Engine derived compressor, power calculations and motorcycle donor

[normspanners]

the other issue is starting a direct drive engine / compressor if you are trying to do it by hand it is hard work------ electric starter --------maybe

 

[Johnseat]

Not really if the head isn't coolant cooled, especially if it's an air cooled motorbike engine.
If it is cooled with water then it would be an easy task to drill the block along the coolant track to the head and fit a union to join on to to return the coolant flow.

 

[eSCHEn]

This can be done to make a 58 cfm unit by using a vw aircooled flat 4 but it is reasonably expensive to buy the kit, http://www.dunnrightinc.com/ baring in mind you have to also get the good donor engine which is going to cost but there is something about this set up that makes me want one.
whereby two of the cylinders are turned into compressors and it runs on the other two as a compact large compressor. This might explain it easier

Two things gave me the idea for this one is the dunnright kit but as you say Norm, not exactly cheap and you also need a Beetle engine, which I don't have.

You could do away with the camshaft, change the springs on the inlet and exhaust valves to lighter ones so they become poppet valves and use the spark plug holes as the outlets.

Or just change the springs on the inlet valves so air is drawn through the air filter.

You'll need non return valves in the outlet pipes.

On a 4 clyinder 4 stroke connect 1 & 4 and 2 & 3 together via the plugs, air will be sucked in the inlets and out through the exhausts. You can keep the camshaft which may be necessary for the oil pump to work, depending on the design of the engine.

And this is the second thing. Wendel post here and Roger's original post (in another thread) a while ago about making poppet valves using weaker springs and exiting the spark plug was an "Ah ha" moment. Even if I don't know all of what they're saying yet

the other issue is starting a direct drive engine / compressor if you are trying to do it by hand it is hard work------ electric starter --------maybe

Initially it'll be a pull start as that's what it's got. There is a starter/alternator package for the engine so I'll see if I can pick one up if this whole thing shows promise on the prototype.

 

[normspanners]

Not really if the head isn't coolant cooled, especially if it's an air cooled motorbike engine.
If it is cooled with water then it would be an easy task to drill the block along the coolant track to the head and fit a union to join on to to return the coolant flow.

still need to cool the plate though otherwise the heat will build up there is a reason compressors have fins on the head because that is where the main heat is.

 

[Crater Creator]

Would not removing the head and valve assembly completely and replacing it with a thick plate fitted with shuttle valves be an idea.
Just get the engine turning the correct way to keep the oil pump working, the internals stay lubricated, no cams and rockers ect to open and close ect...

Mount valves to allow air in on the down stroke and valves mounted to allow the air out on the up stroke, 4 stroke engine turned into a 2 stroke compressor.
You could skim the block down a little and take some of the piston face of to reduce the pocket within the piston squish area to aid in higher compression and more air but that might work to a disadvantage when the pressure starts to ramp up and back pressure on the shuttle valve.

A large enough motor would spin it up without any issues and that could be controlled via a pressure shut off valve to the motors power supply.

John, the motor mentioned by eSCHEn is a petrol one from a lawnmower, but you bring up an interesting point - how to unload it when it reaches pressure.
And like you say, the compression ratio of the engine will determine the max pressure it could provide. air compressors don't usually have much in the way of a combustion chamber!
So a head with a 10:1 compression ratio could only ever produce 10 bar, no matter how fast you spun it...... so no danger of over-pressurising, but obviously it's going to get very very hot if no air is flowing - bit of a problem there......
If the driving engine is like my hydrostatic mower's one, it is supposed to be run at full revs only - I assume for engine cooling or something, but the manual specifically states not to run it at less than full speed for long periods.

 

[normspanners]

Initially it'll be a pull start as that's what it's got.

take it from me direct drive is a pain in MY wrist, I still have the broken bones, coz you have to get everything moving together all the pistons and two are completely dead and just want to stop it not start it , flywheels, cold oil etc etc and all by a piece of string ? you need to uncouple the drive for starting.

 

[eSCHEn]

It's a diesel (315cc) not petrol but the point is very valid. I've had some nasty pain from kickstarts on larger two strokes

The drive and output will be connected using a slipping belt. A lever with an idler pulley will provide the clutch to disengage the input from output. The wee engine has an automatic decompressor < 300rpm (IIRC) so should be a cinch to start.

 

[Chris Stevens]

It's a diesel (315cc) not petrol but the point is very valid. I've had some nasty pain from kickstarts on larger two strokes

The drive and output will be connected using a slipping belt. A lever with an idler pulley will provide the clutch to disengage the input from output. The wee engine has an automatic decompressor < 300rpm (IIRC) so should be a cinch to start.

