Date: Fri, 11 Sep 2009 10:08:42 -0400
Reply-To: mcneely4@COX.NET
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Dave Mcneely <mcneely4@COX.NET>
Subject: Re: Rolling resistance
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On Fri, Sep 11, 2009 at 12:59 AM , Kim Brennan wrote:
> low mass of the vehicle is (curiously enough) not a big factor except
> for energy used in acceleration
In urban and suburban driving, vehicles spend a large part of their
driving time and distance either accelerating or decelerating. This
fact contributes (there are other factors) to the substantially lower
gas mileage that almost all vehicles except hybrids get in this driving
mode compared to smooth highway driving. Despite the above statement
about the unimportance of weight, heavy vehicles consistently get lower
mileage than lighter vehicles. Acceleration matters.
> 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".
The transmission described has been in use for years on the Toyota
Prius, and is a major contributor to the high mileage that vehicle
achieves. The most important factor is the engine shutoff when not
needed for acceleration or maintenance of steady speed. The second most
important factor is a combination of excellent airflow over the vehicle
and light construction. But, the transmission matters a lot.
>
> Anyway. As a demonstration how driving habits can make a huge
> difference
> AC (2-3 mpg)
consumer reports says about 1 mpg for "average" drivers. I've tried to
gauge this in my Prius, and I can't determine a difference in highway
driving.
> 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)
Toyota recommends the slow acceleration with the Prius. I notice on the
(highly inaccurate btw, probably a better marketing tool than actually
functional) digital mpg monitor that I have higher mpg reported with
slow acceleration than with speedy. I don't know how this translates to
fuel economy overall, however, since low gear ratios tend to get lower
mileage than higher.
> 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)
Full hybrids like the Prius recover the energy of coasting by engaging
the electric motors in reverse during this driving phase, thus storing
the coasting energy in the batteries.
>
> 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.
So, back to my original question, are the bigger overall dimensions of
tire and wheel combinations of today compared to a few years back on
fuel economic vehicles a part of the "fashion" in that direction, or are
they engineered to deliver better mileage? I think Toyota claims the
latter, but Toyota claims a lot of things, like all manufacters.
David Mc
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