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Turbo cylinder?

Posted: Thu Jan 16, 2014 12:59 am
by sockmonkey
The threads on combining a turbocharger with a pulsejet gave me this idea.
It's halfway between a turbocharged engine and a jet engine in that while the air/fuel mix in pumped into a cylinder equipped with intake and exhaust valves to be burned, there are no pistons and the whole force of combustion is used to spin the exhaust turbine. Any power extracted for work would come from the turbine shaft.
The point being to achieve high air pressure for combustion without needing all the complex compressor stages that make normal gas turbines so expensive and hard to make.
The valves would be timed to allow combustion to be total for maximum efficiency rather than the compromise that conventional piston engines must use. Likewise with exhaust.
In the animation below I gave it two alternating combustion chambers to smooth out the power pulses.

To further clarify, aside from the simplicity, the advantage is that even though the intake/compression/ignition/exhaust cycle is intermittent like in a piston engine, it's timed according to what is best for the fuel burn rather than needing the movement of a piston.
The fuel and air isn't ignited until it's perfectly mixed, the exhaust valve doesn't open until everything is burned, and the next cycle doesn't begin until all of the exhaust has exited and the incoming air has scavenged every bit of it from the chamber.

Re: Turbo cylinder?

Posted: Thu Jan 16, 2014 1:50 pm
by metiz
That's actually a pretty good idea. Something to take into acount, I think, especially if the mechanical load is placed directly on the axle, is that the pressure pulses will put serious strain on the turbine blades. Metal fatigue and inbalance in the wheel might cause problems.

Still, it's worth trying out! You could build a normal turbojet and try to use pulse-jet valves

Re: Turbo cylinder?

Posted: Thu Jan 16, 2014 5:25 pm
by sockmonkey
I think the turbocharger turbine could handle it since pulsing exhaust is something it encounters in a normal engine and the other stresses aren't much different than what a normal turbocharger experiences anyhow since it only needs to spin at standard turbo speeds rather than jet speeds.
I would use mechanically actuated valves and pressure sensors like in a modern engine as well to maximize the efficiency since it's already using moving parts anyhow.
My only real concern would be how much power can be imparted to and extracted from the rear turbine.
IIRC there was a turbocharged piston engine in use a while back that did extract power from the turbo shaft rather than the crank.
I don't know how well this would fare as a propulsion unit but I think it would do well as a generator.
In terms of sound it would be similar to an ordinary piston engine.

Re: Turbo cylinder?

Posted: Tue Jan 21, 2014 8:31 pm
by sockmonkey
Been pondering this a bit more. Efficiency could be improved even more with an insulating lining for the combustion chamber. Since it doesn't have to deal with any mechanical friction, only heat and pressure, a low-cost ceramic is viable.

Temperature control is easily improved by leaving both valves open for a split second in between cycles so that cold air from the compressor will blow straight through to the exhaust turbine. This also provides complete scavenging of exhaust gases.

If equipped with pressure and fuel/oxygen sensors as most modern engines are, the spark can be timed perfectly as well as providing total flex-fuel ability.

The means of actuating the valves is something I'm still not sure about. Doing it with a camshaft means reduction gears and a fixed timing that can't adapt to changing RPMs. While this isn't a problem in a generator application, For use as a source of shaft power for propulsion it may need solenoid-actuated valves to adjust the timing.
A rotating plate with holes my be a suitable valve mechanism.

As the Intake and exhaust are on opposite ends of the combustion chamber, they can be the same size as the CC itself allowing for easy breathing.

Crap this sounds like an ad.

Anyhow, is a single exhaust turbine enough to harvest a sufficient amount of power?
I know multi-stage types are used for better efficiency, but I don't want to add more complexity and most designs out there aren't made to handle a pulsing flow.

Re: Turbo cylinder?

Posted: Thu Mar 13, 2014 8:39 am
by racketmotorman
It won't work :-(

The radial compressor wheel is a constant supply device , making it intermittent will probably send it into surge , wrecking the turbo .

Also having a "perfect" air/fuel ratio for combustion to occur in the "cylinder" will produce a temperature too high for the turbine to survive.

The irregular supply of hot gases to the turbine will mean the turbine isn't supplying power to the compressor during those "off" periods when combustion is occurring.

Don't waste your time on this one


Re: Turbo cylinder?

Posted: Sun Apr 06, 2014 4:53 pm
by Mike Everman
Very neat speculation and animation! I do agree with racket, though, it is doomed even if it could take the heat and stresses. The outlet of the compressor is blocked for much of the time, and even if you reduce that time of blockage, the average back pressure will never allow the impeller to get up to speed in the first place.
Sorry, I really wish it were a "try it and see" thing, but this unfortunately won't make it through the first turboshaft calculation gate.

Re: Turbo cylinder?

Posted: Thu Jul 03, 2014 9:30 am
by sockmonkey
Hmm, good points there. What about multiple combustion chambers so there is always at least one providing thrust and another taking in fresh air from the turbine?
How do normal turbochargers handle the intermittent intake and exhaust pressures? Is it just by the length or the tubing in the manifold?

Re: Turbo cylinder?

Posted: Thu Jul 03, 2014 9:45 am
by racketmotorman
Turbos don't have a problem with "intermittent" flow to an IC engine unless the engine has less than 4 cylinders , if on a 2 cylinder engine then a large volume plenum is recommended to dampen out pulses ,I think I used a plenum of ~1.5 times engine capacity , the turbo rpm do change slightly as each exhaust pulse hits the turbine wheel , but turbine wheels work OK with pulses , compressor wheels don't

Re: Turbo cylinder?

Posted: Thu Jul 03, 2014 4:58 pm
by sockmonkey
Yeah, a plenum chamber on this thing could also function as an intercooler for the compressed intake air.
The animation does use two combustion chambers and between cycles the intake air blows right through to the exhaust turbine both for scavenging and cooling.

Re: Turbo cylinder?

Posted: Sun Jul 06, 2014 7:12 pm
by ace_fedde
In fact you are simulating a car engine with a turbo mounted... Yes, you could see a car engine as the equivalent of a flame tube!!
And however that is a very heavy "flame tube" :wink: it is a pressure gain "flame tube"!!
I think your car engine substitute won't last very long. Use a loudmouth free piston instead!! I posted plans somewhere on this forum.


Re: Turbo cylinder?

Posted: Mon Jul 07, 2014 5:54 am
by sockmonkey
Indeed. The cylinder assembly wouldn't be nearly as heavy as that of a turbocharged piston engine though since the lack of moving pistons or a crankshaft means that not only will it be much smaller, it also doesn't need to cope with the mechanical stresses caused by having to keep all those parts on track.
There were in fact engines close to this at one point. They had a turbocharged piston engines that produced more torque from the turbine shaft than the crank so they used that as the power take off rather than the crank which wasn't attached to anything.
I don't see this as a case where I think I'm smarter than all those aircraft engineers by coming up with something that they didn't think of.
It's more along the lines of them skipping a few steps ahead since work on straight jet engines was going on at the time so that's why nobody else hit on this particular design before.