(had to ask what many are thinking)
Well 50 years of really, really live active following “the progress” of IC piston engine replacements. Been easy enough to go back 15 years before that with in print promotion and documentation.
Stirling, minigas turbines, microsteam, Wankel , ect, ect, ect. Where are they in actual deployment now?
Past all of the conspiracy theories of being shelved and sunk is thermal efficnecy realities.
These first two WERE engine power reliant on regenerative cycles of heat expansion gases and even the heats themselves.
All shaft power making heat engines rely on maximizing the difference between a cold side and a hot side. Simply put the hotter the hot and the colder the cold and more time you can cycle between these the greater your fuel/heat to shaft conversion effency.
70 years of multi modern intense effort of engineering and Stirlings peak stuck at 29% heat value of fuel put in to shaft power out. That’s for a very high tech exotics materials $100,000 engine. My $200 and $500 Honda IC piston engine lawn mower can do this handily.
Same story for the microgas turbines.
Why? These two must maintain constant heat materials exposures to 3X and 4X the levels as the Honda IC pistons engines. For them either APPLIED heat makes pressure force, or flow reactive force.
The Honda’s you have a suck in cooling charge cycle. Bit of heating up compressing cycle. Then a heat burst resulting in a pressure rising/falling short duration cycle. Followed by a let the un-converted to shaft power remaining heat and pressure out cycle. Repeat. Repeat. Repeat.
Air rifle cross check this. Pump-up and stored pressure types. Make lots of gas compression heats. This is all bled off and lost before application. NONE gets used in the power release. Cycle rapidly and the receiver will actually released gases cool reducing seen pressure, therefore velocity power.
Barrel cocker spring-air. Has an open ended to atmophere (nitrogen) piston. Your muscles heat a bit cocking/setting. Spring heats a bit too. These lost. Minor. Releasing the cocked piston slam compresses the air heating it for even MORE of an energy power release. Cock and release rapidly and the receiver WILL heat. That heat gets to the cocked cylinder “gases” transfered. Your delivery power goes up. Your velocity rises. Really see this cycle heating on cartridge repeaters. Those YOU must slow cycle rates to cool or external cool OR materials damage, and “cook-offs” Will occur.
Micro steam with two hundred of years developement stuck at 16% heat value of fuel in to shaft power out conversion. THATS with triple reuse steam expansion. Last very serious efforts to improve on this by a brothers pair had them running cylinder temperatures so high the the metals cylinder were hot translucent and the moving piston could be photographed inside. Same materials destructive temperatures then. Valving sealing then became the stopping point. So again the drive up into exotics and costs. Easily costs powered instead with IC piston engines. And No small steamer ever really had usable water recovery recycling. Use much more water than fuel.
Back to when your piston face pushing is a let-in externally made pressure release there is too much of the pressure making/delivery “system” subject to too high of constant temperatures and pressures. Sure way to drive up your costs, overall complexity and have higher non-deliverable energy losses.
This should drive home point out that even though in an internal combustion piston engine cylinder that the combustion temperature may spike at 2200-2500F this is the center of the combustion globe. Edge temperatures do not run that high. And this temperature spike is made inside/out and of a short duration. WITH it inside/out made much easier to get materials forgiving rational HEAT to shaft power converting. Old way was cast iron pistons. New way ecludic coated pistons.
Now Wankels. You can read much about compression ratios limitations. Well THAT does hammer internal combustion possible efficients hard. Even piston crankcase pressurized 2-stokes have this limitation.
Here is another Wankel set back. Manufacturing costs.
Take a nail or pin. A piece of string. A pencil. EASY to make perfect circles. Shortened the string. Easy to make perfect concentric circles. Now make me perfect paralle lines. Then make a pair of these for a perfect square. Now YOU try making a Wankle internal shape.
Same thing in metals.
Another design limitation. Rotor edge and tips sealing. Lobe tips can really only have a single gasses areas pressure seal to be able to follow the wankle curves.
My 13,000 RPM round piston Stihl engines use two pressure sealing piston rings. The B&S, Kohlers and the gasoline vehicles use three piston sealing rings. My much higher compression diesels use four piston pressures sealing rings. Ha! Now you know why you never will see a compression ignition diesel Wankel.
So . . . in this past 70 years IC piston engines have just refined better and better. Mazda themselves with a manufactured and deployed Miller cycle engines. Complex. But less expensive with better durability than a Wankel.
Honda and Toyota with emulated Atkins cycles deployed driving around worldwide. Honda with a few hundred thousand deployed true Atkins cycle piston engines out now too.
There are many worldwide serious engine suppliers out there that IF they could truely refine out a better way heat engine, they would. They want to put their competitors out of business.
You bet they use in-cylinder expansion gases to the best effects.
Hyundai big ship piston engines at 178 to 300 rpm are said to be putting more motive shaft horsepower for the highest fuels use efficiency conversions on the face of the planet.
They use atmospheric air with the 78% nitrogen.
And lots of it for the extremely heavy wastes oils that has to be steam heated just to be able to “fuel” inject it.
Regards
Steve Unruh