A New Type of Hot Air Engine

[VincentG]

That is promising about the heat generation Tom. My theory is to take advantage of the truly miniscule amount of heat really needed to raise the temperature of air. Perhaps this second stage of compression will add significant air temperature without the excess heat associated with heating a metal hot end.

I have most of the top end components made up. Unfortunately, this is not seamless tubing so my second stage will not be producing much pressure but will still function as a delayed hot air displacer. I'll order some seamless tubing and retrofit it and a new secondary piston later. All mating surfaces have been lapped flat and are either O-ring sealed or will use sealant of some kind.

The assembled top end, minus head bolts and a press fit cylinder head for the second stage. The lower top hat looking part is the primary compression space and can be adjusted with inserts to change its interior volume and thus primary compression ratio.

top end of prototype.jpg (262.25 KiB) Viewed 2596 times

The components that make it up. Everything is affixed by threaded rod so it can all be adjusted as needed. This includes spring pressure, timing and secondary compression ratio.

top end components of prototype.jpg (307.32 KiB) Viewed 2596 times

[VincentG]

The nearly completed hot end, save for the bash valve bumper and possibly the reed valve modification to the bash valve face.

assembled hot end.jpg (330.84 KiB) Viewed 2587 times

I think at larger scale, all of this could be cast from 5000psi concrete, or perhaps a more insulative aircrete mixture.

[matt brown]

If I install a one-way reed valve on top of the large face of the poppet valve (on the hot side of the valve face), it will allow working cylinder air to be compressed into the hot chamber until the pressure of the hot chamber exceeds that of the working (constant volume heating perhaps?). Then when the bash valve is hit, there will be a uniflow back to the working cylinder.

Maybe this will also keep the mass volume of air up in the hot chamber.

I'd call this counterflow since the gas is passing back & forth thru the same 'region', although thru separate valves. Uniflow would have the working cylinder compress the gas and pass it thru one region while the hot chamber returns gas thru another region. Ex: working cylinder compresses gas into a conduit which connects upper (?) region of working cylinder with upper region of hot chamber, then returns per your bash valve. Again, just an example, but this isolates the gas into a one way 'circuit' (would likely require a reed/port valve or two). No doubt, a 'regulated' cycle, but could you imagine any ICE as an unregulated cycle.

As you get further into this, you'll discover various problems and solutions, then wonder if anyone knows such. A Stirling gamma is similar if you consider the heating & displacer motion separate from the rest of the engine (constant volume heating, displacer motion, expansion, then cooling and compression) but the lame-ass phasing results in out-of-phase gas dynamics which rob the peak power period (as you have observed).

Regardless of the gas mass in the hot chamber, one thing is for sure...no gas from the working cylinder can enter the hot chamber until the working cylinder pressure exceeds the hot chamber pressure, hence my buzz: reservoir scheme.

[VincentG]

Are there any engineering papers you can direct me to that cover some high level designs? Most of what I can find just focuses on increased heat exchanger surface area on a basic Stirling configuration. I now don't believe that is the biggest hurdle.

Have you ever heard or seen a sort of super heated air injector scheme? Picture otto direct fuel injection but with air alone.

[Tom Booth]

Your description of hot air injection got me thinking. Similar to the old steam "injection" type engines. In that case though, there would be a huge boiler full of high pressure steam so "injection" would just be a matter of opening a valve.

From there I thought about an old puzzle; what is the value or purpose of compression itself generally?

A steam ejector works by converting pressure into velocity then using entrainment to pick up mass, increasing the velocity and momentum further, then converting the velocity/momentum back into pressure.


https://youtu.be/4wsiXPgfdlI


In a reciprocating engine, I think something similar happens in that the velocity and momentum of the piston are used to drive air molecules close enough together to convert the velocity/momentum into molecular repulsion.

Think of air molecules as little magnets that repel each other very strongly but only when in close proximity.

So if someone has two magnets and they are pushed closer and closer together, at first nothing happens, but when they get very close they move apart.

But to really get a lot of force they need to be even closer, but getting them close enough can be difficult.

Well if you slide one magnet across a table at high velocity, the velocity will carry the magnet closer than it would be possible to just push them together. They then spring apart rather violently.

So, now back to your superheated compressed air.

By a two stage compression the momentum/velocity of the piston is more easily converted back into pressure, the molecules of air are forced together that much more and the rebound is therefore that much more forceful.

Maybe. It's a theory anyway.

Is it necessary to actually generate heat?

A fire piston does the same sort of thing I think. A heavy hand driving a piston inward at high speed to drive the air molecules close together.

Heat is only generated if the pressure is maintained long enough. Otherwise the piston may just fly back out without ignition as in this video:

https://youtu.be/Bjy6m6MR-PQ

But maybe heat/ignition isn't necessary if that is just a side effect of molecular repulsion.

What I'm mainly trying to say is, if the magnets are 4 feet apart and pushed slowly together nothing much happens, but if accelerated from the same distance they would fly apart, as the real force is generated in the last 1/8 of an inch.

A piston compressing air may be similar. So your two stage compression might be like that last 1/8 inch.

Acceleration is easier at a distance.

