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Achieving a combustion chamber shape with a low surface area for its volume usually results in either an impractical bore/stroke ratio or an impractically low compression ratio.
This can be remedied by designing the engine as a supercharged engine with a low static or geometric compression ratio from
the start. Combining a bore-stroke ratio of, say, .7:1 with a static CR of, say, 5:1 produces something close to the equivalent of an ellipsoid with a height equal to half its diameter.
If used in a conventional pent-roof or hemispherical ohv set-up featuring a dished piston crown, this would deliver a good surface-to-volume ratio but would rely only on very limited induction swirl to produce turbulence, because there is no opportunity to introduce squish. The available valve area is also limited in relation to the actual charge volume.
A side-valve (flathead/L-head) arrangement would give squish but not enough valve area, even if there is ample lift clearance. An inlet-over-exhaust (F-head) arrangement would meet all the criteria. For instance, two exhaust valves in the block and two or three intake valves in the head, the latter operated by an overhead camshaft, would provide an efficient chamber shape, ample valve area, and plenty of squish.
As a result a higher degree of supercharge may emulate a higher effective compression ratio than one would otherwise aim for. It should make for an efficient engine that is relatively insensitive to fuel quality.
Illustration of F-head engine
http://www.motorera.com/dictionary/FH.HTM
[Ned_Ludd, Aug 22 2007]
Other experiments with combustion turbulence
http://www.popsci.c...4eecbccdrcrd/2.html
[Ned_Ludd, Aug 23 2007]
Ricardo SV combustion chamber described
http://www.old-carb.../1927-Dykes-083.htm
[Ned_Ludd, Aug 23 2007]
More Ricardo
http://www-g.eng.ca...do/#7.%20COMBUSTION
Scroll up and down for a bio on Sir Harry [Ned_Ludd, Aug 23 2007]
Quite a good 'basics' type article
http://www.popularh...ression_ratio_tech/
[Ned_Ludd, Aug 24 2007]
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Most of the high-performance cars around here are driven by f---heads. |
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it's difficult to understand your design but if i get this right you would have the exhaust valves placed in the area of the cylinder that the piston rings pass over. wouldn't you run into the same problem as in 2 stroke engines than? that there's a lot of blow-by between the piston rings and the sleeve of the block. |
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No, it's a normal F-head, that is, inlet-over-exhaust set-up, only with more valve area and supercharged to compensate for compression-ratio limitations. See link. |
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OK, I understand what type of engine head you want in your proposal. But I don't see this adding any higher performance than just putting a supercharger on a regular ohc engine. |
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If you are looking for a ellipsoid combustion chamber wouldn't that be ruined by your own F-head proposal? (meaning the engine not you...) |
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I guess I don't see how low surface area in a combustion chamber would help. |
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The problem with a degree of supercharge great enough to bring about a near-spherical combustion chamber with a conventional hemi or pentroof head is that what little quench geometry there was is lost. It is regained, and then some, by moving the combustion chamber half-off the cylinder bore, by introducing a substantial quench area over the remainder of the bore. The same move introduces the opportunity to put valves in the bottom as well as the top of the combustion chamber. The result is a lot of port area for a very compact and turbulent, and therefore efficient, combustion chamber. |
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I don't understand the term "quench area." In my experience I don't have any reference for quench outside of metalworking, and that isn't too well defined, either. |
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You want a turbulent combustion chamber why? is that to ensure more complete combustion of the air-fuel charge? I always thought a smoother flow was better, but perhaps you're seeking to homogenize the charge more prior to ignition. |
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How do you add valves in the bottom of the combustion chamber? Are you talking about the bottom of the combustion chamber as formed when the piston is at top dead center, or do you mean lower down in the actual cylinder? Because that would be crazy. |
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[elhigh], check out the illustration. It should help. |
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Ned, I'm not really sure I see the original idea here. How is this different from a low-compression F-head engine with a supercharger? |
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The idea is really an unexpected application of the engine type. IOE engines have in the past been used in contexts that have called for durability and smoothness, e.g. '50's Rolls-Royce, Rover, and Aero-Willys. Where they have been supercharged (though I cannot think of an instance) the supercharger was probably a small manufacturer's attempt to gain bhp against new, bigger engines from larger competitors, and thus something of an afterthought. The performance potential peculiar to the IOE type has never, to my knowledge, really been explored. |
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[elhigh] and [F_R_O_G], combustion chamber design basics: keep it compact, keep flame-front travel distance short and from hot to cool, keep the mixture moving to help flame propagation, keep the heat in. Most combustion inadequacies in practice stem from failure to satisfy one or more of these requirements. |
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Quench = squish. The latter is as much a technical term as the former. |
