Office of Steam Forum for Model & Toy Steam Gas & Hot Air Engines
The Regular Stuff: Chat, Buy, Sell, Off Topic, etc. => General Discussion - Scale Model Gas Engines - Hit & Miss - Throttle Governed - Non-Compression – etc => Topic started by: ShadetreeMotorcycle on November 17, 2020, 07:48:41 pm
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I have not had time to tune on my engine as my new job has long hrs and sizeable commute...but... my Mind is always tinkering!....SO....our very own Gil posted a YouTube video of a very nice running M92...I was pleased because he shared in detail just what he had done which besides a softer spring was remove .105 from piston face plus dish piston approx .075
I was thinking that lowering compression makes the flywheels able to carry us thru the compression stroke for a longer time...BUT also lowers the power stroke...I am not really concerned about putting This engine to work but more a nice slow display speed...However....I Do want to make the most of the power I Do make....got me to thinking about dished pistons!
I did some math and the photo will show my Very Crude sketches of 3 pistons....
Top A = stock
Center B = full half round dished piston
Bottom C = closer to my proposed mod
The arrows on the left point to the surface top of piston...B has Double the surface area of stock but is not practical as the Volume of material removed would equal shaving .288 off the top of piston if it where kept flat....that is approx double the total amount Gil removed....also the edges of top the piston would be sharp and not ideal or recommended.
My point of all this is: If I just dish the piston rather than follow Gil's recipe...but try to match or get close to his total material removed...Am I correct in thinking if you increase the total surface area of the top of the piston...does it not push harder using same amount of theoretical combustion pressure of a flat top piston?....now I am kind of splitting hairs because Gil also dished his piston a small amount but if piston B could be made to match Piston A's compression reduction....would piston B push a bit harder on the crank?... Not sure if I have explained it well or if my crude sketch helps...But would love to hear what the experts have to say!
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My thinking is that within a closed cylinder with immovable walls and top, the expanding gasses will move the piston to lower the pressure without significant regard to crown shape. Old time flatheads often had pistons with a cylindrical bowl cut out of the middle, (think ash tray) but they were flat heads with side valves. At low compression, as was used in pre-ethyl lead gas engines, such as the model T at 4:1, heavy flywheels meant they could idle very slowly.
I think the larger hit and miss engines benefit from scale, and from well oiled plain bearings. Ball or roller bearings are more durable, but not as silky smooth running as a bronze or even Babbitt bearing that is well worn in on a smooth shaft.
A big honking iron flywheel thumped vigorously by a large volume, low velocity push from a mildly compressed charge is like pushing a bus with a car. Our little engines with comparatively low mass, relatively rattly bearings, larger tolerances per unit of size (.005” isn’t a lot on six inches. It’s a ton on six mm), and relatively small, perhaps more energetic firing as occurs when the resistance to initial movement is less in the small chamber of a light engine, ends up more like hitting the car with a sledgehammer than pushing it. The initial movement of the piston and rod and crank journal doesn’t translate into the same kind of low velocity, high momentum push as the big motor gives its flywheel.
And once in motion, our parasitic drag is probably higher per unit of momentum, which degrades coasting more quickly.
I understand what you’re trying to do. I think it won’t amount to much if any repeatable improvement. I like Gil’s method. Frankly I think the slight dishing and significant shortening is a fair compromise in not reducing wall length or material thickness too much. I don’t think the gasses pushing against it care, so long as it’s not a needle, shoving the gasses past the rings instead of moving the piston.
I say this as someone with decades of experimentation with expanding gasses within relatively cylindrical chambers with varying piston shapes at varying rates. It’s accumulated knowledge from designing bullets for handguns with varying burn rates from lightning fast to black powder slow.
I think another area to explore might be the Governor components. Slick and repeatable, rather than rough and clattering mechanisms eat far less.
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I do plan to polish the governor mechanism and brass gears as well as change out the bearings for bronze...as well as add functional oilers, but I kind of like to make one mod at a time to try to document what changes are the " biggest bang for buck" or effort...I now have 3 engines to tune...the M92 sure is pretty!
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One change at a time, that’s the only way to “science” it. Have fun.
