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Old 12-21-2012, 09:52 PM   #3206
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Originally Posted by def View Post

I would surmise that manifold pressure plays more than a minor factor. Fuel RVP is a critical factor in fuel quality and combustibility.
Here's some more info on fuel vaporization.

The thing is, in the intake tract, gasoline turns to vapor primarily via two processes. In one process the liquid gasoline is heated and turns to vapor from being in direct contact with the intake port wall. In the other process the liquid gasoline vaporizes due to the low pressure of the intake port during the intake stroke.

There are some reasons why 'heat vaporization' plays a more significant role than 'low pressure vaporization' in vaporizing the liquid gasoline.

1. 'Heat vaporization' happens during the entire 720 of the combustion cycle. That is to say, liquid gasoline vaporizes during the whole time it's in contact with the intake port wall. I mean, once all the liquid gasoline 'boils off into a vapor' the heat vaporization process is complete of course. But as long as liquid gasoline is present on the intake port wall the process happens continuously.

2. 'Heat vaporization' is permanent. Meaning that the heat-vaporized fuel doesn't re-condense to a liquid at some point further along in the 720 combustion cycle.

3. 'Low pressure vaporization' is semi-permanent say, and primarily for the two reasons described in item 3 and 4. On an Independent Throttle Body motor, low pressure vaporization only happens during a fraction of the 720 combustion cycle. Specifically, it only occurs during the period of time the intake port is at a pressure low enough that the gasoline can no longer remain a liquid. In practical terms, that's about 120 of crankshaft revolution, less than half a turn. As compared to the two revolutions (720) of the full combustion cycle.

4. 'Low pressure vaporization' is in some ways a temporary phenomenon. Some of the liquid fuel vaporizes during the low-pressure period of the intake stroke, only to re-condense into a liquid when the manifold pressure returns to the local barometric pressure. As mentioned in item 2 above, heat vaporization is more 'permanent'. In that once the fuel is heated to the point of vaporization it stays vaporized.

Originally Posted by def View Post

Low RVP fuels don't do well in piston engines especially those with high CRs.
Diesel fuel is a very low 'vaporization pressure' (RVP) fuel, much lower than gasoline. The super low RVP of diesel fuel operates in the high compression ratio (CR) of a compression-ignition (aka diesel) motor.

I'm not trying to point out an exception. Only that the law-mandated increases in the vaporization pressure of liquid gasoline (done by oil refiners during the higher temperatures and higher barometric pressures of summer months to reduce fuel evaporation emissions) has less of an effect on the combustion process than a host of other factors.

Which is a good thing if you think about it, because the end user has no control over the refining process.


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