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What's the dirt on diesel engines?

Fabian Schmidt
September 22, 2015

Diesel car engines may be more efficient than their gas-powered siblings, but when it comes to emissions, the former are still filthier. DW looks at the difference between the two types of machinery.

Image: picture-alliance/dpa/P. Pleul

There are two crucial differences between diesel fuel and gasoline. The former is far less flammable, and as a result it contains more energy. A cubic meter of diesel will yield around 9800 kilowatt-hours of work, and the same amount of gasoline yields just 8760 kWh.

This means, rather obviously, that diesel engines are more efficient, because less fuel is needed to run them. Additionally, diesel is cheaper than gasoline, because there is much more of it at refineries when compared to other natural gas products, for instance, kerosene and benzene.

The low flammability of diesel makes the fuel more secure in car engines; however, it also means that diesel engines must be designed differently. While a gas-powered engine is injected with a mixture of air and gasoline by the carburetor itself, the pistons of a diesel engine are filled at first with air or a combination of air and exhaust. This is compressed to 20 bars of pressure and heated to 900 degrees Celsius, and then the diesel fuel is injected into the combustion chamber. Fine droplets of diesel are evaporated and ignited.

The higher efficiency of a diesel engine is due not only to the fuel's higher energy density; the compression of a diesel engine bay is much higher, which also accounts for its higher degree of efficiency.

Deutschland Volkswagen Abgasuntersuchung
Due to the emission scandal, VW can't sell some diesel engines in the US for the time beingImage: picture-alliance/dpa/P. Pleul

A host of filters

Although diesel engines emit lower levels of carbon monoxide, they have always had a dirtier image than gas engines. One of the main reasons for this was the black clouds that used to be spouted out the exhaust pipes of diesel cars, before carbon-particulate filters were made mandatory.

When diesel engines are accelerated, the size of the droplets injected into the combustion chamber is increased. The burning becomes inefficient and dirty, resulting in black smoke clouds that contain particles and nitrous oxide.

Modern particle filters have only been around in serial production since the turn of the century. At the same time, a number of car manufacturers attempted to further reduce particle emissions by increasing the cleanliness of diesel combustion. All this resulted in, however, were even finer particles that were more harmful - and probably more carcinogenic.

Every diesel car produced these days is equipped with an extra fine particle filter, in addition to the carbon-particulate filter. Around 95 percent of the particles produced by a diesel engine are caught by the filter - but not other harmful emissions.

Four-way catalytic converters are the way forward for reducing emissionsImage: BASF

Fort NOx

Since the introduction of three-way catalytic converters and unleaded fuels, gas-powered engines now emit essentially no harmful exhaust fumes. With the introduction of four-way catalytic converters, the traces of oxygenated hydrocarbons and oxides of nitrogen - known as NOx emissions - will further diminish. Most new gas-powered engines already meet strict United States regulations of 50 milligrams of NOx per mail with ease.

It's much more difficult for diesel engines. That is one reason why Europe's regulations for diesel engines appear far more lax when compared to the US - where far fewer diesel cars are driven per capita. In September, the limit of 180 milligrams per km was lowered to 80 mg, but that is still twice as much as in the US.

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