I DESIGNED this vacuum engine a few years ago. Just recently, I decided to make some improvements as the original design suffered from rust deposits in the cylinder. This new version includes many changes including:

1. •Cylinder made from stainless steel.
   

2. •Piston and internal valve made from graphite.
   

3. •Increase diameter cylinder, piston and valve.
   

4. •Simplified mechanical construction.
   

The absence of rust and the self greasing property of graphite now ensure a 100% reliable performance with little or no  maintenance being needed. Enlarging the cylinder capacity by about 50% makes the power significant greater, contributing significantly to a less critical engine behaviour.
The number of parts is reduced and some parts are more distinct such as the valve catcher and the stroke pin behind the piston. Now it only necessary to fix this stroke pin on the pusher rod on the right place to give a very small clearance when the piston is on its ultimate back position.

Actually the principle of a vacuum engine is very simple. However, the classic construction with the external head valve (slide) that closes and opens the hole in the cylinder every cycle, is proportionally complex. But above all, the adjusting of this valve with its driving mechanisms is rather critical. Small deviations easily result in an engine running badly or not at all. These include:
1. A too high spring pressure on the cam disc acts as a brake on the engine. On the other hand, if this pressure is too low the system starts floating, and the timing of the head valve becomes incorrect. So adjusting the right spring pressure on the cam disc is rather critical;
2. The spring pressure transverse to the movement of the head valve is also critical. Too high means detrimental friction, too low causes a leakage between the surface of the valve and the cylinder;
3. When the working piston moves to the cylinder head (as a result of the under pressure) the flame gasses are compressed at the same time. Some time before the piston reaches the cylinder head the pressure in the cylinder becomes equal to the open air pressure. It grows to a restraining over-pressure if the cylinder hole is opened too late by the head valve. On the other hand, freeing this hole too early leads to a loss of engine power. So, one has to determine experimentally the width of the cam because this stipulates the movement pattern of the head valve. Besides that the moment where the pressure equals the open air pressure depends also to the cylinder temperature. This temperature varies but the movement of the head valve doesn't.
These three disadvantages gave me the idea to employ an internal head valve instead of an external one. In fact, this is a second piston that makes a small stroke, thereby closing and opening the cylinder hole on the inside instead of on the outside of the cylinder. This construction also makes it possible to drive the valve directly by the working piston in a very simple way.

Working principle of the design
When the working piston has reached the extreme position it has pushed the head valve over the cylinder hole by means of the small pusher that can move horizontally through a bore in the outside cylinder wall. At that moment the hot flame gasses are locked-up in the cylinder and start to cool down. This causes a partial vacuum in the cylinder and consequently the force on the working piston that pushes it back. In fact, this delivers the working stroke of the engine. This vacuum also works on the internal head valve and keeps it in the right position, so that the hole in the cylinder remains closed.
When the working piston moves back the gases in the cylinder are compressed as well. At the moment this pressure becomes equal to the open air pressure the head valve is automatically pushed open. In that way it works also as an automatic relief valve. 
A small cam on the working piston pushes the head valve mechanically to the ultimate position when the working piston is driven further by the flywheel. With that the way is cleared for the flame gasses to enter the cylinder when the working piston moves again due to the flywheel effect. This cycle repeats itself when the working piston has arrived again in the ultimate position.

Advantages of internal head valve
This design has several advantages compared to the previous one with the external head valve:
1. The rather precarious mechanism with camshaft, tumbler and springs is eliminated completely and, with that, the critical adjustments of the cam disc, the spring pressure on cam and on the head valve and the timing of this mechanism;
2. The airtight sealing of the internal head valve is simply a matter of making a good fitting in the cylinder just like the working piston;
3. A restraining over-pressure in the cylinder cannot occur because the head valve opens the cylinder hole at the moment the pressure equals the open air pressure;
4. The friction of the internal head valve in the cylinder is considerably lower than the that of the complete mechanism for the external valve and no spring pressures are needed;

5. The temperature of the internal valve is lower than that of the external one. The latter is radiated directly by the burner, as well as the spring that must keep this external valve tight to the surface of the cylinder;
6. The adjustment of the parameters to run the engine is inherent in the design. In other words, there is nothing left to adjust at all and nothing can drift away;
7. Because of symmetry this engine runs to the right as well as to the left. May be not an explicit advantage, but remarkable anyway;
8. This design is extremely simple and robust with a minimum of accessories. It is a nice model for inexperienced model builders.

Some specifications
-The piston and the internal head valve are made out of graphite. The most important reason is that that this material is self greasing and it has a low coefficient of expansion. This avoids jamming of the piston and/or the head valve in the cylinder. For the cylinder I used stainless steel to prevent any rust deposits. The bore in the cylinder must be exactly cylindrical and smooth. The diameter of the piston as well as the head valve must have minimal clearance (0.03mm or less)  so that they can move with very low friction and yet seal well.
- The pusher for the head valve is a steel rod with diameter 2mm and glides easily in a bore through the cylinder wall. On one side there is a catcher, driving the internal valve with some free space; this to avoid possible wriggling. On the opposite site of the pusher a small accessory is attached against which the piston strikes in the utmost right position to close the hole in the cylinder by the internal valve. This ‘striker’ must be fixed to the rod with a 2mm screw so that there is a very small clearance to the piston when that is in its ultimate right position. Only by unscrewing this striker is it possible to dismantle the rod easily and the piston and the head valve, for instance, to clean these parts and the inner surface of the cylinder if necessary.

Next time, Jan goes on to describe the adjustments that can be made to the engine and provides a trouble-shooting checklist and a full set of drawings to build the engine.

See part two here          See part three here