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Compressed Air V6 Engine Design

By John and Whit

A few months ago, instead of studying for exams, Whit built a small 3 piston (1/2" bore) radial engine powered by compressed air. It generated about 15 in-lbs or torque @ ~7000 rpms. It was a fun and simple project. Seeing his air powered engine made me want to build a small compressed air engine, but I wanted a bit more power than the one he had designed. One alternative would be to go to CO2 as a power source since liquid C02 produces a pressure of about 800 psi at room temperature. We decided against this for two reasons. The first reason is that CO2 isn't free (even though I can get it free from the local paintball stores/fields) and the other reason is that when we calculated the run times, the engines would have a very finite run time before the CO2 would run out. This engine was going to be used for display purposes, and we wanted something that would run for long periods of time.

There's only three other ways to increase the power output without moving away from compressed air. The first way would be to increase the diameter of the piston. Another way would be to increase the length of the stroke. The third way would be to increase the input pressure. We decided to go with a 1" diameter piston and a 1/2" stroke. The reason for keeping the stroke so short was to reduce air consumption in order to maintain a reasonable RPMs given the flow rate of a non-industrial compressor. A standard Craftsman 6hp compressor available from Sears ($330) can put out 150psi @ 6.4 cfm, so we went with those numbers when designing. In reality, we will be using the school's compressed air supply lines which can output 200 psi at an extremely high flow rate, but the following calculations are based on the Craftsman compressor.

For a 1" diameter piston, the surface area of the piston is:

Surface Area = π * r² = 3.14 * (½ )² = .785in²

Given an input pressure of 150 psi from the compressor, the force produced on the piston head is:

Force = Pressure * Surface Area = 150 psi * .785 in²= 117.8 lbs

The stroke of the piston (length traveled) is twice the offset of the connecting rod on the crank shaft. The moment (torque) applied to the crank shaft given a 1/4" offset for the crank rod is:

Moment = Force * Offset = 117.8 lbs * .25in = 29.5 in-lbs

This gives us an engine 2x more powerful than the previously built radial engine. The v6 design was chosen just because we thought it would be cooler and because it will deliver the power more smoothly. The next step is to see what sort of theoretical speed we should be getting from the engine. This is calculated using the flow rate available from the compressor.

Flow Rate = displacement of engine * rpm

where the displacement of the engine is:

Displacement = Stroke * Surface Area * # of pistons = .5 in * .785 in² *6 = 2.355 in³ = .00136 ft³

Plugging this back into the flow rate equation gives us the theoretical rpm as:

RPM = flow rate / displacement = 6.4 cfm / .00136 ft³ = 4700 rpms

In reality, this number will be a bit lower due to friction, compressed air blowing by the pistons, flow restrictions within the engine and it's fittings, the amount of extra space between the piston and the top of the cylinder, and the volume of space between the valve itself and the cylinder. Overall, we decided this design was acceptable, producing a reasonable amount of power and giving us the cool factor that we were looking for. Also, running off the school's air supply, we should actually be able to develop closer to 40 in-lbs of torque @ 5500 rpms.

Continue on to the piston design & machining...