Date: Fri, 11 Sep 2009 01:59:05 -0400
Reply-To: Kim Brennan <kimbrennan@MAC.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Kim Brennan <kimbrennan@MAC.COM>
Subject: Re: Rolling resistance
In-Reply-To: <056601ca328d$4438ef80$6401a8c0@PROSPERITY>
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I didn't phrase it particularly well. For best fuel economy you want
lightweight tires/wheel/rotating mass. The rest of the unsprung weight
is irrelevant towards fuel economy (though yes, you are correct it has
a lot to do with handling and ride.)
Simply put, a low mass rotating body is much easier (i.e. uses a lot
less energy) to spin than a higher mass rotating body. Mass always
matters. If we lived in a world without friction, Newton's laws of
motion would rule supreme. But we live in a world with friction, so
what is in motion does not stay in motion. And to keep it in motion,
requires energy. This applies both to vehicles traveling at a steady
speed, as well as vehicles accelerating.
As to your assessment on what it requires for high fuel economy...
On Vanagons, while our aerodynamics are better than loafs or splitties
(especially for cross winds) they are still far from an ideal shape
for high fuel economy. On our vanagons, this is the single biggest
factor affecting our fuel economy.
low mass of the vehicle is (curiously enough) not a big factor except
for energy used in acceleration (there isn't a lot of difference
between a 3000 pound vehicle and a 4000 pound vehicle in terms of
energy consumption when traveling at a constant speed.
RATE of acceleration is a much bigger factor in how much energy is
used. The way you accelerate also is a substantial factor (there are
tricks you can use that dramatically improve fuel economy, but most
drivers don't know them, and most traffic situations, don't allow for
them.)
While a small engine can be more fuel efficient, by itself, it's
really not all that critical in our vanagons.
Very tall gearing. A big problem for our vanagons is that we only have
4 gears (3 in automatics) for traveling about. Engines work most
efficiently in terms of fuel consumptions, at a steady speed (rpm).
This is most noticible with diesels, but it is true on gas engines as
well. For best fuel economy lots of closely spaced gears would be
best, so as to try to keep the engine at a constant speed.
For years, auto manufacturers have been working on the infinite
variable transmission. Audi has actually sold this (though only in 2
wheel drive vehicles, as it is too complex to work with their quattro
system.) The concept is simple. Run the engine at a (more or less)
constant speed, and change the ratio of engine rpm to transmission rpm
transfer. Think of two tapered cones, with a belt between them. move
the cones in or out, to vary where the belt is located. Sometimes the
cone from the engine has the belt on the small part of its cone, when
the cone on the transmission side has it on its big part. This is
effectively low "gear". As you move the belt towards the large side of
the engine cone, you also move it towards the small side of the
transmission cone. This is high "gear".
Anyway. As a demonstration how driving habits can make a huge
difference. In my commuter car (a Jaguar X-type manual transmission
3.0 V6), I can get anywhere from 18 mpg on a tank of gas, to 30 mpg on
a tank of gas. Same car, same tires, same roads, same time of day
driving. Here are the things that make a difference:
AC (2-3 mpg)
drag racing acceleration (pedal to the floor, rpms to 4500 before
shifting), versus slow acceleration (barely touching the pedal, shifts
at or before 2000 rpms) (10 mpg)
coasting out of gear (this is a one that is sometimes difficult to do
but keeps the rpms low and you can coast a long way.) (5-6 mpg)
Back to the Vanagon world, on the GoldBrick, I have a tiico engine,
which never "seemed" get the stated fuel economy capability that Tiico
claimed. In the end, it was the wheel/tires I was using. When I switch
to stock sized tires (205/65R15), I started getting the 22mpg that
they advertised, compared to the 18mpg I had been getting with my
preferred 215/75R15 tires. (fuel economy corrected and gps verified.)
For the Tiico, the biggest factor was the fuel used in acceleration.
Spinning the bigger tires, just took a lot more energy, and hurt fuel
economy much more that the minor extra distance those larger tires
traveled on each revolution.
On Sep 10, 2009, at 11:09 PM, Scott Daniel - Turbovans wrote:
> I can't really agree that low unsprung mass is helpful especially
> for fuel
> economy, or that cars are built with less unsprung mass to help with
> fuel
> economy.
> Low unsprung mass is all about handling and ride, , and has very
> little
> affect on fuel mileage.
> On smooth roads, unsprung mass has no affect, even.
