Sleeve Porting: What does what?

[Jim Allen]

A blowdown timing amount of 30 deg will work very well on a tuned pipe engine making it's peak HP turning 26,000 to 32,000 RPM; if there is sufficient exhaust blowdown area. A transfer timing of 130 deg would need an exhaust timing of 190 deg, to give 30 deg of blowdown. The pipes ability to return the boost at the right time will be made easier if the blowdown time & area are larger.

Raising the exhaust timing always causes an adverse effect on bottom end & mid-range HP amounts. However, this effect can be eliminated if the compression ratio can be significantly raised.

Jim Allen

 

[JoeWScott]

On Rotax snowmobile engines there is an exhaust port modifier. I think that's what it is called. It is a valve on the exhaust port that opens at a given rpm and raises the exhaust timing. Could this be adapted to a model engine?

 

[Charles Perdue]

The teather cars are using a centrifugal powered device to do this.

Charles

 

[dwilfong]

A blowdown timing amount of 30 deg will work very well on a tuned pipe engine making it's peak HP turning 26,000 to 32,000 RPM; if there is sufficient exhaust blowdown area. A transfer timing of 130 deg would need an exhaust timing of 190 deg, to give 30 deg of blowdown. The pipes ability to return the boost at the right time will be made easier if the blowdown time & area are larger.

Raising the exhaust timing always causes an adverse effect on bottom end & mid-range HP amounts. However, this effect can be eliminated if the compression ratio can be significantly raised.

Jim Allen

Jim what would be a good rule of thumb on ex area in the part of the ex port that is open above the the point that the transfers open or in other words the 30% blow down area.

David

 

[dwilfong]

the valves on the ex in larger motors do not realy change the timing because thy do not seal in cylinder. thy change the ex area. thy are more of a baffle. It will not change the CR.

Thy are to change the tune of the pipe.

David

 

[Marty Davis]

I wrote to Brian Callahan asking him "what is your experience with toroidal combusion chambers and he wrote back the following:

"No direct experience. All I have to go by are guiding theories: first you want fast burn, resistance to detonation, and minimal heat loss. Fast burn comes from high charge motion which comes from high squish velocity. Resistance to detonation comes from short burn duration time, which comes not only from fast burn but also short distance to end-of burn. This means a compact combustion chamber with the max distance from ignition source to the edge of the chamber short. This is where the idea of a quasi-spherical chamber comes from. For minimal heat loss you want the minimum surface area for the volume, which leads to the exact same requirements as for detonation resistance.

Counter to this is exhaust gas scavenging efficiency. There you want a chamber easily scavenged, which means radially narrow squish band with a broad, flat combustion chamber.

Finally, you want minimum crevice volume. Because the air-fuel mixture compresses and reduces in volume before the flame reaches it, a disproportionally large fraction packs in the crevices. 1% crevice volume might mean 10% of your charge does not burn. A crevice is anywhere the gas can flow into but the walls are too close together for the flame to reach in there. Threads around the plug, clearance between the piston and liner, the chamfer at the top of the liner, and perhaps the volume deep in the plug cannot be reached by the flame. Reducing crevice volume does not necessarily counter any of the above effects.

From your positive results with deep, narrow chambers it sounds like the effect on combustion efficiency outranks the effect on scavenging efficiency in our engines. This makes sense because we run a fuel with such high requirements for vaporization energy and such short vaporization time available that we are probably on the limit of air and (gaseous) fuel mixture homogeneity. Otherwise we would be able to get away with more nitro. Not sure how crevice volume ranks."

Jim:

I would be interested in a drawing, etc that shows the shape that you have come up with.

 

[Marty Davis]

A blowdown timing amount of 30 deg will work very well on a tuned pipe engine making it's peak HP turning 26,000 to 32,000 RPM; if there is sufficient exhaust blowdown area. A transfer timing of 130 deg would need an exhaust timing of 190 deg, to give 30 deg of blowdown. The pipes ability to return the boost at the right time will be made easier if the blowdown time & area are larger.

