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Old 02-10-2008, 11:28 PM   #1 (permalink)
EcoModding Lurker
Join Date: Jan 2008
Location: Sydney, Australia
Posts: 76

Piaggio Fly 150 - '10 Piaggio Fly 150
90 day: 64.8 mpg (US)
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Engine-off experiment

I've seen the experiment that measured the additional fuel cars use when starting which concluded that 0.2 seconds is all it takes for it to be worth it to turn off your engine. However that experiment didn't take into account the alternator load which I think is the most important factor for this to be worth while. Personally, I have found that I get worse FE when turning off the engine whenever I can compared to using engine-breaking in high gear so the fuel injectors shut out. I am curious about how this time may vary for different cars. My experiment idea is this:

With the car stationary in neutral and close to empty run the motor on idle until the fuel light comes on, then turn off the car. Add a 5L container of petrol to the car (with exact weight measured on a scale for comparison) then run the car till the light turns on again and measure the time it takes to do so. That is the baseline test. The next test uses the same procedure except having the car on for two minutes then off for thirty seconds repeated until the light comes on. The next test is the same but with the off period reduced to 15 seconds off and 2 minutes 15 seconds on. Then the next with 7 seconds on 2 mins 23 secs off. Finally the baseline test is repeated. The time to the fuel light is compared to see which method is the most FE. I might also check the battery voltage at the start and finish of each test to see if there is a drop with all the starting and stopping. I figure for 25L ($30) I will have a pretty good idea of how long you need to turn the car off for. Am I right in thinking that with the car stationary the light will come on at the same fuel level every time? Can you see any other holes in the method?

I wanted to make the test simple and cheap so that different people can do the same test to confirm the results in different cars/ environmental conditions. Would anyone else be willing to do this experiment?

Edit: Oh I forgot, the time between runs would be some constant, say 3 mins so that the starting temperature was the same each time too...

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