As battery technology and electric cars start to get smarter and better, the oil-powered vehicle industry doesn’t sleep either – and that’s for the consumer’s benefit. Engineers from the Technische Universitaet Munchen (TUM) have shown an engine prototype whose emissions are very close to the Euro 6 standard, even though the Euro 5 norm has just been put into force three months ago for all new car models.
By developing a special probe that allows them to take samples directly from the combustion chamber while the engine is running, the scientists are studying how soot forms, for controlling it, eventually. The probes only enter the combustion chamber for a fraction of a second, take the samples and exit the chamber, since keeping them permanently inside would hurt the combustion process.
The TUM researchers had a two ton LVK engine running in a hall, and the smell of exhaust fumes was barely sensed, even though it was running at full power. The LVK engine is a central part of NEMo (Niedrigst-Emissions-LKW-Dieselmotor), which aims to build the “lowest-emission truck diesel engine”, without using a catalytic converter. Their concern is justified: the Euro 6 norm imposes an emission of only 5 milligrams of soot and 80 mg of nitrogen oxides per kilometer – a fifth of the soot and a quarter of the nitrogen oxides imposed by Euro 4, for comparison.
Nitrogen oxides are formed in an engine when diesel fuel burns in the air of the combustion chamber. Air consists of 21 percent oxygen and 78 percent nitrogen. Diesel fuel reacts with oxygen, producing carbon dioxide and water. This happens in a very fast reaction resulting in combustion chamber temperatures so high that the oxygen also starts to react with the nitrogen in the air, forming nitrogen oxides.
To combat this effect, modern diesel engines recirculate part of their exhaust back into the combustion chamber after cooling it down, together with the fresh air (the EGR system). In this mixture, carbon dioxide and water from the exhaust gases moderate the combustion process, keeping the temperature in check. As a result, fewer nitrogen oxides are formed, albeit at the price of increased soot production since the proportion of oxygen in the air-exhaust mixture is lower.
The TUM researchers designed the LVK test engine in such a way that the air-exhaust mixture is injected into the combustion chamber under high pressure. The engine’s turbo-charger compresses the mixture to ten times atmospheric pressure (measured in bar) — more than double the pressure mass-production vehicle engines can handle. Compressed in this way, the air-exhaust mixture contains enough oxygen for the diesel fuel to burn completely.
Diesel engines are already running in Europe for decades, and research has made them more efficient every year. Fuel consumption has dropped to 60-65 mpg, while emissions have been kept to a minimum. If you smell the exhaust of a Euro 4 TDI engine, for example, you will hardly feel any smell – imagine how these new engines will do.