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1994-2002 [Subscribe to Daily Digest] |
...for a given temperature. So from purely an aerodynamics standpoint, you should actually get better mileage when it's humid versus dry.
However, as Dean mentioned, wind changes everything. So does temperature variation. It's generally (except in Houston) colder when stormy. Colder air is denser. It may be cold enough to offset the decrease in air density from the humidity effect. I don't have my thermo book handy to check :-)
Also, I'm not sure how actual raindrops would affect the aerodynamics, as opposed to saturated air without rain. That could be a complicated problem, also a function of the wind.
I don't know much about changes in rolling friction due to wet surfaces. More slipping = more wasted energy? I'm not sure you necessarily have more slipping though.
The way I understand it, T7 (which your '02 car has) uses an air mass flow sensor to control the drive-by-wire torque output. Your throttle response (and especially turbo lag) will be adversely affected by thin air (takes longer to suck in necessary air mass to get desired torque), but I think it should still be able to adjust to give the proper air/fuel mixture. I believe there's an O2 sensor, or other means by which to read the relative humidity, and adjust for it. You should burn the same amount of air/fuel at a given torque level regardless of the ambient air conditions.
However, if there wasn't a way to adjust for relative humidity, then the computer would think it had more oxygen in a given air mass than it really did (since some of that mass is water). Your injectors would then spit out more fuel than could be combusted. Less fuel would be combusted, and less torque generated, than the computer expected. If there's a feedback loop on the torque output (is there?), then the computer would respond by sucking in more air/dumping out more fuel, and your fuel economy would suffer. So I guess it's conceivably possible, but I really doubt that the combustion is running open loop like that. Dean, please correct me if I'm wrong.
I see the biggest variable as the SID. Remember it's a running average, and not an instantaneous measurement. For a real test using the SID as your gauge, you'd have to drive the exact same route, at the exact same speed, under two different conditions, resetting the SID at the beginning of each journey.
Tire pressures, potentially due to temperature variations, are a big suspect, as Dean also mentioned. Or maybe you just drive more aggressively in the rain? :-)
Interesting problem,
'Roo
'01 9-3SE
'89 900S
posted by 130.76.6...
, Thu, 13 Mar 2003 15:08:36
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