Originally Posted by Chevvron
Surely a diesel Wankel would get round the lubrication, economy and emissions problems?
Fraid not, Chevvron. In a 4-stroke an oil film adheres to the cylinder wall, and actually survives combustion - although this is how the diesel combustion particulates get into the oil. The inlet/exhaust flow doesn't interfere with this oil film, so remains clean. In either 2-stroke or rotary the oil film is constantly swept into the ports, so either escapes directly in the exhaust or becomes a product of incomplete combustion.
That said emissions on aircraft are nowhere near as strict as autos - i'm not sure what the exact requirements are. The problem with rotary diesel is mainly a maximum 12:1 compression ratio, limiting autoignition - good injection mixing is also harder to arrange.
Originally Posted by Dave Jackson
... fuel consumption of a Wankel varies greatly over the normal automobile operating range but the fuel consumption is the same as a reciprocating engine at its optimal rpm.
Compared to an avgas Lycoming i can believe it, Dave - Low RPM gives more time for heat transfer into the cooling system. If a new engine project was being started specifically for a helicopter (ie high RPM) i think rotary would be in the concept investigation, but various factors would rule it out - total loss lubrication for the rotor seals would be my worry, since there are various ceramic coatings that can help with heat loss.
The VW soln for aluminium bores...
Mart