Slow running 4 stroke

[Rudy from Norway]

Hi Guys and Gals,
I’m closing in on my current project and my mind is already searching for the next. I’m thinking an old style, open valve train 4 stroke. I want this engine to be the slowest running engine imaginable. This calls for design features that aids these properties.
From big engines I have learned that this is achieved by low compression, long stroke, narrow intake channel, conservative valve timing, short duration (?), heavy flywheel and probably more. However, when I see some model engine plans I’m noticing an extremely aggressive cam duration. Like on the Farm Boy Hit'n miss engine I built. This makes me wonder and think maybe the full-size design is not directly transferable to a small model engine. So, I’m wondering if some might have some trade secrets up their sleeves about what makes a model engine slower. That would be helpful. I imagine there is possible to start with an existing design an do modifications to it.

Rudy

 

[bluejets]

Many instances, too much emphasis is placed on the cams for model engine.
As long as it has a lump on it and you run around 115 degrees overlap induction/exhaust, it will run.
The "lump" needs to be around 1/8 of the valve diameter.
For other specs, the large flywheel is a definite, the low compression not necessarily but if you make a side valve it is already taken care of.
Long stroke would also be one of my considerations but I have a 6.5cc side valve engine with 20mm piston, 20mm stroke and a large flywheel in a model boat where one can count the propeller strokes on low revs as it chops the water.

 

[Brian Hein]

Hi Guys and Gals,
I’m closing in on my current project and my mind is already searching for the next. I’m thinking an old style, open valve train 4 stroke. I want this engine to be the slowest running engine imaginable. This calls for design features that aids these properties.
From big engines I have learned that this is achieved by low compression, long stroke, narrow intake channel, conservative valve timing, short duration (?), heavy flywheel and probably more. However, when I see some model engine plans I’m noticing an extremely aggressive cam duration. Like on the Farm Boy Hit'n miss engine I built. This makes me wonder and think maybe the full-size design is not directly transferable to a small model engine. So, I’m wondering if some might have some trade secrets up their sleeves about what makes a model engine slower. That would be helpful. I imagine there is possible to start with an existing design an do modifications to it.

Rudy

Hey Rudy,
I am intrigued with the hit and miss engine you mentioned. I'm not sure if my skill level has reached that point yet, but, I do want to build one someday. Did you machine all the pieces from stock or make castings? Are there prints available?

 

[70Rcode]

Hey Rudy,
I am intrigued with the hit and miss engine you mentioned. I'm not sure if my skill level has reached that point yet, but, I do want to build one someday. Did you machine all the pieces from stock or make castings? Are there prints available?

Hi Guys and Gals,
I’m closing in on my current project and my mind is already searching for the next. I’m thinking an old style, open valve train 4 stroke. I want this engine to be the slowest running engine imaginable. This calls for design features that aids these properties.
From big engines I have learned that this is achieved by low compression, long stroke, narrow intake channel, conservative valve timing, short duration (?), heavy flywheel and probably more. However, when I see some model engine plans I’m noticing an extremely aggressive cam duration. Like on the Farm Boy Hit'n miss engine I built. This makes me wonder and think maybe the full-size design is not directly transferable to a small model engine. So, I’m wondering if some might have some trade secrets up their sleeves about what makes a model engine slower. That would be helpful. I imagine there is possible to start with an existing design an do modifications to it.

Rudy

Hi Guys and Gals,
I’m closing in on my current project and my mind is already searching for the next. I’m thinking an old style, open valve train 4 stroke. I want this engine to be the slowest running engine imaginable. This calls for design features that aids these properties.
From big engines I have learned that this is achieved by low compression, long stroke, narrow intake channel, conservative valve timing, short duration (?), heavy flywheel and probably more. However, when I see some model engine plans I’m noticing an extremely aggressive cam duration. Like on the Farm Boy Hit'n miss engine I built. This makes me wonder and think maybe the full-size design is not directly transferable to a small model engine. So, I’m wondering if some might have some trade secrets up their sleeves about what makes a model engine slower. That would be helpful. I imagine there is possible to start with an existing design an do modifications to it.

