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Backside supercharger

  [vote for,

This post will clearly illustrate my relative ignorance of automotive technology.

As I understand it, a supercharger uses a mechanical drive (from the engine) to blow air into the cylinders at above- ambient pressures, increasing the power output of the engine. A turbocharger does the same thing, except that the blower is powered turbinically from the exhaust gases.

In each case, engine power is being tapped to pump extra air into the cylinders, with a net gain in power.


Now imagine a conventional cylinder in an engine. The top end (ie, above the piston) is of course where everything happens, and is closed off by the cylinder head. The bottom end (below the piston), however, is open.

What would happen if the bottom end of the cylinder were closed off, by providing a second (upside down) cylinder head on the bottom of the engine? Ignore, for the moment, the piston rod. In this case, obviously, the air in this bottom space would be compressed as the piston was on its downstroke (ie, during the power phase).

With a cunning system of interconnects and valves, therefore, it should be possible for one piston's power stroke to provide (from its new under-cylinder) compressed air which could be fed into another cylinder during its intake stroke.

Now back to the piston rod problem. The piston rod normally exits from beneath the piston, and moves laterally (ie, crank-like) as well as up and down. However, with an additional linkage, the piston rod could move only up and down, with lateral movement in a second linked part. Thus, the piston rod could be a simple cylindrical rod, moving only axially. If so, then it could pass through a hole in the new, second (upside down) cylinder head.

Obviously, the power to compress the air in the under- cylinders comes from the power stroke (ie, it's not free), but this seems no different in theory from using a mechanical tap or a turbine to blow the engine in the conventional ways.

MaxwellBuchanan, May 03 2014

http://venturebeat....erates-electricity/ [xenzag, May 04 2014]

https://en.wikipedi...wiki/Stelzer_engine [Laughs Last, May 05 2014]

Paut Engine https://www.youtube...watch?v=HFUsnKJtMYQ
a bit more complicated... uses double ended pistons [pashute, May 07 2014]

Dual sided piston motor patent http://www.google.com/patents/US6854429
I think this patent describes your idea [pashute, May 07 2014]

Swing piston https://www.youtube...watch?v=lUop8sZ2B94
Same idea, but in a circle [pashute, May 07 2014]

Wikipedia: Opposed Piston Engine https://en.wikipedi...posed-piston_engine
and its history, beginning at 1900 and running a car in 1904 [pashute, May 07 2014]

Scientifically presented double faced piston engine http://www.scienced...i/S0306261910004447
[pashute, May 07 2014]


       The region below the piston is not just where the piston rod is located; it also is where the piston rod connects to the crankshaft. So, the "crankcase" is the section of the engine containing all the connected piston rods (and the lubricating engine oil). All those spaces under the pistons are connected to each other. When you have multiple pistons, you usually have some going up their cylinders, and an equal number going down their cylinders. Net effect: total volume of space in the crankcase does not change, and so can't be used as a pump.   

       There are some exceptions, such as the "Boxer" engine, and variants thereof, as described in another post somewhere around here not so long ago ("Ghost Cylinder Engine" by [FlyingToaster]).
Vernon, May 03 2014

       Erm, is this not a two stroke engine?   

       ie the bit under the bottom of the piston (in this case - the crank case) has a one way valve, so when the piston descends it compresses the air/fuel to go into a (in the case of a 2 stroke - its own) combustion chamber.
not_morrison_rm, May 03 2014

       //All those spaces under the pistons are connected to each other.//   

       Perhaps my explanation wasn't clear. I'm proposing adding a second cylinder head, inverted, at the bottom of the cylinders, with the piston rods (which will have to be doubly-linked) passing through holes in this second head.
MaxwellBuchanan, May 03 2014

       //Erm, is this not a two stroke engine? // That's entirely possible. See first line of idea.
MaxwellBuchanan, May 03 2014

       Yup, you've invented the two stroke, sort of. The other advantage of the two stroke is that the work done by the piston (the underside) is used to both intake and compress air, then force out the exhaust gases. You need fewer "strokes" per complete cycle. So you effectively double the power density of the engine. Unfortunately, the whole set up is not conducive to good lubrication. Also, in multi cylinder engines, one piston is going up as another is coming down. Airflow in the crankcase is tuned to optimize this.
bs0u0155, May 03 2014

