Re: Potential Lawn-Boy tweaks - hand-lap reed, double crank seals, crankcase stuffing
TheDodgeGuy wrote: "You could also do some work to the transfer ports, perhaps grinding a little "booster" port on the top edge so the piston would uncover this part first, and get a high velocity stream going before the rest of the port was uncovered. Model airplane engines have done this for decades with considerable increases in top end power."
The top ring on F-Series pistons is so close to the piston's top surface that I don't think we can prudently remove enough metal to make a noticeable difference. These top rings are exceptionally close to their piston top. I'm also reluctant to alter the factory's port timing because port timing so strongly influences the torque curve. The periods when I most want more power from this mower is when I'm trying to drive the blade through too much load, causing an overload which lowers rpm. When that happens, I want more medium-rpm and low-rpm torque. I don't want to rebalance the factory torque curve. I'm sure the factory engineers spent a lot of time trying to artfully balance that trade-off.
But I'd love to improve transfer port flow characteristics which would help at all rpms. Again, I didn't mention this earlier because I didn't want to cause information overload with one long post. We have an easy opportunity to reshape those ports with low risk and high probability of improvement. Go-cart racers running Lawn-Boy engines would be well advised to consider the following strategy. The factory ports were formed by two intersecting machining operations. Their side machining cut passed through the cylinder side, through the vertical port, then through the steel cylinder liner. If these were racing engines, that "Y-shaped"-port intersection would be filled with something like JB Weld, then smoothed to form a nice smooth curved port shape. Instead, Lawn-Boy just capped off that side hole into the cylinder side with a metal plug. The intersection is rough, crude and probably creates turbulent swirls like a toilet bowl. I'd be willing to bet that filling that capped side hole to form a smooth tapered port would fatten the entire torque curve. So instead of REMOVING more port material, the first thing I'd do is ADD port wall material to fill that side breach which must cause lots of turbulence that reduces those charging pulse flows which are less than 0.01 second long. We want a nice smooth shot, not a toilet-like swirl just before the port dumps into the cylinder.
"I still don't know about the double seal idea. If it fits, try it. It certainly can't hurt much, but remember the secondary seal isn't going to get much, if any lubrication if the primary seal is functioning correctly. You'll end up just burning it up and still having just one seal functioning after a while."
Your point is well taken. Perhaps we can pack some wheel-bearing grease between them. Eventually that grease would leak out, but
if that's occurring, I'd think the inner seal would be seeping enough lube to keep the outer seal lubed enough to prevent burning.
This is pretty speculative since we haven't tried it yet. Maybe a year from now we will have field experience feedback. Thanks for pointing out that lubrication issue.
According to discussion group posts, many Lawn-Boy mowers have suffered from crankcase sealing pressure losses caused by crankshaft seals migrating outward along the crankshaft until they are no longer captured within the outer case bore hole. When that happens, the seal leaks at its outside rather than at its inside. I know that I'm new here, but I have a solution for that, and my solution would also hold double seals in their bore holes as well as stock single seals.
Observe that both upper and lower case sections have two support "webs" formed to provide side support for their extended side sections which contain the side seals. If we drill a tiny hole through the inside corner of each of those web castings on each side (total of 8 tiny holes), we can run aircraft safety wire through the top web base, out to the end of the seal carrier, then diagonally over the outer crankshaft seal, then down where it would be wired to the bottom web base hole. So we would run two safety wires over the crankshaft seal from top to bottom on each end of the crankshaft. You can probably cut the holes and safety wire them in place in less time than it took me to write this. There's no way in hell those crankshaft seals can migrate out of their case bores if they are safety-wired in place this way. This would NOT significantly weaken the support webs and would eliminate a too-often cited seal failure mode. Many Lawn-Boys have been set out with trash at curbs because one of their crankshaft seals migrated out of its bore. Besides, this way you can definitely run double seals at each crankshaft end without concern that either can migrate out of its case bore. How do you guys like this proposal? Ten minutes of extra work and you've just eliminated one of these engine's most commonly cited failure modes.
"Regular old brake fluid works well to soften and rejuvenate old oil seals. Basically, these ATF "conditioners" are not much more than that."
Interesting idea. Perhaps we can look for discussion by people who many have tried adding brake fluid to automatic transmissions to see
if seal performance has been extended. I know that brake fluid can soften seals. I'm just not confident that it would leave these seals in as durable a condition as ATF seal leak products would. I hope you're right.
"While your heat transfer ideas and cylinder coating are great for high-load high-RPM engines, on an engine such as this, I see it being kind of a moot point. Torque is more important than gross horsepower, and that's the one area a typical 2-cycle engine lacks."
If we DON'T cool combustion gases as much while they can push against the piston top, they will push harder because that higher gas temperature within an equal volume also causes higher gas pressure against the piston top.
Going back to the ideal gas laws:
PV = nRT
P = absolute pressure
V = volume
n = number of moles
R = universal gas constant
T = absolute temperature
Thermally insulating the piston top and cylinder head bottom will increase combustion gas temperature, increase gas pressure, increase piston pushing force and increase crankshaft power output at ALL rpms, not just at high rpms. Granted, increased combustion temperatures will slightly increase nitrous oxide formation. The torque increase will occur across ALL rpms, not just at high rpms.
These tweaks will give slightly more crankshaft work output for slightly less fuel input. I doubt that we can tweak an F-series engine to become as powerful as a DuraForce which has a larger air cleaner, an extra transfer port and more displacement. But I'm confident that we can narrow their performance difference. In a way, I feel that the F-series engine is more elegant. How can you improve on casting the head and cylinder together as a single piece? That gives as nearly a perfect thermal lock between those sections as is possible without liquid coolant. And with no head-to-cylinder parting line, the F-series engines can never develop head gasket leaks.
LoveMySan wrote, "I wonder if powder coating the surface would work."
I think that powder coating would have a lousy cost/benefit ratio. Here's my reasoning. To "reject heat" (as thermal engineering classes describe it), we want the surface to have as much hot surface area as possible, and to be an ideal emitter, which is flat black, NOT SHINY black. Power coatings melt as they are baked on and become shiny. That reflective quality is only possible because it smooth. Flat blacks are rougher at a microscopic level so they present more exterior radiating surface area per square inch of fin material. Some "wrinkle finish" coatings can have about 2 square surface inches for every square inch they cover. We want lots of radiation not smooth low-radiation-rate surfaces. Also, keep the covering thin rather than thick so the coating does not become a significant insulating barrier. Flat-black high temperature barbecue spray paint works well and can be purchased cheaply.
Just some of my opinions.
Last edited by LoveLearn; 06-08-2009 at 10:20 PM.