Diesel remains a dirty word for many car buyers in North America, where it is associated with trucks belching out black smoke from chimney-like exhaust stacks, even though that image has not matched reality for many decades. As fuel prices continue to rise, however, there is a gradual realisation that a diesel engine’s superior fuel efficiency, compared with a gasoline equivalent, makes diesel worth a closer look.
Those car buyers who take a ride in one of the latest diesel-powered sedans now available on the US market, especially a model engineered in Europe, most obviously by Mercedes-Benz or Volkswagen, are often surprised – not only by the invisible, odour-free exhaust but also by the vehicle’s quiet, smooth and lively performance.
Manufacturers of light-duty diesel vehicles, including pick-ups and SUVs developed specifically for the North American market, have faced tough challenges in meeting US Tier II Bin 5 emissions legislation, which crucially makes no distinction between gasoline and diesel power.
But manufacturers of light-duty diesel vehicles, including pick-ups and SUVs developed specifically for the North American market, have faced tough challenges in meeting US Tier II Bin 5 emissions legislation, which crucially makes no distinction between gasoline and diesel power. Its limits are tighter than those of Euro 6 diesel rules and they have to be met while running on poorer quality fuel – with a lower cetane number – than that required by law in Europe. Forthcoming, more stringent, LEV-III regulations will add to the diesel challenges.
Refinements aimed at cutting diesel NOx and particulate emissions to achieve US compliance are the subject of key development programmes by VW, Mercedes, Ford and independent researchers such as FEV at Aachen in Germany. At this year’s Vienna Motor Symposium, VW’s head of diesel development Jörn Kahrstedt said that although its 2.0-litre diesel-powered Golf and Jetta cars were EPA certified, there were compliance problems with an identically-engined Passat. The vehicle’s heavier weight meant the engine had to work harder, which raised peak exhaust temperatures and hence NOx levels.
Induction and injection systems received new attention. A move from air- to water-cooling for the charge air cooler, with accompanying revisions to the high- and low-pressure EGR circuits, reduced engine intake temperatures as well as induction pressure losses. Meanwhile, by making the exhaust manifold and the turbocharger turbine housing an integrated casting, thermal inertia as well as cost were brought down. Engine-out NOx was accordingly reduced, but not sufficiently to ensure the Passat diesel’s Bin 5 compliance.
Truck and bus diesels have led the way, not only on SCR deNOx technology, but also on increased injection pressures, allowing more fuel to be injected over a shorter crank-angle.
More elaborate deNOx aftertreatment was also necessary, replacing the diesel Golf/Jetta’s lean NOx trap with a more costly full urea-fed SCR system, like that employed by Mercedes for its E-Class diesel cars sold in the US. Two separate SCR catalyst substrates are employed, one behind the other, and the upstream ‘brick’ becomes active at a lower temperature, neutralising NOx emissions more rapidly after start-up.
Reduction of particulates, necessarily an accompaniment to fuel efficiency improvements, presents other challenges for car and van diesels. Truck and bus diesels have led the way, not only on SCR deNOx technology, but also on increased injection pressures, allowing more fuel to be injected over a shorter crank-angle. Maximum nozzle pressures in car and van diesels look set to rise to as much as 2500bar ahead of the next step-down in emission limits. Larger-diameter injectors cannot be accommodated in ‘valve crowded’ cylinder heads, so the search is on for stronger materials at a tolerable cost.
The opinions expressed here are those of the author and do not necessarily reflect the positions of Automotive World Ltd.
Alan Bunting has a background in engineering, and has been writing on commercial vehicle and powertrain related topics since the 1960s. He has been an Automotive World contributor since 1996.
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