Alpha type engine Idea/need input

[derwood]

If you will read the quote you posted you will see that I said pressure was not moved but that air was. the air is moved from on side to the other (hot side to cold side). This happens because the displacer piston occupies space and the air has to move (air is displaced) to the other side. When the displacer piston moves to the hot side, the air (hot air) that occupied that space has now been displaced to the cold side. at this point the air is cooled, now the whole system (every bit of unoccupied space inside the engine) experiences low pressure (vacuum) and the power piston is pulled down/in. Next, the displacer piston moves to the cold side and that air (cold) is displaced to the hot side. The air is heated and the whole system experiences high pressure, which forces the power piston up/out. This process is repeated over and over. Hot air expands and cold air contracts. If go outside to play some basketball in the winter, you will notice that the ball bounces good at first. This is because the air inside the ball is warm (assuming you had the ball inside before). As the air inside the ball starts to get cold, it contracts and the pressure inside the ball lowers and it does not bounce good anymore. If you pump it up while you are still outside it bounces good again. When you take the ball back inside it bounces too good because the air inside the ball has warmed (expanded) and the pressure has increased. the pressure inside the ball is now greater than it was before you went outside. This is the most simple way I can explain it to you. I hope this helps you understand how any stirling engine functions. I appreciate your input. A good debate helps me to realize when my assumptions are correct or wrong. In this case I believe they are correct.

[Ian S C]

derwood, I assume that you have read some of the books on stirling engines by authors such as: Robert Sier, James G. Rizzo, Andy Ross, Roy Darlington, and Boyd's Tin Can Stirlings. Its worth looking at Hirata's stirling engine (web site) Lets build model stirling engines.
I suppose you have built a few conventional engines before moving on to something more sophisticated. Ian S C

[PKM]

Your diagram illustrates horizontal pistons moving in tandem and in derwoods diagram vertical cylinders with pistons move 180 degrees. I agree they will shuttle air but with no volume change then where is the pressure change?

At the power cylinder, which can move in and out just like a conventional gamma engine. The two large pistons are performing the same function as the displacer in a traditional gamma engine- they move air from a location where it is heated to a location where it is cooled, subsequently causing it to heat and expand or cool and contract.

Now you can heat the one cylinder and have a vessel (cylinders) full of hot air higher than external atmospheric pressure to push that mid point mounted power piston up its bore but without an explanation of why a cool down and pressure change at that point where the power cylinder is in the air tube there is no Stirling here. You have a cork (power piston) in a bottle (hot/cold cylinders) being heated up to pop its cork.

Remember the power piston and the large pushrod are connected by a crank- I didn't draw it because it would have complicated my drawing but the power piston pushing out moves the large pistons to the cold side, the air cools down, the power piston moves back inwards, the cycle repeats. This is how it could use a single crank:

threepiston_crank.PNG (21.04 KiB) Viewed 11510 times

Forgive the lo-fi approach but I don't have the wherewithal to create a proper animation right now- these diagrams should clarify operation.

threepistonengine_small.PNG (82.26 KiB) Viewed 11510 times

You can't "tee off" a power cyl.

Actually, if we assume that all the air inside the engine is at the same pressure at any given point in time, the power cylinder can be wherever you want (assuming its size and contribution to air temperature is small compared to the hot and cold parts). I've deliberately drawn the power piston small so it is clear it makes no real contribution to changing the temperature of the air.

All: please forgive the size of my drawings; I go large to try to make them clear, and a picture can be worth a thousand words if it explains something hard to visualise or describe.

[Longboy]

If you will read the quote you posted you will see that I said pressure was not moved but that air was. the air is moved from on side to the other (hot side to cold side). This happens because the displacer piston occupies space and the air has to move (air is displaced) to the other side. When the displacer piston moves to the hot side, the air (hot air) that occupied that space has now been displaced to the cold side. at this point the air is cooled, now the whole system (every bit of unoccupied space inside the engine) experiences low pressure (vacuum) and the power piston is pulled down/in. Next, the displacer piston moves to the cold side and that air (cold) is displaced to the hot side. The air is heated and the whole system experiences high pressure, which forces the power piston up/out. This process is repeated over and over. Hot air expands and cold air contracts. This is the most simple way I can explain it to you. I hope this helps you understand how any stirling engine functions. I appreciate your input. A good debate helps me to realize when my assumptions are correct or wrong. In this case I believe they are correct.

You are doing OK but with reservations. I made the quote because its a red flag. Air moved but not pressure is incorect. For if you are moving heated air you are moving a pressure differential. Energy flows from hot to cold (heat flow) and energy flows from a higher pressure to a lower pressure (expansion). This expansion can sometimes be witnessed when lighting the burner and seeing the power piston rise moving the flywheel in a model engine. The pressure is not the same at all points within the sytem for the temperature is not the same in all parts of the engine. If it was (equalibrium) you would not have a running engine for the Stirling depends on a lower preassure at the cold cylinder as well as a volume change with the rise and fall of the power piston. Can we say then that air moving at a changing temperature and pressure (hot to cold, high to low) from the hot cylinder back to the cold cylinder causes a change in pressure to lower than atmospheric causing the power piston to be pushed back down its cylinder (power stroke) that we have a running engine then? For this is Stirling principle.

