Hybrid Car – More Fun with Less Gas

Question about MPG - Page 8

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Posted by Horse's Ass on August 15, 2006, 10:30 pm

You and Bill are wasting your time.  He isn't paying attention.

Posted by Steve Pardoe on August 16, 2006, 2:03 pm



With you so far.

The last two cars I've owned (one petrol / gasoline, one diesel) consume no
fuel at all on the over-run (but I agree they are not conserving any energy
while slowing down).

Yes, although a diesel engine consumes a remarkably small amount when idling
compared with a gasoline engine, since it can run very much leaner.

I think it's fair to say that whether or not a gasoline / electric hybrid
such as the Prius is "a better overall compromise of a design" depends on
what kind of driving you mainly do.  If you mainly drive in the city and
spend a lot of time stopped at intersections, it makes good sense.  Several
European manufacturers make city cars with a stop-start system with a
conventional engine.  On a long steady journey with little opportunity to
regenerate, the savings must be very slight, if any, compared with the same
car without the 'hybrid' bit (but for the sake of argument still using the
very efficient Atkinson-cycle engine and planetary transmission), and so
under such conditions there's no reason why the car should be any better
than a diesel, which was my original question.

Well made point, and all credit to them, and indeed to the far-sighted
customers who care enough about all this to buy and run a Prius.  Iadmire
them, especially since I now understand that diesel isn't a viable
alternative in much of the USA.  However, as in my first post, I was really
more interested in a straightforward user-based mpg comparison.

Thanks anyway,


Posted by Bill on August 16, 2006, 3:57 pm

Again, which vehicle are you comparing the Prius to?  Until you compare EPA
mileage figures, this is speculation.

Which user?  I presume that would be you.  We are back to my earlier
suggestion that you rent a Prius to compare with the mystery vehicle to
which you constantly allude.

Posted by Bob Wilson on August 17, 2006, 9:39 am

There is one other Prius operational mode not yet discussed called
Electric Vehicle (EV) or "stealth mode." At speeds below 42 mph, the
vehicle can operate on just the electric motor with the engine off. The
distance increases significantly at speeds below 30 mph. For example,
the last 1-2 miles into work or returning home, I take streets with
25-30 mph limits. This last mile is run with almost no gas consumption
unless I have to react to other traffic. This includes 2-4 m. rises,
which can be taken at 10 mph without the engine.

This EV mode is a unique to the stock Toyota and Ford hybrid systems.
Some models of Prius in Europe and Japan come with the EV button and in
the USA, there is an after market kit to fit an EV button. So, are any
other vehicles able to climb 2-4 m. without turning on the engine?

My 03 Prius has one well documented EV mode, reverse. In fact, even
cold, the engine can not run when I back out of my driveway. The reason
has to do with the planetary gears and engine. I can also climb steep
hills in reverse.

It is possible to use the EV capability to considerably extend the
"glide" part of "pulse and glide" driving. What this means is the speed
decay is countered by the EV so the hybrid vehicle 'glides' further than
an engine-off non-hybrid. For example, two vehicles starting at 40 mph
start a glide down to 30 mph. The non-hybrid will go about 1/2 mile
before reaching 30 mph and having to start the engine to speed back to
40 mph. The hybrid vehicle can stretch that energy-off glide to nearly a
mile using a small amount of electrical power. The pulse back to 40 mph
quickly add that energy back and you're back in glide again. But in the
meanwhile, the hybrid has gone a considerably further distance.

Hybrids are also designed for frequent engine off and on operation. This
means the start-engine fuel burn is extremely low and efficient. There
is no Bendix gear that has to reach out and engaged the flywheel but
effectively a directly connected motor that engages electronicly. This
minimizes starting friction losses and engine wear. Unlike traditional
12 VDC starter motors drawing a hundred or more amps, the hybrid motors
are 200-270 VAC motors drawing a few tens of amps. This lower current
minimizes the resistance losses and heat suffered by ordinary starter

Bob Wilson

Posted by richard schumacher on August 16, 2006, 3:22 pm

He meant that, when you're coasting, the motors/generators are taking
energy from the wheels and putting it back into the battery.  One does
not have to be using the brakes to be regenerating.


It's a net savings because when the engine runs to recharge the battery
it does so in its most efficient operating range, which produces much
more power than is needed to run the A/C and other accessories.  When
the engine is off is obviously uses no fuel at all.  Overall, even with
the conversion losses, it's more fuel efficient to run the engine

The beauty of Toyota's HSD system is that it can use a variable fraction
of the engine's output with a direct mechanical connection, and allows
sizing the engine to something much less than the maximum power

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