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Screwdriver
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Yes it is and yet for some reason pointing out some of the obvious issues with this makeshift modification appears to be controversial.
There is a reason an engine works one way and a compressor another and those reasons are surely where the interest lies? I was hoping to learn something about either compressors or engines or hear from someone who has done this mod. It is the efficiency that is the key issue for me. Can a modified engine really generate reliable volumes of compressed air without it costing an arm and a leg in terms of energy. I can see how it might be handy if you don’t have an electricity supply or a regular compressor but is there any advantage other than the ready availability of cheap old engines.
I suppose if you had an old engine lying around and no access to a compressor it might make sense but are they so cheap you would really chop up a V8 or a VW engine?
anto44
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This thread has got me thinking now. I have a few diesel engines knocking around to use as donors, would using one 4cyl diesel engine to drive a separate engine which is converted to a compressor be a better idea, most of the potential problems seem to be caused by trying to use one engine as both driver and compressor.
I have most of the stuff there to do it, but I'd like to be getting at least 80 cfm for it to be worth the effort.
I have most of the stuff there to do it, but I'd like to be getting at least 80 cfm for it to be worth the effort.
Al Strachan
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hi, i'm heavily involved in commercial diving equipment, both saturation and surface diving.
breathing air piston compressors normally have physical separation between the oil and the piston rings since you dont really want hydrocarbon based gas/contamination in yer lungs. also the chance that combustion gasses would share the same crankcase as a breathing air compressor with no segregation is a concern.
Breathing nasty gasses on the surface is bad enough. they have a significant and amplified effect when you consider them compressed to up to 30bar (general limit of commercial diving)
Obviously not relevant to this thread, but just saying!!
breathing air piston compressors normally have physical separation between the oil and the piston rings since you dont really want hydrocarbon based gas/contamination in yer lungs. also the chance that combustion gasses would share the same crankcase as a breathing air compressor with no segregation is a concern.
Breathing nasty gasses on the surface is bad enough. they have a significant and amplified effect when you consider them compressed to up to 30bar (general limit of commercial diving)
Obviously not relevant to this thread, but just saying!!
hotponyshoes
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80cfm is 2265lpm.
So a 1litre engine(compressor) driven at 2265RPM should do it.
Providing its running at 100% efficiency
But if you leave a 4stroke engine as standard only half the strokes will be usefull so you would need a 2liter or 4500rpm to do it.
I bet you would be lucky to get anything like 75% efficiency either.
You would have to work out how much power is required to drive it as I can see you might need a 5litre v10 diesel on the input side.
Using something like a small capacity v8 on the compressor side would reduce the input power needed as a smaller amount of air would be under maximum compression at any one time.
I don't know if you used a v12 for the compressor if you could get a higher output from sending the air from one cylinder to another?
I think that's how the dive compressors mentioned above work although I think they have a lot of coolers between pistons and possibly the pistons are varying sizes?
Al Strachan
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Yup, 4 stages are common. 1st stage is normally a big high volume cylinder and getting progressively smaller with cooling between stages, but this is for high pressure compressors- 200-300 bar. for low pressure and high volume you'll need to run in parallel with as much swept volume as possible.
I thought most standard type compressors were just cylinders with a piston 
Ok piston shape will affect efficiency but does that really matter that much?
Most home user compressors run on a 13 amp plug so max about 3000 watts or about 4hp.
Petrol ones in the 5-10hp range???
Even a badly running 4 cylinder running on two cylinders will do that easily.
Ok piston shape will affect efficiency but does that really matter that much?
Most home user compressors run on a 13 amp plug so max about 3000 watts or about 4hp.
Petrol ones in the 5-10hp range???
Even a badly running 4 cylinder running on two cylinders will do that easily.
mtt.tr
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The shop i i worked at 16 was a dive shop, i used to fill bottles those compressor were damn Impressive and loud
outofthefire
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Am I completely missing something here, or are people over complicating this. Surely you only need a reed valve on the inlet port and block off either the spark plug hole or the exhaust port and fit another reed valve on the unblocked hole. The piston goes down and sucks air in past the reed valve on the inlet port, the piston goes up and pushes air out the other, just the same as a normal compressor. I don't get why it has to be any more complicated and certainly not to the extent that some seem to be going to
hotponyshoes
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You could just whip the camshaft or rocker arms out and fit light springs to the valves so they are opened by suction alone.
Use the spark or injector holes for the outlet.
