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Old 08-17-2009, 11:24 AM   #1
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Default "Thump-8" 4 stroke 1/8th scale engine

I figured I would make a thread for my new engine prototype instead of cluttering up the FS26 thread, so here goes!

I named the thread (and for now, the engine) the "Thump-8" because I'm not sure what the final displacement will be. The initial prototype is looking like it'll be about .39ci, but that number can easily change with slight variances in bore and stroke without changing the overall design, depending on how much power and RPM I can wring out of it.

The goal of this design is to end up with a 4 stroke engine that bolts directly into any place a current .21 two-stroke fits, and is able to use the same (or at least very similar) gearing and clutch hardware. The reality may be slightly different than that, but that's the point - to find out if it's possible. Nobody has ever attempted this before (that I know of). It will need to be compact, VERY high-revving and if I can manage, I would like it to not require a cooling fan... Pie-in-the-sky stuff for sure, but the project should be at least lots of fun.

I haven't made metal chips fly yet, but I have discovered Google SketchUp, which allows me to model my engine in 3 dimensions. It's pretty easy to use, and I've already modeled a good portion of the valve system. As I continue to learn how to use the program, I can see it's going to benefit the design and engineering process in a big way. Much better than drawing in 2 dimensions and just trying to imagine how it would work in 3-D.

Tonight, I get to pick up my prototype piston, ring, wristpin, and sleeve. Once that stuff gets digitized, I'll have something concrete to base some dimensions off of.
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Damon Palm's 2011 Race Rigs (the ones that have all the guts in them):
Thump RC 4 stroke "GSC10"
X-Factory X-6 Squared
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THUMP RC -The only 4 stroke race conversion-
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Old 08-17-2009, 06:38 PM   #2
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Originally Posted by DP-buggyboy View Post
I figured I would make a thread for my new engine prototype instead of cluttering up the FS26 thread, so here goes!

I named the thread (and for now, the engine) the "Thump-8" because I'm not sure what the final displacement will be. The initial prototype is looking like it'll be about .39ci, but that number can easily change with slight variances in bore and stroke without changing the overall design, depending on how much power and RPM I can wring out of it.

The goal of this design is to end up with a 4 stroke engine that bolts directly into any place a current .21 two-stroke fits, and is able to use the same (or at least very similar) gearing and clutch hardware. The reality may be slightly different than that, but that's the point - to find out if it's possible. Nobody has ever attempted this before (that I know of). It will need to be compact, VERY high-revving and if I can manage, I would like it to not require a cooling fan... Pie-in-the-sky stuff for sure, but the project should be at least lots of fun.

I haven't made metal chips fly yet, but I have discovered Google SketchUp, which allows me to model my engine in 3 dimensions. It's pretty easy to use, and I've already modeled a good portion of the valve system. As I continue to learn how to use the program, I can see it's going to benefit the design and engineering process in a big way. Much better than drawing in 2 dimensions and just trying to imagine how it would work in 3-D.

Tonight, I get to pick up my prototype piston, ring, wristpin, and sleeve. Once that stuff gets digitized, I'll have something concrete to base some dimensions off of.
any clues to the stroke ? 16.4mm from the 26or smaller ?
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Old 08-17-2009, 08:34 PM   #3
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Wasn't the now defunct FS-40SC supposed to be the "1/8th" scale 2 stroke subsitute?
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Old 08-17-2009, 10:19 PM   #4
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Wasn't the now defunct FS-40SC supposed to be the "1/8th" scale 2 stroke subsitute?
Yeah, in the same way that a bottom of the line Honda with a Type R sticker slapped on it would make a good substitute for a race car. While the "R" on the sticker seems to indicate it would be competitive in a race class like the GT2 class in LeMans, it really can't because it was never meant to.

