Under the Hood: Modern, 'clean' diesel a far cry from older systems

April 7, 2014

QUESTION: I just read your recent column on automotive emissions. You didnít mention diesel particulate emissions, and I wish you had. It seems hard to find info on the current performance of diesels. I hear talk of "clean diesel" but donít know if that just means low-sulfur. I donít much like the idea of tiny particles burying themselves in my lungs and would like to educate myself as to whether this is an irrational fear or not.

óBill DeVore

ANSWER: Great question, Bill. "Clean diesel" generally refers to diesel engines built since 2007 that use ultra-low-sulfur diesel fuel and employ additional technologies to enhance engine efficiency and reduce exhaust emissions.

Ultra-low-sulfur diesel, or ULSD, has sulfur concentrations of 15 particles per million or less, which is about 97 percent cleaner than the previous 1993 standard. Besides greatly reducing emissions of sulfur dioxide, which can combine with water to produce acid rain, ULSD allows the use of a diesel particulate filter and selective catalytic reduction.

I should also mention modern diesel engines typically employ common rail direct fuel injection, which also improves performance, allows quicker starts and reduces emissions and noise, compared to earlier methods. "Common rail" means all of the engineís fuel injectors have fuel pressure available at all times and are electronically operated, allowing precise and highly atomized fuel delivery. The engine management system employs a variety of sensors to allow very smart fuel delivery functions.

Letís return to the exhaust: An oxidation catalyst cleans up carbon monoxide and hydrocarbons. Next is the diesel particulate filter, which collects and stores soot. The engine management system can tell when itís time to clean the filter and adds fuel to add heat/burn off the build-up.

Next, a urea-based solution called AdBlue or DEF (domestic diesel exhaust fluid) is sprayed into the exhaust just ahead of the selective catalytic reduction unit. Exhaust heat converts the urea to ammonia, which reacts with oxides of nitrogen inside the unit, transforming it into nitrogen gas and water vapor.

Vehicles using selective catalytic reduction have a 5- to 7-gallon DEF tank, which needs to be refilled approximately at each oil change. High-mileage cars may go further; the DEF is metered at about 2-6 percent of fuel used. DEF varies wildly in price, from about $3 per gallon in bulk to about $5 at Wal-Mart ó a great price ó up to about $30 per gallon at a luxury car dealer. The cost of the DEF is largely offset by greater engine efficiency, compared to a system that doesnít use this system.

The bottom line: Modern diesels run cleanly and smoothly, and are surprisingly quiet. Iíve had a chance to drive some of the European passenger car models. With the exception of exhilarating low-end torque, a lower-than-usual tachometer red line, and great mileage, you might not realize it wasnít a gasoline engine.




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