The Rand Cam engine is a very simple, powerful and potentially very efficient rotary internal combustion engine, currently under development by a company called Regi Technologies.

One of their first prototypes, which is about 6 inches across and weighs only about 18kg, runs on diesel fuel and develops about as much power as the engine in our little Fiat Punto. Not too shabby.

It's valveless, and has (depending on how you count them) of the order of 13 moving parts, a high power to weight ratio and is incredibly quiet in operation. So, naturally enough, Regi have scooped up a lot of funding for development from the military sector for use in unmanned spy planes.

And, like all of the truly great ideas, it's simple enough that it makes you want to smack your forehead and say "Hey, why didn't I think of that?"

Principle of operation

So, how does it work? The fundamental operation of the thing is still the good old-fashioned 4-stroke Diesel variant of the Otto cycle: induce air into a variable-volume chamber, compress the air by contracting the size of the chamber, introduce fuel and ignition to increases temperature and pressure in the chamber and thus exert force against a load while the chamber expands, and then the exhaust is evicted from the chamber.

Reciprocating IC engines have a cylindrical chamber, with a piston which moves in that chamber to alter its volume and achieve compression and power extraction. The Rand Cam engine has none of that, so I can't really compare it to a reciprocating engine. Instead, I'll compare it to a Waltzer.

Yes, a Waltzer.

If you've ever been to a fairground, the chances are you've ridden on, or at least seen a Waltzer. The Waltzer is a ride with rotating carriages mounted on a segmented floor which undulates up and down as the ride turns round: the supports for the floor ride over a static but sinusoidally undulating track, or cam, and as the floor surface rotates, it follows this track up and down, getting closer to the ceiling as it reaches the top of the cam, and falling further away from the ceiling as it falls back down to the low point of the cam.

And this is the essence of the Rand Cam engine's operation.

The engine, just like a waltzer, is generally cylindrical, with a rotor turning on a central axis. The bottom surface of the combustion chamber is fixed and doesn't rotate, just like the track on the waltzer. The rotor has a disk which corresponds roughly with the position of the ceiling of the Waltzer, forming the top surface of the combustion chamber. Held in slots in the rotor, 12 vertical vanes, which divide the engine radially into 12 chamber segments, like the hour markings in a clock. As the rotor turns, these vanes move up and down in their slots, riding along the bottom surface of the chamber (the cam), carrying with them a quantity of gas which is compressed into a small volume as the vanes ride to the high points of the cam, and expands again as they ride to the low points.

I'll attempt to illustrate through the magical medium of ASCII art. If we were to 'unwrap' the engine along the surface of the cylinder, here's an approximation of what we'd see:

         .              .              .
         |  |  .     .  |  |  .     .  | ←---- vertical vanes
        -|--|--|--|--|--|--|--|--|--|--|-  ←-- rotor disk top surface
        _|__|__|__|__|__|__|__|__|__|__|_  ←-- rotor disk bottom surface
        ----|_ |  |  |_----|_ |  |  |__-- ←--- bottom cam
              --__|_--       --__|_--

The engine's chamber is formed between the bottom surface of the rotor disk, and the top surface of the bottom cam, and the 12 vertical vanes divide this up into 12 individual chambers.

The cam has two peaks and two troughs (think 'sin(2θ)') around its circumference. Starting at the first peak, as the rotor turns the vanes will ride downwards, causing the chamber segment to expand. So, between the first peak and the first trough of the cam, there's an intake port cut into the cam through which air is drawn: making this the induction stroke of the Diesel cycle.

As the trailing vane nears the trough, it passes the end of the intake port and the chamber is once again sealed.

Riding up the cam towards the peak, the air in the chamber segment is compressed, heating up in accordance with Boyle's Law.

Just after the second peak, when the air is at its most dense, there is a fuel injection port in the cam. Through this, diesel fuel is sprayed, igniting in the already hot air and increasing the temperature and thus pressure of the gas in the chamber. As the leading vane of the chamber is further down into the trough than the trailing vane, still close to the peak, it has more exposed surface area, resulting in a net force pushing the rotor in the direction it was already turning: and so this forms the power stroke.

The rotor continues turning through the power stroke to the bottom of the second trough of the cam, where the chamber stops expanding. The segment of the cam which defines the slope back up towards the first peak has an exhaust port cut into it, so as the vanes ride up the slope, the chamber contracts and pushes the remaining exhaust gas out through the exhaust port, until the chamber reaches the top of the first peak, and the cycle begins again.

Since the vanes divide the engine into 12 chambers, there are 12 such cycles per revolution of the rotor, hence the claim of 13 moving parts: a rotor and 12 vanes.

You may have spotted one or two little problems with this so far. First, there's the small matter of the top ends of these vanes. The bottom ends ride along the surface of the cam, but what keeps them against that surface?

The answer is that the engine is approximately symmetrical: the rotor's top is another disc, through which the top ends of the vanes protrude, and their top surfaces ride along a cam, the mirror image of the bottom cam and rotated through 90 degrees so that the top cam's peaks correspond to the bottom cam's troughs, keeping a constant distance (equal to the total height of a vane) between the two cam surfaces. So, as the vanes ride the downward slope of the bottom cam, they are pushed firmly against the cam surface by the 'upward' slope of the top cam, and vice versa.

Clever? I thought so.

Crows and pawns

Thus the engine has a total of 24 combustion chambers (12 on the top, 12 on the bottom) and a total of 24 combustion and exhaust events per cycle, making for a very smooth, very quiet system.

Far simpler than a piston engine, it has no valves (only ports), and because the fuel injection port is not connected to the compression part of the cycle, fuel injection can be almost continuous, vastly simplifying the mechanical components of the fuel injection system. This also makes it incredibly clean-burning.

The weak point, much like the Wankel rotary engine, is the sealing between the cam and the vanes: these surfaces continually move against each other, and are pushed together by the inertia of the vanes themselves with a force proportional to the mass of the vanes and the speed of the rotor, so will be liable to wear, and degradation of the seal between these will cause power and efficiency losses.

At least one independent company is working with Regi, investigating materials-oriented solutions to these problems. Because the characteristics of the Rand Cam engine are so different from those of reciprocating engines, different materials may be used. In particular, since combustion can be continuous, percussive ignition is eliminated, making it viable to use low-friction, hard-wearing ceramic materials which would be considered too brittle to be used in a piston engine.

Since there are no valves, and the fuel handling system can be vastly simplified compared to today's existing systems, a Rand Cam engine can be incredibly flexible about its choice of fuel. The prototypes have been running on diesel fuel, but with minor modifications to the fuel system, it could burn SVO, ethanol, LPG, hydrogen, or almost anything else that might come to hand. Hell, even petrol if there's any of that shit left.

Although prototype engines so far have been normally aspirated, there's absolutely nothing to stop a Rand Cam engine from being fitted with a turbocharger or super turbo.

The first prototype of this engine was only fired up in 2005, and things look to be moving quickly, so we should expect to see a lot more interesting development during the next year or so.

More information and propaganda than you could shake a pinch of salt at is available on Regi's website, including pictures, diagrams, and the rather impressive videos for drumming up investment capital, featuring a couple of actors and a bloke in a garage and a baseball hat, who I presume is the inventor of the thing.

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