[Tom Booth]

If you've ever played with a gauss gun, the difference between ordinary compression and a high compression that utilized molecular force might be apparent


https://youtu.be/xoUNyGUCzqs


I'm not sure what use high compression would be otherwise, as it would seem that you could never get back more than what you put in.

Atmospheric pressure accelerates the piston and at the last moment the velocity is converted to high pressure, but why should a very high pressure, like in a diesel engine be more efficient than low pressure?

[VincentG]

I think of it more as using molecular activity as a force multiplier. If you compress a spring, you only get back what you put in minus other losses (heat, etc). But if you remove that last btu from 33 degree water, you get the power of expansion into ice.

While this is not the same effect as a phase change, I think there may be similarities. Heat engines work by expanding air due to temperature change. It seems to me that we get a bigger return than our investment. For example, given a large enough piston, we could lift a car with a bic lighter expanding the gas. Name another process that could use the potential energy of a lighter to do that much work. I might be way off base with that theory. But its just that. A theory.

Note that this is not perpetual motion of course. But maybe more akin to tesla ambient energy scheme where energy is needed to start the process. And even then, I think the advantage is banking the temperature extremes generated by a partial heat pump cycle, and then using those extremes for a short term burst of power.

[VincentG]

Or maybe explained in another way...if we have an engine of fixed size that runs happily on 300 degrees and 1btu, but our heat source is 300 degrees with 10 btu available. Why can't we convert the 9 extra btu into a higher temperature at 1 btu again. Our fixed sized engine doesn't want the extra btu's, but it would love the higher dT.

[Tom Booth]

VincentG I think we are pretty much on the same page.

The biggest issue I've been facing for the past decade or more has been the prevalence of the Carnot idea that a heat engine is powered by heat "flowing through" the engine like water powering a turbine. That just does not seem to be the case at all. At least my experiments to test that theory (insulating the "sink" etc.) seems to suggest some other mechanism of operation is at work.

The scientific/engineering community in general however has been sold on that idea for the past two centuries, so there is no real openness to any alternative theories.

My observations, however have lead me to consider possible alternatives, because the traditional assumptions just don't seem to hold water.

As you seem to describe, the reality is more like accumulators or capacitors in an electrical circuit. Heat being stored up by a kind of heat pump action then released suddenly.

[VincentG]

I'll propose one more example. A piece of unburnt wood is like a gallon of diesel fuel. Just a store of potential energy. The diesel can power a 1500hp pulling tractor but it is nothing without first putting energy into starting the process.

In this way, although the thermal efficiency of the diesel engine is only 35%, in effect its total efficiency is nearly infinite based on the input energy required to start the process.

Perhaps the hot air engine can behave in a similar manner with a well designed system.

[VincentG]

I agree Tom. And so does the Chinese government, with their megawatt stirling engines powering ships. And the kokums and newer submarine...etc...etc....

We just need a breakthrough on the diy front.

Speaking of diy, how advanced was the JD #6? Because 2hp at 200rpm sounds pretty encouraging to me.

[matt brown]

Hot Potato.png (8.13 KiB) Viewed 2531 times

Vincent, I found a screenshot of Tom's Hot Potato on my HD while searching for something else. The thing I like about this is that the gas isn't heated until it passes thru the matrix, kinda like a regenerator. Per previous comm, I favor a conduit from working cylinder to 'heater' chamber with some type of valve arrangement.

[Tom Booth]

..., how advanced was the JD #6? Because 2hp at 200rpm sounds pretty encouraging to me.

In principle, a Gamma, same as your LTD but with TWO displacers driving one double acting piston. Also pressurized to 150psi

Other than that I don't know much of anything about it except what can be found online.

The heater heads are fairly sophisticated looking. Lots of surface area for heat exchange.

jd6fs-4.jpg (80 KiB) Viewed 2520 times

http://www.starspin.com/stirlings/jimd6.html

[Tom Booth]

Hot Potato.pngVincent, I found a screenshot of Tom's Hot Potato on my HD while searching for something else. The thing I like about this is that the gas isn't heated until it passes thru the matrix, kinda like a regenerator. Per previous comm, I favor a conduit from working cylinder to 'heater' chamber with some type of valve arrangement.

Re-reading that old thread: viewtopic.php?f=1&t=488&p=1292

I just now realized that idea is not much different than my current Ringbom conversion concept.

It also became apparent how long I've been dealing with the heat needing to be removed to a sink assumption:

Have you considered having a 'cold potato' at the sink end, too?

The whole point was/is, a "sink" is not needed

hot_potato_engine.gif (72.02 KiB) Viewed 2513 times

Imagine simply welding a bolt to the top of the piston in an IC engine as a conversion poking up through the spark plug hole into the add-on "hot bulb" hot potato chamber.

I think I've probably forgotten many more ideas I've had on this forum than I've actually implemented.

[VincentG]

I should be able to prototype that hot potato fairly quickly. What was you idea for the regenerator material? And I imagine the valve was meant to be a perfect seal? Or was it just a flow restriction? Also how much importance do you place on the regenerator moving with the valve vs being stationary in the hot chamber?