With diesel engines and their ultra-low-sulfur fuel now green enough even to meet California rules, one remaining environmental objection is that the fuel still starts from a barrel of nonrenewable petroleum.
A possible next step is renewable biodiesel made from feedstocks including soybean oil, animal fats and even recycled fry oil. But some diesel-car manufacturers are wary of a patchwork of state mandates calling for higher concentrations of biodiesel at the pump. Mercedes-Benz dealers, in fact, have stopped selling diesel models in Illinois, which is subsidizing a biodiesel blend that the automaker believes could muck up its engines or emissions systems.
As governments push renewables, several states have mandated varying levels of biodiesel, from B2 (2 percent biodiesel) to as much as B20 (20 percent), which Minnesota has mandated starting in 2015. To spur biodiesel output, Illinois eliminated its 6.25 percent sales tax on fuel with at least 10 percent biodiesel. Given that subsidy for producers, the state projects that half of its diesel fuel is now B11 — too rich for Mercedes, whose models are designed and warranted for no more than B5.
Biodiesel ages and degrades more quickly than gasoline, said William Woebkenberg, fuels policy director for Mercedes-Benz USA. Poor-quality biodiesel, especially if it contaminates engine oil, can damage engines or fuel systems, he said.
A Mercedes spokesman, Christian Bokich, said dealers don’t want to be liable for mechanical damage that might not be covered under warranty.
The company is looking to resolve the issue, perhaps by shortening service intervals so dealers can keep a closer eye on customer’s engines. For now, Illinois consumers can still buy Mercedes diesels in neighboring states including Indiana.
Chevrolet has simply worked around the problem: its Cruze Turbo Diesel is the nation’s only passenger car designed to run on blends of up to B20; G.M.’s heavy-duty diesel pickups can also safely burn B20.
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