Re: The magic of a Prius automobile


"Old Wolf" <oldwolf@xxxxxxxxxxxxxx> wrote in message
news:1185327923.237214.314310@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
: On Jul 24, 9:43 pm, "Androcles" <Engin...@xxxxxxxxxxxxxxxx> wrote:
: > In fact if you drive to minimise your mpg at any instant, it DOES
: > minimise the overall fuel consumption. Drive slowly. (Try it).
: > You don't seem to know what a fact is, but it is a fact that
: > less fuel is used per mile at low speed than at high speed,
: > because E = 1/2 mv^2.
: > The faster you go, the more energy is needed. In fact.
:
: What I said was: if your goal is to accelerate
: to a certain speed (let's say 50mph), you use
: less fuel (per distance covered) by accelerating
: briskly to 50, than by accelerating slowly to 50.
:

Perhaps you can point out in the message below to where
you said that, I must have missed it.

Even then the energy required is the same regardless of acceleration,
although as a practical matter there would be a trade-off between
the air friction and the internal friction of the car. A greater
acceleration would involve higher engine RPM as the vehicle is
held in low gear longer. The maximum acceleration you can obtain,
as with the maximum braking, is when the tyre starts to slide on
the road surface leaving skid marks. Vehicles today are equipped
with automatic braking systems (ABS) designed to prevent that
happening.
You'll use more fuel overcoming internal friction resistance than
air resistance at low velocities, but at higher velocities the reverse
is true. As a rough guide, consider that the internal resistance
is more or less constant (in top gear) but air resistance increases
with velocity. You have only to put your hand out of the window
to feel the force, and then multiply that by the area of the windshield
and divide by the area of your hand to realise it is considerably
large.

As to the hybrid:

Toyota describes HSD-equipped vehicles as having E-CVT
(Electronically-controlled Continuously Variable Transmission).

If you understand a differential transmission (animated here:
http://www.androcles01.pwp.blueyonder.co.uk/Differential.gif )
then consider running it in reverse, i.e. turn the wheels to drive
the mainshaft rather than turning the shaft to drive the wheels, then
a constant RPM internal combustion engine and a variable speed
electric motor replacing the wheels will vary the speed of the
mainshaft which can then drive another differential (not shown).
If the mainshaft is held stationary then any rotation of the half shaft
produces a counter rotation in the other half shaft, the motor
becomes a generator and the engine can charge the batteries.

So for in-town driving, stop the engine and run on the motor only.
Advantage: No gas emissions, quiet, smooth running.
For highways, run the engine and also recharge the batteries.
Hook in a computer to do the controlling and that's all there is
to it. You get the added bonus of recharging the batteries when the
car runs downhill, energy recovered that would otherwise be lost
as heat in the brakes. You then use that to assist in going up-hill.
Advantage: energy is conserved, high mpg.
Disadvantage: Higher initial cost, extra weight of batteries, expensive
maintenance requiring specialist knowledge. Mechanics have
to be re-trained in electrical systems.
Americans are in love with big powerful engines with lots of
acceleration, although they don't drive all that fast. They are
also patriotic (and parochial), notice their love of Harley-Davidson
motorcycles, a machine that for years was inferior to Japanese
products. Only when Ford, GM and Chrysler develop the
hybrid will it have any significant popularity.

As an aside, arguing with Blind Poe is futile. He believes a bus
cannot stop in the same distance as a car from the same initial
velocity because his government has decreed it. His knowledge
of physics and his reasoning powers betray an intelligence
in two figures only, he believes what authority tells him.
A bus takes longer to stop than a car because the driver is
courteous to his standing passengers who are about to disembark
the vehicle and not for any physical reason. Blind Poe will use
that as argument without disclosing the reason for it. In
other words he's an idiot troll and all-round bigot who cannot
tolerate being proven wrong. You'll find many just like him.

I find it ironic that the GW brigade actually believes CO2
emissions have any impact whatsoever on the axial precession
of Earth in relation to its perihelion, but if they are right
then Neanderthals must have been charging around in 4x4s
and SUVs 130,000 years ago when the last warming took place.

http://www.roperld.com/graphics/LIAInsolation.jpg

We'll just have to be ironic at each other.



----- Original Message -----
From: "Old Wolf" <oldwolf@xxxxxxxxxxxxxx>
Newsgroups: sci.physics
Sent: Tuesday, July 24, 2007 4:02 AM
Subject: Re: The magic of a Prius automobile


: On Jul 24, 5:11 am, Randy Poe <poespam-t...@xxxxxxxxx> wrote:
: > I tend to use a summary mpg display. That feedback does
: > tend to train you in good habits. Nothing like seeing your
: > instantaneous mileage go down from 42 to 20 to make you
: > get your foot off the accelerator.
:
: ITYM, lighten your foot on the accelerator. Note
: that for conventional engines this ends up costing
: more fuel overall: if you drive to minimise your mpg
: at any instant, it does not minimise the overall fuel
: consumption. In fact you get the best average mpg
: by accelerating briskly to your target speed. (Try it).
:
: The reason for this is that conventional engines are
: the most thermally efficient with a free air-flow (i.e.
: open throttle) and in the middle of their RPM range,
: and least efficient when they have to suck air around
: the side of an almost-closed throttle plate. The
: thermal efficiency reflects how much of the energy
: stored in the fuel gets translated to kinetic energy.
:
: (Note the caveat that at fully open throttle, most
: engine computers change the timing for greater
: power and less efficiency, so for the most efficient
: acceleration you should use a nearly, but not fully,
: open throttle).
:
: Having said that, I'm not sure how much of this
: applies to hybrid engines, since I don't know when
: and how they use their two engines.
:
: A secondary concern, which applies to both
: hybrids and conventional engines, is that slow
: acceleration impairs the flow of traffic, which
: results in more people waiting at traffic lights
: or in traffic jams.
:
: Any gains you might get from slotheration,
: you lose due to the increased traffic congestion
: caused. Or should I say, the non-hybrid cars
: caught up in the congestion lose because they
: all have to do extra idling or creeping along at
: idle speeds, getting basically 0 mpg.
:
: I find it ironic that the anti-GW brigade promotes
: driving slowly, an action that directly leads to
: greater traffic congestion and thus a far higher
: total output of CO2 as well as other harmful
: emissions (closed throttle = car not breathing
: = poor combustion = side products like CO).
:
:


.

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