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Old 08-07-2008, 10:49 AM   #11 (permalink)
Mechanical Engineer
Join Date: Jun 2008
Location: Richmond, VA
Posts: 190

The Truck - '02 Dodge Ram 1500 SLT Sport
90 day: 15.15 mpg (US)

The Van 2 - '06 Honda Odyssey EX
90 day: 22.63 mpg (US)

GoKart - '14 Hyundai Elantra GT base 6MT
90 day: 30.67 mpg (US)
Thanks: 0
Thanked 7 Times in 6 Posts

In multi-port fuel injected engines there is not a substantial amount of fuel sitting in the manifold. Port fuel injectors spray into the intake port onto the closed intake valve. The only way for this fuel to get into the manifold would be as it evaporates and then exhaust reverts out the intake valve at initial intake valve opening it may push some fuel vapor up the intake runner, but then the piston will draw this vapor back down the runner into the cylinder as the cylinder fills.

When an engine shuts down it doesn't instantly stop. It spins down to a stop over time as engines have substantial rotational inertia. When I EOC my truck I have to key-off for almost 2 seconds (from 2000 rpm or below) because after just 1 second if I key back on the engine still has enough speed to restart itself. At just 1000 rpm an engine rotates more than 16 times in one second meaning that each cylinder goes through at least 8 intake cycles. If you kill your ignition but not your fuel injection you will be injecting enough fuel onto your intake valve to be sucked into the cylinder for about 8 combustion cycles per cylinder that will not be burned before the engine stops. This fuel will wash oil off the cylinder walls, dilute the oil, and some of it will go out the exhaust as unburned HC and/or will collect in the catalytic converter where it will oxidize and heat up your cat. The fuel does not just "sit" in your intake manifold because your engine still pumps air as it is spinning down to a stop. The spinning doesn't stop instantly, so if you kill just your spark the fueling continues as the engine spinning continues even though you don't feel the jerkiness of sputtering combustion.

You are correct that any fuel in the oil will light off as the oil temperature heats up. This also takes time at temperature and if someone is frequently EOCing and minimizing engine load rather than P&G the oil may not get very hot and may not stay hot long enough to light off frequent additions of fuel from a power-on shutdown. Bottom line: it's fine if you want to cut the ignition to shut off your engine, but for pete's sake cut the fueling as well!

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