Raising the exhaust timing always causes an adverse effect on bottom end & mid-range HP amounts. However, this effect can be eliminated if the compression ratio can be significantly raised.

Jim Allen

Jim what would be a good rule of thumb on ex area in the part of the ex port that is open above the the point that the transfers open or in other words the 30% blow down area.

David

David:

I can speak to that.....

We ran extensive dyno tests on the "top hat" style exhaust port shape and found that the top hat needed to be slightly less than 1/3 of the width of the port. If you went 1/3 the engine lost a ton of power. Slightly less than 1/3 was optimum. This is not a guess, it was documented with 50 or so pulls on the dyno with both .21 and .45 engines.

 

[Marty Davis]

A blowdown timing amount of 30 deg will work very well on a tuned pipe engine making it's peak HP turning 26,000 to 32,000 RPM; if there is sufficient exhaust blowdown area. A transfer timing of 130 deg would need an exhaust timing of 190 deg, to give 30 deg of blowdown. The pipes ability to return the boost at the right time will be made easier if the blowdown time & area are larger.

Raising the exhaust timing always causes an adverse effect on bottom end & mid-range HP amounts. However, this effect can be eliminated if the compression ratio can be significantly raised.

Jim Allen

Jim:

I agree 100% with all your points here. We have also documented the 30 degrees of blow down as optimum on all engines that we have tested. I think that the initial suggestion that 30 degrees was optimum was provided by Dave Marles and it was "spot on".

An earlier reply saying that props, etc were as or more important than engine performance. I agree with this to an extent, BUT you must have an engine that performs well to optimize the other factors. Most engines (nitro) that are available to us are fairly well optimized and time should be spent in boat setup, props, pipes, etc BEFORE optimizing the engine. A ton of performance is available just in selection of the correct pipe.

 

[Marty Davis]

A couple of very important things here in this regard:

The fit of the head button into the liner is very important in terms of crevice volume. If the fit is not snug, there is a very large volume and performance will suffer greatly. Also, the best solution is to have the head button sit on top of the liner, BUT the gasket must be EXACTLY the same size as the bore and perfectly centered. This crevice volume, in this area, will greatly affect performance if the fit is not snug.

The glow plug should be exactly the right depth .180" in order to not create some additional crevice volume.

As you can see, there is a lot happening with the head button.

 

[Jim Allen]

"Finally, you want minimum crevice volume. Because the air-fuel mixture compresses and reduces in volume before the flame reaches it, a disproportionally large fraction packs in the crevices. 1% crevice volume might mean 10% of your charge does not burn. A crevice is anywhere the gas can flow into but the walls are too close together for the flame to reach in there. Threads around the plug, clearance between the piston and liner, the chamfer at the top of the liner, and perhaps the volume deep in the plug cannot be reached by the flame. Reducing crevice volume does not necessarily counter any of the above effects."

This is exactly why, for maxium HP to be developed, the suqish band must be closed to the minimum distance possible (until the piston touches the head at TDC during maxium RPM's), no matter what fuel is used. My .90 engine uses .000" to .001" deck clearance for 80% nitro fuel. In the case of my toroidal heads, there is a radius on the top edge of the piston which creats a "crevice volume". This radius & the one on the top inside edge of the exhaust are there to eliminate opening window turbulence. This increases the mass flow out & into the exhaust window by forming a moving DeLaval nozzle.

My experience with using a toroid shows a 50% squish area ratio combined with closing the deck until it touches gives the best power, when combined with plug cooling in the plug's thread area. The maxium compression ratio is set by the amount of over-rev needed, not by when detonation begins.

The heads work on square & oversquare engines even though the interaction of transfer stream relationships are changed because of the distances those streams must travel accross the piston crown in engines of equal displacement. Changes in bore size always affect port flow directions, but they appear to have little affect on toroidal head shapes.

