Slight drop in MPG

[sawood12]

ORIGINAL: Diver944

I'm no science geek but are we not forgetting that our cars have a simple air flow meter with a flap that measures air flow and adjusts fuel according to it's deflection. It has no clue about air density and when its at full deflection it will dump in the same amount of fuel regardless of how dense the air is.

This is right, the AFM measures air volume which is useless in terms of calculation of fuel requrements, but the mapping on the ECU will approximate fuel requirements from the volume of air measured by the AFM. Also the maps are adjusted according to the temp sampled upon startup. This is a very crude approximation of Mass Flow as the cars don't have the necessary sensors to measure the parameters required to directly calculate mass flow. So even though we have crude AFM's there is an attempt to approximate mass flow by the ECU. Obviously a MAF directly measures mass flow.

Again, i'm not sure how much the density of fuel reduces due to temp, but either way if the engine is demanding less air due to colder conditions then the demand on fuel delivery will be correspondingly less.

With regards RAM air. This is not really a realistic proposition on normal cars. F1 cars can only manage a small meagre RAM effect with hugely efficient intakes designed in wind tunnels. Aero engines only get a modest RAM effect at 500mph. The SR71 needed to be well into Mach 2 in order to get a RAM effect that was able to create meaningful compression of air - below Mach 2 it was running on afterburning turbojets before switching to RAMjets.

 

[944 LIL]

Oli

I've not started measuring mpg.

I'm thinking if I start now I'll be really pleased by mid summer []

 

[zcacogp]

OK, thinking back to some A-level Physics, I seem to recall that (P1*V1)/T1 = (P2*V2)/T2, where P=pressure, V=volume and T=temperature, for a given mass of gas. (Universal Gas Law)

Assuming that T is absolute (i.e. in Kelvin), and we assume that the air pressure is the same (big assumption, but necessary), then V1/T1 = V2/T2.

So, comparing the volumes at 0degrees C (273 kelvin) and 30degrees C (303 kelvin) - roughly comparing a cold winters day with a warm summers afternoon, you get the following:

V1/V2 = 273/303 = 0.9009

Therefore, the air will be appromitately 10% more dense on a winters day than on a summers day, requiring approximately 10% more fuel for a given volume.

This 10% is also funnily enough pretty much the amount of increase in fuel consumption we are all observing on a winters day. Now my maths isn't up to calculating the additional drag from an increase in fluid density (I'd have thought it was more to do with fluid viscosity, which is a different property, but I failed my degree in Mech Eng a long time ago now) but if that worked out to be around 10% as well then I wouldn't be at all surprised ...

Colin, yes, now is the time to start measuring!


Oli.

 

[RC18B 911 turbo]

On the subject of mpg I was off to take my Gran home earlier and put £15 in (cheap skate I know) and I used it all up before I even got out of Exeter, granted I did get caught up by every single light being on red, but surely it wouldn't use that much up, would it?

 

[Diver944]

£15 should be roughly 4 gallons at todays prices so even at the worst town driving consumption of around 15mpg in an S2 you should manage 60 miles or so

 

[RC18B 911 turbo]

I think I managed about 5 or 6 miles, although most of it was in 1st or 2 with the occasional chance to get into 4th when I was lucky, I'm thinking there maybe something wrong there then. What is really odd though is that once on the country road and later on the Motorway going back home, it hardly used any fuel up at all and would occasionally go back towards where it was after I filled up so I'm wondering if the float in the tank might be getting stuck or something like that.

 

[zcacogp]

Jon,

If you are thinking of measuring fuel levels using the fuel gauge then I'd be cautious - they are not very accurate, and pretty primitive (in terms of measuring a repeatable level.)

Float sticking / fuel sloshing around / thick (inaccurate) needle on small gauge / etc etc etc. I'd view it as a "guide" and nothing much more (i.e. when the needle is near the bottom you have less fuel than when it is nearer the top. And not much more sophisticated than that.)


Oli.