"Thump-8" 4 stroke 1/8th scale engine
09-06-2009, 09:35 AM
#61
The HB rotary valve four strokes had a shaft that drove the valve rotor. The shaft tube in the front reminded me of a bevel drive Ducati. I never saw one of those engines disassembled...
I don't remember seeing any OPS four strokes. Their aircraft engines weren't popular here for some reason. The only OPS engines I have any experience with are their 2 stroke car and boat engines. That square head .90 was a BEAST!
09-06-2009, 11:33 AM
#62
Tech Initiate
It was in the late '80s that I saw that YS IIRC. Big roots type blower behind the backplate, similar to the supercharged OS four strokes. Makes sense that those belt drive Webra's had rotary valves. The head wasn't tall enough for there to be rockers up there...
The HB rotary valve four strokes had a shaft that drove the valve rotor. The shaft tube in the front reminded me of a bevel drive Ducati. I never saw one of those engines disassembled...
I don't remember seeing any OPS four strokes. Their aircraft engines weren't popular here for some reason. The only OPS engines I have any experience with are their 2 stroke car and boat engines. That square head .90 was a BEAST!
The HB rotary valve four strokes had a shaft that drove the valve rotor. The shaft tube in the front reminded me of a bevel drive Ducati. I never saw one of those engines disassembled...
I don't remember seeing any OPS four strokes. Their aircraft engines weren't popular here for some reason. The only OPS engines I have any experience with are their 2 stroke car and boat engines. That square head .90 was a BEAST!
On the louvered gate valve you'll want to check what the flow coefficient of that might be. Doesn't sound good to me.
09-06-2009, 11:53 AM
#63
I just remembered that the HB valve drive shaft (and the tube it runs in) is in the rear of the engine (not the front like I said earlier), and the carb is in the front (aimed downward at an odd angle). The HB was also available in a .25 car version, with big vertical fins on the head. Never saw one of those in person.
I agree about the louvered gate valve. So many port edges mean a lot of turbulance. High speed airflow will be lousey I'm afraid.
09-08-2009, 10:25 AM
#65
That's pretty sweet that some of the YS engines supercharge that way. I have to admit, I just assumed they were using an external supercharger.
I had also wondered about the extra turbulance that a slotted port design would introduce. It does greatly simplify (and compact) the valve assembly, allows the glow plug to be centered, while allowing a non-linear opening and closing. I wonder if these advantages would matter...
I can totally see how flow would be impeded by small slots vs one large open hole. ...especially with liquid flow, as surface tension would probably really kill it - BUT, we're not working with liquid. There's got to be a way to make a rig to test gas flow through a few different arrangements of slots, holes, etc... I'm sure one already exists for physics type experiments.
I should really do some experimentation with that sort of rig before deciding how I want to do the valve. It shouldn't be that hard since all I want to test is relative flow rates - I don't need hard numbers, just "is this one better than that one" tests.
I had also wondered about the extra turbulance that a slotted port design would introduce. It does greatly simplify (and compact) the valve assembly, allows the glow plug to be centered, while allowing a non-linear opening and closing. I wonder if these advantages would matter...
I can totally see how flow would be impeded by small slots vs one large open hole. ...especially with liquid flow, as surface tension would probably really kill it - BUT, we're not working with liquid. There's got to be a way to make a rig to test gas flow through a few different arrangements of slots, holes, etc... I'm sure one already exists for physics type experiments.
I should really do some experimentation with that sort of rig before deciding how I want to do the valve. It shouldn't be that hard since all I want to test is relative flow rates - I don't need hard numbers, just "is this one better than that one" tests.
09-11-2009, 09:32 AM
#66
So, I think I've figured out how to fairly easily build a flow tester of sorts.
I can use water as long as I keep the surface being flow-tested submerged - that eliminates surface tension. So, the "test rig" consists of a 2 liter bottle with a few different caps - each with a different "port" cut in it. Drill a hole in the bottom of the bottle as a vent. Plug vent hole. Fill the bottle with water. Fill a sink partially with water so the bottle cap can be submerged during the test. Screw the test cap on and cover the "port" with thumb. Turn bottle upside down, submerging the bottle only to the neck. Pull the vent plug. Uncover the port and start the stop-watch at the same time. Stop the timer when the water level reaches the neck. Keep the bottle's neck level with the water in the sink the full duration of the test.
My high school physics is a little rusty, but I think this will give me a good comparison of flow rates of different types of ports.
I can use water as long as I keep the surface being flow-tested submerged - that eliminates surface tension. So, the "test rig" consists of a 2 liter bottle with a few different caps - each with a different "port" cut in it. Drill a hole in the bottom of the bottle as a vent. Plug vent hole. Fill the bottle with water. Fill a sink partially with water so the bottle cap can be submerged during the test. Screw the test cap on and cover the "port" with thumb. Turn bottle upside down, submerging the bottle only to the neck. Pull the vent plug. Uncover the port and start the stop-watch at the same time. Stop the timer when the water level reaches the neck. Keep the bottle's neck level with the water in the sink the full duration of the test.
My high school physics is a little rusty, but I think this will give me a good comparison of flow rates of different types of ports.
