I’ve been thinking about smoke lately and think that it is not something inherent in the 2 stroke engine, but just the way it passes thru the engine.
My thoughts are that fully combusted oil burns cleanly The key reasoning to this was an observation at Orbital many years ago. This was based on the smoke generation, evident in the exhaust, of the original “rotary” style Orbital engine. What we did to understand this a bit better was to feed raw oil into the intake of a 4 cyl carbureted engine we also had on test at the time. No smoke.
Also, do we see smoke from a racing 2 stroke under full power down a straight. No. This one’s a bit subjective, however we do see clouds of smoke when a 2 stroke bike or kart starts up.
My conclusion is that the smoke we see is due to oil being not fully combusted, mainly as a result of short circuiting out the exhaust (and not following the scavenging path) and then being only partially oxidized by the heat of the exhaust. I’m sure everyone at some stage has poured oil onto an open fire and watched & smelt the smoke.
So, the trick is than to ensure that no oil can readily short circuit out the exhaust. One thing we did at Orbital on the 3 cyl DI 2 stroke auto engine was to incorporate a crankcase drain system. This essentially was a small check valve at the bottom of each crankcase, this feeding through a small passage into the intake (upstream of the reed valve) of the adjacent “leading” cylinder. I guess this had some capacitance such that any accumulated drained oil took a little while to get into the system. Net result was very little or no smoke or even smell.
I think we’ve all seen or been behind a diesel that is badly tuned/overfuelling/dirty injectors or whatever, the smoke can be obnoxious. Obviously incomplete combustion.
"Success is the ability to go from one failure to another with no loss of enthusiasm.”
Bookmarks