How to Make your own CAE from Home (Mike Smyth's Design)

Here’s something fun we “Air-Heads” can try at home. About 2 years ago I came across a website by engine designer, Mike Smyth, that showed how to make a very simple compressed air engine. I was actually able to make a scaled-up model at around the same time and created a CAE of my own (which I will be posting on this blog soon). The only real difference between Mike’s design and mine (aside from my engine being scaled-up) was the intake valve. Here he used a simple rotary pipe intake valve while I used a single overhead cam shaft to control the intake valves. However, my engine design will be discussed on this site some other day. For now, let’s discuss Mike Smyth’s design…


Mike Smyth’s V-Twin Air Engine:

Configuration: 70deg V-Twin
Bore: 0.563 in
Stroke: 0.750 in
Displacement: 0.373 cu in
Intake Valves: Rotary
Intake Duration: 167 deg
Exhaust Valves: 0.125 in dia. port at bottom of stroke
Exhaust Duration: 122 deg
Timing Shaft: Gear drive

In Mike’s V-Twin, the bottom end of the engine (pistons, connecting rods, and crankshaft) is very similar to engines found in everyday automobiles. However, the top end normally consisting of multiple poppet valves, springs, at least one camshaft, rocker arms and push rods (unless it's an overhead cam design like my engine) is replaced by a rotary valve that controls the timing of airflow into the cylinders. All of the above mentioned components are replaced by a single hallow shaft with cutouts in strategic locations.


The picture below shows the timing shaft, timing gear and the intake manifold.


Compressed air is injected at the left end of the intake manifold tube. When the timing shaft is turned so the ports do not line up with the cylinder ports (the two tubes on the side of the main tube) the intake valves are closed and no air flows because there is nowhere for the air to escape (the end of the timing shaft by the gear is plugged). When the timing shaft is turned so that either of the ports is lined up with the cylinder ports, the intake valves are open and compressed air flows into the respective cylinders. By changing the location of the ports in the timing shaft, the firing order and relative timing can be adjusted. Changing the mesh of the timing gear and the crankshaft gear allow intake timing adjustment relative to the crankshaft position. Fine adjustments to intake timing can be made by rotating the intake manifold in the plastic friction mounts in either end of the block. This can be done while the engine is running.

The exhaust ports are simply tubes at the bottom of the piston stroke that open to the atmosphere to relieve the pressure in the cylinder. This is very similar to a 2-stroke engine. With this design, the exhaust timing is dependent on the location of the port in the cylinder and the duration is a function of the diameter of the port (a larger diameter port will have a longer duration).

Though Mike’s engine design is sound, it is still very sensitive to the air pressure used. There is plenty of torque on the compression stroke even with very low air pressure, but because there is only an exhaust port at the bottom of the stroke much of the power is wasted as the pistons are traveling up. The high cylinder pressure from the power stroke is relieved once the exhaust port is opened, but when it closes off as the piston travels up, the remaining air in the cylinder is compressed; robbing energy. This problem is made worse by the small amount of air that leaks into the cylinders around the rotary intake valve. Not only is the air in the cylinder being compressed, the leaking air also exerts downward force on the cylinder robbing more power. The main drawback to this design is the power loss on the upward stroke of the piston caused by the exhaust valves. On a 2-stroke engine, the upward stroke is the compression stroke. In this case, it's necessary to compress the fuel/air mixture and the compression is a good thing. The energy lost compressing the fuel/air is more than compensated for by the additional energy gained by igniting the fuel under pressure rather than at atmospheric pressure. However, in a compressed air engine, no additional energy is gained if the air in the cylinder is compressed on the upward stroke of the piston. Any additional energy gain due to the higher pressure on the power stroke can't be greater than the energy required to compress the air in the first place.

This initial drawback, however, was mostly solved when Mike redesigned the valve system to relieve the pressure in the cylinders on the upstroke of the pistons. Below are revised engine specifications and pictures with the new valves.


Mike Smyth’s V-Twin Air Engine with Improved Valves:

Configuration: 70deg V-Twin
Bore: 0.563 in
Stroke: 0.750 in
Displacement: 0.373 cu in
Intake Valves: Rotary
Intake Duration: 175 deg
Exhaust Valves: Rotary
Exhaust Duration: 175 deg
Timing Shaft: Gear drive

One way to alleviate this drawback would be to increase the exhaust duration by using a bigger exhaust port. This would allow the piston to travel up farther before the port is closed thereby reducing the power loss. However, this would also reduce the effective length of the power stroke because once the exhaust valve opens; there is no longer cylinder pressure to force the cylinder down.

To reduce power loss caused by simply increasing the exhaust duration, Mike, then, added exhaust valves to the timing shaft in addition to the intake valves. The picture below shows the redesigned valve. This configuration allows the exhaust valves to be closed during the entire power stroke and also allows them to be open through the entire upstroke.


The new timing shaft is plugged in the center between the intake and exhaust valves and is open at both ends. Air flows in the left side of the timing shaft and is distributed to the cylinders as before through the two left intake ports. Air is exhausted through the two ports on the right and out the gear-end of the timing shaft. There is also now a second port in the top of each cylinder that connects to the new exhaust ports on the manifold. On the upward piston strokes, the exhaust valves are open until the piston is nearly at TDC (top dead center). This corrects the drawback described above and allows wider adjustment of the exhaust timing and duration. With the redesigned valves, the engine is not sensitive to air pressure and will easily run just by blowing into the intake manifold.

I hope this wasn’t too technical! Anyway, I’m sure you air-heads out there can have some fun with this design. Also, if you scale-up, like I did, you might even be able to power a small generator set that could even be used to light up some parts of your house. With a little ingenuity, who knows what else you can do with this?

Source: Mike Smyth's Compressed Air Engines