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Latest Luxury Diesels From Mercedes, Audi, BMW Add Stop-Start Page 2

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2014 BMW 328d xDrive Sports Wagon

2014 BMW 328d xDrive Sports Wagon

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The A8 TDI achieves 24 mpg city, 36 highway, while the A6 TDI and A7 TDI, are rated at 24/38 mpg and the upcoming Q5 TDI gets 24/31 mpg. Each of these Audi models has the same engine, a 3.0-liter turbo-diesel V-6. Meanwhile, the 2014 BMW 328d gets 32 mpg city, 45 highway, according to the EPA. Ratings haven’t yet been released for the Mercedes-Benz E250 Bluetec, which is powered by a new 2.1-liter turbo-diesel in-line four.

The Mercedes-Benz GLK250 Bluetec, which was introduced earlier this year with the same engine as the E250, doesn’t have the feature yet; but officials confirmed that it soon will.

Stop-start for diesels...Really?

While stopping and starting diesel engines repeatedly might seem counterintuitive to longtime diesel fans at first thought, it makes sense now. It didn’t always, acknowledged Bert Herring, general manager of product planning for Mercedes-Benz USA; but Herring points to major advances in electronic controls that help make sure that start-stop only operates when the engine is fully hot, and that it keeps the engine within an even narrower temperature window than with gasoline engines—hot enough for emissions catalysts to operate within their peak range.

What that means is that if you get a diesel vehicle with start-stop, you might not notice the feature working at all in cold winter weather; while out in a Southern California traffic jam, it might save you quite a bit of fuel.

Surprisingly smooth for new Mercedes-Benz diesel four

2014 Mercedes-Benz E250 BlueTec - First Drive, August 2013

2014 Mercedes-Benz E250 BlueTec - First Drive, August 2013

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You're less likely to notice start-stop in a diesel engine as well, counterintuitively. Diesel engines are typically heavier and have intimidatingly high compression ratios, nearing 20:1, which might make you think that there would be even more of a ‘judder’ as a diesel stops and starts, to save fuel at a traffic light, for instance. But smoother combustion from precise high-pressure fuel-injection and spray patterns help tremendously—perhaps more so than relying on a single spark in a gasoline engine.

True enough, in our recent drive of the E250, we noticed a surprisingly isolated vibration as the purring of the diesel four stopped and started at a stoplight.


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