This is What Happens When Pistons and Rotaries Make Babies - Birotary Engine Explained and Reviewed
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Today, on the menu we have something very interesting. A genetically engineered offspring of pistons and rotaries, a completely novel creation whose anatomy allegedly inherits only the best genes from its parents. An engine that runs and has already proven itself on the test bench. An engine that has already taken flight.
Ladies and gentlemen, allow me to introduce you to the Birotary Engine. In today’s video we will explain how it works, we will explore the benefits and drawbacks, we will compare it to a direct competitor traditional piston engine and finally we will discuss it’s potential to spread beyond small aircraft, which are the current target market for this engine, we will see whether this engine has the potential to be used in motorcycles and/or cars.
The birotary engine consists of three pistons attached to a crankshaft. The pistons complete their strokes inside a round cylinder block. This round cylinder block rotates inside a housing. Here we have our first benefit. The Cylinder block rotates in the opposite direction to the crankshaft, which means that they partially cancel out each other’s reaction torques and gyroscopic effects.
Something else a rotating engine block does is that it makes it possible to get rid of valves and camshafts and all of their associated components. If you have a rotating cylinder block then all you need is holes in the housing to do intake, compression, combustion and exhaust. The birotary fires every 180 degrees if you don't count the counter-rotating block rotation. Alternatively, it fires every 240 degrees of rotation if you do count the block rotation.
Why does the crankshaft spin three times faster than the engine block? Well that’s because this is an aerial combustion engine which means that it’s designed to spin a propeller, and propellers needs to spin slower than a combustion engine in order to remain efficient.
The birtoary engine incorporates a planetary gearset with a reduction ratio of 3:1 between the crankshaft and the rotating cylinder block. The cylinder block is what the propeller’s connected to and this is why it most rotate slower than the crankshaft. Ok, now let’s compare this engine to a direct competitor, the Rotax 912S, probably one of the most popular engines in the small aircraft market for more than 2 decades.
So these engines have the exact same power output, but they have different displacement and different dimensions. As you can see the Birotary manages to put out the same power with just over half the displacement and a noticeably smaller size. No matter how you look at it the Birotary is more power dense, which is most certainly an impressive feat. What about the weight, well weirdly enough the birotary is heavier, although I’m not sure on this one. Rotax clearly explains on their website what’s included and what’s not included in their dry weight, whereas birotary just says…”dry weight”, we don’t know what’s included. To me it seems a bit weird that the birotary is heavier. I did message them on their website a week ago asking some questions but I never received a response so unfortunately I can’t give you any more details on the weight.
Ok, the Birotary wins in the power density department. What about noise, vibrations, fuel economy and time between overhauls? The birotary likely has less vibrations, but it probably creates more noise and that’s because it revs much higher. Fuel economy is anyone’s guess, I’d say that it’s likely similar or slightly worse than the Rotax. We have just over half the displacement but almost twice the rpm. We have an increased number of seals and thus potentially higher friction in the Birotary but the Rotax also has the friction associated with the valvetrains of two cylinder heads. I’ll leave this one undecided. Time between overhauls is probably higher in the Rotax since it doesn’t need to rev as high.
The increased power density, the lack of cams and valves and the high redline reminds me of a two stroke engine, but with a fuel economy, emissions and TBO comparable to that of a four stroke. If this is true, than the Birotary offers a very interesting set of benefits that would make it an attractive option in it’s target market.
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00:00 Intro
01:26 Birotary Engine Balance
03:30 Four Strokes Without Cams or Valves
05:44 Built-in Reduction
07:01 10.000 RPM
10:25 Sealing
13:05 Birotary vs Rotax
15:12 Birotary for Cars and Motorcycles?
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