In Europe, standards are about to get tougher, though, forcing diesel makers into a race against time.  One option is to use catalytic converters to remove pollutants, but they degrade over time and more importantly damage performance.  However, meeting the new Euro 6 caps standard without a catalytic converter is a steep challenge.

Researchers at Technische Universitaet Muenchen (TUM), located in Munich, Germany, have developed a new engine that just may be able to meet the tough new standards without necessitating a catalytic converter.  The engine, which may usher in a new era of green performance weighs in at two tons, has hardly any odor of exhaust fumes even when running at full blast.

To design the engine, researchers had to go back to the drawing board.  Modern diesels meet Europe's Euro 5 Norm for emissions by recirculating the polluting nitrogen oxides produced from the combustion of diesel fuel back into the engine.  Mixed with fresh air and re-reacted at a lower temperature, the result is less nitrogen pollution, but more polluting soot.  The new standard calls for a reduction in soot, as well, making this approach impractical.

Instead, the new engine, dubbed NEMo (Niedrigst-Emissions-LKW-Dieselmotor, the German acronym for "lowest emission truck diesel engine), uses a combination of turbocharging and direct injection to burn the fuel more completely.

During recirculation the exhaust gases are compressed to ten times the pressure of the Earth's atmosphere (10 bar), giving enough oxygen for the diesel fuel to burn cleanly.  While the pressure is over twice what current engines can handle, NEMo can handle it with ease.

Improved direct injection also plays a pivotal role.  Traditionally injectors spray large droplets.  Fuel is burned from these droplets, stripping away a layer of fuel akin to "peeling a layer off an onion".  As you peel off more layers, the fuel becomes more dense and compressed, and it becomes difficult for the oxygen to react with the fuel.  The result is soot.  By using higher pressure direct injection, the engine sprays finer droplets, leading to more complete combustion and less soot.  NEMo ups the DI pressure from 1,800 bar to 3,000 bar, allowing this cleaner burn.

Led by Georg Wachtmeister from the Chair of Internal Combustion Engines at the university, the engine team has tune NEMo to almost meet the upcoming standard.  The emissions from the engine are so low they had to invent a new in-engine probe to measure them more accurately.

The team wants to improve their design even more.  They're currently trying to better understand how soot particles form.  To do this they've created the aforementioned probe -- a speedy sample collector that sucks up a tiny sample of combustion chamber gas in one millisecond.  Using the gas sampling valve, 13 samples can be taken during a single ignition, giving the researchers a wealth of new data to better understand and combat soot growth.