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I feel like I'm talking to a person in riddles. I still don't get how the combustion chamber helps. It looks pretty boxy and crude to me. It's an old model from the 40s and 50s. It's nothing more than getting high compression in a small chamber that can early be accomplished with a high compression hemi with better results. I still don't get the purpose of the supercharger. It's not going to help in any type of swirl in the combustion chamber. any performance increase that may somehow come out of this I think would almost be canceled out by the use of pushrods. |
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I don't understand where your coming from. what would this engines purpose be? truck, car, race? maybe you can tell we where this idea came from and if your 6 years old or have a Ph.D in engineering. |
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[F_R_O_G], I am middle-aged and broadly educated, both formally and informally. The application I see for this engine is in a high-performance passenger car. As always, and as befitting my username, I try to avoid reliance on electronic control systems, and to restrict that ephemeral technology to auxiliary information devices like gauges, because the human mind is capable, in a pinch, of making educated guesses even when the electronics have gone 'fzzzzt'. Thus having denied myself the facility of bodging things right with electronics, I look for things that are right in themselves, or can be made so. |
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A modern hemispherical combustion chamber is neither modern nor hemispherical. It isn't shaped like half an orange; it is shaped like half an orange peel. It is hollow, as it were. There is little chance for quick, direct flame propagation, because there is a domed piston crown in the way. |
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A high-compression hemi is great for accommodating huge valves. Otherwise it's a bloody awful shape for a combustion chamber. To get it to work satisfactorily, one needs either extremely knock-resistant fuel, like the highly leaded stuff the musclecar engines of the '60's were designed around, or some sort of control system that will retard the ignition as soon as the engine starts trying to tap the main bolts out of the block. |
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Thus you may see that the satisfactory operation of today's hemi/pentroof combustion chambers is due neither to any particular advance in combustion chamber design, nor to the mere passage of time (when will people abandon that superstition which Jacques Maritain called "chronolatry"?), but rather to new ways of 'cheating' though the use of electronics. |
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By the way, where do you see pushrods in the design I've described? |
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If I understand you correctly Ned, you are putting a hemi head with some kind of OHV mechanism just off to the side of the combustion chamber, and then putting one or two extra valves in the block. Maybe you should make a sketch of how the valvetrain would work, because I can't quite see it. |
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This would almost certainly work, but I'm not sure if it would offer any real advantage over other designs. |
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I don't see any kind of significant advantages to be had in this. |
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It would require two camshafts - one in the block, one in the head - or pushrods to actuate the overhead valves, it introduces a choke point for gas expansion out of the combustion chamber into the cylinder, and appears to increase the surface area of the fully compressed combustion chamber. Am I mistaken? |
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Also, to have an offset combustion chamber implies that you will have portions of the chamber that aren't readily accessed by turbulent flow, that they will essentially be stagnation points that neither contribute to the combustion sequence nor be well scavenged during the exhaust cycle - such spaces contribute nothing to the actual function of the engine, but add volume to the cylinder and reduce its overall compression ratio. Or is that the point? |
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The added complexity seems like a high price to pay for a small increment of increased output or higher efficiency. |
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My truck has hemi heads and gets fabulous fuel mileage and decent power. Perhaps hemis aren't such a bad thing. |
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[elhigh], what CR are you running? and what sort of ignition/engine management/etc? |
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Is it one of the newer Dodge Rams with the 5.7L Hemi V8s or is it an older Hemi engine? The design of the F-head looks kind of like a flat head with more valves. But then again I know very little about different head designs. |
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Oh and I don't really get what [F_R_O_G] is talking about. Even I understand that A.) A supercharger is used to increase compression because of the the added volume of the head design that the piston can't get to for high compression and B.) Pushrods don't cancel out the benefits of superchargers. In fact almost all of the most powerful engines that I've heard of are supercharged OHV types. |
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acurafan07, you must of not heard of to many engins... i never said the superchager wouldn't add power just that it wouldn't help any more than adding a supercharger on any other engin. as far as you saying that i said that the use of pushrods makes superchargers ineffective. i was refering to Neds desine not a superchagor |
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the diagram that you liked to shows that use of pushrods, so i asumed that was your engin setup. |
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ok these are the questions i still have.
