SuperShadow said:
SuperShadow, on 29 Mar 2013 - 13:12, said:
Okay guys, I didn't say it took 15 minutes for the vehicle to reach normal temps. I was just throwing out an arbitrary number to make the point that idling (even extended idling) or reaching normal temps would not create a 50% decrease in fuel economy.......period!
Wrong. Of course it does. Idling gives you zero miles per gallon, averaged into your trip. I've worked with trying to characterize and improve the factors in fuel economy for over 35 years, and idling and cold engine operation are by far the two most influential factors.
Example: My truck's thermostat is stuck open, and instead of the usual 3 miles, it takes over 5 miles to warm up. So on a 38-mile commute, 29 miles of which is highway driving, mileage went from 20 to 18.4 - an 8% drop.
Now look at the fact that my last car consistently got 34 mpg highway at 65 mph, and got 36 mpg at 55 mph. Commuting with 10% of the trip in cold engine mode gave me 36 mpg. Driving with less than 1% of cold operation (700 miles in one day, with about 3 very, very brief stops and essentially hot engine after the first 3 miles) gave me 40 mpg. Driving trips with 2% of cold operation gave me 38 mpg. It's very consistent and easy to prove with algebra that during cold operation, gas mileage is 1/2 that of warm operation. And it's verified by the instantaneous readings in my EVIC.
A/C compressors cut fuel economy by about 1-3% at most, especially today's more efficient models, such as those with scroll compressors.
The best fuel economy can be achieved by starting the car, and once the oil has filled the lifters and circulated enough to get to the cam (10-30 seconds), drive off moderately. That gives the fastest warmup. Idling the engine when cold will extend warmup time and give zero mpg for a longer period.