Ran across another nicely balanced article on Yahoo. In some ways it mirrors the US experience. Many years ago when emission controls were getting started on gasoline engines, diesel engines were praised for their low emissions of carbon monoxide and unburned hydrocarbons. Turned out no one was looking at NOx or particulates. NOx eventually attracted interest and led to exhaust gas recirculation in gasoline engines. It was known all along that diesels emitted NOx, but no one worried about it. Now, things are changing. Diesel: How it changed Europe and how Europe might change back | Ars Technica
The push for cleaning up car emissions that lead to the clean cars we have now started in California. L.A's smog problem wasn't because of all the diesel cars. So the focus was on cleaning up the emissions of gasoline engines. If it were the other way around, we might be discussing a gasoline gate now where a company's cars are emitting too much carbon monoxide. Diesels were given a pass in the US because they are a tiny portion of private cars on the road. They still sell less than hybrids. They are also important for commercial traffic, but the US taxes the fuel higher at the federal level. Once ULSD became available, the cars were held to the same standards as gasoline models.
Europe decided that CO2 was more important than NOx. So some areas taxed diesel fuel less which, with higher fuel priced overall, made private diesel cars more attractive. Euro6 still lets diesels emit more NOx than gasoline there, and also both in the US.
Diesel fuel itself isn't the source of the higher NOx, but the lean running engine cycle. Lean running gas engines also make a lot of NOx, and Honda's first Insight had a NOx trap similar to what diesels now have. Gasoline spark ignition engines can just add more fuel to burn more oxygen in the cylinder. This just means they burn more fuel overall when compared to a similar diesel.
The basics of how to clean up diesel emissions is known. Until recently, there just hasn't been any pressure to develop the technology, and for the US market, the pressure wasn't a gradual one. Diesel passenger cars basically had to go to today's standard from one in the 1980's. If the same were to be required of gasoline cars, here would be temptation to cheat with them too.
I read an article recently where they drove two similarly sized cars the same route, one was a Volvo and the other a Ford Focus. The Volvo had a petrol engine and auto box, the focus had 1.5 diesel. After the route was completed, the fuel was measured. The Volvo only got 28 MPG (probably not helped by the auto box) and the focus got almost 70 MPG. I know for some odd reason there's less of a gap between the petrol V's diesel split in the US (not helped by the smaller gallon) but 250% difference is probably enough to persuade people to opt for the diesel.
As we're on the theme of "nicely balanced articles" I read another story suggesting petrol powered engines actually produce more N0x as time goes by, making older higher mileage vehicles just as harmful as diesels anyway. Again, the Internet is full of stuff like this, but if you want to continue your homework, it's good to look at both sides of the spoon.
From the thermodynamic point of view, the efficiency of heat engines is limited by the difference between the highest temperature in the engine and the lowest. Since both internal combustion engines (spark ignition and compression ignition, gas turbines) and external combustion engines (steam, Stirling) are all heat engines, and since the lowest temperature cannot go lower than ambient, the efficiency is limited by the highest temperature. This is why engine builders keep working for high-temperature alloys in piston crowns and turbine blades. Trouble is, as long as air is used for the oxidizer, oxygen and nitrogen are exposed to those high temperatures, nitrogen is literally burned in oxygen, and NOx is formed. Diesels and turbines never burn all the available oxygen, so their NOx production tends to be higher. Exhaust gas recirculation (EGR) reduces available oxygen by diluting incoming air with gas from which considerable oxygen has been removed, which reduces flame temperature (and efficiency) and helps reduce NOx. Even burning a carbon-free fuel such as hydrogen will not change the NOx problem. Steam engines do not themselves produce pollution, but the fuel combustion in the boiler does. This is where nuclear power has an advantage, but it has plenty of other problems.
I know you're one of the seemingly many that believe everything they read on the Internet is Gospel, but there's a lot of crud in that article, including fundamentaly this
"But the higher compression of diesel engines and the less refined nature of the fuel, compared to gasoline, ensure that diesel cars produce more of several pollutants, including nitrogen compounds, hydrocarbons and carbon monoxide and dioxide"
Where do these idiots get thier data? Diesel produce far less hydrocarbons, especially on cold starts, and virtually zero carbon monoxide, not to mention significantly lower carbon dioxide. In fact a petrol car with a catalyst produces 20 times more carbon monoxide than diesels without, 6 times more hydrocarbons and around 20-30% more C02 too. Just another untrue unfair over biased diesel bashing article, no wonder people's idea are twisted if they're opinions are based on lies. You always seem to find articles on this nature, highlighting the 2 or 3 problems diesel cause, and leaving out the 20+ reasons why they're better...
One thing to consider is European diesel fuel is different (better) than what we get in the USA. The lower CO2 levels of diesel is mostly because of the higher mileage they get ( higher engine efficiency ). However the carcinogens they emit are not converted back to oxygen by plants like CO2 is so give me a gas engine any day!