Low City Mileage

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#11

06-06-2007, 12:54 PM

SPL

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Re: Low City Mileage

turk — Let me see if I can be clearer. The trip FE in L/100 km is 100*(liters L used)/(kilometers X travelled). Since

L = integral(dL/dx with respect to x from 0 to X)

it follows that

FE = 1/X*integral(100*dL/dx with respect to x from 0 to X)

but this is just the distance-weighted average value of 100*dL/dx, and 100*dL/dx is the instantaneous FE in L/100 km. Hence my statement that to optimize FE one needs to minimize this average. Put another way, it's the distance travelled at a given instantaneous FE that is important for the trip's overall FE, and not directly the time spent at each instantaneous FE. (Of course, the time spent will depend on the vehicle's speed and hence indirectly on its instantaneous FE, but won't in and of itself determine the overall trip FE.) One needs to drive so as to minimize the distance travelled at high instantaneous L/100 km, and maximize the distance travelled at low instantaneous L/100 km. [One can make a corresponding statement in terms of distance in miles and instantaneous FE in gallons/mile, but since no gauge I know of reads instantaneous FE in gallons per mile, you're stuck.]

Droid13 — I don't think you can truly conclude that time spent at a given inatantaneous FE can be used instead of distance travelled, since the car's fuel efficiency is not the same at all accelerations. Were the efficiency the same at all accelerations, then your statement would be correct. Suppose that we start from rest at a constant acceleration 'a' and accelerate up to a given speed 'v' in a time 't,' covering a distance 'x.' Then these quantities are related by the formulas

x = v^2 / (2*a)

and

t = v/a

so that the distance x travelled and the time t taken both depend inversely on the assumed constant acceleration a. Changing a changes both x and t in the same proportion. Eliminating 'a' shows this:

x = v*t/2

Were the car's instantaneous FE the same independent of the acceleration 'a' chosen, then it would be true that one would get exactly the same trip FE no matter how you drive the route, since the work done would be the same in all cases (ignoring varying air resistance), and hence the fuel used would then be the same too.

Stan

#12

06-07-2007, 02:09 PM

rmorrow

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Re: Low City Mileage

Quote:

Originally Posted by SPL

turk — Let me see if I can be clearer. The trip FE in L/100 km is 100*(liters L used)/(kilometers X travelled). Since

L = integral(dL/dx with respect to x from 0 to X)

it follows that

FE = 1/X*integral(100*dL/dx with respect to x from 0 to X)

Agh! Grade 9 calculus! Did I mention "Agh!"?

#13

06-07-2007, 02:52 PM

ymiheere

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Join Date: Jun 2007

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Re: Low City Mileage

Just a thought here..... I didn't see anyone mention one very important influential factor in mileage. Just how much are you cranking up the climate control system. If set in "Low" on a hot day, the gas engine will not shut off since it will be needed to power the air!!!

My two cents!!!

#14

06-08-2007, 07:38 AM

SPL

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Re: Low City Mileage

ymiheere — The TCH's air conditioning is electric, powered by the high-voltage NiMH battery through a 3-phase ac compressor motor. Only if the NiMH battery drops too low will the ICE need to come on.

Stan

#15

06-08-2007, 03:45 PM

ymiheere

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Re: Low City Mileage

Quote:

Originally Posted by SPL

Right, but the NiMH battery can only sustain the a/c for a short while before it runs low enough for the ICE to take over. I was only considering this as one overlooked possibility as to why someone would get mileage in the 20's in the city, when I, and others like me, have consistently gotten 35+

#16

06-08-2007, 05:42 PM

kettledrum

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Re: Low City Mileage

Quote:

Originally Posted by n8kwx

Short 5 minute trips?

Live in Fairbanks, AK?

Either can effect city mileage significantly. (Unless you are jackrabbiting and slamming on the brakes every block). Trip length and temperature are much bigger factors.

I would be willing to bet that both of these are the culprits.

7 months of ownership puts you buying the car roughly in November so there's a lot of cold weather months to make up for. I bought my Prius at the end of September and I'm still working to get my overall mileage up from the winter months.

Short trips can be an absolute killer too. Anything under 10 minutes and especially 5 minutes is going to cause lower FE.

My mother in law just bought a TCH a couple of weeks ago and my worst fear is that she is going to get low FE for these very same reasons and is going to be disappointed.