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Gas Use at Idle

9741 Views 38 Replies 15 Participants Last post by  james
I've recently done an experiment to determine Insight gas use at idle. (This is useful for a number of reasons, including setting a baseline that can be checked against as the car ages.)

I performed the test twice, on different days, with the car completely warmed up and AC off. Test results were 0.099 and 0.104 gallons per hour. (0.1013 +/- .0024 gallons per hour)

I would be interested to know if this is fairly typical.

Just for reference, the car is a 2003 with AC and had about 7,000 miles on it when I did the test.

Does anyone know if the Insight runs in lean-burn or in balanced stoichiometric conditions at idle?

I plan to repeat the test with the car warm and AC on (which will be easy), and with the car cold and AC off (which will be more difficult).

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Are you saying the Insight uses gas while auto-stopped/

I'm confused. Are you saying the Insight uses gas while auto-stopped at a red light?

It feels like the engine is off.
My 2000 Insight's internal combustion engine is stopped, consuming no gas, when not moving and in neutral.

When coasting and in neutral, without the A/C on, it idles in lean-burn mode. When the A/C compressor kicks in it goes into rich-burn and I can see the instantaneous fuel consumption go up (instantaneous MPG drops, by more than 50%).

How are you determining fuel consumption? If you can get it into idle-stop at the end of your test (during which time it hasn't moved) and then push it around while keeping it idle-stopped, you can use the resulting mileage to find out how many gallons of gas you have consumed. Maybe this will help with your cold idle test. Or just come out here next winter and run your tests in the @#*$%&ing snow.

When the car is stopped and the engine is running, that condition is generally referred to as "idle".

To perform the idle test, drive until the engine is warm, stop the car, but leave the engine running.

Record the distance driven and check the mileage ("double-click" the segment button as you start a stop watch). This is the base mpg.

After a period of time (say, 5 minutes), double-click the segment button again and stop the stopwatch. Record both the time from the stopwatch and the mpg (the mpg doesn't seem to update at one-minute intervals while at idle).

From the base numbers, you can calculate the total number of gallons used before the start of the test: (e.g. 18.7 miles/53.8 mpg = 0.34758 gal).

Calculate the number of gallons used at the end of the test the same way: (e.g. 18.7 miles/52.6 mpg = 0.3555 gal).

Subtract the 1st number from the second number to get the quantity of gas used while idling (0.3555 - 0.34758 = .0079 gal).

Divide the number of gallons used at idle by the time from the stop watch to get the gallons per minute, then multiply by 60 (minutes/hour) to get the gallons per hour used at idle (.0079/5 *60 = .095 gal/hour).

The longer the test, the more accurate the results.

If you repeat the test with the AC (or maybe your 1000 watt amp) on, you can see how that affects gas mileage.
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Your method shows your thoughtfulness, though I suspect you are assuming a level of accuracy far exceeding that which the instruments you are using can actually provide. I've never seen an even digit show up in the tenths place in the mpg reading (carefully avoiding 66.6mpg, for those religiously inclined), so I assume the instrument itself only displays mpg to the nearest 0.2mpg.

This means you should calculate your margin of error by running all your calculations repeatedly with .01mpg more and less than your readout. Even this assumes that the instruments are accurate to 0.2mpg, just because they display that accuracy. We don't know this.

It is impressive if idle actually consumes roughly 1/10th the fuel of pushing the car down the highway at 70mph, especially since the idle speed is barely less than half the engine speed in 5th gear at 70mph.

If your numbers are accurate, then that explains why Honda tends to explain the autostop as less of a fuel saving technique than as an anti-pollution technique, since the area around an intersection is constantly bombarded by spew from idling engines, unless the cars have auto-stop. Compare that to 1/10th of a gallon per hour consumed while the auto-stop is on. That's not a lot of fuel saved.
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I agree about the level of accuracy - I was going to calculate the uncertainty, but I think that most people on this site are not so interested in these technical calculations.

A long test is better than a short test because it decreases the uncertainty, but I haven't done any longer than 5 minutes.

I performed the idle test again last night for 5 minutes, and calculated 0.095 gal/hour, so after 3 tests (all about 5 minutes long), the results are:

0.099 gallon/hour

The uncertainty may be +/- 8%, which would mean the last value, for example, should be 0.095 +/- .008.

All three of these tests were done with the engine warm (18 miles of driving) and AC off.

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" I've never seen an even digit show up in the tenths place in the mpg reading..."

