Post by artorius on Aug 11, 2016 11:16:42 GMT -5
Hello everyone, I am new here and after perusing several threads over the last few days I am wanting to ask some questions.
Has anyone here ever built a motorjet?
My idea is to build a Motorjet. Which is to say, a jet engine that uses a piston engine to drive the compressor and has no turbine.
Theoretically, fuel efficiency could be much higher. But only a few were ever designed and even less built. Those that were built didn't work great. But they did work. Turbojets were simpler and lighter so they motorjet was abandoned. However, the Japanese built one using a 100hp engine that produced 440 lbs thrust. A typical 100hp propeller plane produces 250-300 lbs of static thrust. The Japanese engine could be run with just the fan running and it produced about 325-350 lbs thrust. The afterburner adding only about 90-115 lbs of thrust.
The Campini CC.2 was another failed design. The engine driving the compressor had 900 hp but the airplane weighed 9,250lb, had a wingspan of 52 feet and a length of 43 feet. The engine was much too small. The total thrust created was 1,550 lbs. Not nearly enough for that size airplane.
The Germans in WWII designed a 32 cylinder engine that drove a multi stage axial compressor. The exhaust was dumped into the compressed flow so that the pistons acted as high compression combustion chambers. It was supposed to be much more fuel efficient than the regular turbojets being developed. It was hoped that with afterburning (then called auxiliary burn) it would produce the same thrust at take off but during cruise it would use half as much fuel.
I have seen some of the electric ducted fans and nitro ducted fans fitted with afterburners for added boost but ducted fans aren't the same as a compressor. They work differently. Most of the motorjets seem to have more in common with ducted fans than with turbojets and I think that is a mistake.
Thus I want to try using an old air-cooled VW engine of 60hp to drive a two stage centrifugal compressor and add fuel in regular combustion chamber(s) like any turbojet would use. Then have it go out and into an afterburner from there.
It is also feasible to have a low bypass system to cool the combustion chambers.
This leads to another question; if compressing air increases it's temperature, wouldn't a high compression single stage compressor add significant amounts of enthalpy to the air causing rapid expansion and ending up with some level of thrust increase even before the combustion chambers? That said, if you are wanting to get the most energy from the fuel burned, wouldn't intercooling the compressed air before it goes into the combustion chamber(s) be beneficial? It would slow the air down and increase density, a slight loss of energy, offset by an increase in density. So I want to experiment with intercooling the inflow air by having a low bypass system that flows through the intercooling system.
Anyway, I wanted to bounce this idea off of you guys as you seem to be the only group I have found that doesn't mind thinking outside the box.
Even if this doesn't work...it will be fun!
(I have pictures to go with the info above...but I can't figure out how to attach them.)
Has anyone here ever built a motorjet?
My idea is to build a Motorjet. Which is to say, a jet engine that uses a piston engine to drive the compressor and has no turbine.
Theoretically, fuel efficiency could be much higher. But only a few were ever designed and even less built. Those that were built didn't work great. But they did work. Turbojets were simpler and lighter so they motorjet was abandoned. However, the Japanese built one using a 100hp engine that produced 440 lbs thrust. A typical 100hp propeller plane produces 250-300 lbs of static thrust. The Japanese engine could be run with just the fan running and it produced about 325-350 lbs thrust. The afterburner adding only about 90-115 lbs of thrust.
The Campini CC.2 was another failed design. The engine driving the compressor had 900 hp but the airplane weighed 9,250lb, had a wingspan of 52 feet and a length of 43 feet. The engine was much too small. The total thrust created was 1,550 lbs. Not nearly enough for that size airplane.
The Germans in WWII designed a 32 cylinder engine that drove a multi stage axial compressor. The exhaust was dumped into the compressed flow so that the pistons acted as high compression combustion chambers. It was supposed to be much more fuel efficient than the regular turbojets being developed. It was hoped that with afterburning (then called auxiliary burn) it would produce the same thrust at take off but during cruise it would use half as much fuel.
I have seen some of the electric ducted fans and nitro ducted fans fitted with afterburners for added boost but ducted fans aren't the same as a compressor. They work differently. Most of the motorjets seem to have more in common with ducted fans than with turbojets and I think that is a mistake.
Thus I want to try using an old air-cooled VW engine of 60hp to drive a two stage centrifugal compressor and add fuel in regular combustion chamber(s) like any turbojet would use. Then have it go out and into an afterburner from there.
It is also feasible to have a low bypass system to cool the combustion chambers.
This leads to another question; if compressing air increases it's temperature, wouldn't a high compression single stage compressor add significant amounts of enthalpy to the air causing rapid expansion and ending up with some level of thrust increase even before the combustion chambers? That said, if you are wanting to get the most energy from the fuel burned, wouldn't intercooling the compressed air before it goes into the combustion chamber(s) be beneficial? It would slow the air down and increase density, a slight loss of energy, offset by an increase in density. So I want to experiment with intercooling the inflow air by having a low bypass system that flows through the intercooling system.
Anyway, I wanted to bounce this idea off of you guys as you seem to be the only group I have found that doesn't mind thinking outside the box.
Even if this doesn't work...it will be fun!
(I have pictures to go with the info above...but I can't figure out how to attach them.)
