Air Engine Linkage Design
You may have noticed that the valve and power piston assemblies go through their cycles independently of one another in all of the animations, as if by magic. The next step in designing the air engine will be to devise a means for driving the valve piston, by using the motion of the power piston. There are a number of possible ways to achieve this, but some of the more practical ones are:
1. Attach gears to the valve crank and power piston crank, and allow these to mesh with each other. The gears need to be the same size, so that the valve and piston cranks move the same speed. A major disadvantage of this design is that it requires gears large enough to span the space between the power and valve cylinders, and these gears can be expensive (and noisy!) Play the animation above to see the geared linkage in action.
2. One way to overcome the disadvantage of the gear design is to attach sprockets to the power and valve cranks, and use a timing belt or chain to connect them. Both the gear and chain designs require on precise angular orientation of the valve crank to achieve the desired timing. The designer must take special care to lock the gears or sprockets tightly to the shafts, because any slippage will result in the engine being mistimed. Note that the valve crank spins in the same direction as the power crank with a chain, but in the opposite direction with the gears. This will affect the timing of the engine, of course. Play the animation above to see the chain-driven linkage in action.
3. The design used in automotive engines is, of course, the camshaft. A cam is a an object that rotates, usually not about its center, and makes contact with another object which it moves. In the animation above, the large, purple, egg-shaped object is the cam. As it rotates, it presses against the follower attached to the end of the valve piston. The eccentricity in the shape of the cam pushes the valve piston back and forth. Note that the design in the animation would not work, the animation is only for showing the functioning of the cam.
The cam design can be made to mimic the chain-driven crank design simply by using a circular cam, and mounting it off-center. In this way, the valve will trace out a sinusoidal path, much like the crank design. There are a few difficulties with the cam design for the student or home designer:
- The cam itself may be difficult to fabricate. It should be of sufficient hardness to withstand the wear of rubbing against the follower. This may be alleviated somewhat by using a roller bearing for the follower. CNC machinery may be required for complicated (i.e. non-circular) cam shapes.
- A spring for keeping the follower pressed against the cam is required. This spring must be of sufficient strength to keep the follower in place, even under the large inertial loads of the valve piston when running at high speeds. If the spring is too strong, it creates friction between the cam and follower, and drains excessive amounts of torque from the power piston. If it is too weak, the follower will lose contact with the cam at high speeds (a phenomenon known as 'valve float').
- The valve rod must be exactly aligned with the center of rotation of the cam, otherwise the cam will exert a strong side load on the valve, resulting in excessive friction and wear. This requires a very precise alignment of the valve cylinder and camshaft bearing, which may be difficult to achieve with manual machinery.
4. A simple, but effective, design is the linkage-driven valve piston, like the one shown in the animation above. In this design, the valve crank (light purple) is attached to the same shaft as the power crank (green). The valve crank drives a connecting rod (dark purple) which is connected to a rocker (fuschia). The rocker has a slot, into which a pin at the end of the valve slides. The rocker is pinned to 'ground' near its midpoint, so that its motion is a simple rocking back-and-forth. The length of the valve crank determines the range of motion of the valve, and the angle it makes with the power crank determines the timing. Since all of the parts in the assembly are simple links, this design is well within reach of the student or home designer. First, a couple of observations:
- As with the geared design, the valve piston moves in the opposite direction from the power piston. This must be taken into account when timing the engine.
- As with the geared design, the angle between the valve and power cranks must remain fixed, in order for the timing to be correct. There are a few different ways of achieving this, which will be discussed in the engine timing page.
- The connection between rocker and valve piston must be a slot, since the end of the rocker traces out a circular path, but the valve traces out a straight line. If you try to pin the rocker directly to the valve, the rod will bind in its guide bushing.
There may be other methods for actuating the valve piston, but the main ones have been presented here. If a reader has a suggestion for a different type, I'd be happy to try to animate it! The next step is to design the timing for the engine, which can be found on the Engine Timing page.