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Well...I got to mod my Piston today!...first pic is fresh off the lathe... second pic I have polished the dish and top half to about 1000 grit...there was a distinctive zipper or record player groove texture to the piston...for now I have left some of the texture on the bottom half for oil to get into...but may fully polish later.
She runs slower!...I need a tach now to tell how much, but need to work on the Governer springs first to get more fine control.
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Only one pic attached...this is the piston dished .225 deep with a .875 ball nose cutter....but before polish.
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That is some nice work. I don’t know that polishing was necessary, but it’s sure pretty. I like the dome dish idea. It preserved full piston length, which I think makes them run smoother, while reducing compression.
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Now that's a "Lathmaster" & a well up-graded piston !! You've probably reduced the factory's hi C/R down to 'bout 5:1 & combined with those friction reducing, oil retention "record player" skirt grooves should really help getting the revs down to a more realistic full scale "hit"/coast speed....Fantastic machining job... Can't wait for you to post some video action ! ........tom.
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Really need to see a vid of this engine running now.
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My engine came with (2) o-rings on the piston....it seems the general consensus is one is better...I removed the one farther from the spark plug.
It still has Plenty of compression even with the deeply dished piston....I did not really want to remove more from the piston, so I dished the Head!...I noticed the intake and exhaust are deep in the spark plug hole....I dished the head to lower the compression a bit more And try to get the path for the intake mix a bit shorter....to my surprise the dished head seems to have made Very little difference in performance!
The photo is the head machined...with the grind on my ball nose cutter combined with the spark plug pilot hole caused quite a bit of tooling chatter. I can polish it out but it was kinda cool looking so I left it for now...lol
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Really need to see a vid of this engine running now.
Agreed ;)
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I did try to post a video straight from my phone...its about 130MB and am sure that is over the limit for file size, I just got the little spinny "Working" circle....of death, and it never loaded....sorry Guys!
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I did try to post a video straight from my phone...its about 130MB and am sure that is over the limit for file size, I just got the little spinny "Working" circle....of death, and it never loaded....sorry Guys!
G'day Ben, its pretty easy to set up a Youtube account to upload a vid to.
Vimeo is super easy also -
https://vimeo.com/
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Thought about this thread when I was watching a documentary about engines. They mentioned hemispherical combustion chambers as being important in the development of high revving, high output multi cylinder engines, in order to get more efficient air/fuel mixing by swirling the charge gasses and reducing the average distance between the gas particles and the ignition spark, and insuring a more even mix, so the charge wasn't stratified, with some leftover exhaust gasses layered like a cake with the incoming charge.
The upshot I get from that is if anything, the hemi head will be more efficient at higher revs, where scavenging and getting a full charge into the chamber is important, but at lower revs in an engine with a deeply buried spark plug won’t likely make a helluva lot of difference, and if anything, improved mixing will somewhat offset the reduced compression. That would be under continuous firing. In a hit and miss at idle, it gets ample swishing around of air post ignition before the next hit, as even with only the exhaust valve open, it’s pumping and tending to clear exhaust gasses even before sucking the next charge.
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Hi Ben,....How much engine Rpm or "hits" per min difference from your compression ratio mod have you noticed ? .....I've been considering dropping my M90's 9.5 C/R down to 4.5 or so & was wondering if the resulting lowered engine speed & reduced hits per min is noticeably worth the effort ?? .... thx, ....tom.
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Hello Tom, I have been all over the map and tryed several things...the 27 oz I temporarily added to the flywheels really slowed the engine down and I kinda liked how it ran....but I felt I was putting quite a bit of load on the engine at that weight....I am a bit concerned as there is only tiny set screws holding the flywheels on and over time they can work loose. I took All the weight off for now and am playing with compression reduction...Something to keep in mind is when I had lots of weight the engine Did run Much more slowly...But it still has to Hit quite often to keep the flywheels moving! I have had hits per min of over 55 and my current lowest count was 33 hits per min on a well lubed and warm engine..others have achieved much better than me....But I have Many more things to try!....So far the 2 single best improvements I have made are:
Remove One of the piston o-rings!...this seems to improve oiling as the space between 2 o-rings is wiped dry...this worked so well I am going to try removing BOTH o-rings!...my flame licker runs without Any rings!...this engine just seems to Want to run!....I feel 90% sure it will run without any o-rings but will report my findings yet this weekend!
