The Rotax is easily the most used engine in the ultralight flying machines. It is Australian built and an excellent efficiency throughout the range. It is also come to light that BMW has recently built an even better engine that produces one hundred and five horsepower at an attractive price, so it might be used instead.
The Rotax 447 being the starting model is a 40 horsepower, two stroke air cooled unit while the top of the line model is much better in stats. It is the four stroke Rotax914 turbo which produces a high end 115 horsepower with a water cooled unit.
The high speed and dual seat versions of the ultra light flying machines often have a two stroke or 4 stroke Rotax unit which is water cooled. The water cooling has an eminent advantage over the air cooled variant as the working temperature stays in a limit optimizing power, fuel efficiency and markedly reducing the carbon dioxide deposits on the inside walls of the engine chamber.
The Rotax ultralight flying machines have a problem in flying in a wide range of altitudes as they lack in even the basic adjustment options. They are usually calibrated for a particular altitude range and if they fly out of that then the engine efficiency drops considerably which makes it difficult to fly. So if the pilot decides to increase or decrease the flying height by more than three thousand feet then the engine temperature can’t be maintained at a specific value which leads to poor combustion and decreased throughput in terms of the power delivered to the fuel consumed. The engine chamber is coated with excess carbon dioxide which needs to be cleaned. For overcoming this, the carburetor needs to be opened to fix another size jets which is not a good option in the long run. A turbocharger can do much better.
The above problem could easily be resolved with a modification allowing the pilot to alter the fuel mixture. Obviously, the Rotax engineers must have this in mind but they left this out as for this plane is used for recreational purposes only and also training the pilots to use it would also be a problem.
The new BMW engine on the other hand may be an exciting prospect with computer monitored fuel injection that takes the responsibility of the pilot’s shoulder and provides adequate engine power at all altitudes.