Friction
As highlighted previously our engine suffered from extremely high friction levels upon first assembly. After examination three major contributors to the high friction were diagnosed in the engine; Rough working piston stroke, flywheel support assembly and bent displacer shaft.
Taking a look at each individually we can see how these issues were resolved in an attempt to increase the chances of a fully functioning Stirling engine which could later be tested and compared with the dynamic ESL model.
Rough working piston stroke:
Clearly in a simple design like our Stirling engine a balance between air tight working piston and low friction movement can be difficult to achieve. When the components in question were first completed they suffered too high friction. Throughout the 22mm stoke length of the Stirling engine there was sections where the piston would snag. At these moments it was difficult to even continue the stroke manually by hand which would clearly mean that the engine would not operate efficiently, if at all.
The decision was made to use extremely fine wet and dry sand paper to smooth over the working piston. With this decision came the knowledge that slightly too much off the working piston would lead to the possibility of a poor seal. After much delicate sanding the result was a far smoother working piston, however we had clearly lost some of the vital seal between the two materials. The balance between friction and air seal had now switched to the other side. This was a necessary risk to give the engine a chance of running.
A solution to aid the air tight seal of the working piston was soon found. By applying heavy duty grease more commonly used in mountain bike suspension forks the integrity of the working piston was increase and the almost perfect balance of low friction and solid air tight seal had been reached.
Flywheel support assembly:
The first method of attaching each support to the hardwood base was by using screws. This seemed to be a fairly easy choice which should not have affected the engines functionality in any way. However soon after screwing down the flywheel support assembly it was clear that this method was not ideal. By screwing the supports onto the wood without first bedding the holes the residual wood rose to the surface and did not allow the supports to sit flat on the base. This resulted in them bending away from each other which lead to a bending moment on the axle. This bending moment resulted in the bearings to be forced at an angle to the axle which essentially seized them.
To solve this problem each support was mounted to the base via bolts. The use of bolts also gives us the flexibility to make alterations to the final placement of each section if the need occurred.
Bent displacer shaft:
When the components were given to us we were aware that the threads on displacer shaft which are used to attach to the clevis/displacer were off centred. This was taken to be a minor detail which would not cause any trouble and so was ignored and the engine was assembled. It was only after attempting to run the engine when fully assembled after improving the two other major friction factors discussed above that the engine still was not running completely freely. Our thoughts then returned to the squint threads on the displacer shaft as a potential source of friction within the regenerator. The small offset angle on the displacer shaft thread was accentuated by the length of the displacer so that it caused a large problem.
The displacer was designed to have a sliding fit with the regenerator, which in itself is an increased cause of friction within the Stirling engine. But when the displacer was mounted at an angle onto the shaft it essentially was increasing the frontal surface area of the displacer. This therefore became extremely difficult to move through the inner diameter of the regenerator and caused high levels of resistance.
In order to resolve this issue a new displacer shaft was manufactured.
Taking a look at each individually we can see how these issues were resolved in an attempt to increase the chances of a fully functioning Stirling engine which could later be tested and compared with the dynamic ESL model.
Rough working piston stroke:
Clearly in a simple design like our Stirling engine a balance between air tight working piston and low friction movement can be difficult to achieve. When the components in question were first completed they suffered too high friction. Throughout the 22mm stoke length of the Stirling engine there was sections where the piston would snag. At these moments it was difficult to even continue the stroke manually by hand which would clearly mean that the engine would not operate efficiently, if at all.
The decision was made to use extremely fine wet and dry sand paper to smooth over the working piston. With this decision came the knowledge that slightly too much off the working piston would lead to the possibility of a poor seal. After much delicate sanding the result was a far smoother working piston, however we had clearly lost some of the vital seal between the two materials. The balance between friction and air seal had now switched to the other side. This was a necessary risk to give the engine a chance of running.
A solution to aid the air tight seal of the working piston was soon found. By applying heavy duty grease more commonly used in mountain bike suspension forks the integrity of the working piston was increase and the almost perfect balance of low friction and solid air tight seal had been reached.
Flywheel support assembly:
The first method of attaching each support to the hardwood base was by using screws. This seemed to be a fairly easy choice which should not have affected the engines functionality in any way. However soon after screwing down the flywheel support assembly it was clear that this method was not ideal. By screwing the supports onto the wood without first bedding the holes the residual wood rose to the surface and did not allow the supports to sit flat on the base. This resulted in them bending away from each other which lead to a bending moment on the axle. This bending moment resulted in the bearings to be forced at an angle to the axle which essentially seized them.
To solve this problem each support was mounted to the base via bolts. The use of bolts also gives us the flexibility to make alterations to the final placement of each section if the need occurred.
Bent displacer shaft:
When the components were given to us we were aware that the threads on displacer shaft which are used to attach to the clevis/displacer were off centred. This was taken to be a minor detail which would not cause any trouble and so was ignored and the engine was assembled. It was only after attempting to run the engine when fully assembled after improving the two other major friction factors discussed above that the engine still was not running completely freely. Our thoughts then returned to the squint threads on the displacer shaft as a potential source of friction within the regenerator. The small offset angle on the displacer shaft thread was accentuated by the length of the displacer so that it caused a large problem.
The displacer was designed to have a sliding fit with the regenerator, which in itself is an increased cause of friction within the Stirling engine. But when the displacer was mounted at an angle onto the shaft it essentially was increasing the frontal surface area of the displacer. This therefore became extremely difficult to move through the inner diameter of the regenerator and caused high levels of resistance.
In order to resolve this issue a new displacer shaft was manufactured.