The the other biggest game changer:
Find any and all parasitic drag!!!....I discovered this quite by accident...having had my Piston in and out of the engine 3-4 times...after one re assembly...I noticed my engine running MUCH more poorly...I have learned to hold the exhaust valve open and give engine a couple hand spins If I think I have flooded it...if with a stout hand flick your engine will not coast MORE than 12 revolutions...you have drag issues!! I took my Piston and rod assembly back out and the flywheels would coast more than 30 revolutions...so well in fact I could not count that fast!...so I checked piston fit to bore...smooth as butter...next checked the rod main bearing and it felt gritty!...the rod main bearing is plain yellow brass...and it is made into two "C" shape half rounds to enable assembly to the crank....I believe When made, the bearing is assembled into the rod and reamed...it is possible (and likely) to reassemble and flip one of the "C" halfs end for end....if the machine work was absolutely Perfect...this would not be an issue...but the world is not perfect and flipping half a bearing caused my rod bearing bore to not be round...I found a metric dowel pin the same size as the crankshaft journal and proceeded to try all the different combos of assembly...both with the rod end cap and the bearing C halfs, there where combos that worked better than others...but All combos produced some drag.
The next step I took was I assembled my best combo and marked a tiny center punch mark on the rod cap to rod...I put the dots up so I can see them...next I locktite each bearing half in place...one in rod...one in rod cap, carefully centering them..when cured I reamed them .001 over with a slip fit meteric reamer...carefully re assembled everything in its proper place and my engine now coasts 20 revolutions with one stout hand flick and exhaust valve held open!!
So far this repair is the single best thing I have done to improve my engine to date!....I don't really care for the plain yellow brass rod bearings and have big plans to change out all bearings to oil-lite bronze...when I do that I would like to pin the bearing "C" halfs in place as I don't really trust the loc-tite on a half bearing long term.
Bottom line ....I don't know if I should post all my little experiments on the forum as many of them are not helpful...or...with my assembly being wrong the first time I could not even notice a difference in the way the engine ran with lower compression!...with my bearing temp fix it does run slower than before...and....It IS super fun to share my adventures....I suppose folks can learn just as much from my mistakes as my wins...but ANY form of drag is NOT your friend when trying to get such a tiny hit and miss engine to run slow....
I guess I really did not answer your question, but something to keep in mind is: If you know you want to put your little M90 to work and power something, you may not want to lower your compression...or lower it only a little...I own 3 of these little engines and I want at least One of them to run super slow!...So I am willing to do most anything to achieve my goal and I don't care if I can stop it with my little finger...as long as it runs well and runs slow!...I will build one of my other engines for powering implements...your mileage may vary!
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Hello again Ben,....Running my stock, factory M90...haven't removed cyl head or piston, so I guess I have 2 Rubber O-rings worth of Friction....Compared to your Flame Lickers Compression-less event, our M90 based engines have basic design free turning High Friction issues....Plz continue your Helpful M94 mods update postings !! ..I'm interested in your "Ringl-less" piston idea & other parasitic frictional losses reduction thoughts...Even if 1 P-ring is required, an improved version would help "coasting" time & the stock engine has many other "improvable" frictional issues without getting away from the Classic antique H&M simplistic cast iron charm.....tom.
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Hello Tom!....I am glad a few folks are following along in my adventures with tiny hit and miss engines,....I Will continue to post both my Wins AND my losses....as we Can learn from when things get Worse as well.
I have not removed the second o-ring yet as I have obligations today....but got the idea from a forum post of the full size engines...they where talking about slowing down full size hit and miss engines and retarding the timing often came up...but one guy said he likes to find an engine that is " loose" with well worn piston and bore and noticeable blow by of the rings...this is a lower drag engine and the ring blow by also has the effect of lowering the compression ratio of an all ready low C/R engine!.... The piston to bore fit (on china engine) is fairly close in my opinion for a hit and miss engine....I am hopeful that Just the film strength of MM oil will give me enough seal to run.....I suppose a thicker oil would greatly improve seal but that would not help with the low drag...I don't know if there is anything out there as thin as MM oil that has lower drag characteristic.
