Though I doubt anyone reads this blog so carefully (hidden assumption: anyone reads this blog at all), it's possible that someone could have wondered, reading earlier posts, how I estimated the idling fuel consumption of my Grand Cherokee. When coasting in neutral down a mild grade, the car will sometimes reach an equilibrium speed. When this happens for long enough for the instant gas mileage reading on the display to stabilize, the speed in miles per hour divided by the mileage in miles per gallon gives the consumption in gallons per hour. I think this is reasonably accurate since the car is only burning fuel to maintain idling r.p.m. (650).
The number is typically pretty close to 0.38 gallons per hour. However, coasting down the typical grades where I check that, I've noticed lately that I'm burning closer to 0.48 gallons per hour. Now, although my most recent fill-up indicated an improved mileage of 22.1, my previous post was pretty much of a rant about my declining gas mileage of late. Is the increase in idling fuel consumption indicative of a systemic problem that could cause my overall decrease in efficiency? Perhaps it's a symptom of dirty injectors or a clogged air filter.
But another possibility is air conditioning. I try to minimize its use, but it's been quite hot in Southern California and I've had it on quite a bit of late. Could it be that this is the difference I'm seeing? If so, it would indicate that the air conditioner uses about 0.1 gallons per hour when it's on. It certainly seems reasonable that air conditioning fuel use would be a constant function of time and independent of vehicle speed, therefore, the faster I'm going, the less deleterious the effect would be on my fuel consumption per hour.
Let's see if this makes any sense at all. A gallon of gasoline contains heat energy of about 1.25*10^8 joules. Thus, 0.1 gallons per hour would be 1.25*10^7 joules per hour, or about 3500 watts. But the internal combustion engine wastes most of that heat energy - something like 75%. So that means only about 25%, or 875 watts (a tad over 1 horsepower) would be available as mechanical energy to cool the air. This squares niceley with my "gut feeling" about air conditioning.
If it is the air conditioner, I can calculate the effect of a.c. on my gas mileage at , say, 55 miles per hour. Let's see what we find: when driving at 55 m.p.h. on stretches of freeway that appear to be level (i.e., 0% grade) I typically see the instant mileage display oscillate between 31 and 32 miles per gallon. Let's say it's 31.5 m.p.g. This indicates I'm burning 1.75 gallons per hour. But if the air conditioner then burns an additional 0.1 gallons per hour for a total of 1.85 gallons per hour, it should reduce my gas mileage to about 29.7 m.p.g.
This number should be quite noticeable and I don't notice it. It could mean that the 0.1 gallons per hour useage noted above comes from some other cause (e.g., the clogged injectors or air filter) or it could mean that the fuel consumption of the air conditioner is not merely a constant function of time. Or finally, it could be an indication of the error in my determination of the relevant numbers while also performing driving tasks (tuning the radio, answering the cell phone, checking my schedule on the pda, etc.).
I guess I need to get a tune up, see about getting the injectors cleaned, and change the air filter. Then it would be reasonable to check the numbers, followed by a day trip out to the desert somewhere to do some experimentation under more controlled conditions. Stay tuned...