I think any old engine in good mechanical condition would be worth more than a new machine mart compressor pump though.
anto44
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I have a 2.0tdi vw engine there, I was thinking of removing the standard head and replacing it with a thick steel plate fitted with a Reed valve setup. The pistons are pretty much flat topped with a combustion chamber in the centre, people regularly weld up the combustion chambers in these pistons and re machine them for higher performance and they work fine. With the combustion chambers filled I'd have 2000cc of swept volume with a tiny squish area.
I have a 1.9 diesel engine to drive it.
I have a 1.9 diesel engine to drive it.
hotponyshoes
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I dont think you would need to weld the piston up?
It's a fixed sweet area so what you loose in compression you gain in suction.
It's a fixed sweet area so what you loose in compression you gain in suction.
the less volume you have at at top dead center or whatever kind of piston you have, the more efficient it will be?
anyone left over it just wasted space?
Screwdriver
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Yes, I am overcomplicating it. i thought that was the point of the question. Clearly a converted engine is not going to be as efficient as a compressor engine. Some of the more difficult problems beyond just getting it to work at all are issues regarding lubrication, overheating and compression ratio.
Most cylinder heads will have a squish band which can't be good for compressed air generation, it will unnecessarily heat it up for a start. If you stick oil in the intake it might even start running! If you don't stick oil in the intake, how is the cylinder going to be lubricated? Its why they (used to) put lead in the petrol.
Efficiency is the big issue for me though. unless there is a particular advantage over using a regular compressor beyond "oh I just happen to have a spare V8/VW engine I am happy to sacrifice", I don't see the attraction. You can turn a compressor motor over easily by hand, an internal combustion engine is a different matter. It is not optimised to run at the speeds you will use for air compression, my gut feeling is any engine would be too "tight" and half the energy will be lost in just driving the crankshaft.
Emergency use only would be my conclusion.
Al Strachan
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to put this in context, here's an example of one of the compressors i mention (this one is actually a wet piston breathing air compressor!) Note that its obviously purpose built for the job, but its essentially a V4 engine, crank, sump pistons etc etc...
Generally compressor valves aren't lubricated and handle pressures way in excess of what you'd need in your workshop. lubricated in exactly the same way as an engine , and some simple solutions are being presented that will work well. Efficiency wise, proposing air sucking in, squeezed and rammed into a collection cylinder is pretty basic and simple.
I've never seen or heard of a compressor firing itself up in all my time in the industry.
Al Strachan
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just to add, i like the idea of removing cams, light spring on the inlet valves, use spark plug/injector holes for the outlet. NRV's on each bank of outlets. Unloader valve tied into the stop circuit. If using an engine, i'm not sure how you would deal with cut-out. maybe just use the unloader to dump excess when the pressure is reached. cooling the head via its own water pump or via the prime mover.
Screwdriver
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Some good info in there. How does that sort of v4 in your post compare to a typical garage 150 litre Clarke jobbie?
The self ignition comment is aimed at the hack engine mod where I understand the idea is to drip oil into the air inlet.
Out of interest, what size motor powers a compressor of that size?
Al Strachan
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11kw (2 stage) to 75kw (4 stage). 68m3/h to 271m3/h (1000 - 4500lpm roughly)
a clarke 150l 3hp compressor will kick out around 350 lpm
regarding oil drip, piston compressors tend to suck a fair bit of oil into the cylinders, so i doubt that would be necessary. a catch can/seperator would be required even without additional lube.
i see where you are going, and tend to agree, but if you dont have a 3 phase supply or a bottomless wallet, this conversion has the potential to be cheap and workable for some. And lets face it, you only live once. Imagine the forum Kudos points!!!!!
best thing you could do for efficiency is keep the piston as high compression as possible and the NRV (or outlet valve) as close to the cylinder as possible, since any remaining pressure left in the deliver side at the top of the stroke is wasted energy.
a clarke 150l 3hp compressor will kick out around 350 lpm
regarding oil drip, piston compressors tend to suck a fair bit of oil into the cylinders, so i doubt that would be necessary. a catch can/seperator would be required even without additional lube.
i see where you are going, and tend to agree, but if you dont have a 3 phase supply or a bottomless wallet, this conversion has the potential to be cheap and workable for some. And lets face it, you only live once. Imagine the forum Kudos points!!!!!
best thing you could do for efficiency is keep the piston as high compression as possible and the NRV (or outlet valve) as close to the cylinder as possible, since any remaining pressure left in the deliver side at the top of the stroke is wasted energy.
