Short trips & bad mpg?

[the other pc]

Originally Posted by JohnnyDaJackal

.... As for where they drive it's mostly residential, with maybe 1-2 traffic lights/stop signs on their whole ride. ...

Probably a big chunk of it right there (in addition to all the stuff already mentioned).

1-2 traffic lights/stops doesn’t sound like much, but if the whole trip’s less than a mile with 2-3 total minutes those stops are a huge percentage of the overall trip.

Everybody knows that you get worst mileage when accelerating. To accelerate you must pour energy into the car’s motion. The energy of motion is called kinetic energy. It has to come from somewhere and that somewhere is the fuel. The fuel contains (chemical) potential energy until it is burned and converted to kinetic energy by the engine.

So more stops and starts means more accelerate/decelerate cycles and more fuel burned.

In a standard internal combustion powered car all that kinetic energy is simply thrown away in the form of heat whenever the car has to stop. The car’s brakes convert the kinetic energy directly into heat dissipated into the environment (which is actually a form of kinetic energy too, for the air molecules around the car).

Some electric and hybrid cars use regenerative braking, which converts the car’s kinetic energy back into potential energy (charging batteries) that’s stored for later re-use. A car with regenerative braking can get better mileage in town than on the highway.


pc.

[jDizzle]

and engines PCM operates on a seperate set of parameters when started up and when cold. this is called closed loop.. after the vehicle warms up it goes into open loop, which means the computer uses more sensors and uses the input to get u as close to ur stochiemetric air fuel mix (14.7:1 which would in a perfect world, give you perfect combustion)... but thats beside the point.. when your engine is cold, and you are in closed loop, you are running rich, as others have stated.. your idle is higher.. almost doubles in some cases. which it controlled by your IAC (idle air control... think it as the choke on a carbuetaror, kinda..) which allows your pcm to control how much air gets in when your throttle plate is closed ... .... when not at idle, in closed loop you run rich, your engine wants to get up to operating temperatures quickly so it can get youinto open loop and closer to your ideal air fuel mixture, and thats how it does it...engines are not build or designed to be as efficient when cold, theyre designed to operate effieicntly at operating temperature, so your engine wants to get there as quick as possible... im starting to prattle, so ill stop. ha hope it helps

[the_invisible]

My many years of experience suggests that short trips does equate less miles per gallon. However, with my newer cars I purchased in the last five years, the M45, G35, and 335i, the economy difference between short trips and long trips is virtually less than 5%. My result is by no means scientific, but I keep track of my fuel economy every time I fill up, and I do long highway trips two times a week, while the remainder of the week I just do grocery shopping.

[artm3]

short version:
fuel delivery is controlled in 2 ways:
by a MAP, the ECU provides preset amounts based on throttle/load/etc, when cold or full throttle
by the O2 sensor fine adjusting the MAP when the engine is warm

when cold or full throttle more fuel is needed, to keep it running stable or generate power without going lean...

when warm and cruising the O2 sensor adjusts the air/fuel ratio (MAP) for maximum efficiency...

in my experience it's significant:
my e46 M3 averaged 24 when commuting 40 miles on the HW
commuting 5 miles HW yeilded 18...>30% delta

my RS4
HW 22
short HW commute 17 or so

my 951
HW 24
short HW <20
the turbo improves efficiency and the control system is much older so less of an influence overall...