Propwash October 2019 web.pub
With the rich mixture, raw fuel droplets can quench the plug. The double plug design was the first try, but the titanium plug shield worked better. Placing the squish area on the exhaust side helps cool the hot side of the piston without blocking the boost port flow scavenging the combustion chamber. He feels that a plain dome with no squish band may work as well since scavenging seems to be more important than the squish effect. The next part in the series will explore the flow through the engine. I will examine intake, transfer, and exhaust design. The second picture shows what severe detonation can do. Note the signs of overheating on the exhaust side of the holed piston pictured. PROPWASH October 2019 7 Light Frosting from detonation on a K&B Keeley head copy Holed Piston from detonation Head with Plug Shield Double Plug Head All the above discussion has been for head designs that give the most power. Control line acrobatic engines have very different requirements. They need to maintain constant rpm to maintain constant speed under varying loads. That has been described as a 2-4-2 pattern where the engine alternates between firing every and firing every other revolution. To achieve this they run rich with the muffler or pipe design helping to maintain constant rpm. One Swiss engine builder has experimented with several head designs for these conditions. The challenge is to keep the engine firing while the very rich mixture cools the plug.
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