quattro Digest, Vol 17, Issue 33

[E. Roy Wendell IV]

> I haven't kept much of an eye on hybrids, so maybe I have missed 
> something.

>  Since there are two "power conversions" involved, I
> don;t understand why the efficiency is not lower.

>
> Is it just better matching of the power produced to the power needed? 
> i.e., a
> car traveling at 60mph is probably not using all of the power 
> generated by
> the engine, so, producing only the power "needed" might be more 
> "efficient". A
> car sitting at idle is producing more power than is needed, if it were
> operating a generator then, the wasted power would be stored for later 
> use.
>
> Tom Faust
>

You haven't missed anything. Your analysis is correct for the most part 
although the conversion of chemical to electric energy to mechanical 
energy is actually much much better than the conversion of chemical 
energy to mechanical via an IC engine. Think about how much of the 
energy in gasoline ends up as waste heat. Batteries don't get hot when 
properly used and variable frequency AC motors are very efficient. 
Sizing the gas engine to be just large enough for cruise power along 
with designing it for max efficiency account for a large part of the 
fuel economy gain. In the case of the Prius, idle efficiency approaches 
100% because the gas engine shuts off at traffic lights. It also uses 
regenerative braking where the car decelerates by way of charging the 
battery rather than converting the kinetic energy to heat via the 
brakes. The brake pedal is actually connected to two systems at once. 
It acts as both a force sensor for the regen braking as well as a 
source of hydraulic pressure for the friction brakes.

The issue of fuel economy of hybrids in general has a lot to do with 
driving conditions. In steady state cruise a hybrid is only marginally 
better than a properly designed economy tuned conventional drive train. 
As has been discussed here many times, a TDI can be better for highway 
driving. It's in the city driving cycle with lots of stop and go where 
the hybrid shines. Also, the higher the GVW the greater the advantage 
of the hybrid drive train versus the conventional one.

One could always go the next step and use a diesel as part of a hybrid 
drive train but for marketing reasons they seem to be competing 
strategies.

Roy