[Longboy]

Actually, if we assume that all the air inside the engine is at the same pressure at any given point in time, the power cylinder can be wherever you want (assuming its size and contribution to air temperature is small compared to the hot and cold parts). I've deliberately drawn the power piston small so it is clear it makes no real contribution to changing the temperature of the air.

All: please forgive the size of my drawings; I go large to try to make them clear, and a picture can be worth a thousand words if it explains something hard to visualise or describe.

Thanks PKM, I can't agree about same pressure and nobody puts the power cylinder midway between the hot and cold cylinders in Stirling. There is good reason for that for in these experimental engines there is not going to be a pressure drop at that point sufficient to push down upon the power piston. That hot air is just going to blow right by into the cold chamber. Your illustrations are great! Here is a theory: replace the regen with a venturi under the power cylinder. The air pressure is lower passing thru a venturi. The power piston will be pushed down.........but will we have a continuous cycle, a running engine then?

[derwood]

In my explaination I realize that there is no vacuum inside. I only mean low pressure.( Lower than atmospheric). I also agree that the power piston goes back in due to this reason. But, it does produce power on this stroke. Also, when you say "the hot air would just blow by the power piston" That is also true. Cold air wil blow by it too. This movement of air is not what causes the power piston to work, it's the change in pressure.

[derwood]

Longboy, you use the term "equalibrium". That is a very good explaination of the system. When the air is cooled the whole system experiences a low pressure equalibrium almost instantaneously. When it is heated a high pressure equalibrium is achieved almost instantaneously. This is the basic reason why the stirling engine runs. since the displacement pistons are 180 deg. off and the pressure inside is equalized, Their motion is unrestricted no matter what the pressure inside is. The power piston simply reacts to the change in pressure and turns the crank shaft.

[PKM]

nobody puts the power cylinder midway between the hot and cold cylinders in Stirling.

Erm... I did, and my engine runs fine. The venerable walking beam engine has the power cylinder mid-way between the hot and cold ends. I can also confirm that the power piston does exert a force on the outwards stroke- when building my engine I moved the displacer by hand and watched the power piston shoot in and out of its cylinder without the crank attached.

As for the pressure being different at different points in the engine- I can't say for sure that it's exactly the same everywhere as I haven't done the measurements, but I am convinced that changes in pressure affect the entire engine in mostly the same way, so if the power cylinder isn't contributing to heating/cooling the air (as in a gamma design) it doesn't much matter where you place it.

I'm tempted to build an engine to this design to prove it can work- I realised that the large pistons don't even have to be fitted pistons, they could be diaphragms (if the material could handle the heat) reducing the engine to only needing one true piston. If the hot/cold cylinders had their open ends facing each other it wouldn't even need any pushrod seals like a gamma displacer does- this might be the most mechanically low-tech design I've seen yet!

[derwood]

I did the same test about a month ago. and my results were the same. Also I have started building one similar to the drawing you posted. The one where all three cylinders connect to the same spot on the crank.

[Longboy]

Erm... I did, and my engine runs fine. The venerable walking beam engine has the power cylinder mid-way between the hot and cold ends.

No! Your power/cold cylinder is mounted midway up the "hot cylinder". I do see that you mean that the hot cyl has one end heated and the top of it water cooled. I got lost if you don't describe the Stirling parts properly. I don't model the tin can engines but my "Zephyr" fan engine has the displacer moving across the air passage way to the cold cylinder. In the tin can beam Stirlings the cold cylinder is mounted midway to the hot cylinder. My point was elseware in that the power cylinder is the cold end and where the airway ends.

[Longboy]

Longboy, you use the term "equalibrium". That is a very good explaination of the system. When the air is cooled the whole system experiences a low pressure equalibrium almost instantaneously. When it is heated a high pressure equalibrium is achieved almost instantaneously. This is the basic reason why the stirling engine runs. since the displacement pistons are 180 deg. off and the pressure inside is equalized, Their motion is unrestricted no matter what the pressure inside is. The power piston simply reacts to the change in pressure and turns the crank shaft.

No, it doesn't explain Stirling dynamics but explains heat energy flow and Stirling depends on this flow. The term "equalibrium" means no change or movement in pressure/temperature. Your use of this word describing what happens inside a running engine is incorrect. For if the pressure inside of a Stirling engine is at equalibrium..........it won't run! Your explanation regarding an "instantaneous change in pressure" thoughout the engines internal airways does not happen and is imposible for it to happen that way. You are missing the "flow" of heat energy. From the hot to cold side and there is a time period involved. I believe you have a handle on a pressure drop causes the engine to run but not on the science to explain whats happening inside to make it so.

[derwood]

I also believe that a beta type could be split up in a similar way. Thus eliminating a seperate power cylinder.

[Ian S C]

The first plans i came across , over 20 yrs ago, was in a 1944 copy of Mechanics inc English Mechanics&the World of Science, The Amature Mechanic & work. the motor is a gamma type, with the take of port to the power cylinder half way along the displacer cylinder, I started to build it, then found one in Model Engineer with the power cylinder at the cold end, I changed to that design, it seemed more logical. Ian S C

[derwood]

From what I have read about stirling engines over the last few months, I have come to the conclusion that the only reason for placing the power piston at the cold end, is just to keep as many moving parts as possible away from the heat source. This makes it easier to seal the power piston and piston material does not have to be as heat resistant.

[Ian S C]

Putting the power cylinder at the cold end ensures the greatest temperature difference, every little bit is important. Ian S C