My theory is that most companies that already sell 2 stroke RC car engines have no reason to make competing 4 stroke engines. Here's a hypothetical: Let's say one of the popular companies makes a 4 stroke that is actually competitive with their 2 strokes. They now have to make 2 varieties of engine, parts support, keep more inventory on hand, and it takes sales away from their already successful 2 stroke engine lines. They would likely lose money, especially once another company decides to compete, prices drop, and sales are spread thinner among the competing companies and engines.

This is why I think the 4 stroke "car" engines we've seen are all slightly modified airplane engines that fit squarely into the novelty market, not competing directly with the mainstream RC car engine market. Yes, with enough work, you can make them competitive - but it was never their intended purpose so it costs more than it should and it's not easy by any stretch of the imagination.

The only companies it could possibly make sense for would be companies which don't currently sell to the RC car market, such as Saito or YS... but even they would be taking a big chance trying to create a market for an entirely new (and expensive) product. If they wanted to do that, I think they would have by now.

I'm doing this for a completely different reason. I'm not motivated by money. I simply want what I want - it's my passion, my hobby. I want to do it right from the very start. I'm not starting with an existing engine. I started with a blank sheet of paper, some design goals, and a big cup of coffee. I don't have a corporation to answer to. No deadlines. No investors. No politics. Most importantly, no risks. It's a hobby for me, so if it doesn't work... Eh, no big deal. I'm sure I would have found another way to spend that money, and it may not have been as fun or satisfying.
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Damon Palm's 2011 Race Rigs (the ones that have all the guts in them):
Thump RC 4 stroke "GSC10"
X-Factory X-6 Squared
Traxxas Slash w/ STRC LCG conversion
THUMP RC -The only 4 stroke race conversion-

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Old 08-17-2009, 11:59 PM   #5
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I would like to see the outcome as well. Good luck on your endeavor.
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Old 08-18-2009, 06:00 AM   #6
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The sales of engines in r/c car market is driven very much by the horsepower ratings published by the manufacturer . I know when I started out eons ago, the I remember asking the store owner, "Gimmie the fastest, most powerful engine you got!". And I'm pretty much sure that's how many still buy their engines.

By their nature 4 strokes can never bost the same hp figures as the 2 strokes, in addition 2 strokes are cheaper to build, have fewer moving parts, need less parts support etc.

I guess it boils down to this.... r/c car and boat people want hp/rpms which = 2 stroke screamer. Plane people need (notice I said 'need' vs' 'want) torque = thumpers.

Personally, I'm now way past sky high hp figures & I go for realism/novelty. I don't race competitively so this is usually not a problem. Right now, I'm at the stage that if I can squeeze a thumper in place of a 2 stroke, I'd do it!

So DP, if you do come up with a new reliable 4 stroke, I'd certainly be willing to buy one from you.
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Old 08-18-2009, 08:21 AM   #7
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Quote:
Originally Posted by DP-buggyboy View Post
The goal of this design is to end up with a 4 stroke engine that bolts directly into any place a current .21 two-stroke fits, and is able to use the same (or at least very similar) gearing and clutch hardware. The reality may be slightly different than that, but that's the point - to find out if it's possible. Nobody has ever attempted this before (that I know of). It will need to be compact, VERY high-revving and if I can manage, I would like it to not require a cooling fan... Pie-in-the-sky stuff for sure, but the project should be at least lots of fun.
You know the term "floating valves" ?
With surch high rpm's a very strong return spring is needed to close the valves folowing the camshaft. Having such a pressure and so resistance on the camshaft you will loose power.....

The only thing to solve this is using rotating valves. Once I have seen an engine using 2 rotating tubes above the cylinder, one with an hole to the cylinder to let the fuel through the tube in and one tube with a hole conected to the exhaust.
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Old 08-18-2009, 08:31 AM   #8
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Originally Posted by Roelof View Post
You know the term "floating valves" ?
With surch high rpm's a very strong return spring is needed to close the valves folowing the camshaft. Having such a pressure and so resistance on the camshaft you will loose power.....

