I have looked into everything. I personally don’t like belts as much as they need to maintain a proper tension, they wear out, and stuff could possibly get stuck in it. On the other hand it would be easier to make and lighter weight.
With modern technology it basically does need to be a 2 stroke. I forgot where I saw it, but one team of researchers was able to make “sliding ring valves” to make a 4 stroke opposed piston engine, but the technology is unproven, it compromised the shape near the combustion chamber, if I remember correctly the valves had issues sealing, and the mechanisms needed to control the valves were complex and heavy. In addition the scavenging on a 2 stroke OP engine is efficient enough that switching to 4 stroke has no real advantage.
Very true. The 21 horsepower jetcat spt15 burns fuel at about twice the rate of a 25 horsepower vittorazi moster engine. The one intriguing part is that the turboprop engine is so light weight that a paramotor powered off of one filled with 5 US gallons of fuel will still weigh about 10 pounds less than a comparable vittorazi moster powered paramotor with US 2.5 gallons of fuel
I disagree on this one. As of a few years ago the most efficient gasoline engine in the world was the Mercedes 1.6 liter V6 F1 engine at over 50% thermal efficiency. That equates to 267cc per cylinder. Additionally the most efficient vehicles ever built exceeding 10,000 mpg (no that is not a typo) are typical powered off 50cc 4 stroke engines of around 35-40% thermal efficiency. The size does limit the efficiency of an engine a some primarily due to the surface area to volume ratio, but not nearly as much as you would think.
The opposed piston design also represents a non-interference design, so if the belt skipped time there wouldn’t be any damage. There might be some weight savings if the gears that tie the cylinders together also contain the prop hub and reduction gearing - but you could do that with belts, too. I’m not sure why you’ve ruled out crankcase pressure induction - you would just need a crossover pipe to tie the crankcases together.
Comparing a typical engine to an F1 engine is a bit of an apples and oranges thing. The F1 engine is going to be extremely over-square, much larger valving, high RPM, and the one you mention is turbocharged - which changes things. They also spend a bajillion dollars on getting tolerances perfect. Most road-going engines are near square, the Prius engine is under-square and naturally aspirated.
Those ultra-high fuel economy vehicles as I mentioned before can overcome some of the efficiency losses by operating the engine in a particularly efficient band. It’s a delicate balance between speed/air resistance and power output. It’s the same reason, under typical conditions, you can have a 50cc scooter that gets the same fuel economy as a 250cc motorcycle and a 125cc motorcycle that gets better fuel economy than the 50 or 250. If you look at a Brake Specific Fuel Consumption chart you can get an idea of where the most efficient loading is on a particular engine.
The Prius went from a 1.5 liter to 1.8 liter displacement and improved efficiency and you’ll often find 1L 4Cyl engines and even 1L 3Cyl engines being trounced in fuel economy and performance by 2.0L 4Cyl.
Of course, for something that you need to be able to carry on your back and not break a leg while taking off and landing, efficiency is not necessarily the top priority.
The reason I have rules out crank case pressure induction is that it can only fill the cylinder about 70% of it’s volume. 2 strokes overcome this by using an expansion chamber to assist the crank case, but the in turn gives them their peaky power band. They also need to make sacrifices to the bottom end if they want decent top end power. I have given a lot of thought into trying to run the engine off the crank case induction with a crossover pipe, but the engine would end up with a less efficient cruise, it would be louder, have a less linear power band, and would be much harder to have an effective and efficient scavenging process.
That was the plan, but it would not allow for a centrifugal clutch due to the low rpm and high torque output.
What I was trying to point out is that a properly developed small displacement engine can be extremely efficient. This is not typically the case, but it really comes down to how the engine was designed.