Gasoline Usage at Idle with and without Air Conditioner Running

I logged gasoline usage with my car running at idle.  I varied the RPM to see how that affected it.  I turned the air conditioner on and off to see how that affected it.

The car is my 2005 Mercedes CLK320A.  This has a 3.2L V6 gasoline engine.  It is supposed to use "premium" fuel (91 Octane in the US), but I have been unable to determine any performance difference using what is sold as "regular" fuel (87 Octane in the US) in San Diego, where I live.  So I run it using regular fuel, and the results here are with regular fuel.

My set-up for testing this stuff is described in this post.

For the test, I used a warmed-up engine.  I ran the engine in my driveway and applied the accelerator pedal to achieve different RPM levels.  The RPM levels and fuel usage rates shown are those logged by the setup described in the other post throug the car's OBD2 connector.

For "air conditioner off" condition, everything in the car is turned off including the climate control.  The car does have running lights that stay on in the daytime, these were on.  No attempt was made to determine the state of the alternator, whether it was charging the battery or not during this test.

For "air conditioner on" condition, the climate control in the car cabin was turned on.  The thermostatic control was set to 60° F in order to keep the air conditioner on constantly.  The fan blowing air in the cabin was set to its highest setting.

Results

Results are shown above.  Minimum mean square error line fit to the data are shown.  At lower RPM, results are pretty nice and linear.  

Idle Fuel Usage with Air Conditioner turned off

Without applying the accelerator, this engine idles at about 600 RPM.  The fuel usage at idle is about 0.25 gallons per hour, with the air conditioner turned off.  The idle fuel usage rises to about 1.0 gallons per hour at 2500 RPM.  The fitted line, with air conditioner turned off, nearly passes through 0 fuel usage at 0 RPM.  Thus for all intents and purposes, the idle fuel usage is:

(1 Gallon per Hour) * ( Engine RPM / 2500 )

What is the meaning of 0.25 gallons per hour of idling loss?  This is a car which gets about 22 miles per gallon averaged over my driving.  So an hour of idling uses the same amount of gasoline as driving about 5.5 miles.  At $4/gallon (as I write this prices in San Diego are as low as $2.40/gallon), idling costs $1/hour or 1.6 cents per minute.  My conclusion is that in regular use where idling more than a few minutes would be very unusual, idling is a very inexpensive process and not worth great efforts to avoid.  

Fuel Usage by Air Conditioner

The caption of the figure shows the linear fit equations for the fuel usage with air conditioner off and on.  The difference between these two linear fits is 0.16 gallons per hour.  This suggests that when the air conditioner is running, no matter what the RPM of the engine is, it is demanding fuel at a rate of 1 gallon every 6 hours.  

If the car gets 22 mpg with the air conditioner off, adding fuel flow of one additional gallon every 6 hours would correspond to moving the car at about 4 miles per hour.  Now this is with the air conditioner running constantly (it was set to 60° F for this test).  Since it cycles off and on in actual use, actual usage with air conditioner on will be less than this.  

At $4/gallon, air conditioner on constantly costs about 67 cents/hour.  Since in actual use air conditioner cycles on and off, a better rule of thumb might be 25 cents/hour.  

Gasoline Usage in your Car, the tools I use

You can learn a lot about gasoline usage in your car by, well, by logging gasoline usage in your car under a variety of conditions.  You can then analyze the logged results and figure out an astonishing amount of stuff!

In this post I just list the tools I use for getting results.  Following posts will show some of those results.

The Hardware

This is what I use.  It plugs into your OBD2 connector.  On my cars, this is located somewhere near where the hood release for the car is located, under the dashboard on the left side of the steering wheel.

The Software

Torque Pro is available for Apple and Android, I use the Android version.  Torque Pro has amazing real time gauges for displaying many things it can read from your car.  For this work, the real time displays are not so important.  What is important is the ability to log these data into spreadsheets, which spreadsheets can then be opened and analyzed later.  

After I have the spreadsheets of logged data from my driving runs, I write scripts in Matlab to do the analysis.  I have used Matlab for decades in my job.  It is a powerful and complicated tool.