alt.energy.renewable

"Paul Ciszek" wrote in message
news:fs7bgc$baq$1@reader2.panix.com...
>
> In article ,
> ecarecar wrote:
>>A post or two above inspired me to ask
>>
>>Why are not hybrid diesels in vogue.
>>
>>I can see that some of the efficiencies produced by hybridization do not
>>appear in a diesel-hybrid. Is that the reason?
>
> Could you elaborate on that please?
>
> Diesel-electric drives are common in other sorts of vehicles, so they
> must have some merits.
>

Quite true, but it isn't an efficiency issue in the engine. Diesel electric
drives provide for hi efficiency transmission between widely different
speeds. RR Locomotives and marine applications are two that come to mind.
The engine can run at fixed speed (or only a couple of 'steps') and the
traction motors / propeller drive can be turned at a variety of speeds.

Otto-cycle (gas) engines have a lot of losses when run at part-load. The
biggest is the 'pumping power' of sucking air into the cylinders against
intake manifold vacuum (>25 inhg) and discharging the air to atmospheric
pressure in the exhaust system. The hybrid gas system avoids this by
running a smaller engine more nearly at its ideal, most-efficient operating
point (or shut off).

Diesel engines don't suffer from this loss because the intake is not
'throttled' when under part load. If the RPM is allowed to drop off at part
load (as it is in automobiles), then the part-load losses are not much
different than full load on a percentage basis. Fuel consumption for
diesels follows almost a linear relationship from low-load to full-load. So
a 'big' engine that is just loafing along driving around town is not as
wasteful as a 'big' gas engine under the same operation.

So while a diesel hybrid is certainly *possible*, it wouldn't see the kind
of improvements over conventional diesel fuel economy.

daestrom