Mike Smyth's Compressed Air Engines 

Welcome to my compressed air engine page! Below you'll find information about the V-twin and radial air engines that I designed and constructed.

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Air Engine FAQ

SF376

Configuration: 5 cylinder radial

Bore: 0.4375 in (7/16")

Stroke: 0.500 in

Displacement: 0.376 cu in

Intake Valves: Rotary

Intake Dia.: 0.0938 in

Intake Duration: 44 deg

Exhaust Valves: Rotary

Exhaust Dia.: 0.125 in

Exhaust Duration: 60 deg

Weight: 6 oz.

 

 

This is a 5 cylinder radial engine that has the same displacement as the V-twin below. The radial cylinder configuration has the advantage that at least one cylinder is on a power stroke at all times. This allows the engine to self start from any position and it also allows the engine to run at very low RPM without a flywheel. The valve design is similar to the V-twin except the valves are integrated with the crankshaft. This eliminates the need for the external timing gears. The intake and exhaust timing are independently adjustable.

The construction of the SF376 is very similar to the V-twin. All of the metal parts are made from brass tubing and sheet from a local hardware store. The white parts are made from a plastic cutting board. All of the parts were made with basic tools - drill press, belt sander, bench grinder, hacksaw, dremel tool and a vise. The brass parts were soldered together using a small butane torch and plumbing solder.

The videos below show how the engine performs under different conditions. All the videos except the first one include sound. The first video shows engine with one side of the crankcase removed. The crankshaft is slowly rotated through a full rotation to show the connecting rod configuration. If the video player is put on continuous repeat it will look like the engine is turning continuously.

In the second video, the engine is idling at only a couple hundred RPM on about 1-2psi. without a flywheel. The third video shows the engine revving with no load, and the fourth shows the engine spinning an 18" 3-blade propeller. Even with the fairly heavy load, the engine accelerates rapidly. The maximum pressure in this video was about 45psi. The engine would easily generate enough thrust to lift it's own weight vertically.

Video 1 (340Kb) Cutaway view showing the internal movement

Video 2 (1.8Mb) Engine idling slowly with no flywheel

Video 3 (2.0Mb) Engine revving with no load and no flywheel

Video 4 (2.6Mb) Engine powering an 18" 3-blade propeller

I am working on a set of plans with step by step instructions for the SF376. These will be available from this web site for a small fee.

 

V-Twin

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

 

 

The bottom end of the engine (pistons, connecting rods, 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) 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).

This engine design worked (barely) the first time, but is 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 cyclinder 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 cyclinder 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 valving. 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.

Because of this major drawback, I redesigned the valve system to relieve the pressure in the cylinders on the upstroke of the pistons. One way to alieviate 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.

Because I did not want the power loss caused by simply increasing the exhaust duration, I 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 may make a small dyno with a DC motor to measure the actual power output of the engine at various intake air pressures. Below are revised engine specifications and pictures with the new valves.

 

V-Twin 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

 

 

 

Not shown in the pictures above is the flywheel. The flywheel I've been using is simply a bar with a bolt in each end for weight. It serves the purpose, but is pretty ugly so I didn't include it in the pictures. Once I turn down a nice steel flywheel on the lathe I'll post some pictures with the flywheel. With the improved valve design, the engine will actually run without the flywheel, but it will need one if I do any dyno testing.

The engine could also use some crankshaft counterbalance to make it run smoother. I may try to add some weight to the crankshaft and maybe offset the flywheel weight a little.

Here is a video of the V-twin engine running with a simple flywheel. It is simply a piece of plastic with a large nut and bolt at each end. It's a little crude, but it offers a significant load as the engine accelerates. The picture below shows the flywheel.

Video 1 (1.4M) Engine accelerating with flywheel.

 

 

 

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