       The other interpretation of this title, is some form of powered bowel pressurisation mechanism. For more rip-roaring bottom-burps.
bs0u0155, May 03 2014

       This would be much much less efficient than a regular supercharger or turbocharger. And apocalyptic from a maintenance standpoint.
DIYMatt, May 03 2014

       Like a 2 stroke then....
bs0u0155, May 03 2014

       split-cycle, using one physical cylinder/piston.
FlyingToaster, May 03 2014

       //That's entirely possible. See first line of idea   


       Don't worry about not having an encyclopedic knowledge of the internal combustion engine, you've missed nothing...
not_morrison_rm, May 03 2014

       I was thinking more of the line after that one.   

       What is astonishing about the internal combustion engine is how so many variants have failed to solve a very simple problem. I am proud to have added to this tradition.
MaxwellBuchanan, May 03 2014

       and I'm embarrassed that in my yoof I apparently permanently gave brain space to Schneurle and his twin-loop scavenger system...
not_morrison_rm, May 04 2014

       See last link.
xenzag, May 04 2014

       There is (currently) only one link, so it's also the first.
not_morrison_rm, May 04 2014

       I think the basic model of this idea would be a 1cyl 2-stroke that self-scavenges, with intake flapper-valves on the piston and exhaust valves on the cylinder head. The size of the crankcase determines the scavenging pressure. Oh, and a flapper valve on the crankcase.   

       It's also the direct opposite of one of my recent posts, but I can see my way past that [+]
FlyingToaster, May 05 2014

       If you think of this as a two-stroke, then there is no aspect of supercharging because the volume of air pushed by the back side of the cylinder is the same or slightly less than the volume needed to fill the piston during the intake stroke (unless you have a stepped piston engine which is a whole new mess of possible optimizations.)   

       On a four stroke engine it could be arranged so that two pistons are compressing air in the back side area at the same time that one piston is in the intake cycle. This could theoretically result in nearly doubling the mass of air in the cylinder for each cycle. I don't know how much additional compression is useful, but if you were satisfied with a boost that was significantly less than 2x (maybe 1.2x?), it seems like a 2 cylinder boxer engine could accomplish that with very little modification. I assume a 4 cylinder boxer would need to be modified to split the crankcase into two sealed chambers.   

       It seems to me that putting a joint in the piston rod is going to really hurt performance. Any gains due to supercharging will be lost in max RPMs. However, if you divide up the crankcase into a chamber for each cylinder with seals around the crankshaft separating these chambers, then fill the space so each rod is running in a narrow slot (maybe even make the rods narrower), the total displacement of each cylinder due to piston movement will be significantly larger than the dead space around the crank and rod, getting you much closer to the theoretical 2x.
scad mientist, May 05 2014

       The problem with what you've just said, [scad], is that it appears to be based on a sound understanding of the subject.
MaxwellBuchanan, May 05 2014

       I previously said that I figured a jointed rod would hurt max RPMs. But then I realized that if you have the part of the connecting rod that is attached to the piston constrained to movement in 1 dimension, you no longer need the wrist pin at the piston. In addition, my understanding is that the height of a piston is determined partially by the need to make sure it doesn't tip as it moves up and down. With a piston and half the rod as one solid unit, the piston is much better constrained against tipping, so the piston portion might be smaller. Maybe, the reciprocating mass could even be reduced in this arrangement. The engine would be a bit bigger because the rotating part of the connecting rod would probably need to be the same length as the entire connecting rod before to avoid putting it at a larger angle.
scad mientist, May 06 2014

       [scad] you mean a "crosshead" shaft, used on larger diesels to prevent sideloading on the cylinder walls.
FlyingToaster, May 06 2014

       // milking from one power unit to the next // I interpret this comment to mean that you don't think this would be useful because it's just using power from one cylinder boost another cylinder. If that's what you're saying then you're missing the point of a supercharger.   