As Marty said, there are a lot of things happening in single cylinder two stroke engine!

Jim Allen

 

[Marty Davis]

Jim:

Please talk a little about how compression ratio affects the ability of the engine to reach maximum rpm (over-rev). Compression ratio acts like a governor to top rpm's. Why is this?

Also, why do you use 50% for squish band area? What is the basis and why not more squish band area? I test all my ideas and findings on the small engines because effects are greater with less excess HP. I have found that 70% squish band area, or a little more, is best for my use. Anything less does not give me enough "launchability".

 

[Jim Allen]

Jim:

Please talk a little about how compression ratio affects the ability of the engine to reach maximum rpm (over-rev). Compression ratio acts like a governor to top rpm's. Why is this?

Also, why do you use 50% for squish band area? What is the basis and why not more squish band area? I test all my ideas and findings on the small engines because effects are greater with less excess HP. I have found that 70% squish band area, or a little more, is best for my use. Anything less does not give me enough "launchability".

The high compression ratio acts like a governor when using hemi style heads because the engine goes into detonation, even when you increase the squish area to 70% & use large deck clearances (.005"+). The oppisite thing happens with the toroid because there is no detonation. The geometric compression of a toroid can be twice what would be possible with a hemi style head. The increase in bottom end & mid-range HP will be so large, that any propeller where your throwing the boat to get started, will now be launched at low throttle by setting the boat in the water. Tuned pipe design will also change to very aggresive baffle & diffuser angles with long, small ID stingers. Much shorter tuned lengths will be possible with easy launchability because of the high compression ratio. If the toroid is working properly the needle will be much richer than what can be used with the hemi style head.

I use the 50% squish area because it allows the bowl shape I want for 1.125" to 1.339" bores. As yet I have not tested various squish areas on the toroid & I may not until the detonation reappears. I am still testing increasing compression ratio amounts. I also used a larger squish area (65 to 65%) on engines that had hemi style bowls.

During this discussion, consideration of where the incomming transfer streams are being directed & the shape of the piston crown should not be overlooked. The piston crown is exposed to the heat of combustion. That heat must go into the piston crown & the piston skirt must absorbe a large part. Therefore, the cooling of the piston crown becomes very important in any high HP engine. Notice the extra water cooling area around the plug's thread area on the toroidal head. IT IS IMPOSSIBLE TO GENERATE HIGH HP AMOUNTS WITHOUT ADEQUATE COOLING!!

Jim Allen

 

[Jim Allen]

All head drawings are drawn at a ratio of 1 to 10.

Jim Allen

 

[dwilfong]

Some great stuff here.

I have bin taking in all the info that has bin presented on early postings and have come up with a recipe on a head button for a large nitro eng that I have bin using.

I have taken the 50% squash and used this for the MSV calculations.have bin able to make a flat top chamber with a depth of .120 and still keep a 9.2 CR.with 195 ex duration. after pulling the plug element it resides in the middle of this chamber. Their is a 1/8 radius in the transition from the roof to the side making a strait side to the chamber. the mixture rolling of this sharp 90 deg edge I feel will tumble the flow in to the chamber. this is all done with a .006 squash clearance.I would go tighter but don't want to push my luck.

This shallow wide flat chamber should make for very clean scavenging.

I think when looking at a flame front in the chamber where the kernel starts and the speed for it to propagate are controlled by the area square of the front. a tall front will travel slower than a small front. you would have to do the math but in a radial progression of the small front to start will expand quicker to the out side. also a even hight will make for a more liner movement to the front.

I hope I explained this right. I can see it in my mind but it is hard to explain.

Also what about the boundry layer on the surface that is not burned? how can this be adressed?

David

 

[Marty Davis]

Jim:

I would be super happy to be able to get away from detonation on my little engines. They don't last very long that way. I tried to use NE as a detonation inhibitor, but that engine did not like to stay lit with it. It is also very bad to breathe.

