Recently I received an email containing the following sentence:
“I am light on the accelerator both under acceleration and at cruise speeds (although I did take note of your optimum rpm/acceleration/fuel economy discussion in the December '01 newsletter suggesting better fuel economy with higher accelerator settings and lower rpms - very interesting).”
This comment urged me to confirm my experimental conclusions of December 2001, now that I have equipment that will make it easier to confirm the conclusions. The recent purchase of a Scangauge has made it quick to get rough estimates of experimental results that normally took me long drives or complicated calculations to determine.
With the Scangauge set on trip mpg operation, I ran numerous acceleration tests of 0-50mph over a couple of different test tracks. The first tests were done on a ½ mile stretch of fairly flat and straight road. After doing a number of test runs I decided I needed a longer test track so I could test really slow accelerations. I chose a straight flat stretch of .75 miles. The hypothesis of the experiment was that better mileage would be achieved when acceleration was done under conditions of low rpms and high throttle settings, because under lower rpms there is more time for the fuel to completely burn, and the higher throttle setting gets more air and fuel into the cylinder making the molecules more crowded during compression and combustion, causing better reaction.
Test procedure was to line up at the starting line at idle and dead stop. Then the reset button on the Scangauge was pushed as I accelerated from dead stop to 50 mph. When 50 mph was reached the throttle was set to maintain the speed until the finish line was reached. As I crossed the finish line I read the Scangauge’s trip mpg reading. I recorded the acceleration manifold pressure, the acceleration rpms, and the trip mpg. Thie data was arranged in a table with manifold pressure on the left axis, and the rpm range during acceleration on the bottom axis. The trip mpg readings were recorded in the appropriate position on the chart that described the acceleration conditions.
Two things were concluded from the results of this data:
This leads to a few other conclusions:
1. The method of acceleration is not nearly as important in high mileage driving as other factors.
I find it important to note here that the Scangauge does not calculate actual miles per gallon. The Scangauge does a calculation based on the assumption that the air/fuel mixture is always the 14.7 to 1 goal of closed loop operation. This is fairly accurate for a warmed up engine under normal loads, however during warm-up and during full throttle operation the air/fuel mixture is richer than 14.7 to 1. This is why I intentionally disregarded the full throttle accelerations that were done. These full throttle accelerations indicated higher mpgs on the Scangauge when in fact lower mpgs were experienced. I don’t know what manifold pressure triggers an ECU command for a default rich mixture for wide open throttle, to prevent extreme exhaust gas temperatures from burning exhaust valves. I believe the highest throttle setting used in the chart mentioned above, only 2 in. Hg on my vacuum gauge measuring manifold vacuum/absolute pressure, was within the range of the 14.7 to 1 fuel ratio operation.
Since I concluded that acceleration technique was not as important as other driving techniques, I also did a few tests to examine deceleration techniques. This was actually quite simple to analyze. My test procedure was to hit the starting line at 50 mph while resetting the Scangauge in trip mpg operation. One test was done at 50 mph cruise until just before I was ready to turn onto a side street just past the finish line of the test. Moderate braking brought me down to a safe speed for making the turn. This was what I would consider normal driving technique for most drivers. For this test the Scangauge recorded 50 mpg.
The next test run procedure was to hit the starting line at 50 mph and to coast in neutral with the engine running until I got to the spot where I needed to brake for the turn (actually there was no need for braking because I can turn the corner at 25 mpg). The Scangauge recorded a trip mpg reading of over 100 mpg.
To round out the deceleration tests I did a final run over the same course staring at 50 mph and coasting with the engine off. The Scangauge recorded a trip mpg of 565 mpg but that is not actually correct, since no fuel was used the actual miles per gallon was infinite.
Conclusions that can be drawn for these tests are the following:
25 mpg accelerating over .75 miles
50 mpg cruising at 50 mph
43.24 mpg to accelerate to 50 mph then coast to 25 mph at idle 1.5 miles total
50 mpg to accelerate to 50 mph then coast to 25 mph with engine off, average speed 43.75 mph
cruising at 43.75 mph should achieve 55 mpg
note: cruising at 30 mpg achieves 60 mpg, according to the Scangauge.
Lessons to be learned from these tests: accelerate moderately under low rpms and high throttle settings. Accelerating any slower than this only backs up traffic and is unwarranted. When cruising leave plenty of room between your vehicle and the vehicle in front of you, so that if he stops to turn left you have plenty of room to coast and not have to use your brakes. When going through the city with traffic lights, try to time your lights so you never have to use your brakes. You do this by adjusting your speed by coasting and accelerating. Turn off your engine when you don’t need it, especially at a traffic light. My vehicle uses 0.25 gallons (about one quart) of fuel per hour idling when fully warmed up (0.3 to 0.4 gph when warming up). For those of you who see stop and go bumper to bumper traffic daily, learn that idling in first gear is a lot more efficient than jerking ahead and stopping behind the car in front of you. Give the car in front some distance and idle in first gear (3-5 mph or whatever speed it takes to not have to stop). Coast to a stop whenever doing so does not hamper traffic. Save your brakes for emergencies.
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