Diesel Hybrids: Why They Don't Make As Much Sense As You Think

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Mercedes-Benz E300 Bluetec Hybrid sedan

Mercedes-Benz E300 Bluetec Hybrid sedan

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Volkswagen's line of TDI diesel cars has passionate fans.

Now that the company is launching the 2013 VW Jetta Hybrid, it will likely gain some hybrid advocates too.

But diesel and hybrid fans are very different groups, as the company's marketing surveys show.

So why couldn't Volkswagen simply add a hybrid system to its diesel, thereby getting the best of both worlds?

Many diesel fans have suggested just that, but there turn out to be valid reasons that automakers believe will largely keep diesels and hybrids separate.

In discussions with engineers from Volkswagen and other diesel makers, three main hurdles come to the fore:

(1) COST

First and foremost is the issue of cost. On average, a diesel engine costs about 15 percent more to manufacture than a gasoline engine of equal output.

Add to that the $1,000 or more for a high-voltage battery pack, power electronics, and one or two electric motor-generators, and you've created a very pricey powertrain indeed.

In European countries, where diesel fuel and gasoline can cost $7 to $10 a gallon, buyers will pay a considerable premium for more fuel-efficient cars.

That's less true in the U.S., where gasoline averages less than $4 a gallon--and where diesel fuel is often more expensive per gallon than gas.

And it's one of the reasons that diesel passenger cars pose a thornier equation in the States.

While their torquey driving characteristics make diesels appealing (just like electrics), the fact that both the cars and the fuel are more expensive makes the payback from diesel's greater fuel efficiency more challenging to compute.

(2) NON-COMPLEMENTARY TORQUE CURVES

Most hybrids, especially those from market leaders Toyota and Ford, use gasoline engines specially tuned to run on what's called the Atkinson Cycle.

This highly efficient tuning gives them maximum power output at the top of their range, but almost no torque at lower speeds.

That is perfectly complemented by a hybrid's electric traction motor, which develops peak torque at 0 rpm, neatly compensating for the gutless gas engine until it runs up to speed.

A diesel, on the other hand, produces all its torque down low--as does an electric motor.

That means a diesel hybrid should have boatloads of torque off the line, but may require extensive gearing to ensure highly efficient running at speed.

There's likely a more technical explanation of the relative power and torque curves of the three different sources, but we're not going to attempt it here.

2013 Volkswagen Jetta Hybrid

2013 Volkswagen Jetta Hybrid

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(3) LESS IMPRESSIVE IMPROVEMENT

Finally, one of the reasons that hybridizing gasoline engines works well is that they're less fuel efficient to start with.

Gasoline engines convert 25 to 30 percent of a fuel's energy content into forward motion at the wheels; the rest is wasted as heat and noise.

By contrast, a diesel converts 30 to 35 percent of the fuel's energy into forward motion--hence the higher fuel efficiency figures.

But that leaves less "headroom" for improvement.

In the end, say diesel engineers, a diesel hybrid would add a very expensive electrified system to an already-expensive engine ... but produce a less impressive overall increase than in gasoline cars.

Aha, but you say, there are already three diesel hybrids on the market in Europe!

Well, that's true. But only one of them pairs the diesel engine and electric motor together into a single powertrain: the Mercedes-Benz E 300 BlueTEC Hybrid.

The other two--the Peugeot 3008 HYbrid4 and the Citroën DS5 Hybrid--are what is known as "through-the-road hybrids," meaning that they have one powertrain on either end.

Those two can use the electric motor alone for lower-speed trips, the diesel engine for high-speed travel, and combine for highest performance driving by transmitting power through all four wheels.

It's probably significant that Mercedes-Benz, which has sold diesels in the U.S. for many decades, has no plans to sell the world's sole diesel-electric hybrid powertrain here in the States.

We'll see how it does in Europe once it's been on sale for a year or two.

Meanwhile, we invite engine and powertrain engineers to weigh in on these issues.

Leave us your thoughts in the Comments below.

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