Rudy

Hi Guys and Gals,
I’m closing in on my current project and my mind is already searching for the next. I’m thinking an old style, open valve train 4 stroke. I want this engine to be the slowest running engine imaginable. This calls for design features that aids these properties.
From big engines I have learned that this is achieved by low compression, long stroke, narrow intake channel, conservative valve timing, short duration (?), heavy flywheel and probably more. However, when I see some model engine plans I’m noticing an extremely aggressive cam duration. Like on the Farm Boy Hit'n miss engine I built. This makes me wonder and think maybe the full-size design is not directly transferable to a small model engine. So, I’m wondering if some might have some trade secrets up their sleeves about what makes a model engine slower. That would be helpful. I imagine there is possible to start with an existing design an do modifications to it.

Rudy

Hey Rudy, assuming your engine has atmospheric only (no cam linkage) intake valve & exhaust valve only cam bump lift point at about piston Bdc to Tdc close for about 190 deg exht valve duration,...why is that considered aggressive valve timing ? ..just wondering, tom.

 

[Rudy from Norway]

Tom, just by looking at the cam lobe on my Farm Boy Hit'n miss engine I see a huge difference compared to a car engine. The lobe opens very fate and stays open for a very long time, and shuts equally fast. This is of cause possible because of the low rpm, but anyway, the duration is what I would say "aggressive"?
When I think of it, maybe the slow speed has to do with it. Very low compression and long time to empty the exhaust. Combined with sparingly filling with new air/gas. I'm just guessing..
Rudy

 

[70Rcode]

Tom, just by looking at the cam lobe on my Farm Boy Hit'n miss engine I see a huge difference compared to a car engine. The lobe opens very fate and stays open for a very long time, and shuts equally fast. This is of cause possible because of the low rpm, but anyway, the duration is what I would say "aggressive"?
When I think of it, maybe the slow speed has to do with it. Very low compression and long time to empty the exhaust. Combined with sparingly filling with new air/gas. I'm just guessing..
Rudy

 

[ninefinger]

If you get to the fundamental question of what keeps an engine running at a constant speed its that there is enough power stored as inertia in the engine / flywheel to get it through to the next power stroke (usually after delivering some form of output), otherwise it will keep slowing down and quit. (single cylinder anyways). At the very slow end it needs enough inertia to overcome friction for the exhaust phase, intake phase and then to have enough left over to compress the next charge or it will completely stall. Now in my mind that means minimizing losses, especially friction. Usually the faster something moves the more loss due to friction. Also, heavier is typically related to more friction. So all parts must be free running, and use ball bearings where possible (crank / flywheel), even get rid of the seals if the bearings have them. Also, use a really thin lubricating oil. On a 4 stroke the valve train is an additional drag so the lighter the valve spring pressure the better, and roller followers for the cam not sliders. Lighten all reciprocating weight (the connecting rod and piston, crank journal, etc.). Lastly, don't make the engine do unnecessary work, such as working against an overly restrictive intake or exhaust. And as high fuel or thermal efficiency is not the intent then lower compression as has been said - this will help with less inertia needed to compress the charge and likely less friction as the sealing requirements are lower.

Mike

 

[70Rcode]

Hi Rudy,..i agree the exht lobe profile looks to have very steep, fast ramps but with little to no actual intake valve over-lap, i think it's just a very quick but mild duration timing. My cam lobe has the same aggressive Look but the actual cylinder compression starts just as the piston starts upward makes me think it's good for the slowest possible running rpm we both want. It's close to the ultimate square lobe profile that's mild seat to seat total open duration (good for low rpm) But fast banging the exhst valve open & later slamin it closed. I just have no various H&M cam profile experience to go on. If you find a profile that gives slower reliable rpm let me know..thx,...tom.

