OT: Hybrid cars

[guy keren]

On 09/17/2013 09:07 AM, Oleg Goldshmidt wrote:
> guy keren <guy.choo.keren at gmail.com> writes:
>
>> "watching the numbers occasionally" is not a proper experiment. you
>> need to reset the computer before you start the "drive under test",
>> and check the value after - and the length should be enough to even
>> out the fluctuations.
>
> I don't know what your car shows you. Mine directly shows the fuel
> consumption at the moment (that jumps around) and a running average over
> some period of time that is updated every few seconds. I don't remember
> what the averaging period is exactly, but if you drive at a steady speed
> for a while (I have cruise control, too) you will get a pretty stable
> number on the screen.

watching the "current consumption" numbers can be quite missleading, 
since' during a lengthy period of drive, the number is usually not 
stable, and the assumed summing up of the numbers isn't necessarily the 
real summing up of the numbers.

> These are two different screens on the dashboard that I can switch
> between with a button on the steering wheel. The computer screen is
> right next to the speedometer, so I can watch the speed (even without
> cruise control) and fuel consumption simultaneously.  In principle, I
> think there is another screen that reports your running average speed,
> but I don't think I used it for this purpose.
>
> I don't know what you mean by "resetting" the computer. I assume you
> reset the "trip distance" counter. I don't even need it to watch the
> fuel consumption numbers.

one  of the fuel consumption parameters the Prius gives (and it also 
existed in the renault megan i had on year 2000) is the "average fuel 
consumption since the last reset" - and you can manually reset this 
counter whenever you want - so it allows you to reset the counter, 
perform a drive of any distance you wish (1km or a million km - doesn't 
manner) - and get the actually (computed, not guesstimated) fuel 
consumption you had across the entire drive. to me - this is the *only* 
number that counts, since the other numbers are not steady enough across 
a long drive.

>
> It sounds like you took a trip with a full tank, guesstimated your
> average velocity, and topped the tank again to see how much fuel you
> spent. If I misread, sorry. If this is roughly what you did, then I am
> sorry to say I am not particularly impressed with the methodology (I
> realize this is the only thing you may be able to do - no offence meant
> at all - it is better than nothing). It cannot possibly be close in
> precision or reliability to direct observation of km/l or l/km.

as i said - i let the car's computer (together with the resetting i 
mentioned). since i also have cruise control in the car, i can also 
assure a fixed speed across a given distance.

and by the way, this fixed speed does NOT generate fixed fuel 
consumption across a long drive. very tiny changes in the road's slope 
(even on what superficially appear to be a flat road) bring it up and 
down quite dramatically.

in fact, driving without cruise control and adjusting the speed to the 
changing road conditions allows you better fuel consumption then using 
cruise control. since the Prius's speed meter is digital rather then 
analog, you can see exactly how a change of speed of even 1km/h 
sometimes has a dramatic effect on the fuel consumption. even more - 
sometimes keeping the same speed but slightly changing the pressure 
level on the accelerator - can change the fuel consumption considerably.

>
> I hope the above gives you a good idea how I know.  This is the least
> "theoretical" approach mentioned so far. All my "occasional
> observations" disclaimers mean that I didn't obsessively do it over
> dozens of trips, write down the numbers, run F-tests or whatever...

my observations showed me that what i guessed to be the fuel consumption 
based on watching the "current consumption number", and what i actually 
used across a 5 minutes period, or across a distance, can be completely 
different number, and thus the former is not a useable measure if you 
want to know how much fuel you've eventually used across a distance.

--guy