Description: This piece was made for a Surrey Association of Woodturners competition entitled “Something that moves”. The design is based on a steam engine but uses a vacuum rather than steam to power the engine. The valve gear is based on the Stephenson linkage used on many steam trains and includes a simplified reversing mechanism.
Wood: Mahogany (Base and pistons), Lignum Vitae (linkages and cranks), Leadwood (Fly wheel), Ebony connecting pins.
Status: Not for sale
Size: 17cm diameter by 5cm high
Made: July 2020
Working Video
The video below shows the engine working from a workshop vacuum.
Design and principles of Operation
The air engine is based on a single cylinder that operates on vacuum pressure being applied alternately to the front and back of the piston. The vacuum pressure is controlled through a piston valve whose timing is controlled by a Stephenson gear linkage as illustrated in the image below.
The valve timing is controlled by a link (red in the diagram) attached at either end to two eccentrics (green and blue) 180 degrees out of phase with each other and at about 90 degrees phase to the piston. This means that if the link is raised, the phasing of the valve shifts by 180 degrees and the flywheel reverses direction.
Wood choices.
The wood used for the bulk of the Air engine was mahogany (or probably sapele). This was very dry and stable and of sufficient size to be able to make the matching base as well as thick enough to turn the piston cylinder. I also had a piece of leadwood from my father. This is a very dense (and hard to acquire now) South African hardwood so seemed an ideal choice for the flywheel. The eccentrics and linkages were made from Lignum Vitae. This is also a very dense wood from Central / South America but has a natural oil in it which makes it very good for sliding surfaces. The pins for connecting linkages to pistons, flywheel and eccentrics were made from ebony as a very hard wood contrasting to the Lignum Vitae.
1 Piston Assembly –
1a Cylinder Mahogany 10cm x 5cm x5cm square blank. The cylinder blank has two 2cm deep squares left for mounting the cylinder, with a 5cm 4.5 diameter cylinder between and 1cm 4.5 diameter end to bear the piston rod. The cylinder was then bored using a Forstner bit to 3.5 cm internal diameter and the piston rod hole drilled to 1.3 cm. The piston is then turned to fit these dimensions. Air inlet and exhaust holes were bored in the mounting squares 1cm diameter – effectively 7cm between centers. Two rectangular pieces were then cut and glued on to extend the height of the mounts. The end cap was turned from the same piece of wood to be a push fit into the cylinder bore.
1b Piston Mahogany 5cm x 5cm square blank finished length 12 cm.
The 7cm spacing between ports allows a 1cm thick piston about 6cm of travel between TDC and BDC. At TDC (piston out) the piston inner edge is 2cm from the outside of the cylinder and we need 3cm clear of the cylinder at BDC to allow space for the piston connecting rod joint. This leads to an overall piston and piston rod length of 12 cm including a 1cm thick piston and a 1cm thick piston support. The piston blank was mounted in a chuck with piston rod end to the tailstock. The piston rod was turned to diameter 1.3 cm so it was a reasonably sliding fit in the cylinder hole. This can be checked from the outside of the cylinder. Excess slack will mean air escapes round the rod, not enough slack means the piston will bind. When the rod has been profiled to 10 cm long, the piston and support can be turned to fit. In this case the open end of the cylinder is put over the piston and the rod inserted through the bearing hole until the piston presents against the bore. This allows the piston to be turned to a good fit and sanded before parting off. Note that polish is not used on piston bearing surfaces as it can cause binding.
1c Piston connecting pipes – Mahogany. The connecting pipes reduce the spacing between the piston inlet / outlets from 7cm down to the 3cm needed for the valve. They also provide the support for the valve. The support was made in two pieces allowing 45 degree holes to be drilled in a 9 x5x2cm thick piece of wood taking the spacing from 7 to 3 cm. A second piece of wood of 5x3x1 is drilled with holes at 3cm pitch is then glued on so the ports line up. This additional piece was needed to get enough spacing for the flyweel and axle brackets between the piston and valve rods. This assembly is then profiled to fit the cylinder and the valve casing.
2 Valve Assembly
The piston connecting block passes air from the valve casing through two feeds to the main cylinder. The feeds lead to the front and back of the main piston and are controlled by a valve piston. The end of the valve cylinder is sealed by a cap.
2a. Valve Casing. The valve casing provides a single hole in the centre opposite which takes the vacuum feed and routes it to either end of the piston according to the valve piston position. At each end of the casing are exhaust holes that let air into the cylinders according to the valve piston.
2b Valve Piston. The valve piston contains two pistons spaced so that in mid position, they cover the feeds to the piston. When moved to the right, the vacuum from the center feeds to the right hand end of the piston sucking the piston to the right and the left hand end of the piston can exhaust through the left hand valve hole. With the valve piston to the left the situation is reversed and the piston moves to the left away from the flywheel.
3. The Flywheel Assembly. The flywheel assembly consists of the axle brackets (3a), the eccentric axle (3b) ,the flywheel (3c) itself, and the connecting rod (3d).
3a Axle brackets. The axle brackets are made from mahogany with Lignum Vitae bearings to minimize friction and keep the eccentrics and the flywheel at the right spacing from the pistons. The brackets will eventually be positioned vertically so the flywheel axle is level with the centerline of the pistons. This means that the flywheel extends below the top surface of the base in a cutout. The distance of the axle from the pistons is also crucial and had to be designed to allow enough piston stroke and still retain the flywheel within the extent of the base. The axle brackets then sandwich the eccentrics on the axle to keep them in position.
3b Eccentric Axle. The eccentric axle is turned from Lignum Vitae to minimized binding. The two eccentrics axles are turned offcenter before turning the axle bearing down to the design diameter. The axle protrudes the other side of the inner axle bracket to hold the flywheel.
3c Flywheel. The flywheel is turned from a piece of leadwood passed on to me by my father. This is very dense and makes an excellent choice of wood for the flywheel. As can be seen above the fly wheel is recessed into the base. The key decision is then the radius on which to position the connecting rod. This must match the stroke of the piston allowing a full range of movement and maximizing power transfer.
3d Connecting Rod. The connecting rod is the final piece of this assembly and links the piston to the flywheel. Ebony pins are turned for the joints and an ebony spacer separates the connecting rod from the flywheel. The connecting rod must be long enough so that the joint with the piston does not interfere with the flywheel. Once this length is decided, the axle distance from the piston can be determined ensuring that the protruding piston rod is adequately long to support the stroke but not so long as to bind.
4 Stephenson Eccentric Assembly. The eccentric assembly provides the valve timing that applies the vacuum to either end of the piston. It consists of the Eccentric rods (4a), the link (4b) and the reversing lever (4c)
4a Eccentric Rods. The eccentric rods fit of the eccentric axle 3b and convert the rotary motion of the axle to a radial movement – each of which is 180 degrees out of phase with the other and about 90 degrees out of phase with the piston.
4b Link. The eccentric rod ends are pinned to either end of the link. This has a pin in the center that can be used to raise or lower the link. The link also contains a slot that holds the end of the valve piston (2b) allowing the link to be raised or lowered without changing the mean distance from the axle.
4c Reversing lever. The reversing lever has a cam attached to it so when it is rotated through 90 degrees, it lifts the link on the central pin shifting the phase of the valve by 180 degrees and reversing the engine.
Duncan Clark Woodturning 