hit&miss

1. The idea
After designing and building three 4-stroke and three 2-stroke engines I had the idea to build a hit&miss engine.
With a 4-stroke hit&miss the exhaust valve is blocked with a governor system on the fly wheel at a certain high revolution speed. At that time the exhaust valve is kept open and, as a result, there is no power because there is no compression and no vacuum for sucking-in the fresh gas mix in this ‘miss’-phase. The speed goes down up to the moment where the exhaust valve is unblocked again and the normal compression, the sucking-in and the ignition of the gas mix, occurs again. In this ‘hit’-phase the speed is increasing and the hit&miss process is repeating. In fact, it is just an unusual way to limit the speed of the engine.
The sound of these hit&miss engines is very characteristic: a distinctive "POP-whoosh-whoosh-whoosh-whoosh-POP" as the engine fires and then coasts until the speed decreases and needs to fire again to maintain its average speed.
Hit&miss engines were in vogue in the years 1900 to about 1930, mainly for pumping machines in agriculture environments. At that time one didn't have disposal of the modern techniques to stabilize a stationary engine so the hit&miss process was used.
These engines were rather heavy with low speeds and, mostly, two heavy fly wheels to keep them going on during the miss-phase. It is the very characteristic "POP-whoosh-whoosh-whoosh-whoosh-POP" sound that makes these engines still popular for model builders who remember this sound from walking through farms. Look and listen to the video for a nice demonstration. The practical usefulness of this kind of engine is gone now but the nostalgia around it remains and that keeps the building of hit&miss model engines popular.
Two-stroke I/C engines don't have an exhaust valve at all, so it is not obvious to think about a hit&miss two stroke engine. It is even comprehensible to think that it is not possible at all to make a hit&miss two-stroke engine. As far as I know there was only ever one, the May Tag, but there only the ignition spark is disabled during the miss-phase, while the compression and the sucking-in of the fresh gas mix is going on. Not the real deal, if you ask me, because the engine must wrestle with the compression during the miss phase and the fresh gas mix burns in the hot muffler causing a lot of smoke and pollution.
But I always like challenges and realized that a ‘real’ 2-stroke hit&miss should be possible using the fuel by-pass system that I developed for my Pressure Controlled two-stroke engine, see also this animation. The trick is to keep the ball in the (upper) intake check-valve lifted during the time between a certain high and a certain low revolution speed. In that lifted position the chamber above the piston is continuously connected with the chamber below the piston. The result is that there is no under- or over pressure in the cylinder during that miss-phase so the engine is turning around freely "as driven by the wind", driven by the absorbed energy of the fly wheel. There is also no fuel consumption during that miss-phase. At the moment ball is falling back on its seat again the normal two-stroke process is recovered (hit-phase) and the engine is delivering power again.
The animation below demonstrates this process.

2. General design
The basis for this engine
Apart from some scale enlargement, the point of departure for this engine was the design for my Pressure Controlled 2-stroke. I was pretty sure that this enlargement was necessary to drive the extra mechanisms for a system to lift the ball in the check valve. I chose a 24mm diameter and stroke for the piston instead of 18mm resulting in a a cylinder capacity of 11cc instead of 4.5 cc. The remaining part of the basic engine was a matter of enlarging all dimensions in proportion. Because the Pressure Controlled 2-stroke engine runs perfectly I didn't need to worry about the behaviour of this engine in its normal (hit) phase.
The mechanism for lifting the ball in the valve
The movement of the mechanism that has to lift the ball in the valve must be derived from the speed of the engine. With all 4-stroke engines the exhaust valve is blocked by means of a ‘governor’ system with weights in the fly wheel moving due to centrifugal forces. These weights are connected to a lever system ending in a mechanism that block the movement of the exhaust valve at a certain high speed to start the miss-phase. At a certain low speed the same mechanism unblocks the exhaust valve again causing the engine to restart in its hit-phase. For me this is a rather complex mechanical system so it brought me the challenge to design something that is easier-to-make.
In fact I had to solve two problems: how to lift the ball in the valve and how to make a speed dependent mechanism for it. I worked out several solutions on paper but I was not satisfied with them. So it was time to brainstorm this with my good friend Jos who always inspires me with smart technical solutions combined with a positive critical approach.
Just before that I had discovered that I could just as well use a steel ball in the valve instead of a neoprene ball. This solved my first problem easily: the ball could be lifted with a Neodymium (NIB) magnet. These are alloys of the rare element Neodymium with iron and borium, and are extremely strong. You can buy such magnets of all sizes everywhere and for very little money see: http://www.supermagnete.nl/eng/magnets.php?group=discs_big. For lifting the steel ball I used a magnet with diameter of 6mm and length 8mm. It can lift about 1 kilogram, which is far more than needed here.
The last challenge was to design as simple a mechanism as possible to let the magnet move in front of the ball valve depending on the speed of the engine. After a brainstorm with a good friend we got the idea to apply an ‘Eddy Current’ clutch like that used in speedometers for (old) automobiles and other motor vehicles. The spring loaded pointer is coupled contact-less via such an eddy current clutch to the cable that rotates with the speed of the wheel axis of the vehicle, and that was exactly the system that I needed here.