The only thing to solve this is using rotating valves. Once I have seen an engine using 2 rotating tubes above the cylinder, one with an hole to the cylinder to let the fuel through the tube in and one tube with a hole conected to the exhaust.
Yup. I've seen all the aircraft, motorcycle, and automotive non-poppet valve designs (that I can find online), and all the RC non-poppet valve 4 stroke designs. What I'm drawing is much different than anything I've seen before. The only reciprocating thing in my engine will be the piston.

I've had this design kicking around in my brain for years but I assumed it wouldn't work, or that I would never have the means to make one. Now that I'm seeing it in 3 dimensions, I know it will very likely work and that it isn't outside my abilities and means to at least make some prototypes.
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Damon Palm's 2011 Race Rigs (the ones that have all the guts in them):
Thump RC 4 stroke "GSC10"
X-Factory X-6 Squared
Traxxas Slash w/ STRC LCG conversion
THUMP RC -The only 4 stroke race conversion-
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Old 08-18-2009, 08:47 AM   #9
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Originally Posted by Roelof View Post
You know the term "floating valves" ?
With surch high rpm's a very strong return spring is needed to close the valves folowing the camshaft. Having such a pressure and so resistance on the camshaft you will loose power.....

The only thing to solve this is using rotating valves. Once I have seen an engine using 2 rotating tubes above the cylinder, one with an hole to the cylinder to let the fuel through the tube in and one tube with a hole conected to the exhaust.
we've been over this it's pneumatic valves springs for high rpms.
a metal spring horribly slow compared to pneumatics.
drag is usually reduced by rollers on the cam, rocker or, lifters
a big tube with holes lol that sounds so silly but yet probably the easiest to seal the valves
you'd till have the drag of a chain, belt or gear, in that.

just make sure we can use turbo plugs on this compatibility is the key Dp-buggy wants here
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Old 08-18-2009, 09:03 AM   #10
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just make sure we can use turbo plugs on this compatibility is the key Dp-buggy wants here
HMMM! I hadn't thought of that before. I wonder if there's such a thing as a 4 stroke turbo plug. I have been drawing everything with an O.S. Type F plug so far.

Does anyone know what the other 4 stroke manufacturers use, and if they have anything that is equivalent to a turbo plug?
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Damon Palm's 2011 Race Rigs (the ones that have all the guts in them):
Thump RC 4 stroke "GSC10"
X-Factory X-6 Squared
Traxxas Slash w/ STRC LCG conversion
THUMP RC -The only 4 stroke race conversion-
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Old 08-18-2009, 09:32 AM   #11
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HMMM! I hadn't thought of that before. I wonder if there's such a thing as a 4 stroke turbo plug. I have been drawing everything with an O.S. Type F plug so far.

Does anyone know what the other 4 stroke manufacturers use, and if they have anything that is equivalent to a turbo plug?
regular plugs with no idle bar. the turbo plug is 5/8" while a normal plug is 1/4" in diameter.
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Old 08-19-2009, 01:54 PM   #12
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A rotary disk valve or tube valve will certainly work and best of all with our fuel there's no problem with exhaust valve's cooling and lubrication, but if you want ultimate power you need a cam, only in a cam profile you can obtain really high area under the curve - aka WORK, since it gives a non-linear relation between valve position and crankshaft. With a rotary type valva you can only go to a certain point dictated by the linearity of a 360deg circle - the crankshaft. That's the reason you dont see the F1 guys messing with rotary valves, and maybe thus even 2-stroke engines it would actually rob them a ton of power.

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Old 08-19-2009, 05:22 PM   #13
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Originally Posted by maxflo777 View Post
A rotary disk valve or tube valve will certainly work and best of all with our fuel there's no problem with exhaust valve's cooling and lubrication, but if you want ultimate power you need a cam, only in a cam profile you can obtain really high area under the curve - aka WORK, since it gives a non-linear relation between valve position and crankshaft. With a rotary type valva you can only go to a certain point dictated by the linearity of a 360deg circle - the crankshaft. That's the reason you dont see the F1 guys messing with rotary valves, and maybe thus even 2-stroke engines it would actually rob them a ton of power.

