Engine Air Compressor?

[AKwelder]

There have been several of these rigs on spread that i worked over the years, and they all have one thing in common, different heads. Some had special pistons in the compressor cylinders. They all were used in high flow applications. But by having more compressor than we needed we got high PSI.

I am sure that you could modify a cylinder head to work, but you might want to sure that the engine you are using has good cooling, not an engine that has a history of over heating.

The unloader is a big deal, make sure you have a large enough one.

running of a belt sounds like less work, yet may still be exspensive

 

[Redneck in training]

I guess one of the things I'm shooting for is to lower the power consumption. I was guessing that if each of the pistons were only compressing the air halfway (i.e pistons 1 and 2 are taking it from 0 to 50 psi and piston 3 is taking it from 50 to 100) it would lower the power consumption. It would also lower the output, but it seems pretty well accepted amongst us that for the task at hand this setup would be more than sufficient.

By increasing the dead space to change the compression ratio like you suggest would it also decrease the amount of heat created? Even if it didn't would the heat created be more that normal operating temperatures of an engine. You'd have the radiator and engine cooling system there that could be used to cool.

It will not decrease power consumption. The power equals to mass flow of air times head. You can substitute pressure ratio for head to see that doubling pressure ratio will approximately double the power consumption (Ignoring effect of temperature). The power consumption will go actually up due to higher mechanical losses. High pressure ratio compressors have to be multistage exactly due to temperature control. If we would have materials that can handle high temperatures theoretically we could build single stage compressor for any pressure ratio.

 

[Iplayfarmer]

It will not decrease power consumption. The power equals to mass flow of air times head. You can substitute pressure ratio for head to see that doubling pressure ratio will approximately double the power consumption (Ignoring effect of temperature). The power consumption will go actually up due to higher mechanical losses. High pressure ratio compressors have to be multistage exactly due to temperature control. If we would have materials that can handle high temperatures theoretically we could build single stage compressor for any pressure ratio.

I'm thinking you'd aim for 120 PSI vs. the 100 that we've all been talking as a theoretical limit of a single stage in this system.

The reason I was thinking that power consumption would go down is exactly because of that head that you are talking about. What the trade off would be is giving up volume for a little more pressure and less power consumption. My approach is that none of the pistons in a two stage setup would work as hard as they would have to in a single stage setup.

Of course another option for trading volume for power consumption would be to just run the engine slower.

 

[Egon]

Rod loading.

 

[bobodu]

It's a simple matter of physics...storing energy requires energy.
Lessay you wanna lift a ten ton truck.You can use this jack and give it ten hard pumps or you can use that jack and give it fifty easy pumps....at the end of the day you still would have exerted the same amount of energy....

 

[CCWKen]

I was lucky and found a used Sullair 160 for less than a 1/3 of what I paid for the shop compressor. All it needed was a few adjustments, the cover assembled and a new thermostat. I use it for sand blasting. It outputs 160cfm @ 125psi. The twin screw compressor is powered by a GM 4-cylinder industrial engine. More than enough for even the largest shop and plenty for my blasting needs. The only time it hits full throttle is when I open the air valve to fill my blasting tanks. It can fill an 80 gallon air tank in about 4 seconds.

 

[Bishop]

The Reciprocating Compressor Division is involved with the design, development and manufacture of single and two stage reciprocating compressors. The following are the range of compressors developed at the Reciprocating Compressor Division.

 

[Iplayfarmer]

It's a simple matter of physics...storing energy requires energy.
Lessay you wanna lift a ten ton truck.You can use this jack and give it ten hard pumps or you can use that jack and give it fifty easy pumps....at the end of the day you still would have exerted the same amount of energy....

Understood. I'm using physics too in that if you do less work (i.e. less volume) you use less energy.

 

[BrentD]

Maybe I'm just being thick, or maybe my brain has just decided to burn out from the monotony of my day job, but I keep coming back to the pressure question. I sort of understand what you guys are saying about the 100psi not being realistic, and until this morning I was pretty much sold on the argument. Then I thought about the compression test again.

A compression tester is just a pressure gauge with a schrader valve that you thread into the spark plug hole. The engine cranks a few times and the gauge goes up until it hits a point where the engine can no longer generate enough pressure to open the schrader valve. On the particular 4-cylinder engine I was testing each cylinder tested between 150 and 170 psi.

In the compressor application I understand that the engine would lose a little bit of compression based on the amount of pipe between the spark plug hole and the check valve, but surely it won't be that much.... And the pressure in the line and tank will be cumulative. Shouldn't you be able to achieve a tank pressure equal to at least the compression of the weakest cylinder?

 

[Egon]

With a positive displacement type compressor the only real limit on the compression possible is the strength of the components. No ratio involved.

 

[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.