Using a multicylinder engine as a compressor

[the snooper]

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

Just drill a core plug

 

[hotponyshoes]

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.

The compression ratio is going to determine the maximum output pressure.
A 3-4:1 is only going to give you 50-60psi (if 100% effective)
It's a useable pressure but you would need a pretty big air receiver for it to be useful unless you can run it at high enough rpm to supply whatever you are using from it.

The engine will cope with much higher pressures than this, 1000psi under detonation of fuel (quick google result)
I dont know how much back pressure it can withstand from the exhaust before it stalls?
I remember sticking potatoes up exhausts as a child and making hell of a pop.

I wonder if it might be possible to take a bog standard engine and just use the exhaust gasses.

Get a big cylinder/tank with a plunger or membrane in the middle.
Air into one side, pipe exhaust gas into the other. The exhaust gas will push the plunger and compress the air. Make it up with ports like a 2 stroke so at the end of its travel the compressed exhaust gas gets dumped and a spring returns the pump to start again.
Finding a suitable heatproof seal would be the main issue I think.
Or modify something like a Mazda rotary engine so exhaust gas is pushed into the inlet for one combustion chamber while the next one around compresses air.

It would also be interesting to see the maximum pressure that a turbo can provide.
Normally the boost is limited to prevent pre detonation. If you could take a turbo diesel engine and reconfigure the inlet side to run as normally aspirated you might be able to use the turbo (or a series of sequential turbos) for air output.
I remember something like 5bar earlier on in F1 but again, that might be limited because that's forcing air back into an engine rather than an air receiver.

 

[Pigeon_Droppings2]

If the head is blanked off then would the pressure only be limited by your ability to turn the crank as the combustion chamber would be mostly gone too?

 

[mtt.tr]

The compression ratio is going to determine the maximum output pressure.
A 3-4:1 is only going to give you 50-60psi (if 100% effective)
It's a useable pressure but you would need a pretty big air receiver for it to be useful unless you can run it at high enough rpm to supply whatever you are using from it.

The engine will cope with much higher pressures than this, 1000psi under detonation of fuel (quick google result)
I dont know how much back pressure it can withstand from the exhaust before it stalls?
I remember sticking potatoes up exhausts as a child and making hell of a pop.

I wonder if it might be possible to take a bog standard engine and just use the exhaust gasses.

Get a big cylinder/tank with a plunger or membrane in the middle.
Air into one side, pipe exhaust gas into the other. The exhaust gas will push the plunger and compress the air. Make it up with ports like a 2 stroke so at the end of its travel the compressed exhaust gas gets dumped and a spring returns the pump to start again.
Finding a suitable heatproof seal would be the main issue I think.
Or modify something like a Mazda rotary engine so exhaust gas is pushed into the inlet for one combustion chamber while the next one around compresses air.

It would also be interesting to see the maximum pressure that a turbo can provide.
Normally the boost is limited to prevent pre detonation. If you could take a turbo diesel engine and reconfigure the inlet side to run as normally aspirated you might be able to use the turbo (or a series of sequential turbos) for air output.
I remember something like 5bar earlier on in F1 but again, that might be limited because that's forcing air back into an engine rather than an air receiver.

The average engine compression is 130psi

 

[knighty]

If the head is blanked off then would the pressure only be limited by your ability to turn the crank as the combustion chamber would be mostly gone too?

that's what I was thinking

 

[hotponyshoes]

If the head is blanked off then would the pressure only be limited by your ability to turn the crank as the combustion chamber would be mostly gone too?

I would think the compression ratio would be the limiting factor in practice.
10:1 gives you a maximum of 10bar.
If you had zero space above the piston then you could generate infinity high pressure.
Reducing the compression chamber size will increase the pressure but there must come a point where the input power required will graph up steeply and the losses from blowing past the piston rings etc will do the same.

 

[Pigeon_Droppings2]

I would think the compression ratio would be the limiting factor in practice.
10:1 gives you a maximum of 10bar.

Agree....I think deleting the head gives you a "theoretically" high compression but in practice it would seem sensible to stay within the original design limits of the engine (to avoid damaging the crank/mechanicals)....although I think 10:1 is the basic volume compressed say....I imagine on the ignition stroke where the petrol is ignited the pressure might be higher at point of ignition....but the pistons will be travelling away from the head....no idea how much higher though so maybe it makes sense to stay within the 10:1 limit.

 

[hotponyshoes]

Quick google showed cylinder pressures up to 1000psi on a standard engine under load.
That must drop off rapidly as the piston moves down but no reason to think pushing it up would be any worse for the engine.
And compressing air is a gradual rather than shock loading.

 

[cheb]

There's not much point in compressing the air too much, most airtools work at around 90psi afaik. For diving, 10bar gets you to at least 90m deep, way beyond anything you'd be doing on air. Normal dive regulators are two stage, the first stage drops from tank pressure, up to 300bar, down to 10bar (ish), and 2nd stage down to ambient, 1bar at the surface, and 1bar for every 10m you descend in sea water.

