Re: The magic of a Prius automobile

On Jul 25, 11:29 pm, "Androcles" <Engin...@xxxxxxxxxxxxxxxx> wrote:
"Old Wolf" <oldw...@xxxxxxxxxxxxxx> wrote in message

: 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.
:
Even then the energy required is the same regardless of acceleration,

Therefore, (ignoring friction for the moment),
the least fuel is used by operating the engine in the
mode where it's most thermally efficient. For most
engines, as far as I know, this occurs with mostly
open throttles and in the middle of the designated
RPM range.

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.

I suppose you would also have to figure transmission
losses into it; perhaps they are more or less efficient
depending on the RPM as well.

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.

The A in ABS stands for anti-lock, and it only applies
to braking. The equivalent for acceleration is called
traction control (or whatever trendy name the sales
department came up with now).

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.

I suppose the question is: does the loss due to spending
a higher proportion of the journey at the higher speed,
exceed the gain due to thermally efficient acceleration?

For low speeds (e.g. 40mph or less) the air resistance
would not be significant.

He believes a bus cannot stop in the same distance as a car
from the same initial velocity because his government has decreed it

Well, it seems to me that it depends on the heat dissipation
rate of the brakes, and the grip of the tyres of each vehicle.

I've seen heavily laden coal trucks stop in a damn hurry,
so I don't see why buses couldn't do the same. Of course,
many buses in service are old and have appalling brakes.

.

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