Crude (oil) is composed of thousands of different hydrocarbon (carbon and hydrocarbon) compounds. The carbon atoms link in chains of different lengths, shapes and sizes, and each chain can be separated by different boiling points or distillation.
In a refinery, distillation is the fi rst major process. As the crude heats, different compounds are separated; the lightest (like ethane and propane) come off the top of the distillation tower. The next lightest are used for gasoline, with heavier diesel fuels below.
Diesel contains larger hydrocarbon molecules, with more carbon atoms than gasoline. Distillation alone doesn’t produce enough gasoline and diesel fuel, so heavier fractions of crude oil are broken into smaller compounds by thermal or catalytic cracking, or hydrocracking, to produce higher volumes of gasoline and diesel fuel.
To lower the level of sulphur in the fuel, some of these fractions may be hydrotreated. It’s a refi ning process that reacts a fraction of crude oil with hydrogen at high temperature and pressure with a catalyst to improve colour, odor, and to reduce its sulphur.
Compared with gasoline, the fi nished diesel fuel is heavier and oilier than gasoline. It evaporates more slowly because it is composed of larger hydrocarbon molecules, which have higher boiling points; typically 150 °C to 370 °C. In an engine, this means the fuel can be ignited by pressure alone — making for very effi cient (but loud) combustion. (Props to Petro Canada for their great explanation of diesel fuel.)
How does urea injection work?
In an engine, high compression ratios and a lean fuel mixture make high combustion temperatures, resulting in more nitrogen oxides (NOx) and soot (diesel particulate matter) out the exhaust. Soot can be eliminated by particulate fi lters — Audi’s R10 TDI race car is a perfect example of clean and quiet operation — but NOx is the big challenge.
In the Mercedes BlueTEC system, two catalytic converters target NOx; the fi rst traps NOx, the second converter converts it into nitrogen and water.
But for the more stringent State regulations of California and others, urea is introduced into the exhaust to make the conversion from NOx into nitrogen and water more complete.
Urea is nothing new; it’s used in fertilizers because of its high nitrogen content and because of its highly water soluble nature. Urea itself can be irritating to the skin and eyes, and can cause dermatitis.
Which is why your dealer will handle the maintenance of the urea system for you at regular service intervals.