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Old 08-09-2012, 12:02 PM   #48 (permalink)
EcoModding Lurker
Join Date: Dec 2011
Location: United States
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RebCivic - '12 Honda Civic EX
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Originally Posted by ecomodded;320787

Read more: Driving Tips to Save Gas - Memorial Day Weekend - Popular Mechanics

Driving Tips to Save Gas - Memorial Day Weekend - Popular Mechanics[/url]
Actually, this is the one I was trying to refute:
Coasting in Neutral or Gear to Save Gas - Coasting and Fuel Economy - Popular Mechanics

Here's a bit of it for those who hesitate to follow links:

I've replicated these conditions with instrumented cars, both with scan tools and with an oscilloscope, measuring the leads leading into the fuel injectors. The signal controlling the injector is a 12-volt square wave. It's pulse-width-modulated, varying from 5 percent or so at idle to around 80 percent or so at full throttle. The higher the percentage of on time to off time, the more fuel. There's one on pulse for every cylinder firing, so the consumption also varies with engine speed (rpm). All vehicles show a short pulse width at idle, regardless of whether they're sitting in traffic at a red light or coasting downhill—at idle—in neutral. (Actually, they use a fraction more fuel sitting in drive at a traffic light, because of the drag in the torque converter, but I digress).

Almost all vehicles show a pulse width of zero when coasting while in gear. Zero, as in there is no fuel injected at all. Yes, the engine is turning over, the pistons are going up and down, the water pump, alternator and a/c compressor are working, so technically you can say the engine is running, sort of. But it's not consuming any fuel. And that goes for automatic or manuals.

Okay, eventually, at the bottom of the hill or as you creep up to the traffic light, the engine finally will slow to idle rpm—at which point the fuel injection will wake up and start adding fuel to keep the engine from stalling. That usually starts at around 1000 rpm, and if you pay attention, you can sense when it's happening as the engine will rev up slightly. And that's when the scan tool or oscilloscope will show injector dwell rise from 0 to 5 to 10 percent. So you're actually wasting gas by putting your car into neutral.

I hear this argument as well: My car-mileage-information computer goes wild with increased mileage while coasting.

The algorithm the trip computer uses is not based on how much fuel is actually consumed, but on some calculated value based on airflow past the mass airflow sensor, manifold vacuum and engine rpm. And it's not accurate under these coasting conditions. That's why when we report fuel economy here at PM, we never just print the numbers we read off the trip computer's display: We use the gallons pumped into the tank divided by the mileage on the odometer—which we check against a handheld GPS.

I use a Scangauge II for a lot of diagnostics and general tinkering. It's a great tool—but I've learned not to trust the economy or gallons-used function too closely. That's why the Scangauge has a function that allows you to tell it what the engine-idle cutoff is for your particular car to get somewhere closer to the truth.
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