Speaking of oils...I did read your thread about getting down to 38 hits with just fancy oils and a softer Gov. Spring....Gil posted an answer to one of my questions about which model china engine might be the best to tune for slow run and he suggested the M90 model you have....I have one new in box and think I want to get it out!....I saw a video last night of an M90 with head removed and it "looks" like the M90 has larger valves than my M94....I want to measure to confirm this but the combo of larger valves PLUS M90 has valves in the head so the air pathway is the best and shortest pathway possible.
My M94 the exhaust has to travel up the threaded spark plug hole...then turn 90 deg and go down a long smaller hole then thru a brass side exhaust feature....I think the M90 is a better place to start for performance tuning....glad I just happen to have one in stock....lol
As long as we are brain storming....I had another idea....one of the best upgrades you can do to a flame licker engine is make the entire ring-less piston from graphite!... This allows you a self lubricating Very low drag piston....key to making this work is a very well polished bore as the tipical cross hatch scratch pattern will slowly wear away the piston....if the bore Starts out polished and the piston starts out close fit...it Can be a long life piston......I do not know if Solid graphite piston has ever been tried on an internal combustion engine as the forces applyed to piston are much greater than any vacuum engine....however....our little hit and miss run so cool that o- rings live for a long time...and if we lower compression significantly...they run cooler still and with less strain.....I have access to some super hard premium graphite used in the tool and die trade....I want to Try it some day in the future......specialy IF my engine runs well with zero o-rings....thats all I have for now...got to go fill obligations.....
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Hey Ben,..agree con rod Needs a much lower friction bearing arrangement with traditional oiling Hole thru con rod...Also, I presume the shallow piston ring groove Pre-Loads the Rubber O-ring Tension against cyl wall for full time sealing ? ...Even when Not needed for Coasting ! Try something like a Deeper Groove for Less O-ring tension pre-load (and Less friction) & drill piston Top Gas Port Holes thru to Behind O-ring protective thin steel Backer Strip ! You now Only have O-ring Drag during Active Compression stroke or firing "hit" stroke With far Less Coasting O-ring Drag ! ...The Gas Ports would also let your previous suggested, Lower friction, Rigid Teflon piston Ring idea work better ... Piston Gas Ports, ...it's the current hi perf lo drag power plan !!! .....Keep trying ideas,.....tom.
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Hey !!! Just got 33 Hit a min ......Gettin there !!
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Hi Ben,...a steel backed, Ptfe lined bearing 1/2 Shell with Upper shell unit having a 1mm diam or so "oiling" hole lined up with Matching Oil hole thru Con rod would make a good, substantial con rod big end Low drag unit...Something on style of full size auto con rod bearing mating 1/2's....Bore out Con rod so the assembled Cap slightly "pinches" 1/2 shell's "Locked" in oil hole lined up position Without any glue adhesive needed..Ptfe + Lite oil lube makes awesome Low drag bearing ! ...Similar 1 piece Steel backed Teflon (Ptfe) Lined bushing with upper oil hole stabilizes the crank "play" & makes each running session lube a breeze...Fully caged Needle Bearing with Inner Race & upper oil hole (lined up with matching oil hole in Main caps) would also make a good low drag lubricatable Crankshaft Bearing.....tom
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Sadly my employer has kept me so busy there has not been time or energy to work on my little engine...hopeful that will correct soon!.... I did stumble on a modified M90 video and thought I would share with those interested in tuning these sweet little engines...I think he made a different power supply...BUT... Also installed a drip oiler..he bent up some copper tube to run external to lube the piston skirt....I would have drilled the top hole thru bore liner and dripped the skirt from top only...but it is fun to see how other Craftsmen solve lubrication....he has it slowed down pretty good, though not to Gil's engine level of tune I think...thought I would share link.
https://youtu.be/Kc7NJQ0ozTA
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Hi Ben,...That's a creative & safe piston drip oiler design that gets the job done ! ...That factory stainless Steel cylinder Liner is dang near Undrillium Hard requiring considerable PIA effort & plenty of carbide drill bits to "simply" extend the factory provided thread tapped "dummy" bolt hole to implement a Functional drip oiler...Seems the factory could've at Least punched a hole there for owners to add an oiler ! For the average equipped owner, That's an unexpectedly high Risky operation to easily go wrong many ways to Wreck an expen$ive new engine..I would haveta knock down a couple "sixers" to "steady up" my grip !......Cheers,...tom.