 

[Pigeon_Droppings2]

Quick google showed cylinder pressures up to 1000psi on a standard engine under load.

That's pretty impressive.....the pistons will mostly experience that pressure in "free fall" as the gases expand rapidly so the crank isn't having to resist that force I would think for any great amount of time.

Still mighty impressive engineering I reckon....I remember it being high but not that high.

 

[hotponyshoes]

There's not much point in compressing the air too much, most airtools work at around 90psi afaik. For diving, 10bar gets you to at least 90m deep, way beyond anything you'd be doing on air. Normal dive regulators are two stage, the first stage drops from tank pressure, up to 300bar, down to 10bar (ish), and 2nd stage down to ambient, 1bar at the surface, and 1bar for every 10m you descend in sea water.

Compressing it over using pressure just gives you a reserve.
At 100m deep a 10bar bottle will only give you a few breaths. A 300bar bottle might give you half an hour.

That's pretty impressive.....the pistons will mostly experience that pressure in "free fall" as the gases expand rapidly so the crank isn't having to resist that force I would think for any great amount of time.

Still mighty impressive engineering I reckon....I remember it being high but not that high.

If the piston was in free fall I dont think there would be much pressure at all.
Pushing a car up a hill and the pressure is generated by the explosion being restricted by the piston pushing back against it.

 

[stuvy]

The average engine compression is 130psi

bang on, our 6cylinder race engines make 160 to 240 depending on who built them and how many races they have had a 4 is pretty much bang on 130 from the last time in pressure tested one a few weeks back

 

[anto44]

The compression ratio of the vw engine with the standard head is 23:1

 

[knighty]

with a "flat plate head" on the compression ratio will be miles better than a running engine...

at TDC the piston is level with the head, so you're going from the volume of the full cylinder to the volume of those little dimples in the top of the piston (idk the name for these.... the bits to aid fuel/air mix/turbalance)

got to be closer to 100:1 than 10:1

with reed valves on the air output it doesn't really matter what pressure the cylinder/piston can make as long as it's more than your air tank... as soon as pressure inside the cylinder inside the cylinder is higher than the air on the output side the reed valve will open and dump air out of the cyliner anyway?

 

[Pigeon_Droppings2]

If the piston was in free fall I dont think there would be much pressure at all.

What I meant by "free fall" was in the normal combustion cycle the instant the fuel ignites there is a massive pressure increase (instantaneous almost)...but as the gas expands it's pushing against a piston which is turning a crank and flywheel that is already in motion so it's adding a nudge to the rotation of the crank. Its not having to hold that 1000psi of force as would be the case in the compression cycle (with a flat head replacing the cylinder head)

Been too long since I did all the maths behind this stuff so I'm just shooting from the hip on gut feeling....which probably means I could be wrong!

 

[R Kraft]

The compression ratio of the vw engine with the standard head is 23:1

That would be about right for a diesel.

 

[outofthefire]

I really don't understand why your all getting bogged down with compression ratio's, your sucking air in through one hole and forcing it out through another. Surely it's displacement that's important.

 

[Screwdriver]

What I meant by "free fall" was in the normal combustion cycle the instant the fuel ignites there is a massive pressure increase (instantaneous almost)...but as the gas expands it's pushing against a piston which is turning a crank and flywheel that is already in motion so it's adding a nudge to the rotation of the crank. Its not having to hold that 1000psi of force as would be the case in the compression cycle (with a flat head replacing the cylinder head)

Been too long since I did all the maths behind this stuff so I'm just shooting from the hip on gut feeling....which probably means I could be wrong!

That's not how I understand it. Google suggests 21 thousandths of a second for the fuel to burn completely (definitely not explode) so that it ignites just before TDC, reaches maximum pressure at or around TDC but is still burning a few degrees after. The gas pressure is pushing the piston at all times until the exhaust valve opens.

Apparently a fuel air charge will expand by about 150 times it's unburnt volume which is interesting to compare with the compression ratio because unless the CR is 150:1 the spent exhaust gas is still at a very respectable pressure when it is released. Makes you wonder if hooking an air compressor system to a turbofan might actually be a more efficient way to compress air.

 

[Screwdriver]

I really don't understand why your all getting bogged down with compression ratio's, your sucking air in through one hole and forcing it out through another. Surely it's displacement that's important.

In an open system perhaps but the displaced air has to fill a compressed cylinder so it needs to be compressed when it comes out.

 

[Al Strachan]

Makes you wonder if hooking an air compressor system to a turbofan might actually be a more efficient way to compress air.

it is and without question. In an industrial environment and in my experience, any high volume/low pressure air requirements would be met with a screw vane type compressor. Piston compressors are only generally for High pressure applications, where piston are used in series to progressively raise the pressure.

Obviously 99% of DIY/light industrial/commercial compressors are mostly piston, because at that size/demand/maintenance it's cost effective.