avatarby Nick HallSeptember 23, 20160 comments

Diesel has become a dirty word in recent times with the VW scandal, but it wasn’t always that way. California never liked the stuff, but there were plenty of claims that diesel could be cleaner if it was treated properly and it was definitely more economical. So why don’t we have diesel-electric hybrids?

On the surface it’s a no-brainer. You get more miles from the diesel and combined with the electric motors that could give us hybrids that could comfortably breach 200mpg in real world terms. Volkswagen, ironically, even revealed a 300mpg diesel-electric concept back in 2014. At one stage it looked like the future.

VW pressed ahead with the Golf TDI Hybrid, Mercedes brought us the E-Class E300 BlueTEC Hybrid and S-Class equivalent and Peugeot, Citroen and Volvo joined the fray. GM, Chevy and even Ford had diesel-electric hybrids ready to go at one point. Now, you can have a diesel-electric Range Rover, in Europe, which delivers 44.1mpg on the combined cycle and Citroen has kept going with the DS5. But most manufacturers have quietly shelved the development and moved on to better things.

The sticker price would be painful

The reasons are long and varied, but the main one is simple cost. A diesel engine costs around 15% more to produce, which in a car that already comes loaded with expensive battery packs and electric motors can be the straw that breaks the camel’s back. Hybrids aren’t cheap in any case, but with the additional cost of the diesel motor then the final price tag would be tooth sucking high and we could be looking at $10-$15,000 on top of the price of an average car.

In Europe, where gas prices are high and diesel is cheaper, this might be a bitter pill that people could swallow. But in other parts of the world where petrol is cheaper and diesel often costs more, the math just doesn’t work. Car manufacturers work in a global economy and have to produce a one-size fits all approach. Petrol works, diesel just doesn’t.

Then what about the infrastructure? The US and Japan really aren’t set up for diesel passenger cars and to achieve this ‘Utopian vision’ of diesel-electric hybrids, we’d have to install diesel pumps at every gas station. The investment that would take would be a nonsense, especially as hybrids are really paving the way for a full electric future.

California, one of the biggest hybrid markets from the early days, would have to overturn strict legislation, too, and it just isn’t going to do that. Especially not now that VW have cast an ugly shadow over the black pump.

The Atkinson Cycle is a vital part of petrol’s power

Get into the mechanics and there is more bad news for diesel power. A petrol engine is a natural bedfellow for the electric motor when it’s tuned to the Atkinson Cycle. This means the petrol unit gives its maximum power at the top of the range, but minimal torque at lower engine speeds.

The electric motor provides all of its torque at lower speeds and the two powerplants provide complementary torque curves. A diesel doesn’t work like that and it provides most of its torque low down the range, so it needs additional gears to really make it work.

That’s not a dealbreaker, cars come with eight speed ‘boxes as a matter of course these days and Mercedes has a 9-speed. But it is a layer of complication and added expense.

Diesel engines are harder to work with

Diesel also gives less room to maneuver. It’s more efficient than petrol to begin, converting up to 35% of the energy into power to the wheels. So there is less room to plug the gaps and the diesel-electric hybrid simply won’t seem as impressive on the road. When you add the additional weight of a sturdier diesel block and the batteries, it’s just harder to work with for the engineers.

Now we’re not banishing diesel-electric hybrids to the technological wastelands just yet. Range Rover has made it work and diesel-electric hybrids are standard practice with ships and locomotives.

It’s a common powerplant for buses, too, but they just never found a home with cars. With the level of development heading into petrol-electric hybrids we can’t see that changing anytime soon.

Diesel-electric hybrids are a great concept, in theory, but the number of them out there tells its own story. For a variety of reasons, they just don’t work.

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