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Great work guys....!!!
Please continue to update us.
Gil
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Hello Tom...I do not currently have a tachometer...I would like to get one. Gil is currently getting excellent results with little more than bore polish and shave top of piston...its a very easy task IF you have access to a lathe? They sound SO much closer to the full size size versions I feel it's worth the low effort. However...if you can not easily gain access to a lathe, I Would wait till U can...your mileage may vary.
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Hi Ben,...a Tach IS nice but Not required for our H & M Common interest Improvement measured progress purposes...Your Existing cell phone device using Digital Stopwatch mode & Counting Hits per min + Video Camera Record mode at 1/4 or 1/8 Slo-mo speed & counting Audible "Huffs"(coasting strokes) between Hits does same + let's you Observe subtle Dynamic nuances (intake valve action, governor weights & Latch-out action, etc actions...The instruction sheet specing 95# (octane) fuel is a clue of near 10:1 CR way too high for realistic H & M Rpm as I don't plan to drive any accessories...4.5 CR is probably near optimum power hit to Flywheel inertia pump over & that's gonna require a good size piston top cut...I plan to fit the piston skirt with some small Teflon clearance "Buttons" which combined with the upper O-ring, should Totally remove all piston to liner Metallic physical contact friction & sliding scuffing.....tom
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I Did see Gil's slow-motion video and I think that is a Great test to actually count revolutions between hits...this is probably the truest test of friction reduction and also how your other settings are working, for example: timing settings.
I have not took the time to try to calculate the compression ratio by actually measuring the swept Volume or the area that's left when piston is at top dead center....I do not place a lot of trust in the China instructions asking for high octane fuel as a gauge for compression ratio...many have run there engines on 87 but the engine does have what I would describe as snappy compression stock...I have reduced my compression perhaps 70% of max can be achieved by milling piston top and also the material I removed from the head. If I took the dish out of piston and made it flat with perhaps only .050 thickness left on piston top I would be at max reduction from shaving piston....I wish they offered spare pistons as purchase parts so I could get a little Wild with my experimental tests...another thought would be to leave the piston top stock and ream a new wrist pin hole closer to the top which would have the effect of moving the piston further back in the bore by quite a bit....next time I have my piston out I will take some measurements to see if this is possible...likely have to trim a bit from the skirt as the piston may travel to far rearward. I have the ability to make a new piston from scratch of my own design, however finding time is another matter.
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Note on octane: several years ago I was moderator of a group of scooter enthusiasts. The engines our scooters employed were ordinary OHC four stroke made in Taiwan. The manufacturer spec’d them for 95 octane. I did some research and reached out to the representatives in Taiwan about this, as many of our US owners were concerned.
Turns out, there are more than one way octane is measured. Bog standard 87-89 octane US gasoline is exactly the same as the 95 octane Asian measurement. It’s just measured using a different scheme.
Ordinary pump gas is all you need, and it’s likely the nominal compression of our little engines is more along the lines of 8 or 8.5 to 1. That would be assuming a running engine with ideal dynamic ring sealing. Hand cranked testing would likely be lower.
Note on tachometers. The inexpensive laser optical handheld tachs available for twenty bucks or less work great.
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Yep, I forgot Asia uses the numerically higher RON octane standard compared to North American AKI (Averaged Knock Index) octane rating
standaed. ...Not familiar with "Bog" standard US gasoline ?...maybe you can help me out on that rating standard...tom
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Bog standard, a slang I picked up from my English motorcycling friends, refers simply to ordinary pump gas, or may refer to the very common, dirt cheap 15W40 motor oil often sold in bulk for Diesel engines. The Brits I knew were all about saving a penny when they could. If I had to guess, the reference might be from back in the day, when cheap coal, only a step up from peat might have been so named. It would be synonymous with “garden variety”.
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I fired up my brand new M90 this weekend. It is a MUCH stronger running engine than my M94...I believe the overhead valves Right in the head are what makes it a Stronger running engine and likely THE best choice for engine tuners!...I am not ready to sell my side valve engines...but thought I would post for those considering purchase for tuning.