quote:
Originally posted by Jason Troxell:
No modern fuel injected vehicle should have fuel dilution, ever.
Here's the issue. Most people, including myself at one time, think fuel dilution is inevitable with carbureted engines but modern FI engines virtually eliminated this problem because of their precise metering capability.
But here is the reality. When fuel is injected from a fuel injector it "wets" on the intake port, intake valve, combustion chamber, piston top, and cylinder walls. The amount of fuel that evaporates off these surfaces will of course vary with the temperature of these components along with other variables. As an example, even if these components are at operating temperature, if the engine is operating at mid-high RPM there's less time for the evaporation to take place. Now granted, the amount of fuel that doesn't evaporate and get burned, or get flushed out of the cylinder on the exhaust stroke is small, but it persist none the less. Here are some rough numbers to quantify things.
Amount of gasoline used
3000 miles / 20 mpg = 150 gals
5% fuel wetting
150 gals x .05 = 7.5 gals = 960 oz
1% of wetted fuel makes it way pasts the rings
960 x .01 = 9.6 oz
Sump capacity
5 qts x 32 = 160 oz
Fuel dilution with no evaporation
9.6 / 160 = .06 = 6%
Now the reason we don't see that kind of fuel dilution in the oil is because most of it evaporates out of the oil, especially if the oil gets to operating temperatures on a regular basis. (As an experiment, mix a 50/50 solution of oil and gas in a cup and let it sit in your garage for 3 months. Even when not at elevated temperatures most of the gas will evaporate out of the oil.)
How are carburetors different? Well the fuel is atomized in the venturis and a lot of the wetting will take place in the manifold itself and will enter the engine uncontrollably, thus the difficulty meeting tight emission standards. This is where fuel injection shines. Even though fuel wetting still takes place, it takes place in a different place and in a predictable manner. (Carburetors may have other problems during idle or leaks, etc, but we're ignoring these for the sake of the discussion)
Just from a logical point of view, it's one of the reasons why emission controlled engines need catalytic converters, to fully "burn" all of the incomplete combustion products, including the vaporized "wetted" fuel.
And Jason, this isn't BITOG arm chair engineering. I do this for a living.