1) whats the use of the supercharger for beond creating higher compression? any swirll created by the superchager will die down by the time the you get a spark acording to your "old-carb" link. |
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2) what improvment have you been able to make over the origonal F-head desine? are you changing the combustion chamber shape making it a modifide F-head? |
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3) where did the use of electronics come in? |
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Lol because I misunderstood you has nothing do do with my "engin" knowledge. Search through my ideas and you might find I know a thing or two about engines. |
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[F_R_O_G], to answer your questions: |
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1. Supercharging isn't about increasing compression as such, it is about increasing the mass of air-fuel mixture at the same compression as on a naturally-aspirated engine. In other words, it gets the engine to act like a bigger engine, because it's pumping as much air as a bigger engine. |
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To achieve the correct compression just before ignition, the combustion chamber volume on a supercharged engine has to be in the right proportion to the mass of air the engine and supercharger between them pump. The resulting bigger chamber volume translates into a lower "static" compression ratio. |
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What I'm after with supercharging is to increase the volume of the combustion chamber in relation to the bore in order to get the surface-to-volume ratio more like that of a sphere and less like that of, say, a sheet of newsprint. The practical, physical, chemical, and thermodynamic desirability of this has been amply established. Consult any textbook on combustion chamber design. |
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The problem with this is that, with a bigger combustion chamber (given a conventional ohv/ohc head), the piston-to-head clearance at tdc is likely to be too big to induce any useful turbulence, the desirability of which has, I think, also been established. The L- and F-heads provide ample turbulence, but make high compression ratios difficult to achieve - but here we're looking for a low static CR. Add the possibility of making an F-head breathe better (e.g. adding more valves), and we've got an idea. |
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2. I suppose you might say my proposed engine stands to a Rover P4 engine as an 5-valve Audi stands to a 1955 Alfa Romeo twin-cam four. |
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3. I mention electronics because they are required to get today's huge-breathing but thermodynamically compromised cylinder heads to work. I would like to obviate the need. |
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Some advice, [F_R_O_G]: go and read the sources. Ricardo is a source. Miller is a source. Hillier, Puttock, Norbye, etc. are sources. Even Setright is a source. Your calendar isn't a source (though I think some of the sources didn't understand that themselves...) |
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ok i'll read up a little more but i have one question |
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//Supercharging isn't about increasing compression as such, it is about increasing the mass of air-fuel mixture at the same compression as on a naturally-aspirated engine.\\
how are you going to get more air-fuel mixture in the combustion chamber without increasing compression via a superchager? you either are or are not increasing compression it can't be both... |
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You're aiming to get the charge to about 8 atmospheres just before the plug sparks (depending on the octane rating of the fuel used and several other factors) whether the engine is supercharged or not. This is a gross oversimplification, but you'll understand the principle. With a naturally aspirated engine the compression is done only by the pistons. Air at 1 atmosphere is compressed at a compression ratio of, say, 10:1 and at a volumetric efficiency of, say, 80%: 1 x 10 x .8 = 8 atmospheres. With a supercharged engine the compression is done by the supercharger and by the pistons. Air at 1 atmosphere is compressed by the action of a supercharger which displaces, say, 1.6 times the engine capacity per two revolutions (four stroke, remember), and is then further compressed by the pistons at a compression ratio of 6.25:1. Assuming the same volumetric efficiency, we get 1 x 1.6 x 6.25 x .8 = 8 atmospheres. So what's the point of supercharging? The point is that you've now got 1.6 times the mass of air/fuel mixture, at the same 8 atmospheres of pressure. |
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In other words, you've got an engine that pumps air like an engine 1.6 times the size. |
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I had recently been looking at the implications of far higher degrees of supercharge, corresponding to 3 or 4 (or more) times the engine capacity. This idea was part of that process. |
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