about biodiesel. ULSD is a big step forward but biodiesel is even better in terms of emissions profile. It's also got a number of other things going for it - it can be run in conventional un-modified diesel engines that don't produce high-tech waste, and there's several factors that make its production more desirable than other biofuels.

Does biodiesel running in a properly tuned engine produce much NOX?

I'll start right out by saying that NOx production is very sensitive to engine tuning, as VW has shown. If you run lean you increase your mileage, but you run hot which can increase your NOx output by two-digit factors.

I scanned the literature real quick, and it's pretty mixed. I think stuff before the ULSD switchover pretty clearly favored the biodiesel, because No.1/No.2 were nasty. After the switchover, with a properly tuned engine, it seems to be pretty close with the ULSD having perhaps a slight edge. Without retuning the ULSD seems to be somewhat better. However all of this varies pretty broadly - some people found improved NOx levels just from switching biodiesel. I didn't find a good, modern paper that quantifies the increase, a few looked promising but were paywalled.

http://umexpert.um.edu.my/file/publication/00003184_92278.pd...

I thought this was a good summary of the effects of biodiesel and tuning strategies on various engines. Like ethanol vs gasoline, biodiesel contains more oxygen than its fossil counterpart. It's also not as fully saturated. As such it just tends to run leaner and hotter, and that's why there's a change in average levels. There seems to be a huge variance in the data. Some possible explanations: every manufacturer tunes their engine a little differently, air conditions like humidity and altitude may play a part, and the composition of the biodiesel may not be constant (eg percentage of fossil mixed in, level of saturation in the feedstock, different additive mixtures, etc).

I think manufacturers probably tune their engines primarily for fossil, and to improve mileage they tend to run them as lean as emissions standards will let them get away with. So they may be fairly sensitive to small changes. It would probably be best to add biodiesel to the emissions tests as well, to get them to pay some attention to tuning for it. Maybe there's a happy medium that will work acceptably with both. If not - flex-fuel ethanol vehicles are capable of automatically detecting the level of ethanol and adjusting their tuning to match, we should have something like that for biodiesel. Perhaps a property like acidity or density, levels of particulates, or presence of a taggant in the fuel.

Anyway, hadn't looked at that stuff since before ULSD was out and didn't realize it had changed the picture a little bit. I don't mean to minimize the problem, acid rain sucks and we should absolutely do what we can to address it. But there's no zero-harm solution, if the increase can be mitigated or kept minor with proper tuning there are some compelling benefits to biodiesel over fossil diesel.

Part of the sales pitch is the better fuel economy of diesel cars.

One problem in Europe is that sales of diesel cars have become so high that we are not making good use of all the distillation fractions of the oil.