Engine Air Compressor?

[Ernie Alabama]

That first part about the infinite compression ratio doesn't seem quite right. Any given cylinder is going to have some sort of compression ratio that is calculated by comparing the volume of the cylinder with the piston completely retracted to the volume left at the top of the cylinder when the piston is at full stroke. An air compressor cylinder might have a better ratio than an engine if it has a flat head and recessed valves. The psi any given cylinder/piston mechanism can generate will be limited by compression ratio, quality of valve seals, piston ring seals, and the amount of stress individual components can handle. (In an application like I'm planning, the main limiters are going to be the electric motor and the various seals. If I wanted a higher compression ratio I could switch to a diesel engine for the compressor instead of the gas.

As far as the PSI goes, As long as I can approach 100PSI at enough cfm to drive a sandblaster I'll be satisfied. My sandblaster is about the biggest air hog I have at the moment, and can't really think of anything else I'd ever get that would need more.

The only stock air compressors I've been able to find that come close to the CFM rates of an engine are between $700 and $1000. I can pick up a used running 3-cylinder geo metro engine for around $100 (Heck, a friend of mine once bought an entire running geo metro in good running condition for $25). The biggest cost would be the electric motor if my 1hp pool pump motors won't drive it. Of course, I could always just pick up a second Geo engine and have one drive the other....

I'm thinking that as you approach 125 PSI, you'll need a second Metro engine to turn the compressor.

 

[Mace Canute]

I think you have overlooked the main difference between the cylinders on an ICE and on an air compressor which, as has been mentioned a couple of times, is the fact that an air compressor has a very very high "compression ratio". This is a very important point. In the ICE converted to an air compressor, as the piston reaches the top of it's stroke, the air is at maximum compression. If you use the stock cam on the exhaust valve, remember...it remains open for a certain amount of time after TDC...not very long, but remain open it does. As the piston goes down on the intake stroke, air beyond the exhaust valve will flow back into the cylinder as long as the exhaust valve is open. After the valve closes, the remaining air in the cylinder is still compressed air! Until the piston has traveled down far enough in the cylinder so that the volume has increased to the point that the pressure is at ambient pressure, there is zero possibility that any air can be pushed by atmospheric pressure into the cylinder. The lower the compression ratio is on the original ICE head, the worse this becomes. As pressure builds in the receiver, the volume of air moved will decrease.

IF you close off the original exhaust valve and use a one way check valve in the spark plug port, you still run into the same problem but to a slightly lesser degree. The biggest problem with that is the very small hole you have for the air to flow through! Above a certain volume, this will act as a restriction and pressure will build in the cylinder but air flow will not increase linearly, plus the same problem exists with compressed air trapped in the cylinder at TDC when the one way check valve closes.

Many, if not all, of the commercially built half engine/half air compressors had extensive work done to the heads and camshafts to eliminate this problem.

It's not an inconsequential issue...

 

[Jimbo16720]

What makes a good compressor is an air conditioner pump. I put one on an old Blazer I had. Bolts right up and all you do is run a hot wire to a toggle switch mounted in the engine compartment to turn it on and off. It engages the clutch when it's on so the motor turns the pump. I hooked a coiled air hose to mine that when stretched would reach to the back of the vehicle. You should try and get a pump that has oil in it for lubrication. A small air tank can also be put under the hood, but I didn't have one on mine. The only problem is I had to release the air as it ran so it didn't blow the hose up. You can put a relief valve and pressure switch on it too so it works like a regular air compressor at home.:confused2:

 

[Redneck in training]

I think you have overlooked the main difference between the cylinders on an ICE and on an air compressor which, as has been mentioned a couple of times, is the fact that an air compressor has a very very high "compression ratio". This is a very important point. In the ICE converted to an air compressor, as the piston reaches the top of it's stroke, the air is at maximum compression. If you use the stock cam on the exhaust valve, remember...it remains open for a certain amount of time after TDC...not very long, but remain open it does. As the piston goes down on the intake stroke, air beyond the exhaust valve will flow back into the cylinder as long as the exhaust valve is open. After the valve closes, the remaining air in the cylinder is still compressed air! Until the piston has traveled down far enough in the cylinder so that the volume has increased to the point that the pressure is at ambient pressure, there is zero possibility that any air can be pushed by atmospheric pressure into the cylinder. The lower the compression ratio is on the original ICE head, the worse this becomes. As pressure builds in the receiver, the volume of air moved will decrease.

IF you close off the original exhaust valve and use a one way check valve in the spark plug port, you still run into the same problem but to a slightly lesser degree. The biggest problem with that is the very small hole you have for the air to flow through! Above a certain volume, this will act as a restriction and pressure will build in the cylinder but air flow will not increase linearly, plus the same problem exists with compressed air trapped in the cylinder at TDC when the one way check valve closes.

Many, if not all, of the commercially built half engine/half air compressors had extensive work done to the heads and camshafts to eliminate this problem.

It's not an inconsequential issue...

It is true that ICE have significantly larger "dead" space than purpose built compressors. But it can be overcame by connecting three cyclinders in parallel and the fourth one in series with intercooler in between.