> High mileage cars get their fuel economy from, mainly -
>
> low mass of the entire vehicle
> ( mass is only a factor when changing speeds or direction. When
> keeping a mass moving at a steady speed ins a straight line, mass, or
> weight, isn't really a penalty mass by itself ) ,
> low rolling resistance - tires are a big factor there for sure, mainly
> width, tread design, stiffness of the tire, and tire pressure. On
> cars that
> are really serious about tire pressure............the tires are
> inflated
> with nitrogen, since it expands with heat less than air does.
> small frontal area of the vehicle
> low drag shape
> small engine.
> very tall gearing
>
> large diameter narrow tires equal, more or less, taller gearing.
> Extra tire width would add to rolling resistance for sure.
> and with a 5 or 6 speed car with tall gearing, and close ratios in
> the top
> two gears, it would be interesting indeed to compare fuel mileage
> at say,
> 60 mph in top gear, then measure it at the same speed, but one gear
> lower.
> It's whatever is the least load on the engine ........whether that
> is at a
> higher rpm , or a lower one.
>
> And speed of course, speed is the biggest killer of fuel mileage.
> And it gets worse exponentially as speed goes up. 80 mph is
> drastically
> more-fuel eating than 60 mph is.
>
> Some might find it interesting that in very new throttle by wire
> cars - with
> a V-8 engine - for a given amount of acceleration or power requested
> from
> the engine, if it is more efficient and fuel-economical to run 4
> cylinders
> harder and let four coast along, onboard electronics will make the
> engine do
> that.
> Because - at higher throttle openings ( gasoline car with a throttle )
> there are less pumping losses ........so a fewer cylinders working
> harder,
> with burn less fuel than 8 cylinders working only medium hard.
>
> Supposedly, accelerating at a fairly fast rate will burn less fuel
> than
> accelerating very gradually up to speed - because .......with a wider
> throttle opening, the engine is operating more efficiently during
> acceleration, , and you spend less time getting up to speed - saves
> both
> fuel and time. Though I usually accelerate fairly gradually to be
> easy on
> equipment myself.
>
> Who remembers cars that could freewheel - that is, when you let off
> the gas,
> the drive between engine and trans was disconnected , so you were
> coasting
> any time you let off the gas - the engine would just return to idle
> rpm.
> Until you accelerated again. The Saab 96 with 3 cylinder two stroke
> engine
> had that, and perhaps the V-4 gasoline ones too, and so does a 1956
> Chevy
> overdrive transmission - those can coast in freewheel mode.....
> and the freewheeling device, which is a ratchet basically, also
> could make a
> nice hill holder device. In neutral the car could not roll backwards
> on a
> hill.
> There is far less control when coasting of course, compared to 'in
> gear.'
>
> One subaruvanagon guy was getting ....whatever, 20 mph, and was all
> excited
> about the better fuel mileage he was going to get with either taller
> gearing, or larger rear tires. His fuel mileage went down with that
> change !
>
>
> ----- Original Message -----
> From: "Kim Brennan" <kimbrennan@MAC.COM>
> To: <vanagon@GERRY.VANAGON.COM>
> Sent: Thursday, September 10, 2009 9:07 AM
> Subject: Re: Rolling resistance
>
>
>> Nope, the great misconception. It actually requires more energy to
>> turn those larger diameter wheels/tires. More than you get from them
>> propelling your further. If you look at almost all the high fuel
>> economy vehicles, one thing they'll have in common is small diameter
>> tires. Narrower tires aren't necessarily better, but because they
>> have
>> less mass, they also help. In the end for best fuel economy you want
>> the least amount of unsprung weight (the unsprung weight is the
>> weight
>> of the tires/wheels, brakes and other suspension stuff that is BELOW
>> the springs. The rest of your car/van is sprung weight (above the
>> springs).
>>
>> Of course with our Vanagons, you have a balancing act to get small
>> tires/wheels, with sufficient strength rating to handle the weight of
>> the van.
>>
>>
>>
>>
>> On Sep 9, 2009, at 7:30 PM, Edward Maglott wrote:
>>
>>> I thought a larger diameter would be better for mpg by raising the
>>> effective gear ratio. Engine turns fewer revolutions/mile. 'course
>>> if you are figuring your mpg using the stock odo, smaller diameter
>>> tires would make it look like you had better mpg because the actual
>>> distance would be fewer miles than the odo says. You would also be
>>> travelling slower than indicated on the speedo and saving some fuel
>>> there too. I think that makes sense.
>>> Edward
>>>
>>> At 01:41 PM 9/9/2009, Kim Brennan wrote:
>>>
>>>> Larger DIAMETER in your tires will have a more significant impact
>>>> on
>>>> mileage (adversely) as it will take more energy to spin your tires
>>>> (even though you are traveling further on each revolution, your net
>>>> will be an overall drop in fuel economy.)
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