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All the reasons why a diesel engine is NOT good for a hybrid system

Seen on Jalopnik:

I believe the problem with the diesel-electric hybrid is the law of diminishing returns. What a lot of people don't seem to realize is that the best gas-electric hybrids are not running an otto cycle ICE, they're running atkinson cycle, or maybe one day miller cycle. These combustion cycle offer efficiencies much closer to a conventional diesel, and can be used thanks to the electric portion of the powertrain compensating where the ICE portion of the powertrain is weakest. There wouldn't be much additional benefit to using a diesel over an atkinson cycle engine

Hybrids, even the super mild GM variety, are somewhat reliant on turning the ICE on and off with great frequency. A diesel is NOT well suited to this, as it requires an even bigger starter, and more warmup time. The Prius uses a coolant thermos to try to keep the engine warm so that they minimize the number of cold starts. Having to heat a gigantic diesel block with some leftover hot coolant would be an even more significant challenge.

Diesels also cost extra money over a gasoline ICE. This number is definitely coming down, as gasoline engines have started to adopt things like direction cylinder injection, and diesel engines production has ramped up, but diesels are also significantly heavier. Given the marginal benefit at best by combining the diesel and electric drive-trains, the added weight and cost of a diesel hybrid over either a gasoline hybrid or a straight diesel, really makes the whole thing uneconomical. The only time this sort of thing seems to make sense is in super heavy duty equipment, where the weight of the powertrain is relatively small anyway, and they're generally already using diesel engines - like buses, for example.

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Reply to
Elmo P. Shagnasty
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These are all great points but there is another one as well. Most diesel fuel is obtained from crude oil and the portion of most crude oils that consists of diesel-range material is quite small (as it is for 'gasoline' range material as well). Oil refineries have invested hundreds of billions of dollars in very expensive equipment to convert larger molecules in crude into smaller 'gasoline' molecules. For the most part, the processes used to produce gasoline cannot be converted to produce the slightly-larger 'diesel' molecules. As a result, producing a much larger portion of 'diesel' from crude oil than at present would require the investment of additional hundreds of billions of dollars as well as the development of new processes to accomplish the task and is therefore unlikely to happen. For the foreseeable future, most crude oil is going to be converted into 'gasoline' rather than 'diesel' so if a significant increase in diesel demand occurs, the inevitable result is a much higher price for diesel fuel. For this reason, it would be extremely foolish for auto makers to plan a future product line-up which depended on running on inexpensive diesel fuel. Diesel fuel has some advantages: 1) slightly better energy storage density, and 2) safer storage and handling due to decreased vapor pressure but it also has some serious disadvantages: 1) more likely to solidify in very low temperatures, 2) produces many more particulates during combustion, and requires much heavier combustion equipment due to its higher pressure during combustion. A gasoline-electric hybrid with a large battery seems like the best long-term type of transportation power plant.

Reply to
David T. Johnson

But if I can take a gasoline-engined Accord that gets 35mpg on the highway and turn it into a diesel-engined Accord that gets 68mpg on the highway...

Reply to
Elmo P. Shagnasty

The real-world benefits of diesel are not as good as that. The 2009 volkswagen Jetta with a 4-cyl 2L gasoline engine has an EPA mileage rating of 22 while a 2009 Jetta with a 2L diesel engine has a mileage rating of 29. The increased energy content of diesel fuel accounts for about 4 mpg of the improvement and the improved efficiency of the diesel cycle accounts for 3 mpg.

Reply to
David T. Johnson

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