I have been under the impression that although diesel achieves significantly better efficiency (mpg or km/l) it's not possible to burn diesel as cleanly as gasoline: one always winds up with soot, or if burned with excess air, then NOx. Gasoline, on the other hand, leaves only vapor HC and CO, which can react cleanly with NOx in catalytic converters to emit almost nothing except CO2 and water.

Is this true, and if not, can someone please clarify? I'm trying to understand the environmental tradeoffs with burning diesel, and whether or not this European trend is a "good thing".

Thanks.

[q->t to email]

Without catalytic converters Diesel emits much more particulates and NOx. But catalytic converters fix that.

Adam: it depends what you mean by "burn cleanly". If you mean combustion of fuel to CO2 and H2O, diesel is much more efficient. From a NOx perspective a modern diesel is also at least as clean as a modern gas engine without a catalytic converter. The problem is that diesel exhaust is oxidizing (diesels run with an excess of oxygen, gasoline engines run near stoichiometric) and diesel fuel in the US has too much sulfur, making 2-way catalytic treatment of the exhaust more difficult. The second of these problems will be mandated out this year; now that high sulfur diesel is being phased out, the first of these should be fixed soon as well (Mercedes Benz just announced Bluetec, eg)

Diesels put out more soot and coarse particles than gas engines, although that should change soon as diesel particulate filters are starting to come online, at least in Europe. Fine particles are a different issue, and neither diesel nor gasoline do particularly well here. Filters will help trap exhaust particulates, but will do nothing to solve secondary aerosol, as those are formed from gaseous precursors. It's not yet known which is worse in this respect, but I'd guess gasoline (at least after we go to ULSD): it has a higher vapor pressure, and gas engines have higher VOC emissions.

Catalytic converters have been a huge help but of course do not work with perfect efficiency. It's only been recently that we've realized that cars are a significant part of the increase in N2O, a greenhouse gas (not the same as NO2, an air quality pollutant), because the reduction of NOx in a catalytic converter, while efficient, is not perfect. Catalytic converters also do not work very well until they reach operating temperature, that is, on short trips.

Right now, gasoline engines are cleaner with respect to air quality, because of the cat; diesel engines are cleaner with respect to climate, because of their efficiency. A gasoline hybrid will be cleaner wrt air quality and similar with respect to climate; biodiesel shifts the climate advantage back to diesel. Nothing is perfect, although the best near-term solution may be just around the corner, with Citroen and others annnouncing diesel hybrids, and MB announcing 2-way cats and DPFs for diesel.

Fundamentally European emissions regulations are slightly less strict on NOx than the US and fuel economy is much more of an issue. Auto manufacturers assoications in Europe(ACEA)have voluntarily signed up to reduce fleet CO2 targets for 2008 (140g/km) and into the future. They're not lobbying to PREVENT changes they're volunteering to make them.....

Fuel economy is just much more important over here. Hence a lot of people drive modern diesels.

Contrast and compare a modern European Diesel eg a Ford 2.2Turbo in a Mondeo (Contour in US) with the gasoline version and you'll see why.

Also unless you've driven a modern common rail diesel, you'll not realise how much of a huge advance over previous mechanically injected versions they represent for driveability, performance, emissions and NVH.