Oil fill cap with air/oil separator

1. An oil fill cap for an internal combustion engine, said cap comprising a cap body for mounting to said engine for receiving an air/oil mixture therefrom, said cap body defining a plurality of passages therethrough, including an upstream passage and first and second downstream passages meeting at a separation junction, said cap body defining a separation zone at said separation junction and receiving said air/oil mixture from said engine through said upstream passage and sending separated air to said first downstream passage and sending separated oil to said second downstream passage, said first downstream passage directing said separated air to a first exit port from said cap body, said second downstream passage directing said separated oil to a second exit port from said cap body, wherein fill oil passes into said upstream passage without passing through said separation zone, and wherein said fill oil flows into said upstream passage in the opposite direction of flow of said air/oil mixture in said upstream passage, wherein said cap body has upper and lower necks, said lower neck being a hollow tubular member having inner and outer surfaces, said inner surface defining said upstream passage, said upper neck being a hollow tubular member having inner and outer surfaces, said inner surface of said upper neck defining a fill passage communicating with said upstream passage through said lower neck to provide an oil fill for said engine, said upper neck being closeable by a second cap, said fill passage communicating with said upstream passage without passing through said separation zone, said fill passage supplying said fill oil to said upstream passage in the opposite flow direction of said air/oil mixture in said upstream passage, wherein one of said inner and outer surfaces of said lower neck and one of said inner and outer surfaces of said upper neck are threaded, said lower neck being thread-mounted to said engine, said upper neck being thread-mounted to said second cap, wherein said outer surface of said lower neck and said inner surface of said upper neck are threaded.

2. An oil fill cap for an internal combustion engine, said cap comprising a cap body for mounting to said engine for receiving an air/oil mixture therefrom, said cap body defining a plurality of passages therethrough, including an upstream passage and first and second downstream passages meeting at a separation junction, said cap body defining a separation zone at said separation junction and receiving said air/oil mixture from said engine through said upstream passage and sending separated air to said first downstream passage and sending separated oil to said second downstream passage, said first downstream passage directing said separated air to a first exit port from said cap body, said second downstream passage directing said separated oil to a second exit port from said cap body, wherein fill oil passes into said upstream passage without passing through said separation zone, and wherein said fill oil flows into said upstream passage in the opposite direction of flow of said air/oil mixture in said upstream passage, wherein said separation zone comprises an impaction/separation zone comprising one or more nozzles accelerating said air/oil mixture against impaction media to separate said air from said oil, wherein each of said first and second downstream passages has first and second segments, said first segment of said first downstream passage being common with said first segment of said second downstream passage, said separated air diverging from said separated oil at a divergence junction of said first and second segments of said first downstream passage and said first and second segments of said second downstream passage, wherein said second segment of said second downstream passage as said second segment of said second downstream passage diverges from said divergence junction is gravitationally below said second segment of said first downstream passage as said second segment of said first downstream passage diverges from said divergence junction, wherein said cap body defines a collection reservoir along said second segment of said second downstream passage downstream of said divergence junction, and wherein said second exit port from said cap body is an oil drain from said collection reservoir, and comprising a baffle in said cap body along said second segment of said second downstream passage and downstream of said divergence junction and upstream of said collection reservoir and directing said separated oil to flow into said collection reservoir.

3. The oil fill cap according to claim 2 wherein said collection reservoir is gravitationally spaced below said divergence junction by a vertical gap, and said baffle spans laterally in said gap to dampen surges of oil in said collection reservoir towards said divergence junction.

4. An oil fill cap for an internal combustion engine, said cap comprising a cap body for mounting to said engine for receiving an air/oil mixture therefrom, said cap body defining a plurality of passages therethrough, including an upstream passage and first and second downstream passages meeting at a separation junction, said cap body defining a separation zone at said separation junction and receiving said air/oil mixture from said engine through said upstream passage and sending separated air to said first downstream passage and sending separated oil to said second downstream passage, said first downstream passage directing said separated air to a first exit port from said cap body, said second downstream passage directing said separated oil to a second exit port from said cap body, wherein fill oil passes into said upstream passage without passing through said separation zone, and wherein said fill oil flows into said upstream passage in the opposite direction of flow of said air/oil mixture in said upstream passage, wherein said separation zone comprises an impaction/separation zone comprising one or more nozzles accelerating said air/oil mixture against impaction media to separate said air from said oil, wherein each of said first and second downstream passages has first and second segments, said first segment of said first downstream passage being common with said first segment of said second downstream passage, said separated air diverging from said separated oil at a divergence junction of said first and second segments of said first downstream passage and said first and second segments of said second downstream passage, wherein said second segment of said second downstream passage as said second segment of said second downstream passage diverges from said divergence junction is gravitationally below said second segment of said first downstream passage as said second segment of said first downstream passage diverges from said divergence junction, wherein said cap body defines a collection reservoir along said second segment of said second downstream passage downstream of said divergence junction, and wherein said second exit port from said cap body is an oil drain from said collection reservoir, wherein said second segment of said second downstream passage includes a drain passage between said collection reservoir and said second exit port, and comprising a one-way check valve in said drain passage permitting oil drain flow from said collection reservoir to said second exit port and blocking reverse flow from said second exit port to said collection reservoir.

5. The oil fill cap according to claim 4 wherein said second exit port drains said separated oil to said upstream passage in said cap body.