Hybrid Car – More Fun with Less Gas

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

register ::  Login Password  :: Lost Password?
Posted by Elmo P. Shagnasty on September 6, 2009, 5:51 pm

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

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.


Posted by David T. Johnson on September 7, 2009, 5:37 pm

Elmo P. Shagnasty wrote:

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.

Posted with OS/2 Warp 4.52
and Sea Monkey 1.5a

Posted by Elmo P. Shagnasty on September 7, 2009, 8:13 pm

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

Posted by David T. Johnson on September 7, 2009, 11:33 pm

Elmo P. Shagnasty wrote:

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.
Posted with OS/2 Warp 4.52
and Sea Monkey 1.5a

This Thread
Bookmark this thread:
  • Subject
  • Author
  • Date
please rate this thread