It becomes more complex when you are trying to make a million engines a year, to meet stringent emissions criteria, so you want to combust at a high temperature for (thermodynamic) efficiency but not exceed 900deg.C to avoid making toxic NOx gases in the exhaust. Collects a Gov't fine if you get it wrong! And complete combustion before the gases expand and cool below about 300C when CO combustion stops. - otherwise you get High CO and a Government fine.
But HOBBY engines are just that. Usually replicas of a real old engine, or a simple engineering principle (Oil burns better at high pressure and temperature). So fortunately we can still run them on proper old recipe oil fuel.
(Unlike Steam engine guys who now have to use smokeless coal and non-coal, etc. in various countries! Wood pellets in a boiler designed for Steam Coal are not good! And my 45 year old Moto Guzzi doesn't do well on today's 95octane 10% Ethanol/Petrol compared to the 101octane leaded stuff it was designed for.).
Enough of my whinging.
WELL DONE for making proper engines that work!
What a gorgeous engine in that video Green Twin.
Here's an engine i saw on Sunday at a show.
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