Hybrid Car – More Fun with Less Gas

The "compromise" in the Prius drive train

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Posted by Ike on October 9, 2008, 2:30 pm
Technology moves along...

As mentioned in another thread, the Prius engine must
operate over a wide range of RPM and load because it
provides a combination of propulsion and
battery-charging. The electric motor means that the
Prius engine will not have to meet the challenge of
acceleration from zero speed, thus limiting its range of
RPM and load and permitting improved efficiency.

Engines can be made extremely efficient when designed
for a constant load and rpm. The Volt, for example, will
charge from the grid and then drive for (supposedly) 40
miles, and then the engine kicks in to begin charging
the system. That engine will always see the same load,
etc. so can be designed to run very fuel-efficiently.

 From an engineering viewpoint, the Prius is a great
achievement. The Volt and the many cars to follow that
model are far simpler - and in engineering, simplicity
is always better than complexity. In fact, there's a
good argument that the Volt is not a "hybrid" - it's an
electric car that carries an onboard charger...

Like most, I hope GM does a good job of executing the
Volt, that Toyota soon introduces its plug-in car, and
that many other car companies follow suit. And I
especially hope that emerging ultracapacitor
technologies overtake electrochemical batteries for car


Posted by Bob & Holly Wilson on October 9, 2008, 3:13 pm

Mostly correct. Both the Toyota and Honda hybrid systems, although
different, are using the electric power system to make up for the
defficiencies of an ordinary heat engine.

Yes and no. The thermodynamic efficiency of the engine should be high.
But it wiill have these energy conversions:
(1) engine->generator ~95%
(2) generator->motor ~95%
(3) generator->battery ~95%
(4) battery->motor ~95%

In the engine->generator->motor conversions:
.95*.95 = 90% (assuming the 95% is maintained)

In the engine->generator->battery->motor conversions:
.95*.95*.95*.95 = 81% (assuming 95% is maintained)

The typical Prius efficiency is a little more complex:
(1) 72% of power is: ICE->transaxle ~95%
(2) 28% of power is: ICE->generator->MG1 (.95*.95 = 90%)

.72*.95 = .68
.28*.90 = .25

We agree and the Prius has been on the roads since 1997.

No argument here. The 'plug-in' is just tapping a different power

Agreed and some of the stuff that has leaked out sounds very promising.
The environmental chamber for the batteries is a very good solution to a
difficult problem.

My current thinking is if the plug-in energy can keep the ICE in a
'thermal warm-up idle' until the block reaches the right temperature, it
will be a very good thing. Beyond that, the rest is keeping the ICE in
the high efficiency range.

My studies suggest that if the ultracapacitor unit handles surge charge
and drain with the traction batteries for sustained loads (aka., hill
climb and warm-up,) we'd have an optimum mix.

Bob Wilson

Posted by Bob & Holly Wilson on October 9, 2008, 3:19 pm

.95*.95*.95 = 86% (assuing 95% is maintained, not a trivial problem)

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