Using a multicylinder engine as a compressor

[mtt.tr]

http://www.dunnrightinc.com/

As old as the hills

 

[hotponyshoes]

I suppose you could put some really light springs on the inlet valves so they could open under suction and removed the rockers on the exhaust valves so they stayed shut then replace the spark plugs with one way valves.
You would want an engine with individual injectors so you could stop fuel going to the compressor cylinders. I would also want one with individual inlet tracts as I would not want to risk any fuel getting into my compressed air!

Not sure what the max outlet pressure would be, say 10:1 compression ratio would suggest a maximum of 10bar, something like 135psi?

Main problem I think would be that the engine would then run out of balance. If you used 2 cylinders on a v12 you might not notice it? Some engines use balance shafts so you might be able to balance one of those up better?
Or something like the Mazda rotary engine might be a better bet?

The simply (and much better in my view) would be to leave the engine alone and use it to drive a separate compressor.
Either the normal pump type (centrifugal clutch/flywheel/belt)
Or you can get dedicated compressor pumps as kits that just bolt onto the engine.
Or you could just use the air-con pump just might need to make up a simple oil lubricant system for it then you can turn it on/off with the switch that already fitted to the dash!

 

[outofthefire]

A friend of a friend 30 odd years ago was an apprentice paint sprayer had a compressor at home made from a small bike engine, but it was powered by an electric motor. No idea now what engine it was now, but it did the job for spraying in a shed, but it wouldn't have powered air tools.

 

[mtt.tr]

I think weak springs is a poor idea, just split the head and use typical reed valves compressors use in a plate


what ever you do will require serious effort, out of balance wouldnt be the end of the world comfort isnt an issue, long term will probably kill big ends how long who knows. If i had a scrap engine id be tempted to build one

compression ratio you can change, skim the block

 

[Screwdriver]

You need to stop telling me what I don't understand...

You started it.

I thought I spelled it out above - flat head, spring loaded poppet valves instead of using the cam - sigh....

You don't need to "spell it out" and clearly a fourstroke engine would operate as a makeshift compressor. But since I need to "spell it out" to you, the interesting part is what actually happens when you do it for all the reasons I state above, plus a few I haven't.

I do not claim to know how scavenging works at a fluid dynamic level and the effect of swirl in what is effectively a shock wave but I do know it is critically important in the design of a cylinder head. The system is optimised to use that feature to both generate pressure and evacuate the cylinder. A makeshift compressor assumes the volume of air will simply be pushed out due to the displacement of the piston.

This is a discussion and potentially a very interesting one. Nobody doubts that it will "work" so it might be more constructive if you were to address those issues rather than attack my comments. My personal interest would be the use of a two stroke engine because I seriously doubt if that would work at all. It might huff and chuff and you might generate a waft but for serious use as a compressor, no chance.

 

[mtt.tr]

screwdriver you dont understand the purpose of scavenging, you also are unaware of how a typical compressor works, it doesn't need scavenging

 

[stuvy]

Has anyone tried this? The Americans used V8 engines, one bank running normally for power, the other purely as an air compressor, no fuel nor sparks, as a air supply for surface supplied air scuba diving

I'm sure there's lots of reasons why it's not done, but it's an interesting concept, well, to me at least.

Some people use the air con compressor on there engine as an air pump have a google as two or three of them in series might be enough from a mower engine with a pulley and a few belts

 

[Seadog]

I do not claim to know how scavenging works at a fluid dynamic level and the effect of swirl in what is effectively a shock wave but I do know it is critically important in the design of a cylinder head. The system is optimised to use that feature to both generate pressure and evacuate the cylinder. A makeshift compressor assumes the volume of air will simply be pushed out due to the displacement of the piston.

Combustion chamber design is for improved combustion. Scavenging is to ensure that as much of the exhaust gas is expelled to maximise charge, and thus power. All we're doing is compressing air and expelling it. I think you are over-thinking the problem. Something I do a lot of

 

[Rig Pig]

This is a commercially available kit, looks impressive for a small package 58CFM @ 100PSI !

 

[mtt.tr]

This is a commercially available kit, looks impressive for a small package 58CFM @ 100PSI !
View attachment 223078

serious kit, uses a redesigned cam

 

[Rig Pig]

serious kit, uses a redesigned cam

It's a clever idea, I'm a cheapskate I'd use poppet valves!

 

[Cobbler]

I had

Way back in the stone age 51 yrs ago out in Germany I visited a Canadian barracks & popped into their forces shop at Zoest which was like an Aladdin's cave .

