Mike Smyth's Compressed Air Engine FAQs 

If you have a question about my air engines, please look through the frequently asked questions below to see if it has already been answered. If your question is not on the list, you can e-mail me at the link at the bottom.

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Q1

I would like to purchase plans for the SF376 radial engine. Are they done yet?

A1

I have a lot of the work done on the plans, but I have been struggling to make time to work on them. I will have a link posted on the website once they are ready.

Q2

I would like to build the V-twin engine, will there be plans available for it?

A2

At this time, I don't intend to offer plans for the V-Twin. However, the construction methods for the V-Twin are the same as the SF376 radial. With the SF376 plans and some imagination virtually any type of engine configuration could be constructed.

Q3

How much will the plans cost?

A3

I have not set a final price, but probably $14.95. The other material should be available from Wal-mart and a hardware store or hobby shop for about $30.

Q4

How is the engine lubricated?

A4

Brass to brass friction is pretty low at light loads with no lubrication when you turn the engine by hand. Because I have run the engine with significant loads, I lightly oiled everything, including the cylinders/pistons, with a thin machine oil. This actually created more resistance when turning the engine by hand, but should help when the engine is running under load. Oiling the pistons also helps reduce the air leakage, although there isn't much leakage dry. There is no pressurized lubrication system that would be required for sustained heavy duty operation.

Q5

Can these engines be used to power an automobile?

A5

As they are, my engines are far too small to power a full-size car. They also don't have a lubrication system that would be required for automobile use. However, engines like the one from MDI (http://www.theaircar.com) have successfully powered cars and the basic concepts are the same as my smaller engines. Getting enough air storage for a reasonable range is a significant challange for an automotive application.

Q6

Could these engines be used to power a small R/C aircraft or boat?

A6

The SF376 wasn't specifically designed to be lightweight, but it has enough power output that it may be able to power a slower moving aircraft with a large wingspan. The biggest issue I see is getting enough on-board air storage to get a usable amount of run time. The best storage area would probably be to pressurize the entire fuselage or hull, but the run time would probably be less than one minute without using exotic materials like carbon fiber air tanks at very high pressure.

Q7

How much power output do your engines have?

A7

I haven't taken measurements yet, but I'm working on a set-up to measure the power output and will post it on the website. I think I can get fairly accurate speed/torque readings (and calculate horsepower) using a home made inertial dynamometer. I estimate a couple tenths of a horsepower at 30-40psi.

Q8

How long will the engines run on a tank of air?

A8

They will run at low speed for a couple minutes at no load on about 0.3ft^2 of air compressed to 120psi. I will have more data at different speeds and loads once the dyno measurements are complete.

Q9

What are the weakest parts of your engines?

A9

The V-twin uses a very crude crankshaft and is not balanced very well (this could be fixed). The radial uses a better crankshaft design that includes a counterbalance. The weak point is the joint between the main shaft and the plate that holds the rod journal. It relies on a solder joint to tranfer the torque to the crankshaft. This could be improved, but so far I haven't had any trouble with it. It should be more than sufficient to run for demonstration or for experiments. I don't have a lot of runtime on the radial, but I have pushed it pretty hard a few times with a large propeller and a large flywheel without any trouble.

Q10

Can I convert an internal combustion car engine to run on compressed air?

A10

I have heard of people converting an internal combustion engine to run on compressed air, but have not done it myself. One of the changes to the engine would be to modify the camshaft so the valves open/close at the correct time for an air engine. The biggest challange would be to get enough air storage to power the engine for a usable amount of time. To get enough air storage to get a reasonable driving range, you either need big tanks or smaller tanks at a very high pressure. For example, the MDI car (http://www.theaircar.com/) stores air in tanks at around 4300psi!

Q11

If air storage is a problem, couldn't you put an air compressor on-board the car powered by a motor and alternator to keep a supply of compressed air?

A11

You could, but it will take more power to drive the alternator and compressor than you would get out of the air that is compressed. This would end up wasting energy.

Q12

If air storage is a problem, couldn't you use a battery to power an on-board electric compressor to keep the car running?

A12

You could, but it will be more efficient to use the electric motor to directly drive the wheels rather than compressing the air first.

Q13

How do you get a good air seal between the pistons and cylinders and in the rotary valves?

A13

The pistons and cylinders are made from consecutive sizes of store bought telescoping brass tubing that have close tolerances. The valves are made from smaller sizes of the same type of tubing. No piston rings or other seals are needed and no precision machining is necessary. A thin coat of light machine oil helps lubricate and further reduce air leakage.

 

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