X
RC 2 stroke engines do use rotary valves for their intake. That's what the hole in the crankshaft directly under the carburetor is.

The design I am developing has lots of area under the curve, and lots of time with the valve ports fully open. I need to plot the curve and compare it to a typical engine's curve to see the real difference, but I think it's going to be pretty dramatic. It also has no transitional time to get the valve moving and bring it to a stop more gradually like a poppet valve engine has to have. If a poppet valve just slammed open and shut, it (and its valve seat) wouldn't last long.

In other words, the bottom and top of the curve in the disk valve engine has sharp angles, and the "ramp" is a constant angle, and it has a substantial flat top too.

Now I really want to graph it to see what it looks like, given the valve timing I've chosen. I'll post it here so you guys can see what I'm talking about.

The one thing that's going to be hard to compare will be the cross-sectional area of the valve opening. It'll be easy to figure out on the rotary valve, but I'm not sure how to calculate it for a poppet valve.... unless you just calculate the area of the outer surface of the cylinder created by the movement of the outside of the valve face, using the valve face diameter as the cylinder diameter...

I'm not thinking the poppet valve stands a chance once the cross-sectional area is taken into account.
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Damon Palm's 2011 Race Rigs (the ones that have all the guts in them):
Thump RC 4 stroke "GSC10"
X-Factory X-6 Squared
Traxxas Slash w/ STRC LCG conversion
THUMP RC -The only 4 stroke race conversion-
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Old 08-19-2009, 08:59 PM   #14
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LOL, what I ment is that maybe thats one of the reasons F1 doesn't mess with 2 strokes either.

That's what I'm talking about, in 2 stroke rotary valve engines like ours for example, the gains that you get from the rotary valve design against a piston port for example are inmense, the same goes from a rotary valve to a cam curve, they are usually worlds apart, I'm not saying you are going to find a "brick wall on that road", im saying i've walked that road and found one! I hope you find a way to go over it though

The thing is that a poppet valve is not moved by the circular motion of the cam's shaft, instead is actuated by the cam, and that allows for independence in the design of the curve, allowing you to plot a curve that is non linear to the camshaft and crankshaft rotation, unlike any rotary valve design on 2 strokes or tube or rotary disk valves in 4 strokes

And thats kind of what I thought, but I even tryed very aggressive engines like hot on-road .21's, the area under the curve is not that hot at all compared to a hot 4 stroke, and if you try to get more agressive you start messing up and mixing the cycle stages.

Also the same thing allows for not needing to have that big of a port/valve area because the valve is fully open more time, and that helps velocity and torque.

Just for help: 70 to 75% of the poppet valve's face area is a good and conservative aproximation , that's what (+/-) a good full scale racing engine flows through a puppet valve like a modern (ok not new LOL) nascar or pro-stock engine. So there you go: valve area x .725

good luck, I would love to see your racing 4stroke project

X
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Old 08-20-2009, 08:52 AM   #15
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Cool! I didn't do much last night - hopefully I'll be more motivated tonight.

I thought about the need to maintain port velocity too. I'll likely end up changing the shape of the ports depending on how it works. The good thing about the prototype design I have is I can easily change the port shape, size, overlap, and timing by swapping out the upper valve seat (which is basically just a flat piece of steel with holes in it).

I even had thoughts of a guillotine-style power valve arrangement to vary the size of the ports depending on RPM. It wouldn't actually be all that hard to do, BUT it probably won't happen since the motto of all good RC engines is KISS... I guess I'll file that one under the "Anything is possible" category for later on.
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Damon Palm's 2011 Race Rigs (the ones that have all the guts in them):
Thump RC 4 stroke "GSC10"
X-Factory X-6 Squared
Traxxas Slash w/ STRC LCG conversion
THUMP RC -The only 4 stroke race conversion-
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