Using a multicylinder engine as a compressor

[hotponyshoes]

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?

More than likely right. I think the depressions in the pistons are more for mixture distribution and burning than the compression.
But say the depression is 100ml and the piston swept area is 1litre, it will pull in 1litre of air on the inlet stroke and compress it then 1 litre of compressed air will go out.
The 100ml area is not going to drop below whatever backpressure you have at the outlet and might actually be usefull for starting the piston off on its downward stroke again although it will work against the new air being drawn in.
There will be 100ml of dead space which (using the numbers above) might give a 10% drop in compression ratio? Or not?

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.

Lubrication and overheating should be easy to deal with. Use the existing oil pump and cooling system.
Compressing air heats it but it cant generate any more heat than the original cooling system can cope with considering that was pumping the same amount of air plus hot burning fuel.
If anything it probably wont run hot enough for the oil to work correctly so might need that changing.

The hard part of turning a normal engine over by hand is the valve gear springs. A v8 block with no heads on can be turned by just a hand on the nose of the crank.

 

[Pigeon_Droppings2]

Quite an interesting idea...I reckon there will be some unforeseen issues to say the least.

I was trying to work this out in my head....the "compression" stroke is actually the 4th stroke ...and the real compression/ignition stroke of the engine doesn't do anything for us I guess because the valves stay shut on that stroke.

One issue I can foresee is the valve springs on the exhaust...if there isn't some sort of device to prevent back flow then the cylinder pressure achieved might just be limited to the strength of the exhaust valve spring and it's ability to prevent the air pushing back on the valve maybe. I doubt those springs were designed to hold pressure out vs the usual requirement which is to shut the valves to hold pressure in.

All guess work of course...

 

[R Kraft]

When I was 12 or 15 I made a compressor out of a horizontal lawn mower engine.
I removed the tappets, so that the valves did not open.

I drilled holes in the cylinder wall so that at bottom dead center they were uncovered .
This gave me an intake port that was opened by the piston.
The outlet was a spark plug with the ceramic removed.
A ball bearing fit in the spark plug shell to act as the outlet valve.
It worked OK.

 

[outofthefire]

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.
.

To someone with a basic knowledge of engines you seem to be describing issues with building an engine. Why are you worrying about compression ratio? overheating and lubrication why are you wanting to drip oil in the inlet, aren't pistons and everything else below the piston lubricated from below the piston, isn't that why there's an oil ring on the piston (and how compressors are lubricated? and if your running the engine your going to keep the cooling system so overheating shouldn't be an issue. If your using reed valves your not really compressing the air in the cylinder it's getting pushed out of the which ever hole you choose to use, the air is being compressed is taking place in the receiver.

 

[mtt.tr]

Quite an interesting idea...I reckon there will be some unforeseen issues to say the least.

I was trying to work this out in my head....the "compression" stroke is actually the 4th stroke ...and the real compression/ignition stroke of the engine doesn't do anything for us I guess because the valves stay shut on that stroke.

One issue I can foresee is the valve springs on the exhaust...if there isn't some sort of device to prevent back flow then the cylinder pressure achieved might just be limited to the strength of the exhaust valve spring and it's ability to prevent the air pushing back on the valve maybe. I doubt those springs were designed to hold pressure out vs the usual requirement which is to shut the valves to hold pressure in.

All guess work of course...

Id drop the valves then at the manifold put in plate reed valves

 

[Al Strachan]

Quite an interesting idea...I reckon there will be some unforeseen issues to say the least.

I was trying to work this out in my head....the "compression" stroke is actually the 4th stroke ...and the real compression/ignition stroke of the engine doesn't do anything for us I guess because the valves stay shut on that stroke.

One issue I can foresee is the valve springs on the exhaust...if there isn't some sort of device to prevent back flow then the cylinder pressure achieved might just be limited to the strength of the exhaust valve spring and it's ability to prevent the air pushing back on the valve maybe. I doubt those springs were designed to hold pressure out vs the usual requirement which is to shut the valves to hold pressure in.

All guess work of course...

No point using the exhaust valves at all. the pressure ability of the exhaust system is enough to discount its use, including the back pressure on the valves as you suggest. far easier to use an NRV on the injector/spark plug port.

 

[Pigeon_Droppings2]

No point using the exhaust valves at all

I initially thought that too...but without the exhaust valve would it suck from the exhaust on the "ignition" stroke...but that could be fixed with other changes of course.

Agree on the exhaust in general....I can't imagine the exhaust gasket holding up too well.

 

[qwakers]

id use a rover v8. have the centre 2 on each back firing and the outside 4 as compressors, take the pushrods out to take them out of the normal valve sequence, leave the exhaust valves in to block the port and remove the inlet, fit poppets to the inlet valves and the spark plug holes. . should work quite well and i cant think of a simpler solution. will obviously need to fit a modified intake manifold too.

 

[Al Strachan]

I initially thought that too...but without the exhaust valve would it suck from the exhaust on the "ignition" stroke...but that could be fixed with other changes of course.

Agree on the exhaust in general....I can't imagine the exhaust gasket holding up too well.

if does open the intake stroke, the welder is getting dragged out!!

 

[Pigeon_Droppings2]

Yea definitely doable with mods I reckon.

My initial thought was diesel because of the high compression....but I reckon it's the sort of thing where it takes the first build to work out how to build it

 

[anto44]

I've decided to build one, I'm going to use one engine to drive another engine converted to a pump.
I've been looking at the specs of various compressor pumps and the doner engine I have is suitably sized. I'm going to ditch the standard head and run it with a reed valve setup, all the purpously designed piston compressors I've found run a flat head with reed valves so it's a tried and tested system.
For lubrication I'm going to use the original oil pump setup. For cooling I'll use the original water pump and a small radiator.
I think the idea has enough merit to be worth trying, if it doesn't work at least I'll know why.

