Engine derived compressor, power calculations and motorcycle donor

[eSCHEn]

So I'm looking to make a compressor based on an engine being turned by another. I'm just at the stage of throwing some numbers about in my head to try and get a grasp on what's possible / needed. Here's my calculations so far, anyone care to sanity check for me (@pdg perhaps mate, I know you like this stuff )?

My power source (props to Mr Migwire) is a Lombardini 15 LD 315 from a lawn tractor; producing 6HP at a continuous service level. I'm sort of rounding the figures a bit to make it easier to play with so don't worry about rounding errors.

From the manual, at 2500 RPM there is ~15Nm of torque. Theoretically an output engine of 600cc displacement (I'm thinking of common motorbike engine sizes) should yield:

2500 * 0.6  = 1500L

1m3         = 1000L

1500 / 1000 = 1.5m3/min (53ft3/min)

But I'm not sure that a wee 6HP motor would turn a 600cc engine, even with gearing 2:1 that still only gives 30Nm - is that enough? I honestly don't have a clue and el google is annoyingly good at providing figures for output but no idea on input.

Assuming I'm not a million miles away, and that's a big assumption, the second part of the question is what engine from the motorbike world would be a good fit for these criteria:

• 400..600cc
• Common - so parts are easy to find and not eye wateringly expensive
• Reliable/rugged - I don't want to have to fettle it constantly
• Four stroke
• Air or liquid cooled but pref liquid
• Any piston arrangement fine but a flat twin is definite bonus
• And of course - cheap!

I'm thinking of occasional blasting of small/medium items. Nothing larger than a motorbike chassis for example and most stuff a lot smaller. I'd also like to do a bit of spraying of items that same sort of size range. It'd also be nice to use some air tools on a good supply so I don't end up pitching them out the window.

I await your musings my fellow peeps

 

[normspanners]

a normal four stroke engine is immediately half the output of a normal comparable compressor because of the cam/ valve arrangement because it is a four stroke.

 

[eSCHEn]

Assume that I'm a complete an utter moron when it comes to the internal bits and how they whirr and dance with the pixies inside. Would you mind elaborating Norm?

 

[normspanners]

A compressor has reed valves that the air pressure opens, so that it compresses every up stroke, but a four stroke engine pumps/ compresses only once, every two revolutions,hense four stroke = suck, squeeze, bang, f&rt. you dont want the bang or the f&rt

 

[normspanners]

A 6hp engine will only run about a 4/5 hp engine running as a compressor--- if you are lucky----- I would think,
so therefore trying to run a 30 hp engine as a compressor is hopeless. IMO you would need a Much bigger driving force.

 

[eSCHEn]

Cheers for the explanation Norm

 

[the snooper]

you wont even get full horsepower from the engine unless you run it at high revs

 

[colnerov]

Hi, You're not so worried about the horsepower, you want the torque which is lower down the rev range.

Colin

 

[eSCHEn]

Hi Colin, yes I think that's what I'm aiming at. Max HP depends on the loading curve on the graphs, taking half ish between heavy and medium gives 6.2 @ 3600 RPM. Scaling back to say, 3000 RPM is a reduction to 5.5HP.

Torque peaks at 15Nm @ 2400 RPM but I was saying 2500 for an easier figure to play with.

 

[Crater Creator]

How are you going to get it to discharge on every stroke? You could do away with the exhaust valves completely and stick a non-return valve on the discharge but you'd need the inlet valves to open on every downstroke as well, instead of every second one.
Off the wall - how about a twin running on just one cylinder, use the non-fired one as the compressor? Nah.... probably not.
I'll watch this one with interest though.

 

[daleyd]

Assume that I'm a complete an utter moron

Consider it done pal!!

 

[Chris Stevens]

So I'm looking to make a compressor based on an engine being turned by another. I'm just at the stage of throwing some numbers about in my head to try and get a grasp on what's possible / needed. Here's my calculations so far, anyone care to sanity check for me (@pdg perhaps mate, I know you like this stuff )?

My power source (props to Mr Migwire) is a Lombardini 15 LD 315 from a lawn tractor; producing 6HP at a continuous service level. I'm sort of rounding the figures a bit to make it easier to play with so don't worry about rounding errors.

From the manual, at 2500 RPM there is ~15Nm of torque. Theoretically an output engine of 600cc displacement (I'm thinking of common motorbike engine sizes) should yield:

2500 * 0.6  = 1500L

1m3         = 1000L

1500 / 1000 = 1.5m3/min (53ft3/min)

But I'm not sure that a wee 6HP motor would turn a 600cc engine, even with gearing 2:1 that still only gives 30Nm - is that enough? I honestly don't have a clue and el google is annoyingly good at providing figures for output but no idea on input.

Assuming I'm not a million miles away, and that's a big assumption, the second part of the question is what engine from the motorbike world would be a good fit for these criteria:

• 400..600cc
• Common - so parts are easy to find and not eye wateringly expensive
• Reliable/rugged - I don't want to have to fettle it constantly
• Four stroke
• Air or liquid cooled but pref liquid
• Any piston arrangement fine but a flat twin is definite bonus
• And of course - cheap!

I'm thinking of occasional blasting of small/medium items. Nothing larger than a motorbike chassis for example and most stuff a lot smaller. I'd also like to do a bit of spraying of items that same sort of size range. It'd also be nice to use some air tools on a good supply so I don't end up pitching them out the window.

I await your musings my fellow peeps

Not very sensible idea, as a 600cc bike engine would produce a great deal less compressed air than a compressor pump with the same displacement (maybe only 10% of the compressor output), and to work effectively at all would need about 10hp to turn it. Add to that weight, bulk, and the fact it would not be anywhere near as reliable as a compressor pump, and you have a ready made recipe for a complete waste of time.

 

[normspanners]

Off the wall - how about a twin running on just one cylinder, use the non-fired one as the compressor? Nah.... probably not.
I'll watch this one with interest though.

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

 

[Wendelspanswick]

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.

 

[Seadog]

If it's a 360 degree twin turn it into a twin stage compressor. one cylinder feeding the second.

 

[normspanners]

If it's a 360 degree twin turn it into a twin stage compressor. one cylinder feeding the second.

then you would need to sleeve one cyl down to half'ish the dia otherwise there is no benefit.

 

[Crater Creator]

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.

Now that sounds more like it. Never thought of that. Might be best to go for an old two-valve head design for less moving parts and less air leakage?
The spark plug hole would also be a lot easier to plumb in to handle HP discharge air than modifying an exhaust manifold to handle 10 bar (!). Might need to enlarge the plug hole a wee bit though for air flow rate.

 

[Johnseat]

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.

 

[normspanners]

Would not removing the head and valve assembly completely and replacing it with a thick plate fitted with shuttle valves be an idea.

Then you also have cooling to consider

 

[rtbcomp]

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.