Pulsejet Engine

From https://en.wikipedia.org/wiki/Pulsejet

Cheap, simple to build, very painfully loud and fuel thirsty or dirty and inefficient, almost like a fuel + air sound converter to make mad levels of sound pressure from absurd amounts of fuel in a dirty low mass innovative cleave pulse jet engine. 

Turn your sound levels down for headphones, speakers, earbuds, or car stereo or phone volume, since this video is loud once his engine starts up. 

Watch JonnyQ90 video about this on YouTube https://youtu.be/1LimWlYIhjw?si=Hh3VgAiFcMHpOk85

Understanding the high fuel consumption or lower efficiency comes from the very low compression ratio, where a diesel engine has a higher compression ratio of 25:1 and gasoline engines are around 11 to 14:1, these pulse jet like very easy piston engines have ultra low compression ratio of 1.2 to 1, meaning much of the fuels heat energy cannot be expelled as useful thrust, but instead are emitted as a deafeningly loud sound from the acoustical resonance compression of the Lenoir Cycle.

Low Compression Ration = Low Thermal Efficiency or Poor Fuel Economy & High Emissions

The reed valves made of spring steel also have a short useful life, meaning this kind of engine limited to hobby aircraft and military applications where the vehicle life not important and the high noise emissions are also not considered important. Use one of these in a neighborhood or populated area where its also illegal to fly a para-motor, and you will piss off your neighbors and someone will call the police or report it as a noise disturbance. 

The high heat and vibrational energy of the noise emitted make pulse jet engines useful as heat sources for industrial drying applications, high output heating systems, to burning biomass or crop waste or waste gas or municipal sewer gas or dump gas recovery gases as alternative energy.

Truly Flexible Fueling, they will burn almost anything flammable if properly fueled, even able to burn coal dust or sawdust or anything ground up fine enough, even aluminum powder and black powder or smokeless propellent, hydrogen, gasoline, diesel, natural gas, propane, butane or low boiling point fluids like DME or dimethyl either. 

Some push tip helicopters have pulse jet engines at the end of the rotor blades such that no tail rotor requires for such aircraft, and since it lacks a central engine, no transmission or driveshafts either, greatly simplifying the design. AHC or American Helicopter Companies XA-5 flew January 1949 as did the later improved XA-6 Buck Private. The Hiller Powerblade hot-cycle pressure jet rotor also the same year. The AX-8 for the US Army, but ultimately even as of the XH-26 the high noise levels and rotor tip drag penalty made it unattractive and contract was canceled. The main advantage was low cost, since a pulse jet rotor trip pusher helicopter 10X less expensive. 

RC aircraft powered by pulse jet engines have hit speeds of 322 km/h or 200 miles per hour, while ram air pressure limits the top possible speed of a valved engine to 450km/h or 280mph. 

See the wiki for a deeper dive TLDR edition writeup https://en.wikipedia.org/wiki/Pulsejet