 

[Screwdriver]

To someone with a basic knowledge of engines you seem to be describing issues with building an engine. Why are you worrying about compression ratio? overheating and lubrication why are you wanting to drip oil in the inlet, aren't pistons and everything else below the piston lubricated from below the piston, isn't that why there's an oil ring on the piston (and how compressors are lubricated? and if your running the engine your going to keep the cooling system so overheating shouldn't be an issue. If your using reed valves your not really compressing the air in the cylinder it's getting pushed out of the which ever hole you choose to use, the air is being compressed is taking place in the receiver.

Good point yes but I am not entirely sure if I am describing a hack engine, the camshaft mod VW or a homemade poppet valve conversion.

I’m just throwing a load of questions out there with potential problems which may or may not apply depending on exactly which variant of mod is under discussion. All problems can be addressed.

The OP was asking about a simple spark plug out, fire it up hack and since then, every possible variation has been under discussion and every solution right up to a purpose built 75kw v4!

I read somewhere that using an engine with a valve train still attached requires lubrication and the trick is to drip oil into the inlet. I know fuel has additives to promote lubrication but I don’t know how a compressor gets around the problem. I guess it has wider tolerances and a different type of piston ring

Probably that’s why a rattly old air cooled VW engine is a good candidate. I am still struggling to see why you’d want to go to all that effort (with any of the modified engines) unless there is some as yet unforeseen advantage over a purpose built unit.

 

[Screwdriver]

I've decided to build one, I'm going to use one engine to drive another engine converted to a pump.
I've been looking at the specs of various compressor pumps and the doner engine I have is suitably sized. I'm going to ditch the standard head and run it with a reed valve setup, all the purpously designed piston compressors I've found run a flat head with reed valves so it's a tried and tested system.
For lubrication I'm going to use the original oil pump setup. For cooling I'll use the original water pump and a small radiator.
I think the idea has enough merit to be worth trying, if it doesn't work at least I'll know why.

Awesome. Place your bets!

My money is on it being fantastically inefficient and expensive to run. Good fun though. Gonna be plenty of internal combustion engines going spare in the near future (sadly).

 

[hotponyshoes]

If you are putting the effort into building some sort of compressor why do someth that has been done before and plenty of better ready made options are available?

Compressed air is a good store of energy, how about doing something a bit more usefull and environmentally friendly.
How about a smaller engine (pump) that could be driven from some small solar panels to generate high pressure output. It could spend all week charging up tanks and give you enough stored for a weekends use in the workshop.
Not only cheap to run but you would not have to listen to the noise of it either!

 

[Rig Pig]

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.

The cylinder would be lubricated by the sump oil the same as always in almost any compressor and any 4 stroke engine, lead was added to prevent the exhaust valves welding themselves to the valve seats it isn't a cylinder lubricant.
I don't see that it's an issue running an engine / compressor single unit, it simplifies the setup and reduces the weight and cost, surely if you fit reed valves the squish will be pushing into the tank as with any other compressor? That along with an unloader would also remove the efficiency issue you mention at start-up, however it's going to get stiff again as the tank fills up as with any other compressor.
The VW unit advertised produces small road compressor output, it's not supposed to be in competition with a 3hp home compressor. I don't have any plans to build one but if I was in the market for a mobile I would certainly investigate further, it would make an interesting project, a turbo and intercooler on the compressor side might stir it up a bit more

 

[knighty]

I've decided to build one, I'm going to use one engine to drive another engine converted to a pump.
I've been looking at the specs of various compressor pumps and the doner engine I have is suitably sized. I'm going to ditch the standard head and run it with a reed valve setup, all the purpously designed piston compressors I've found run a flat head with reed valves so it's a tried and tested system.
For lubrication I'm going to use the original oil pump setup. For cooling I'll use the original water pump and a small radiator.
I think the idea has enough merit to be worth trying, if it doesn't work at least I'll know why.

I'm guessing it'll oil itself ok without a head on... but will it cool via the old water galleries? honestly I've never traced them around an engine, but I've got it in my head that the path of the water flows around the block and the head... so without a head and a flat steel plate where it used to bt you won;y get any flow?

 

[anto44]

I'm guessing it'll oil itself ok without a head on... but will it cool via the old water galleries? honestly I've never traced them around an engine, but I've got it in my head that the path of the water flows around the block and the head... so without a head and a flat steel plate where it used to bt you won;y get any flow?

The coolant feed to the radiator would normally come from the head so I'll have to have a drilling in the new head or drill the flywheel end of the block somewhere for an outlet. The coolant flow needed will be a lot less than what was needed as a running engine

 

[cheb]

The overthinking is part of the fun of threads like this. There's been lots for me to think about. I think I first read about this in 20,000 Jobs Under The Sea by Torrance Parker. I'll dig it out and refresh my memory. It would have been traditional dress (hard hat) diving and using a scrap V8 would have been easier than turning a manual air pump.

Would the holes left by the injector and spark plug on a 1l petrol be big enough for the possible airflow throughput? I'm tempted to see what scrap cars the local garage has, and if I can find a 2t scooter kicking about. The latter to see how bad it would be, very is my suspicion, and effective compression ratio of 3 or 4 to 1.

I don't for a minite think that this will provide a better alternative than a shop bought compressor BTW, though the VW based one looks good.

 

[knighty]

^^ injector or spark plug sized holes will be miles more than big enough

 

[Pigeon_Droppings2]

Would the holes left by the injector and spark plug on a 1l petrol be big enough for the possible airflow throughput?

I hadn't even thought of using the sparkplug holes...makes a lot of sense....now I understand how the head can be blanked off too