 

[70Rcode]

Hi Rudy,..i agree the exht lobe profile looks to have very steep, fast ramps but with little to no actual intake valve over-lap, i think it's just a very quick but mild duration timing. My cam lobe has the same aggressive Look but the actual cylinder compression starts just as the piston starts upward makes me think it's good for the slowest possible running rpm we both want. It's close to the ultimate square lobe profile that's mild seat to seat total open duration (good for low rpm) But fast banging the exhst valve open & later slamin it closed. I just have no various H&M cam profile experience to go on. If you find a profile that gives slower reliable rpm let me know..thx,...tom.

Hi Rudy,..like Mike said to minimize all frictional losses & maximize net post cumbustion pistion psi transfere to flywheel (torque) inertia to overcome next upcoming compression cycle. My current intetest lie in the apparent compression ratio conflict. Yes, lower compression ratio needs less flywheel stored inertia to pump thru the next compression stroke But the frequently used 8:1 CR lies centered on the Non-linear CR to torque production curve where lower CR's generate a Faster reduction rate in torque output per CR unit...So which CR, 3:1 or > 8:1 for lowest rpm to use ??...Another interesting area is the fuel Mixer venturi size & Spray Bar hole(s) orientation direction to fluid-gas flow dynamics for optimum fuel atomization (where your needle valve is right at seemingly A/F Lean miss-fire But it Still feels flywheel & exhaust smells A/F Rich) conundrum !! Also, typical small bore model size H&M's seem run too cold for optimum combustion efficiency...just some of my thoughts & observations that might be way off but thought i would mention,..tom.

 

[Rudy from Norway]

Thanks guys. This is thread actually starts to be very educational for low RPM guys
So far I'm thinking: Low friction, 1:4 CR, Very-very conservative intake valve cam lobe (I would prefer both valves to be cam operated just to ad more nice moving visible parts). Exhaust lobe probably as a Hit'n-Miss engine (appears aggressive, but actually not as long as intake is limited). Flywheel tuned manually to the best result. I also prefer Propane as fuel. Find it easier to get the engine run better and I can run it it indoors for a while without the smell of a lawn mower and black oil from the exhaust. A buzz type ignition will also probably prevent stalling if we are on the edge on other parameters.
Rudy

 

[70Rcode]

Thanks guys. This is thread actually starts to be very educational for low RPM guys
So far I'm thinking: Low friction, 1:4 CR, Very-very conservative intake valve cam lobe (I would prefer both valves to be cam operated just to ad more nice moving visible parts). Exhaust lobe probably as a Hit'n-Miss engine (appears aggressive, but actually not as long as intake is limited). Flywheel tuned manually to the best result. I also prefer Propane as fuel. Find it easier to get the engine run better and I can run it it indoors for a while without the smell of a lawn mower and black oil from the exhaust. A buzz type ignition will also probably prevent stalling if we are on the edge on other parameters.
Rudy

Hi Rudy,...Even with the non-linear lower CR to Hp output curve, i think the required Hp to compress gas is even More Non-linear, so the 3:1 CR should promote lower running Rpm effect ...if i did the math right at this time of day !...tom

 

[70Rcode]

Hi Rudy,...Even with the non-linear lower CR to Hp output curve, i think the required Hp to compress gas is even More Non-linear, so the 3:1 CR should promote lower running Rpm effect ...if i did the math right at this time of day !...tom

I'm fortunate i found your thread Rudy,...it's generated some good engine mod ideas to help my "lowest running rpm" quest !...tom.

 

[Rustkolector]

