[Nobody]

Not in this graph:

http://www.fueleconomy.gov/feg/driveHabits.shtml

The energy which a particular vehicle consumes (in fuel) also depends upon
the efficiency of the engine and drivetrain, and the efficiency of ICEs
varies wildly with both speed and load.

The part of the curve where economy increases with speed is primarily due
to increasing engine efficiency. There isn't any source of opposing force
which decreases as speed increases.

Apart from increasing engine efficiency with speed, the only other factor
which can produce a positive slope on the economy curve is for constant
power sinks (anything which consumes power while stationary: alternator,
water/oil pumps etc).

I've seen a few other mpg-vs-speed curves, and they all look similar.
Looks like aerodynamic drag starts to seriously kick in above 55 MPH.

It kicks in way before that. Even at cycling speeds, you get a very
noticable reduction in effort when riding in the slipstream of a truck.

 

[Nobody]

You are correct at higher speeds. The relationship isn't simple it seems.
http://en.wikipedia.org/wiki/Drag_(physics)

Note that Stokes's drag isn't meaningful for vehicles. Where it meantions
"small objects", "small" means particles whose size comparable to the
gap between the molecules. IOW, it's useful for modelling a cloud of dust
settling, but not for modelling "objects".

Drag coefficient can make collosal differences too.
http://en.wikipedia.org/wiki/Drag_coefficient

Unfortunately current US styling (the trend for truck like 'slab fronts' ) is producing
cars with greater rather than lesser drag.

It isn't just the coefficient. Drag is also proportional to (effective)
cross-sectional area; large vehicles have more drag than small ones. The
sheer size of an American SUV is

The current hybrids also use skinny tyres to reduce rolling resistance. I hate to think
of the adverse effect on road holding.

Tyre width only matters when you get close to the limits of traction. It
doesn't affect total friction until the tyres start to distort
significantly and the force/area ratio exceeds the strength of the rubber.

 

[Nobody]

No thats a cubic law. The energy per mile is the square law one.

Energy = force * distance. Drag force is proportional to speed squared, so
energy per unit distance is proportional to speed squared.

Distance per unit time is (by definition) speed, so power (energy per
unit time) is proportional to speed cubed.

F = k*v^2
E = f*d
E ~ k*v^2*d
W = E/t
W ~ k*v^2*d/t
v = d/t
W = k*v^2*v = k*v^3

 

[Phil Allison]

"Nobody"

The part of the curve where economy increases with speed is primarily due
to increasing engine efficiency. There isn't any source of opposing force
which decreases as speed increases.

** There is one - the " rolling resistance " of the tyres decreases with
speed.

At 100 kph, a car tyre experiences about 200Gs of centrifugal force making
it larger and rounder - this reduces flattening at the point of road
contact and so there is less resistance.

It kicks in way before that. Even at cycling speeds, you get a very
noticable reduction in effort when riding in the slipstream of a truck.

** False comparison - a bicycle plus rider ain't very aerodynamic compared
to a modern sedan car, so air resistance is far more significant at lower
speeds.





...... Phil

 

[JosephKK]

Richard Henry [email protected] posted to sci.electronics.design:

Documentation?

The first question would be: How did you measure it?

Quite simply, miles traveled divided by fuel to top up again. With
trip the odometer it was very easy.

 

[JosephKK]

MooseFET [email protected] posted to sci.electronics.design:

It was likely a combination of coincidence and something very
wrong
with the car. 22MPG at 55MPH is a horrid milage. This is very
like the leaky fuel line case.

It was not that bad in the early 1970's. Please consider the noun
phrase "My first new car" more carefully. Besides, the point being
made was that better fuel milage can occur at mean (and nearly
constant as well) speeds above 55 mph.

 

[JosephKK]

MooseFET [email protected] posted to sci.electronics.design:

No, I disagree. The dual electrical machine design beats the
series
system hands down. Having the engine go straight to the wheels
when it makes sense to do so makes the demand on the electrical
system way less.

The highest demands are during maximum acceleration / deceleration.
Since it would complicate the controls hugely to no gain, there is
less than no point in trying to parallel the mechanical paths.
During cruise the electric drive train is loafing just like a
direct ICE would.

 

[Nobody]

Don't both Honda and Toyota have models with CVT (continuously
variable transmission) which keeps engine RPM's in a narrow/optimum
range?

It isn't just RPM; load torque is also a factor. Optimum efficiency occurs
depends upon both. If there's too little load, increasing the gearing
will move the load closer to optimal but the speed away from it, and
vice-versa.

Engines tend to be designed for optimum efficiency at cruising speeds
(~60 MPH, 3000-3500 RPM in 4th/5th gear). This helps counteract the
inherent losses from increased drag at that speed, keeping the MPG
curve relatively flat.

 

[JosephKK]

Nobody [email protected] posted to sci.electronics.design:

The disadvantage of an all-electric or series hybrid vehicle is
that the maximum total power is constrained by both the ICE and
the electrical system, while a parallel hybrid can potentially
combine the two, or at least provide a more powerful ICE for
motorway cruising without having to size the electrical components
to match.

A secondary consideration is that the generator losses subtract
from any efficiency advantages of the electrical drivetrain.

But mostly it's a case of having to size the electrical system to
the power requirements for motorway cruising even though it has
fewer advantages over a mechanical drivetrain in that context
(i.e. regenerative braking is of no benefit).