09-12-2009, 08:03 PM
#67
Tech Initiate
I think you'll need to account for viscosity and reynolds number effects. Why not use air? Rotameters are fairly cheap on thebay. Vacuum cleaner, throttle, rotameter, dummy cylinder with valve arrangement.
I just ran into a website with some info about the OPS 1.20 four stroke. I don't know if this is current or just informational. Bottom of the page.
http://www.mantuamodel.com/index.php...elli&Itemid=92
Greg
I just ran into a website with some info about the OPS 1.20 four stroke. I don't know if this is current or just informational. Bottom of the page.
http://www.mantuamodel.com/index.php...elli&Itemid=92
Greg
Last edited by dieselpilot; 09-12-2009 at 08:46 PM.
10-05-2009, 09:56 AM
#69
I wanted to let you guys know I still have ideas swirling around in my head about how to make the first prototype - regardless of the type of valve I decide to use.
Since I'll be trying to replicate all the current .21 engine specs (mounting-wise, etc...), I'll just start out with something like a VG-30. There's a pull-start version, so it'll have the nub on the end of the rod journal to drive the supercharger valve (if I decide to go that route) and valvetrain. It also has a fully functioning induction valve and a very standard place to mount the carb if I decide to try supercharging. Most importantly, it's got a long enough stroke for the larger displacement thumper. If I can bore out the cylinder big enough to fit the FS40 sleeve, it'll just be that much easier. If not, I know a guy who could weld a machined cylinder to the crankcase easy enough.
Basically, it's just that much less stuff to design/engineer/prototype/test to get the very first working prototype. I'm sure this method will save lots of money and time.
With the racing season wrapping up this month, I should have a little more time to work on this.
The other day I thought of a variation on the slotted valve idea that would allow almost 2/3 of the valve's surface area to be fully open vs the current limitation of almost 1/2. It requires 2 sliding components per valve and offset sealing surfaces, so it would be much more complicated and expensive to produce... Probably never see the light of day, but it is an interesting concept to think about.
Since I'll be trying to replicate all the current .21 engine specs (mounting-wise, etc...), I'll just start out with something like a VG-30. There's a pull-start version, so it'll have the nub on the end of the rod journal to drive the supercharger valve (if I decide to go that route) and valvetrain. It also has a fully functioning induction valve and a very standard place to mount the carb if I decide to try supercharging. Most importantly, it's got a long enough stroke for the larger displacement thumper. If I can bore out the cylinder big enough to fit the FS40 sleeve, it'll just be that much easier. If not, I know a guy who could weld a machined cylinder to the crankcase easy enough.
Basically, it's just that much less stuff to design/engineer/prototype/test to get the very first working prototype. I'm sure this method will save lots of money and time.
With the racing season wrapping up this month, I should have a little more time to work on this.
The other day I thought of a variation on the slotted valve idea that would allow almost 2/3 of the valve's surface area to be fully open vs the current limitation of almost 1/2. It requires 2 sliding components per valve and offset sealing surfaces, so it would be much more complicated and expensive to produce... Probably never see the light of day, but it is an interesting concept to think about.
10-13-2009, 09:56 AM
#70
Well, I ordered the VG 30 yesterday, so I'll have something to play with by this weekend. First thing is to figure out if the 40 sleeve will fit in that block or if I'll need to make a cylinder to weld to it. Then it's on to fitting the piston to the rod. THEN, on to the valve train/supercharger valve.
THEN, the hard part - the cylinder head.
The way the prototype is designed, I can easily switch between normally aspirated and supercharged. That way it'll be easy to tell if it is worth doing.
THEN, the hard part - the cylinder head.
The way the prototype is designed, I can easily switch between normally aspirated and supercharged. That way it'll be easy to tell if it is worth doing.
10-16-2009, 08:40 AM
#71
Tech Initiate
Cool. I look forward to seeing some chips flying.
Greg
Greg
10-16-2009, 09:55 AM
#72
I did some more exact calculations on how much surface area is available for air flow through poppet valves vs the sliding valve. The slider has a slight "fully-open" area advantage. I based the poppet valve numbers on the biggest diameter valves that will fit in the FS40 cylinder, with a lift of 2.6mm (which I measured from the Alpha 56). Taking into account the very quick opening and closing mechanism I've got planned, area under the curve should heavily favor the sliding valve.
I can't wait to be able to work with more than numbers and drawings.
I can't wait to be able to work with more than numbers and drawings.
10-23-2009, 10:24 AM
#73
Tech Adept
any update.
11-10-2009, 11:28 AM
#75
Little update:
I've begun the drawings for the backplate/supercharger/valvetrain assembly. I also figured out another way to use poppet valves just in case...
It would end up being a 4 valve hemi - head with smaller exhaust valves and a centered glow plug. Very common in full-size engines, but I haven't seen it in small glow engines. Does anyone know if there's a smaller diameter glow plug? That would help with the valve size if I end up designing it that way....
I've begun the drawings for the backplate/supercharger/valvetrain assembly. I also figured out another way to use poppet valves just in case...
It would end up being a 4 valve hemi - head with smaller exhaust valves and a centered glow plug. Very common in full-size engines, but I haven't seen it in small glow engines. Does anyone know if there's a smaller diameter glow plug? That would help with the valve size if I end up designing it that way....