I've seen even tenths, but I've never seen the reading change by just a single tenth. It's always 2/10 or 3/10. I'd assumed that it was actual recording everything in metric units, then doing a limited-precision conversion to mpg.
You may be on to something there. In the past, I'd never seen odd digits in the tenth's place, but just yesterday, I did. There seems to be certain numbers that just won't show, which fits your guess that it's a conversion from metric, which Japan uses, as does most of the world. In fact, the legally defined official American measurement system is metric, though we only use it for things that we buy from other countries...
So for best accuracy, Randy should switch his display to km, use those numbers to compute liters/hour, and convert that back to gallons/hour.
james said:
So for best accuracy, Randy should switch his display to km, use those numbers to compute liters/hour, and convert that back to gallons/hour.
If I remember correctly (I'll have to check when I go home tonight), the liter/100 km display only has one decimal place. If so, the accuracy is far less than the mpg display. For the typical fuel efficiency range of 60 mpg, the equivalent metric and US values are displayed below:

l/100 km = mpg
3.4 = 69.2
3.5 = 67.2
3.6 = 65.3
3.7 = 63.6
3.8 = 61.9
3.9 = 60.3
4.0 = 58.8
4.1 = 57.4
4.2 = 56.0
4.3 = 54.7
4.4 = 53.5

Note that the display resolution at 60 mpg, when using the metric indicator, is +/- 1.5 mpg. If using the US indicator, the display resolution is 0.2 or 0.3 mpg. At higher mpg, the resolution of the metric display is even worse:

l/100 km = mpg
1.0 = 235.2
1.1 = 213.8
1.2 = 196.0

If the metric display goes to 2 decimal places, than the metric and US resolution are approximately equivalent.

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Gas Use with AC On

Last weekend I did the idle test with AC on (60° and fan on full).

I performed the test for 5 minutes with the following results: Total gas use at idle with AC on = 0.229 gallon/hour. Subtracting 0.99 gallon per hour for gas use at idle with AC off means that the AC uses about 0.13 gallons per hour (at full cooling and fan power).

From this, you can calculate the effect of AC on fuel mileage at typical driving speeds.

You're assuming that the A/C consumption of fuel is linear; I would wager that it consumes less fuel when the car is cruising along.

I confirmed your estimates of fuel consumption rate at idle by observing the instantaneous MPG readout while coasting along, car in neutral. At 20mph (+/- 0.5mph) I was getting 120mpg (+/- 2.5mpg). 20mph/120mpg = 0.17 gal/hr, a little higher than what you had (twice) but it's in the ballpark.
I wonder if the different results are due to the different methodologies or are due to differences between the two vehicles? I will try your method if I can find a fairly flat, straight road on which to run the test - I live in the foothills west of Denver and there's nothing like that on my regular commute.

Did you have AC on or off when you did the test? (I'm assuming it was off).

If you get a chance to run the idle test while parked, I would be interested to see the results.

Humm... You could test it on a downhill. Put it in neutral, so the engine idles instead of going into fuel cutoff, hold constant speed for a bit, and look at the mpg readout.
Randy said:
Did you have AC on or off when you did the test?
It was off.

I will try to remember doing your test and report back results... my "test" was more a spur of the moment kind of thing.
amplifiedmouse said:
this thread makes my head go 'ow'
amplifiedmouse, I'm sorry about your head. Don't you find this interesting?

Some science is just done for the pure enjoyment of it. This, at least, has a practical value.

Before I bought the Insight, I assumed that the best fuel economy would occur when travelling slow, say 15 to 20 mph, and thought that slow acceleration was the Way to fuel economy Nirvana. Now I see that I was on the wrong path.

But different people have different opinions regarding the secrets to best fuel economy. I would like to develop equations that would indicate how to alter driving habits to optimize efficiency. Understanding fuel economy at idle is the first step, and understanding the effect of AC could be a second step.

There are still 10 steps to go.

Keep it up Randy. I'm all for learning new secrets on how to get more mpg. I'm kind of an extremist and will try almost anything if I think it will get me more mpg. My personal best one tank mpg was 91. I would like to know the secrets of other drivers that can get mpg in the 90's or 100's on a regular basis.
It's amazing to me that someone can maintain over 90 mpg for an entire tank (or longer). My best one tank mileage is 74.9, and my lifetime average is 66.7 (5 speed with 8,000 miles).

I tried Tim's "idle while coasting" test several times last night on the way home. With the car in neutral, engine running, AC off, at 20 mph, the instantaneous indicator was still at 150 mpg (which really means 150 or higher). It did not drop below 150 until somewhere between 19 and 17 mph, say 18 mph on average.

19 mph/150 mpg = 0.127 gallon per hour
18 mph/150 mpg = 0.120 gallon per hour
17 mph/150 mpg = 0.113 gallon per hour

The idle test with the car stopped resulted in an estimate of 0.095 to 0.104 gallon per hour, so the rolling test result was about 20 percent higher than the stopped test. This could be partly due to the length of the test. A longer rolling test might give the engine more time to reach some steady-state condition.

The rolling test does have an advantage over the stopped test; with the stopped test you are using gas without adding miles, which means that you're lowering your Lmpg by a tiny amount.

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Try your test with the headlights and all other electrical items off. I have a 1970 Subaru 360 that has a top speed of 73. If I turn on the lights, Its 72, if I turn on the brights, its 71. If you reduce the electrical load it might reduce the fuel consumption. I know you get slightly better mpg with the lights off. Louis
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