Fuel in the Oil

I’ve read a number of posts on a couple of Internet forums lately about fuel in the RX3’s crankcase oil.

I had never noticed a fuel smell from the oil before on any of the motorcycles I previously owned, so I asked Zongshen’s engineers about this phenomenon.   They explained to me that it is a normal occurrence.   In fact, they told me a fairly high percentage of fuel in the oil is expected as a normal part of the engine’s operation.

Here’s what’s going on:  In the early stages of the fuel injection cycle, some of the injected fuel does not atomize completely.  When that occurs, a small amount of raw fuel washes down around the rings and finds its way into the crankcase.   Ultimately, as the engine reaches operating temperature, most (but not all) of this fuel will vaporize and either be sucked back through the intake tract, or it will condense and go into the drain tube beneath the oil/air separator.

As a check on the guidance offered by Zongshen, I did a literature search on my own at the California State Polytechnic University (I teach in the College of Engineering there).   There’s not a lot of information on this issue, but what information there is supports what Zongshen tells us.   In particular, here’s the conclusion from a technical paper in which the researchers focused on this topic for both diesel and gasoline engines:

The obtained results are in agreement with the relevant published data for fuel-engine oil mixtures, where amounts of fuels in oils of up to 4 or 5% are foreseen as acceptable levels, while amounts of 7 to 10% of fuels in oils are considered as unacceptable levels.

Someone on one of the forums wondered aloud if my recent blog describing improved smoothness and performance with 20W-50 oil was related to any of the above.  Folks, it was not.   We’re not that devious…if we have a reason for stating something we won’t hide it from you.   Not surprisingly, my literature search indicated there is some viscosity reduction as a result of fuel in the engine oil.  This is acceptable within the fuel-in-the-oil levels indicated above for both 10W-40 and 20W-50 oil.  The literature indicates that a higher viscosity oil (the 20W-50) will have higher remaining viscosity than would a lower viscosity oil (the 10W-40) if fuel is present in the oil.  That’s entirely logical, but it wasn’t the reason I mentioned that my bike runs better with 20W-50 oil.  I mentioned that because, well, the bike runs better with the 20W-50.

The significant point from all of the above is that fuel in the oil is normal, as both Zongshen and the engineering literature tell us.