       Of course the supercharger uses energy that could be used to turn the wheels, but it forces more air into the cylinders. That allows the engine to burn more fuel per cycle, getting more horsepower out of the same size engine. So why not just use a bigger engine? Because most of the time you don't need an engine that big. For example, the Ford Ecoboost engines, as the name implies, are designed to save fuel. They do that by using a smaller engine than one might otherwise want, but with a turbo charger to get the horsepower needed for short periods of time. During hard acceleration, the Ecoboost is probably not as efficient as a larger engine putting out the same horsepower, but most of the time, when the turbocharger is bypassed, it will be more efficient than the larger engine. Similarly, with [Max]'s engine here, I would expect that during lower power demand times the supercharger function would be disabled by opening different values to vent the air rather than pressurize it.
scad mientist, May 06 2014

       "crosshead shaft" Yeah that's about what I was saying. And according to the wiki, those are heavier and therefore only used in large slow moving engines, so I guess I'm showing my ignorance here. But it still seems to me that you can stabilize the piston using less materials with a long narrow arm than with an extended piston. I almost wonder if they are only used in big engines because it adds complexity. The slow part is a result of the big engine, not the crosshead piston. If a crosshead piston is always heavier, why wouldn't they just make the piston in this big slow engine a little larger to deal with the side load rather than going to the complexity to have the crosshead?
scad mientist, May 06 2014

       The piston on a crosshead is heavier because it has a rod welded to it. I don't think they meant that the piston-head on a crosshead system is heavier (all other things being equal). The crank-rod (on the other end of the crosshead assembly) is the same length as it would be if there wasn't a crosshead there.   

       For simplicity's sake, I'm still viewing Max's device as a single-cylinder 2-stroke, where the compressed air whooshes through the top-cylinder, scavenging the exhaust out, though he has the "undercylinder" feeding another cylinder (in a 4-stroke one would imagine), to improve pumping efficiency.   

       Unless I've misunderstood the whole thing and he's actually talking about trying to increase the pressure in the cylinders in which case bollocks, where's the extra air coming from ?
FlyingToaster, May 06 2014

       I have no idea what [Max] is talking about. He's quite unreliable on this entire subject.
MaxwellBuchanan, May 06 2014

       I'm taking this as an "add idea to HB, see what happens, then stir" type event.
not_morrison_rm, May 06 2014

       // trying to increase the pressure in the cylinders in which case bollocks, where's the extra air coming from ? //   

       The extra air comes from the fact that on a four stroke engine, every fourth stroke is an intake stroke on the front side, but every other stroke on the back side can be used for compression. That gets you close to 2:1 increase in air. Perhaps I'm saying more than [Max] originally intended. He said:   

       // it should be possible for one piston's power stroke to provide (from its new under-cylinder) compressed air which could be fed into another cylinder during its intake stroke. //   

       I interpreted this to say: // it should be possible for one piston's power stroke [and intake stroke] to provide (from its new under-cylinder) compressed air which could be fed into another cylinder during its intake stroke. //
scad mientist, May 07 2014

       ahh, so two cylinders worth of air into the crankcase (or external airbox) vs one cyl's worth of 2x (minus the displacement of the shaft) out into a cylinder. Or words to that effect. A crosshead shaft protruding from a double-ended cylinder is standard in steam engines. Higher reciprocating mass and extra friction on the piston-rod(though the rings or whatever on the undercylinder head don't have to withstand any real pressure to speak of). Almost certainly better efficiency (and certainly waaay more power) than a supercharger when it's on, pumping losses when it isn't.   

       Of course there's the equal possibility that he's just gone and cut'n'pasted random enginey words into the text-box.
FlyingToaster, May 07 2014

       // The piston on a crosshead is heavier because it has a rod welded to it // Yes, we're agreeing on the terms. I'm saying that a piston plus crosshead rod might weigh less than a piston design to handle the same side loading without a crosshead rod. I also know that this disagrees with what Wikipedia say, but here's how I see it.   

       If this is a standard piston:

       An equivalent crosshead piston might look like:

       (++ is the area with the rings) It seems like more material would be saved by removing the skirt (---- -) around the large diameter of the piston than would be used to add the much narrower but longer rod (========). Now the rod will need to handle a much larger compression load than the skirt so maybe it would be heavier, but I wouldn't be surprised if the real reason they aren't used in small engines is complexity and engine size.   

       If size and complexity are the real reasons, then adding this system rather than a traditional supercharger might actually make sense.
scad mientist, May 07 2014

       Good point; wouldn't really trust WP on that one: crossheaded engines are usually large, low-speed diesels for which there's no real equivalent... and the crosshead makes for increased engine size: it might not fit into a car.
FlyingToaster, May 07 2014


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