Pressure's effect on spring rate.

[dn24210r]

I have a little project I have been designing for 3 months now off and on. Basically it is a double acting steam engine. No this is not for homework and I am not taking any type of classes. I am just interested in the power of steam lately and believe I can build a steam engine. Yes I am plenty aware of the dangers and am taking caution upon precaution to ensure my safety.

After several design changes this is the layout I have decided to start building. Note these are very crude drawings. I am more of a person who just builds things then one who designs them.

Basic layout;


Valve in detail; With the one on the left being fed steam and the one on the right just being fired and now closed for the steam to expand.


Basically I am taking an ICE inline 6 engine that is internally balanced and removing all the cylinder rings except for one to reduce the friction and retaining the cam to allow the oil pump to feed the short block. The Ice cylinders become dead moving within the cylinder sleeve so that the steam rod mounted to them and above them is able to move along a vertical axis. The head is of course gone and replaced with my version of a double acting steam cylinder.
The steam cylinder does not have any rings. It is designed to be operated at high rpm with very tight clearances to the walls to prevent leakage. I am doing this with a small cylinder with seals and bearings on top and bottom to retain the rod and keep the piston centered. The piston is slightly curved upward towards the edges and has 2 protruding rods. These rods will push up two valves injecting steam into the cylinder. The exhaust is centralized for simplicity allowing higher rpm use.


Now that the introduction is out of the way my issue that I would like help with. The valves are a simple design in which the piston pushes them up. The crankshaft rotates so two pistons are being forced down at the same time. The rate of engine speed is controlled by pressure alone. The maximum pressure I would like to operate at is 1200psig and the lowest a meager 120psig. This is not using a boiler. I am using a steam generator that is capable of maintaining the 1200psig in a short amount of time.

Because I am using springs to hold the valves down I need to calculate that spring rate. But what I do not know is how the pressure varying from 120 to 1200psig pushing against that valve will affect the requirements for springs to keep that valve closed. To add to this the springs will be within a 540 degree environment at 1200 psig so the material must be able to preform in those conditions.
If I choose a spring with to high of a spring rate then the rod mounted to the piston will hit the valve with higher force then is needed to inject the steam. If I choose one with to little spring rate then there is the possibility of the spring not sealing the valve when it should be closed causing engine failure or at the very least a loss in steam right out the exhaust.

To further add to this Because this is a high rpm engine I am trying to eliminate friction where ever possible. The movement and weight of that spring both creates friction and resistance. A smaller spring with a spring rate just enough to keep the valve closed is what I am after. In the pictures I have a spring that is a 1/4" OD and 5/8 long with the valve rod passing inside it being an 1/8".


The cylinder is going to be 2.5 ID and 3.4" stroke with a exhaust port .5" diameter in the center of the cylinder.
The ICE engine is a push rod engine and will retain it's oiling. But the steam cylinder and springs in question will not have any oil due to the need to circulate the water and issue a separator will cause. This will further add to the fatigue.


I am willing to destroy a few short blocks in the process of fine tuning this spring rate. But if I can get a good amount of information through links and pdf's to get that spring rate at a close starting point then that would be very helpful.

[Cake of Doom]

Have you had a nose through the calculators on this site yet? There is loads of useful bits in there.

[dn24210r]

That's the thing.
I can calculate spring rate.

I can calculate the pressure on the head of the valve.

But I need to know if it is simply a manner of subtracting one from the other or if one is influenced. Like for instance a spring is going to get resistance if moving through water. I do not see steam having that effect but there is pressure it is working against.