Comments by "H. " (@LocPH.) on "driving 4 answers" channel.

@iddqd339 Except for the fact that turbines are VASTLY more expensive and far less efficient. Which is why they're not in use for cars.
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@iddqd339 what are you talking about? Small turbines are the absolutely WORST out there in terms of efficiency. They rarely exceed 20-25% while piston engines operated in a specific region can exceed 40% (CI engines). You are confusing small turbines with large, stationary ones. The larger ones have at least twice the efficiency of the small ones.
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@Appletank8 exactly my point
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@brianb-p6586 yes but it uses synchronizing gears that will be heavily loaded during combustion and expansion due to the force direction = reliability issues.
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@clivestainlesssteelwomble7665 They made a video of one of their smaller engines on a drone. The last thing you would call it is quiet. Also the propeller makes the majority of noice. As for fuel efficiency, it's really not special at all. The haven't even passed 35% TE for the gasoline one last I checked.
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@GRAHAMAUS what??? The most efficient ICE operates at 55% thermal efficiency (Wärtsilä ship engine), the second most efficient is a recently launched Chinese diesel engine with 53% and the third most efficient is the Mercedes F1 engine from 2014 with just over 50% (including MGU-H). Lots of commercial diesel engines in trucks and generators can hit 45%. All of this is improving too. Batteries also have a certain charge/discharge efficiency and the same goes for inverters, all of which has to be accounted for. Pumping fuel to an ICE is MUCH more efficient than any of those. Also, the much added weight of batteries considerably reduces overall efficiency in weight-sensitive applications like airplanes and cargo transport. Electric has its place, and so do ICE:s, so quit the whole "electric is the end all be all" mantra.
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Great points, but I think you forgot the most important reason these engines have such high power density/specific power. Size. The square cube law. Area scales with the square of linear dimensions while volume and therefore weight scales with the cube. All else equal, a 10x smaller engine will have 10x LARGER power density/specific power. The same reason why huge ship engines with 20-30 bar BMEP with piston speeds similar to car engines will have extremely low power density/specific power (orders of magnitude lower than car engines). If you could make engines really, really tiny, like molecule sized, they would have absolutely insane power densities (easily thousands or even millions of HP/kg or liter) simply because of the small size.
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@d4a rpm will reduce linearly with increasing engine size, if all else is equal, including piston speed. You should simplify it to power = force x speed. Rpm is not a speed, but a frequency. Hence, it depends on engine dimensions too. If BMEP stays the same, a 10x larger engine will have 10^2 = 100x the force acting on the piston on avarage and move at the same SPEED = 100x the power. It will however have 10^3 times the weight and volume, giving it 10x lower power density/specific power. The mechanical pressures exerted on the engines insides will be the exact same as the smaller engine, while having 10x lower rpm. Don't believe me? Here's another way to look at it, the larger engine internals will have the same speed, but weigh and volume to area ratio will be 10x higher than the small engine. So mechanical load (pressure) should be 10x higher, right? No, as everything also moves a 10x longer distance, so acceleration will be 10x lower. Therefore, mechanical loads will stay the same for the same piston speeds and BMEP, while rpm will decrease. You are comparing engines with vastly different mechanical loads and therefore lifetimes. If the small OS engine you showed had the same MPS and BMEP as the top fuel dragster, it's power density would be way higher.
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@d4a You didn't read what I wrote. Rpm is NOT a speed, but a frequency. "Rotations" has no meaning if you don't know the size. Speed is meassured in distance/time, like m/s. Very important distinction. Simplify it to power = force x speed = Newtons x m/s. If BMEP stays the same, a 10x larger engine will have 10^2 = 100x the force acting on the piston on avarage. Alright? Moving at the same SPEED (meter per second!) = 100x the power. Accelerating to the same SPEED (m/s) in 10x the distance will in fact give you 10x lower acceleration. Rpm will NOT stay the same! For mechanical loads to remain the same (measured in Pascal as I'm sure you're aware) SPEED (in m/s) has to remain the SAME when scaling the engine. Read my previous comment, I broke it down very thoroughly.
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@crezychameau I think the purpose was to use the rotor inside as an overexpansion chamber. But I find that to be really weird - the exhaust will be open to the atmosphere while this is happening and the same force applied on the the wall inside the rotor moving towards rotation will also be applied on the opposite wall - no net force. It's like keeping the exhaust valve open in a regular engine while also making the head move with the piston. How are you extracting work from that?
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@iddqd339 helicopter engines are in the range of a few 100hp to 1000s of hp continuous power. You can find the lists of BSFC numbers and calculate TE easily, and I did just that quite a long while ago. The 55% figure is familiar, it is for one of their engines in the 1000s of hp range, so not a microturbine at all. I don't remember the exact one though. Efficiency sharply drops off from there. For a car you need a few orders of magnitude less than that. No more than 50 hp continuous. There is no turbine in the world in the 100s hp range, let alone under 50 that comes anywhere closer to the 55% figure. They don't even go above 35%. I suggest you Google these tables, Wikipedia is one place to find them. If you somehow find a turbine with around 50% TE around the 50hp mark, please share you source. That would be very interesting to me.
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@GRAHAMAUS Literally 2 of the 3 engines I mentioned are car engines. And one of those 2 engines is a gasoline engine, the F1 engine. Technology that has already started to be applied in road cars. All of the mentioned engines, including the gasoline one, reach over 50% TE. Re-read my previous comment. Diesels generally run lower temperatures, not higher, as they usually operate ultra lean. This results in lower heat conduction losses, which along with higher CR makes them more efficient.
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You are explaining the VE figures as a percentage of the volume of the cylinder that gets filled with air. Maybe you only said that for the sake of being pedagogic, but it is technically not true. The WHOLE cylinder will always be filled with air, there will never be pockets of vacuum. The precentage is really describing the ratio of cylinder air pressure to manifold pressure when the intake stroke is complete. If manifold pressure is atmospheric, 1 bar, and VE is 50 (%), this means the cylinder air pressure when the intake stroke ends is 0.5 bars. Similarly, 110% VE at 1.5 bars manifold pressure will mean 1.65 bars cylinder air pressure.
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