The principle of the Eddy Current clutch.
If a magnet is moving along a metal plate it induces eddy currents in that plate causing counteracting magnetic fields. The strengths of these eddy currents and, with that, the counteracting forces depends on the magnetic field in the plate and the speed of the magnet. If the plate can move freely it will move with about the same speed as the magnet below it. If the plate is loaded with some force this eddy current clutch will start slipping. The slipping force can be influenced in several ways, such as:
- Changing the strengths of the magnetic field by means of changing the electrical current in electro magnets. This is done, for example, to regulate the speed of machine staples. Changing the magnetic field in the plate can also be done mechanically by changing the distance of the magnet to the eddy current plate.
- Changing the speed of the magnet. The speedometers are based on this effect and also the ‘governor’ system for this hit&miss 2-stroke engine.

My experiments with the Eddy Current clutch.
To find out if I could make an eddy current clutch with the required force and geometries I did extensive experiments. I made an aluminum disc into which I pressed Neodymium magnets, and aluminium and copper discs in which the eddy current could be induced. I put the disc with the

magnets in the head of my milling machine and the eddy current disc on a axis below it with a light spring on it so it could only make some angular rotation. In this way I could vary all kind of relevant parameters and determine the influences of them on the angle rotation of the eddy current disc. I will not list all the result in figures here but only my most important conclusions:

1. The material of the discs.

It will be clear that the disc with the magnets must be made from non-magnetic material, so I choose aluminum. For the eddy current discs I did experiments with aluminum and copper. The better the electrical conductivity is the stronger the eddy currents will be and that's what I found: the induced torque force in the clutch was 2 to 8 times higher with copper compared to aluminum, depending on the distance of the magnets to the eddy current disc. During these experiments I varied this distance between 0.5 and 1 mm.

2. The distance of the magnets to the eddy current disc.

Remarkable was the big difference between aluminum and copper in this respect: per 0.1mm distance increase, the torque force reduces with 6% for copper disc but about 20% with the aluminum disc.

3. The number of magnets.

With four magnets in the driven aluminum disc the torque forces were about 60% higher than with two magnets. With three magnets this difference was about 40%.

4. The circumference of the magnets.

With the magnets on a circumference of 44mm the torque forces were about 3.5 times higher than on 22mm; a big influence also. The choice of the circumferences was arbitrary with these tests.

5. The rotation speed of the magnets.

This influence was linear but with a magnet circumference of 44mm it was about 1.5 times bigger than with a 22mm circumference.

The results of all these experiments led me to choose four Neodymium magnets in an aluminum disc with a circumference of 40mm and a copper eddy current disc. The optimal distance of the magnets to the copper disc has to be determined on the engine itself. There are more forces playing a role in this system: the pulling force between the steel ball and the magnet and the force of the spring in the other direction. I replaced this spring by a contra weight later on, with the arguments you can find further on.

Next time we will look at the rest of the design of the hit&miss engine. Go to Part 2 Go to part 3