Rudy,

For what it is worth, in Google Books I found the valve timing of early 325 RPM Titan and Mogul throttle governed engines which were built in single and twin cylinder configurations with mechanical and automatic valves. The intake valve opening duration for automatic intake valves was 150 crankshaft degrees. For mechanical intake valves the duration was 227 degrees with about a 20 degree ATDC opening. The exhaust valves opened 40-45 degrees BBDC and closed 5-10 degrees ATDC. Scaling issues can complicate duplication of getting a model to run at this speed and I cannot vouch for how well IH engines of that era ran. It might have been poor. In my experience, increasing the flywheel weight does not help slow running as much as increasing the diameter of flywheels. If you add flywheel weight it should be at the flywheel rims. Further, you absolutely need very good sealing piston rings, valves, and valve guides to prevent misfires. By good, I mean at least two very round rings per piston and valves that pull down to 25”hg vacuum and don’t lose more than 5”hg in 10 seconds. As far as C/R goes, low compression ratios allow more residual exhaust gas to remain in the cylinder, and more chance for misfire. I think the optimum CR should be 5 to 5.5:1 compensated by larger diameter flywheels. For good running characteristics the carb throat should be small for good fuel atomization. How small depends on the engine bore, but I would say about .100” isn’t too small on a 1” bore engine. Unfortunately, ready made carbs this size don’t exist. I like propane so this isn’t as important. Just my $.02.

I will be interested to see what you come up with.

Jeff

 

[ninefinger]

You could always design it so that the CR is adjustable, something like the model diesel engine with a contra piston or some other device that fills in the void in the combustion chamber (easiest on a side valve design), or just have head shims that can be added or subtracted to adjust the CR. Have you got a rough displacement in mind or vague idea of overall desired size? Apologies if I've missed it. The basic engine design will dictate the method of adjustment, and the lower the CR the larger the physical adjustment factor becomes so it could get quite interesting if you were aiming to adjust from a 4:1 to 3:1 CR!

Mike

 

[Dragons_fire]

Sorry for bringing back an old thread, but I was reading through here and it got me thinking.

Would reducing the compression ratio down to a 4:1 or so also work for slowing down a glow engine?

All I could find on google was about adding more nitro to the mix as the CR gets lower. I also understand that with glow, if the engine is running too slow, the plug can cool off too much between firings.

I'm just playing around with thoughts of a 4-stroke glow engine that would run slower than most glow engines I'm used too. Im thinking about something that would maybe have a few hundred rpm idle and max out around a couple thousand rpm.

 

[bluejets]

This is fairly low compression, around 6:1, (side valve 6.5cc capacity)
decent flywheel (way to go No.1) ,
standard 2 stroke glow plug (not 4 stroke which has a much smaller hole inside)
Standard 4:1 methanol/castrolM (caster oil) no nitro added
Standard air bleed carburettor (old os pet carby)
Battery remains on ( on-board glow driver in the boat) for low down idle (best way No.2)

I think you will find trying to get a 4:1 cr engine to run will be a right pain in the butt.

 

[Vietti]

I too like very slow running engines and long coast times if hit and miss. About to start an R&V hit and miss with the hope to make it a slow runner and long coaster.

Some of my design thoughts are probably not main stream, even controversial but here goes.

!. If its a hit and miss or even a conventional 4 stroke consider 0 rings, advantages have been discussed. Biggest disadvantage seems to be its too easy.

2. Conservative valve timing seems correct, If its a hit and miss, intake timing is not an issue, light spring seems like a good idea. I like a large exhaust valve and passages so exhaust gas can easily be evacuated. Even with the engine coasting seems helpful for the air to move in and out as easily as possible.

3. On hit and miss engines I favor a balanced crank, more rotating mass, better balance, and no balancing holes in the flywheels. Worthwhile balancing any engine I suppose.

4. Take great care that all moving parts are in the proper plane or alignment and at exactly at 90 degrees when called for. ETW said this was one of the biggest problems he saw in others models. Misalignment causes friction and poor running in general.

5. Probably controversial, surely debatable, I believe on a hit and miss engine a higher compression ratio than suggested results in the engine hitting harder and results in a higher initial rpm after firing and therefore a longer coast time. Easy experiment is to attach a removable disc to the top of the piston to raise the compression and see what happens. I've modified some engines by boring and stroking just to get a more powerful power stroke, seems to work. I think within reason this is true of other four stroke engines as well. One caution, they do tend to jump more when firing!

 

[johwen]

Hi Vietti,
A smaller choke size in the carburetor will help with slow running also achieving bettor atomization of the fuel. cheers John.