NO! The electric drive train has to be sized to the maximum
(majority electrical contribution) acceleration in all cases. It
must be sized (possibly much) larger than the ICE.

 

[JosephKK]

MooseFET [email protected] posted to sci.electronics.design:

They usually are bigger and heavier but that is partly because
when you are designing the engine for a train you don't worry
about
weight. The forces in a high performance gas engine are higher
than in a diesel.

Any time you squeeze 100 horsepower (75 kW) out of 1 liter
displacement you will have very high stress levels. I have such a
vehicle.

 

[JosephKK]

Spehro Pefhany [email protected] posted to
sci.electronics.design:

For a given shape, at automotive speeds, I think there is a square
law relationship between air speed and drag.

Best regards,
Spehro Pefhany

The maximum significant contributing order is 4th power (of speed).
Just the same, there are significant contributions from the 3/2
power terms and the 3rd power terms (all of speed). These can and
do add up to specific strange maxima and minima in fuel efficiency
versus speed. Look at the graph again, the shape dictates a
minimum order of 4th power of speed best curve fit.

 

[Richard Henry]

Richard Henry [email protected] posted to sci.electronics.design:






Quite simply, miles traveled divided by fuel to top up again. With
trip the odometer it was very easy.

So you drove 70 mph between fillups?

 

[[email protected]]

Why waste your energy with 10 minutes charging and grid failure. We
can have battery stations like gas stations with efficient chargers.
Just need a removable battery pack on cars and use a fork lift. All
done in less than 10 minutes....?

 

[Eeyore]

The disadvantage of an all-electric or series hybrid vehicle is that the
maximum total power is constrained by both the ICE and the electrical
system, while a parallel hybrid can potentially combine the two, or at
least provide a more powerful ICE for motorway cruising without having to
size the electrical components to match.

At the cost of a vastly more complicated drive train.

A secondary consideration is that the generator losses subtract from any
efficiency advantages of the electrical drivetrain.

With a plug-in hybrid you may not even need to use the ICE/generator on shorter journeys.
Hence no additional losses at all.

But mostly it's a case of having to size the electrical system to the
power requirements for motorway cruising even though it has fewer
advantages over a mechanical drivetrain in that context (i.e. regenerative
braking is of no benefit).

Motorway cruising does not require high power. It's acceleration that requires high power.
You can cruise at motorway speeds with say 30kW.

Graham

 

[Eeyore]

Note that Stokes's drag isn't meaningful for vehicles. Where it meantions
"small objects", "small" means particles whose size comparable to the
gap between the molecules. IOW, it's useful for modelling a cloud of dust
settling, but not for modelling "objects".


It isn't just the coefficient. Drag is also proportional to (effective)
cross-sectional area; large vehicles have more drag than small ones. The
sheer size of an American SUV is

You'll see I covered this point in more detail an another post.

Tyre width only matters when you get close to the limits of traction. It
doesn't affect total friction until the tyres start to distort
significantly and the force/area ratio exceeds the strength of the rubber.

Some of the tyres being sold for this application are also low friction AIUI.

Graham

 

[[email protected]]

Richard Henry wrote:

Ninety-one lines of quoted material, nested twelve levels deep, with seven words tacked on at the bottom.

You ARE allowed to trim the quoted material, you know.

 

[MooseFET]

MooseFET [email protected] posted to sci.electronics.design: [....]

It was likely a combination of coincidence and something very
wrong
with the car. 22MPG at 55MPH is a horrid milage. This is very
like the leaky fuel line case.

It was not that bad in the early 1970's. Please consider the noun
phrase "My first new car" more carefully. Besides, the point being
made was that better fuel milage can occur at mean (and nearly
constant as well) speeds above 55 mph.

If it was an american car made in the early 1970s then it was indeed
like the leaky fuel line case. The "big three" were all making
complete crap in that era. They wasted gobs of fuel. Yes this can
lead to a best milage at 55 MPH but this should no longer be the case
on any car made today.

 

[Spehro Pefhany]

You get 500HP out of your Chevette?

With a JATO, but it's an Impala.


Best regards,
Spehro Pefhany

 

[Michael A. Terrell]

But my favorite gal likes wearing dresses. Oh wait, that wouldn't
be drag then. I guess i will have to tell her not to wear pants.

DRAG was from the theater when all parts were performed by males. It
meant: 'DRessed As Girl' The other parts were DRAB, or "DRessed As
Boy' I worked the stage crew on plays in high school, and the drama
teacher couldn't let a day go by without dumping a lot of theatrical
history on us, while we were working on the play. BTW, I was the sound
man.


So, if you want to keep her in dresses you DO want her in drag.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[MooseFET]

Why's that an advantage ? It also means you can't have 'meaningful' true electric only
operation of it, plus it requiresa gearbox which otherwise may not be needed at all.

Graham

Here's a picture to look at while I explain:

http://www.cleangreencar.co.nz/page/prius-technical-info

At low speeds or when you are running only on the electrics the
electric machine connected to the wheels does all of the work of
moving the car. In this situation the gear box from the engine to the
wheels is disengaged.

When more electrical power is needed, the electrical machine connected
to the engine is used to start the engine. Using this large of a
motor as a starter means that the engine can be started very quickly.

On the highway, the engine gets connected directly to the wheels.
This takes the sosses of the electric machines out of the picture. It
also allows the engine and both motors to be used for passing power.