Making a compressor out of a small engine

[amron]

Hi
Does any of the forum members know if it is possible to make an air compressor out of a small engine (in my case it's a horizontal shaft, cast iron, 4 stroke, about 10 hp)? all the other componnents, like the container, electric motor, presostat , etc, I already have, so I would like to know if it's can be done and if so, some guidence how to modify it, since I have no other use for the engine anyhow.
Thanx.

 

[Shox Dr]

It's recently been talked about in the this thread

 

[eil]

I seem to remember that guys back in the days when compressors were out of the reach price wise for home use some made their own but had them spinning opposite to how they ran normally, cant remember if it was for two strokes or four though

 

[rtbcomp]

There are several ways it could be done on a standard 4 cylinder 4 stroke, probably the easiest is to

1) Disable the camshaft, connect cylinders 1&4 and 2&3 together with pipes (make connectors to screw in the plug holes from old spark plugs).

2) Weaken the inlet valve springs so they act as poppet valves.

3) Connect check valves to the pipes in 1) so that air can only be pushed out through these pipes into your reciever.

4) Remove the carburettor and fit the air filter to the inlet manifold.

Keep the oil pump connected.

Keep the water cooling, you may even be able to recover the waste heat.

 

[stormin]

Basic problem with using a four stroke efficiently is the valve timing and the wasted stroke. With the standard cam shaft you are only getting air out every other revolution, assuming you use something like a modified spark plug with a one way valve as your air outlet. The power and exhaust stroke are wasted in compressor mode.

Options are to make a modified camshaft alter the valve timing or create a new cylinder head with reed type valves.

 

[stormin]

There are several ways it could be done on a standard 4 cylinder 4 stroke, probably the easiest is to

1) Disable the camshaft, connect cylinders 1&4 and 2&3 together with pipes (make connectors to screw in the plug holes from old spark plugs).

2) Weaken the inlet valve springs so they act as poppet valves.

3) Connect check valves to the pipes in 1) so that air can only be pushed out through these pipes into your reciever.

4) Remove the carburettor and fit the air filter to the inlet manifold.

Keep the oil pump connected.

Keep the water cooling, you may even be able to recover the waste heat.

Given it's a 10hp engine he's talking of using I'm assuming it's only a one or two cylinder engine and more than likely air cooled. Also lubrication in small engines is generally splash from a thrower on the crank only.

 

[rtbcomp]

Basic problem with using a four stroke efficiently is the valve timing and the wasted stroke. ......

Which is why you connect cylinders 1&4 and 2&3 externally

Options are to make a modified camshaft alter the valve timing or create a new cylinder head with reed type valves.

Way too complicated! Weakening the inlet valve springs will do this easily, if there are 2 springs per valve just remove the stronger one.

You could also disable the inlet valves and use extra extrernal check valves, this would eliminate the problem of the wasted compression stroke on a single cylinder engine.

 

[eil]

You seem to have four cylinders stuck in your train of thought there Rog.

 

[rtbcomp]

Same principles apply to any number of cylinders.

If it's a really ancient single cylinder engine it may already have a poppet valve on the inlet, in which case just disable the exhaust valve, connect a check valve to the plug 'ole and away you go.

 

[stormin]

Which is why you connect cylinders 1&4 and 2&3 externally



Way too complicated! Weakening the inlet valve springs will do this easily, if there are 2 springs per valve just remove the stronger one.

You could also disable the inlet valves and use extra extrernal check valves, this would eliminate the problem of the wasted compression stroke on a single cylinder engine.

I can see why you may connect cylinders externally, if not using the weak valve spring principal, as an adjacent cylinder may then intake air from it's connected cylinders valve being opened. But I don't see the advantage if you have weak valve springs and each cylinder is effectively acting as a two stroke.

How weak would the springs have to be to not significantly affect volumetric efficiency? I suppose just enough strength to close the valve and no more is the ideal. How easy would that be to achieve in practice.

Have you done one of these conversions then Roger? Would be interested to see detais if so.

 

[rtbcomp]

You need to connect 1&4 and 2&3 so that on the compression stroke (both valves closed) air is passed into the cylinder that's on its exhaust stroke. (assuming that pistons 1&2 rise together, as do 3&4, I guess this will be the case of 99.9999999999% of 4 stroke 4 cylinder enngines).

Also connecting the cylinders this way allows 2 cylinders to share 1 check valve.

I suppose you could join the cylinders, retain the camshaft to open the exhaust valves and connect the exhaust manifold to the reciever, but this would be more difficult to do, there may be valve overlap considerations and making a pressure tight exhaust gasket may be a problem.

The spring could be quite weak, the pressure on the upstroke will force the valve closed, I would guess an inner valve spring on its own would do the job, but maybe the typical engine you'd pull out of a scrappy doesn't have twin valve springs? I'm not au fait with modern stuff!

I haven't tried this, it's on my to-do list, and has been for a long time!

 

[amron]

It is a single cylinder indeed.
stormin - could you please give us more details regarding the reed valve modification?
and yes, it is a cast iron, air cooled engine (I believe it's about 250 - 300cc).
Thanx.

 

[Justme]

assuming that pistons 1&2 rise together, as do 3&4

I thought that the common way was

1 & 3 then 2 & 4

or

1 & 4 then 2 & 3

Dont thin I have ever seen 1 & 2 / 3 & 4

 

[R Kraft]

There is also that,many small engines in the states only have about 5 to 1 compression.
Robert

 

[rtbcomp]

I thought that the common way was

1 & 3 then 2 & 4

or

1 & 4 then 2 & 3

Dont thin I have ever seen 1 & 2 / 3 & 4

OOPS my typos - I meant 1&4 and 2 & 3 rise & fall together

I got it right in post #4

Also I glibbly mentioned disabling the camshaft, you'd probably have to remove it as I doubt whether there is a position where all 4 exhaust valves are closed at the same time.

 

[penfold]

Why not just have a non-return valve fitted to each sparkplug hole? Simpler than plumbing separate cylinders together. That plus replacing the valve springs with weaker ones and removing the rockers, followers and camchain. On most engines you'd need to leave the camshaft in place, otherwise you'd need to block the oilways(assuming a pressurised lubrication system).

 

[rtbcomp]

Why not just have a non-return valve fitted to each sparkplug hole? Simpler than plumbing separate cylinders together. .......

That way you need 4 NRVs. connect the cylinders you only need 2.

It's just as easy to connect the cylinders before the NRVs as it is after them.

Good point about the camshaft. It would be easier to convert an old OHV engine than a modern OHC engine as you just need to remove the pushrods.

 

[rocketroy2112]

Hi
I haven't commented on the forum for a while but I thought I would comment on this subject of the compressor.
I have made my compressor from an old calor gas bottle ( and also made a log burner from one) and a compressor from a fridge/freezer using a blowoff valve set at 120psi. Would be no good to use as a constant supply as the compressor takes too long to charge fully ,but is great for an air gun or blowing up / seating tubeless tyres. If interested I will send a photo in!!

 

[rtbcomp]

Been there, done that, got the tee-shirt!

http://forums.lr4x4.com/index.php?showtopic=9710