I purchased a nifty little device that you screwed in place of a spark plug , left the HT lead off and started the engine . On every compression stroke of the cylinder it was fitted to it pushed a small piston up to compress about 20 cc of clean air .
Good enough to inflate a car tyre in 4 min or so and good enough to operate a small spark plug grit blaster .

Both items came as a set with a small bag of grit IIRC .

I had a similar one Dapph, bought from a show about 50 years ago, but it had an inlet valve on it so it drew fresh air into the cylinder through the plug hole. It used the vehicle cylinder as a compressor, it didn’t draw fuel/air mixture in from the manifold as it didn’t produce a vacuum. I don’t think it would work properly on an injection engine as it would continue to spray fuel, or upset the ECU. It worked really well, it was supplied with a small sort of airbrush spray gun. I think it could still be about somewhere, I’ll have to have a look.

 

[Screwdriver]

Combustion chamber design is for improved combustion. Scavenging is to ensure that as much of the exhaust gas is expelled to maximise charge, and thus power. All we're doing is compressing air and expelling it. I think you are over-thinking the problem. Something I do a lot of

Come on Graham, you know better than that. Of course I know what scavenging is and what it does, what I do not claim to know is how it works at "a fluid dynamic level". That's the interesting part. The material science inside a combustion chamber is a fantastically complex subject. I appreciate a flame front is preceded (or followed??) by a shock wave and introduces a swirl into the chamber allowing or promoting scavenging to take place. It is a key issue because if you want to create a compressor, clearly you cannot allow the valve timing to overlap.

If the exhaust and inlet port are open at the same time (WHICH THEY ARE!) then your compressor function is going to be pants unless the fluid dynamical behaviour of mere air happens to behave in a similar way to an exploding fuel air mixture. I suggest that is unlikely in the extreme and I further suggest anyone who thinks I am too dim to know what scavenging is hasn't actually thought about how it works or understood the implications of that process as applied to a makeshift "compressor".

I think it is interesting but I didn't expect to have to explain how an engine works to make such a simple observation.

 

[mtt.tr]

Google is taking a hammering today

Reed valves
And a perfect example of it working on vw engine, i suppose you know something / Googled something they didn't in the 80s

 

[Seadog]

It is a key issue because if you want to create a compressor, clearly you cannot allow the valve timing to overlap.

Hance my comment about removing the rockers and using another valve system.

 

[Screwdriver]

Hance my comment about removing the rockers and using another valve system.

Indeed and then you run into other issues as I detail above. It's an interesting idea and a bit of fun with some complex problems to solve if you wanted to have a crack at it. I thought the whole purpose behind the question is "what would happen if" so I am not really overthinking it because frankly I have no intention of doing it.

But the bottom line is yes you can do it but if you want a compressor, buy a compressor.

 

[gt6s]

loadsa flow but little pressure......don't bother......ud be better off with a comp from Lidil......
did see a Harley 45engine converted.....again what a waste.....

Sounds too useful a purpose for a Harley POS. Boat anchor springs to mind.

 

[minimutly]

Lol the hardley thing.
I didn't start anthing @Screwdriver.
So here we go, two ways to skin a cat - no need for fluid dynamics or to even mention scavenging.

You isolate two cylinders - one pair, up and down together, so the petrol engine fires regularly. By isolating I mean inlet and exhaust ports, carbs etc.
The two to be compressors you fit a set of reed valves from a compressor. If you choose a flat head engine -ie ford crossflow you have a flat head, so life is easy allready. But you have to find a pair of flat top pistons that will come within a few thou of the head.
There is a problem with the exhaust port - it's likely too big a diameter, but a few minutes with a miller and angle plate should have it ready to be sleeved, but hey maybe you could leave it.
Job done, how simple does it need to be? Oh, you need a governor of sorts, rpm wise I would expect 2000 or so should work. What have I missed? Oh, for @Screwdriver - the engine is still a 4stroke, the compressor is a compressor, a 2 stroke one.

Option 2 is to do with keeping the valves and opening them on every stroke. How? 2 lobes would do it, and there is room for them. A more difficult option would be to run a seperate cam at crank speed, but that's crazy isn't it?
Bit like Rover actually slowing cams down as they rotate....

 

[Screwdriver]

Yeah that’s fine, glad to see you’re up to speed. You’re not telling me anything I didn’t know though I am curious as to why you say there’s no need for fluid dynamics and the exhaust port is too large.

Why is that?

 

[mtt.tr]

Yeah that’s fine, glad to see you’re up to speed.

How insulting!