It is hard to decipher your sketch. Do you have an image of the head button so that we can see the shape?

 

[Andy Brown]

Jim:

I would be super happy to be able to get away from detonation on my little engines. They don't last very long that way. I tried to use NE as a detonation inhibitor, but that engine did not like to stay lit with it. It is also very bad to breathe.

It is hard to decipher your sketch. Do you have an image of the head button so that we can see the shape?

Marty,

Jim's drawing is for a gas engine. Note the spark plug to the left side of the drawing. Jim has several combustion chamber shapes drawn in at the right side of the plug. Also note the angled piston top and the matching squishband. Just need to blow up the drawing a bit so us old guys can see the numbers. LOL

 

[Jim Allen]

I have a drawing of the head I use on the .90 nitro engine. There are no numbers on it & maybe that will be easier to see. The glow plug is .070" away from the piston crown at TDC. The flat top piston has a 1 degree squish band. I will post it soon.

Andy, you have excellent eyesight. The #4 chamber has been dropped down .009" to give chamber #5, raising the compression higher yet.

Jim Allen

 

[Marty Davis]

Jim:

I would be super happy to be able to get away from detonation on my little engines. They don't last very long that way. I tried to use NE as a detonation inhibitor, but that engine did not like to stay lit with it. It is also very bad to breathe.

It is hard to decipher your sketch. Do you have an image of the head button so that we can see the shape?

Marty,

Jim's drawing is for a gas engine. Note the spark plug to the left side of the drawing. Jim has several combustion chamber shapes drawn in at the right side of the plug. Also note the angled piston top and the matching squishband. Just need to blow up the drawing a bit so us old guys can see the numbers. LOL

Andy:

I saw that, but I can not determine the shape at the outer edges. I also saw the angled piston and matching squish band. I wonder if there is enough meat on the top of the piston to put some angle and still leave enough thickness.

10-4 on the size of the drawing for us old folks.....

I am interested in the shape of the outer edges of the chamber. An image would be awesome for visualization.

 

[Jim Allen]

Jim:

I would be super happy to be able to get away from detonation on my little engines. They don't last very long that way. I tried to use NE as a detonation inhibitor, but that engine did not like to stay lit with it. It is also very bad to breathe.

It is hard to decipher your sketch. Do you have an image of the head button so that we can see the shape?

Marty,

Jim's drawing is for a gas engine. Note the spark plug to the left side of the drawing. Jim has several combustion chamber shapes drawn in at the right side of the plug. Also note the angled piston top and the matching squishband. Just need to blow up the drawing a bit so us old guys can see the numbers. LOL

Andy:

I saw that, but I can not determine the shape at the outer edges. I also saw the angled piston and matching squish band. I wonder if there is enough meat on the top of the piston to put some angle and still leave enough thickness.

10-4 on the size of the drawing for us old folks.....

I am interested in the shape of the outer edges of the chamber. An image would be awesome for visualization.

Marty,

Its not necessary to put an angle on the piston top. The chamber I use on the .90 cu in engine has a flat top piston with the squish band angled at 1 degree. All the chamber shapes are made with ball nosed end mills using a rotary table. I think the things could be made in a lathe, but the grinding of form tools would be necessary. Greg Settles has been making toroids for some time now in .21 cu in size motors. He also uses ball nose mills & a rotary table. None have been made for a .40 size motor, but he is seeing the same things that I see in the big motors; no detonation, very high compression ratios, much richer needle settings, excellent over-rev HP & no plug burning problems. I believe he uses Rossi turbo plugs in his engines.

As the engines bore becomes smaller, the making of the toroid becomes more difficult because of the size of the 1/4 - 32 glow plugs OD. Greg has reduced the squish band area ratio to 40%, to accomodate the .625" bore size, but the chamber design still seems to work with the same results.

Jim Allen

 

[Jim Allen]

Marty,

This is what it basically looks like.

Jim