so i've been driving around with the "fuel economy" screen on just to see..

i noticed that when i'm idling the screen shows that i'm using max (15 L/100km)

is this right? do engines consume that much fuel when idling?

 

on the newer EFI engines studies have shown you actually save gas by shutting off the car if you are going to be idling for more than 30 seconds. i dont do it just because i feel like it puts extra stress on other parts of the car that arent worth it.

 

i thought shutting off your car and starting it up puts a lot of stress on the car?

 

I don't think it shows the max because you're using that much fuel but rather because you are getting 0 miles to the gallon. At idle, the car is not going anywhere but still consuming fuel. Thus, 0 miles to the gallon and the worst possible fuel economy.

 

Yea that's why I said I didn't do it, wasn't worth the stress

 

ya that makes sense.

but how much fuel are we actually consuming when idling? say you're idling for 10min.. would you be using less, more or the same as 10min of driving on the highway?

 

lol, sorry..

 

i have heard that 10 min idling is a cup of gas, seems right but don't know if its accurate

 

If you're at idle RPM, much less than the highway. Fuel use is based on engine load and airflow. Idle is as low as you can get on both of those metrics (with the engine running).

 

Pretty simple math would be, the higher your RPM's are the more fuel you are burning. The only reason people tether hwy mpg to be better is because there is no stop and go traffic and you tend to stay at the same speed. If your engine is idling that is probably the most efficient fuel consumption available.

 

Not quite. If you are coasting, off throttle, and in gear while rolling down grade on the highway, your ECU will shut down your fuel injectors and you will essentially have infinite mpg; kind of the opposite scenario as idling at a stop.

 

lol I love fooling the computer with this.

 

For some reason I get bad mpg when I'm at a stop and revving my motor... :s

 

That's not necessarily true. You're assuming volumetric efficiency is a linear scale. It's more logarithmic and doesn't always rise with RPM (or load).

No, they don't. That would require complete engine braking and the transmission running the engine complete, let me know how that works on an automatic... Put an OBD2 scanner on your car and watch your fuel injector pulsewidths. They'll be near idle, but not 0. 1.9-2.0 ms on my car, with DFCO enabled (Dynamic Fuel Cut Off).

 

omg thats too technical for me to understand, but i think you are correct
i think what you are saying possibly relates to when I am flooring it in 4th gear at 26mph but the rpm-meter is only at 2 rpms? so im getting terrible MPG but im still in a low RPM