Be interested to see some pics of progress up to now...............

 

[normspanners]

It's a diesel (315cc) not petrol but the point is very valid. The wee engine has an automatic decompressor < 300rpm (IIRC) so should be a cinch to start.

at the end of the day a standard SMALL compressor takes a 2-3 hp motor ( that is on overload for a few moments) all its time to start just a compressor from cold, the forum is full of people usually in early winter saying they have fuses blowing caps going etc etc, then on top of that you have the effort of starting a diesel engine !! any restriction even a slipping belt can stop it before its started. the belt needs to be totally free from the pulley.

 

[Chris Stevens]

Would have thought the main difficulty with this project is finding pistons for the compressor motor, with much more effective rings, able to seal effectively without the help of combustion pressure pushing them out against the cylinder walls? The stock set up isnt going to seal very well as a pump, and I would guess cylinder pressure possible would reduce by maybe 50%, so a 600cc motor would output the same as 300cc, and would need to be run at relatively high speeds. Take a look at the ring set up on a compressor pump piston, and its easy to see the difference between those and an IC motor whose rings are sealed by combustion process.

 

[Chris Stevens]

Rings on any modern IC engine are very thin section and flexible, and work as combustion pressure forces them out into contact with the cylinder wall. Compression pressure would initially be far lower than purpose designed compressor piston (which has more rings, which are designed to seal without combustion pressure), but with any amount of use the lower ring to cylinder wall pressure, would mean cylinder wall would glaze, and pressure generated would ultimately reduce to close to nothing.

It would be possible to convert an IC engine to compressor pump, but for it to work reliably for any length of time would mean different pistons and rings, and designing and fitting some sort of reed valve system rather than stock poppet valves and camshafts. Very costly to do properly, and to be honest cant see there is much point to it?

 

[daleyd]

Rings on any modern IC engine are very thin section and flexible, and work as combustion pressure forces them out into contact with the cylinder wall. Compression pressure would initially be far lower than purpose designed compressor piston (which has more rings, which are designed to seal without combustion pressure), but with any amount of use the lower ring to cylinder wall pressure, would mean cylinder wall would glaze, and pressure generated would ultimately reduce to close to nothing.

It would be possible to convert an IC engine to compressor pump, but for it to work reliably for any length of time would mean different pistons and rings, and designing and fitting some sort of reed valve system rather than stock poppet valves and camshafts. Very costly to do properly, and to be honest cant see there is much point to it?

The rings must seal to start compressing the fuel/air mixture before its ignited though so I don't see why it wouldn't work? There are lots of other issues to overcome I agree but I don't think the rings sealing is one of them.

 

[Chris Stevens]

The rings must seal to start compressing the fuel/air mixture before its ignited though so I don't see why it wouldn't work? There are lots of other issues to overcome I agree but I don't think the rings sealing is one of them.

Take a look at the piston from a compressor pump, and note that the compression ratio is far higher than an IC engine, and that the ring set up is very different.

 

[the snooper]

Rings on ic engines seal no matter what and piston / ring shape don't allow combustion to force rings out o viously they will seal better with some heat as the expand but I can't see it being an issue there will be other inefficiencies that will outweigh the rings

 

[daleyd]

Rings on ic engines seal no matter what and piston / ring shape don't allow combustion to force rings out o viously they will seal better with some heat as the expand but I can't see it being an issue there will be other inefficiencies that will outweigh the rings

Exactly, they may not be the "best" design for a compressor, but they will work. As shown by the hand cranking/ compression test scenario.

I'd say that as the compression force increases with combustion they will seal better, as they would naturally need to do to contain the higher forces, but at the pressure needed for compressed air they will seal just fine as well.

 

[chunkolini]

How about buying an air compressor, it might just fo the job.

 

[daleyd]

How about buying an air compressor, it might just fo the job.

I see what your saying. You mean to strip the motor off and use that to power the bike engine to fill the air tank, right? I like your thinking!

 

[daleyd]

Or, you could use the bought compressor to power an air-start unit for that diesel engine - it'd save your wrists for sure.

 

[8ob]

I have not read the whole thread because I fell into a coma For starting why cant the engine you are using as a compressor retain its flywheel and starter motor so it can be used to start ( via the belts ) the diesel engine? If the timing chain/gear set allows it you could grind off the exhaust cam lobes or remove pushrods and change the cam speed via sprockets to run the same as engine speed. Thats compression on every up stroke.Job done.

Where`s my cigar?

Bob

 

[rtbcomp]

I see what your saying. You mean to strip the motor off and use that to power the bike engine to fill the air tank, right? I like your thinking!

You could convert the compressor pump into an engine to drive the engine that you converted to a compressor pump.