What about diesel hybrids?

Some form of hybrid electric drive looks like the wave of the future, or at least the near-term future, and a diesel-electric hybrid would seem to be the best of both worlds. That’s because the electric portion of a hybrid powertrain offers its primary benefit in stop-and-go low-speed city driving, while diesels are at their best cruising steadily along at highway speeds. Why not marry them?

The short answer is cost. Two-mode hybrid vehicles have two powertrains, one internal combustion and one electric. The electric portion is the expensive one, largely because of the cost of the big, powerful, exotic battery it must use. Add to that a diesel, which is far costlier than a gasoline engine, and you have two high-cost powertrains on one chassis. (Learn how hybrids work.)

Even in Europe, where diesel cars comprise about half the new-car sales, economies of scale have not brought diesel prices in parity with gasoline engines. Because a diesel must cope with significantly greater operating pressures than gasoline engines, the engine itself has to be much beefier.

On paper, and maybe on the road some day, a diesel electric hybrid could be the best of both worlds. As was said during our Future of the Car day panel discussion: “Combining the technologies can lead to great efficiencies in power and emissions, however there is cost and question if market will bear it.”

What do you think? Would you pay a premium for such a powertrain?

—Gordon Hard

Comments

[Steve]

I remember in 1978 Volkswagen had a diesel version of the Rabbit, and although I can't recall the exact price difference, I don't believe it was that much different. In addition, I believe that VW used the same engine block for the diesel as it did for the gasoline version. Was it beefy enough from the get go, or did it die an early death?

[Cale]

Is some form of hybrid electric drive the wave of the future? What is really known about the reliability of hybrid electric systems outside of the efforts by Toyota and Honda? Have hybrid electric systems stood the test of time long enough to establish what the true cost of ownership will be like when owners drive cars equipped with li-ion batteries or their depreciation will be like when gas prices aren't rising?

Diesel-hybrid electric? What a rush! (heavy sarcasm here) Watching the gas mileage indicator hover around 60mpg instead of 40mpg in my Prius!

I'd rather spend my money in tax dollars that would buy a decent light rail system for my community and the country. Getting rid of one of the cars in my driveway, that's freedom. Freedom is not owning one of the most complex automotive gas-saving appliances.

Just my $.02.

[Billy]

I have always thought there might be one technical glitch with a full, Prius-style diesel hybrid: engine restarts. It takes a lot more ooomph to restart a diesel than a gas engine, and there could be some emissions problems in that process. I'm not saying it's not do-able, it's just that the shuddering of the restart and the amount of motor power that would have to be used to restart might be a challenge.

[Wolfman]

The early Rabbit diesels were plagued with problems. The little gasoline engine had a strong enough block. The heads were the weak point, being extremely susceptible to warping and head gasket failures. Longevity was not much longer than the gassers initially. The new TDI is still evolved from it's early gasser roots, but many of the problems have been addressed, except the use of a rubber belt in a seriously interference engine.

[Wolfman]

Bill, the extra power requirement is workable. I have a Yanmar diesel tractor that can literally be started by wrapping a rope around the flywheel and pulling it. There is a compression release lever on the engine, that opens the exhaust valves just enough to bleed off initiall compression. You hold the lever down, pull hard on the rope to get the crankshaft spinning, and release the lever. The engine will catch on the next compression stroke and start idling. With all of the techno gadgetry available for variable valve timing, it could easily be programmed to work as a compression release to wind up a diesel engine prior to starting.

[Billy]

Wolfman,

Very interesting stuff. Indeed, high compression is the bugaboo and a compression release would certainly take care of that issue. But I wonder about the complexity of such a system in the constant start/stop hybrid application which brings me to a point I didn't realize I was trying to make: the reason there are no diesel hybrids immediately on the horizon is the complexity/cost issue.

A few years ago I did an article about diesels and I was talking to a Mercedes-Benz engineer. I mentioned that I always loved the fact that diesel engines were simpler than gas engines, and he said "not anymore." As an example he said Benz makes two versions of a V6 engine, the gas and the diesel. The diesel with common rail fuel injection, turbocharging and elaborate emissions systems was much more complex, and much more expensive to build. As far as I know at least in passenger car applications, there are no diesels for sale in the US that aren't turbocharged these days and the new clean diesel plumbing ain't cheap. Would a diesel hybrid be likewise as complex and expensive on the diesel side of the arrangement? Maybe that's why we aren't seeing any hybrids with diesels. It's just a cost thing.

I'm just speculating, but I figure there's got to be a reason. In so many ways it seems a diesel hybrid could be amazingly efficient, so I figure there must be something that's keeping it from happening. Thoughts?

[Wolfman]

Cost is most certainly the inhibition to marry diesel with hybrid technology. The complexity would still be the same, as the engine would still be required to meet the same emissions standards as a non hybrid vehicle.

On the compression release, using variable valve timing technology would not be a problem. This technology is already proving to be reliable, and the computers are good enough now to insure that there wouldn't be any piston/valve impact issues.

[R Hartman]

I think diesel/elect hybrids are a great way to go. I think prices can be comparable once we get the scale of production up. A diesel VW today doesn't seem to cost much more than gas. And with diesel, we would have the option of running it on bio-diesel, thereby freeing us from imported oil even more,

[no name]

Wolfman wrote,
I'm just speculating, but I figure there's got to be a reason. In so many ways it seems a diesel hybrid could be amazingly efficient, so I figure there must be something that's keeping it from happening. Thoughts?

At least in America(land of the oppressive gov't), the Feds require all of the expensive and elaborate emission systems on diesels. Just before I left working for Mercedes-Benz, company talk was how MB would be the first manufacturer to sell diesels in all 50 states. Only now have the other manufactures found their own way of lowering emissions, which the Feds made them do, so now the buck gets passed on....

[Dr. Herbert D. Zeman]

Most of the comments about diesel hybrids are referring to parallel hybrids, where the ICE is required to drive the wheels directly and must operate at a range of speeds and power levels. Maybe some of the cost of the diesel engine would go away for a serial hybrid, where the engine is much smaller and operates at constant speed and power output just to charge up the battery. GM is building the Chevy Volt initially with a small gasoline engine, but they have mentioned the possibility of using a small diesel engine instead. I'm waiting to see what happens.

[T. D.]

Diesel/electric is actually nothing new...just walk down to your local railroad track, and see that locomotives use a diesel engine to drive electric traction motors, these electric traction motors then generate the power to the wheels. The diesel engine itself does not power the wheels directly.