Re: The magic of a Prius automobile


"Old Wolf" <oldwolf@xxxxxxxxxxxxxx> wrote in message
news:1185408737.567231.20820@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
: 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.

It would be very convenient to your argument to ignore friction
for the moment, but inserting impossible caveats into the
discussion isn't going to enlighten anyone.
An engine is designed to do work, converting chemical energy
to mechanical energy. Inevitably there are losses in the form of heat.
For motor vehicles, all energy is eventually lost as heat in braking.
If you are going to operate the engine at full throttle, get a small
engine and do not accelerate as greatly, you will save fuel.
Hence the term "gas guzzler" applies to vehicles with larger engines.


: > 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.

Yes indeed. Automatic transmissions have oil coolers
in larger vehicles. The lower the gear, the greater the loss
as heat. Manually operated clutches also run hotter at
higher engine RPM, so again keep the RPM low and
the gear ratio low, 1:1 is better than overdrive.
That equates to low acceleration.
Your argument for high acceleration is time related.
It seems intuitive that one does not need to burn the fuel
for as long a period of time, but the quantity of fuel
increases. There is no overall fuel saving.
Those old steam engines didn't need a gear box, the
piston could move slowly. The internal combustion engine
burns its fuel in a short time interval, typically running at
~2000 RPM = 33 revolutions per second. In a piston engine
that's 66 strokes, one up and one down and takes 1/66 = 15
milliseconds to complete the burn. If the burn takes longer
then the exhaust valve opens and unburnt fuel is wasted.
If it takes less time then it is doing no useful work, waiting
around for the valve to open and be expelled, and so the
internal combustion engine has a short range of RPM
in which it is efficient and that produces the need for
careful ignition timing and a gearbox. Higher octane
fuels can mean higher RPM but are more expensive to
produce. Of course "good mileage" really equates to
"low cost", and so we use cheaper fuel.



:
: > 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).

Then it should be ALBS, not ABS, but I agree its just a
marketing ploy. All I'm saying is you don't accelerate as well
by spinning your wheels, which, without locking the differential,
will occur on one wheel only and the car will turn sideways.

: > 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.

But it is significant. ALL the energy not lost as heat or used
in climbing hills (where it can be recovered by rollng down them)
is used to overcome air resistance.
E = 1/2m v^2...
Let m = 2 (the mass of the car is constant), so that
E = v^2
At 10 mph, E = 100.
At 20 mph, E = 400.
At 30 mph, E = 900.
At 40 mph, E = 1600.
At 50 mph, E = 2500.
At 60 mph, E = 3600.

You need 4 times the fuel to get to 40 mph as you need
to get to 20 mph, not twice as much, and if you then have
to brake at a red light you've wasted it. We all do it, I know,
we are all in a hurry, but we are not being economical.
In practise, of course, nobody checks fuel mileage at 10 mph
and there would be higher losses due to internal friction, heating
and/or airconditioning, but accelerate gently and brake gently.
To the truck owner it's a careful balance between paying for the
driver's time, on-time delivery and fuel costs.
A late delivery means the customer loses money paying idle hands,
an early delivery and the driver is waiting around to be unloaded.


: > 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.

Those are special cases, the physics of the situation is that
a bus has a greater footprint on the road surface than a car, the
emergency stopping distances are the same, all else being equal.
We do not put 13" wheels on a bus, but we give it a proportionally
smaller engine (7 litres typical for a London double decker).

If a child runs out in front of the vehicle, the bus can stop as
quickly as the car. It is a myth that the heavier vehicle will
take longer to stop, a 400 ton airliner can stop as quickly
as any car can from the same velocity and faster than a kid
on a skateboard (which has no brakes) but not as quickly
as a running man. I raced against my 5 litre Camaro once and
won for several yards, my acceleration was greater initially but
I could not sustain it, I'd reached my top speed of 15 mph
in two or three strides. Stopping is more difficult, I almost fell.
A large oil tanker on water takes longer to stop than a rowboat,
but that's an almost frictionless surface and different conditions
entirely. Put proportionally small screws on the rowing boat
and then make the comparison.


.

Relevant Pages

  • Re: The magic of a Prius automobile
    ... :> minimise the overall fuel consumption. ... Even then the energy required is the same regardless of acceleration, ... acceleration would involve higher engine RPM as the vehicle is ... energy is conserved, high mpg. ...
    (sci.physics)
  • Re: 1958 M. Benz 190SL and octane levels
    ... its ignition at a higher temperature to prevent preignition in a high compression ratio engine. ... To that, an acceleration pump is added to kick extra fuel for acceleration, otherwise the air / fuel ratio would become too lean as the throttle is opened. ...
    (alt.auto.mercedes)
  • Re: Injection Limiter adjustment
    ... If it doesn't look like it gets oil from the engine lube ... It might make sense that the engine is using less fuel. ... usage is during acceleration, the engine may not be accelerating when loads ... change and actually saving fuel by coming up to the new load very slowly. ...
    (rec.boats.cruising)
  • Re: Any hope? ST (Itlaian) ST .25 ? (ATTN: LONG)
    ... Super Tigre carburetor adjustment procedure ... The Super Tigre carburetor has two needle valves for adjusting fuel ... left of the carburetor throat (as viewed from rear of engine). ...
    (rec.models.rc.air)
  • Re: Mustang GT and K&N air charger
    ... Even when replacing the air filter ONLY to a higher flow ... I dug out the Ford shop manual for a 1999 Mustang 2V 4.6 Liter engine. ... throttle plate are the MAF sensor and air temperature sensor. ... Fuel Pressure Sensor ...
    (rec.autos.makers.ford.mustang)