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HomeMotorcycleUnfamiliar Concepts in Piston Ring Design

Unfamiliar Concepts in Piston Ring Design



Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>.

Kevin Cameron has been writing about bikes for almost 50 years, first for <em>Cycle journal</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

My first impulse in scripting this piece was to say “new concepts” in piston ring design, however the concepts right here will not be new. They date again to Honda’s third period in F1, which led to 2008, which is now 13 years in the past. Honda engineers confronted the problem of sealing pistons at engine speeds close to 20,000 rpm, and so they discovered present piston ring ideas insufficient.

Associated: Scorching Air, Chilly Air, Delta-T

Since English inventor John Ramsbottom first conceived of them 169 years in the past to be used on railroad steam engines, typical piston rings have developed considerably. As a result of it isn’t possible to suit pistons into cylinders so tightly that they obtain a dependable seal (aside from sure tiny mannequin engines), designers match a springy round metallic cut up ring of roughly rectangular part right into a groove minimize close to the piston’s head. Since they’re usually bigger than the cylinder’s bore, the cut up ring is compressed to suit into it after which expands to seal towards it; the ring’s flexibility is ready to accommodate the cylinder’s growth because it heats to working temperature.

Thoughts the Hole

It appears intuitively right to worry over potential leakage by the ring’s hole on the cut up. Many and varied are the innovations proposed to remove this hole, corresponding to stacking two vertically skinny cut up rings in a single groove, every with its finish hole pegged to maintain the 2 at 180 levels to one another.

It seems that ring-end-gap leakage is kind of small, offered that the hole itself measures near the same old rule of thumb: about 0.004 inch for each inch of piston diameter. In consequence, stacked rings and different strategies of stopping ring-gap leakage have handed out of use.

One early utility through which the Ramsbottom ring emphatically didn’t work was the rotary-radial piston engines utilized in massive numbers of World Struggle I plane. As a result of early plane flew too slowly to push enough cooling air by their engines’ fins, cooling was improved by bolting the crankshaft to the plane’s firewall and having the crankcase and cylinders rotate round it (therefore “rotary”). A typical rotary engine was about 37 inches in diameter, so when it operated at 1,200 rpm, the heads whirled by the air at 165 toes per second (112 mph)—even with the airplane sitting nonetheless.

The large issues have been 1) the cylinders’ main faces have been properly cooled, however their trailing sides remained extremely popular, and a couple of) the cylinders have been comprised of metal, which conducts warmth poorly. On account of this nonuniform cooling, the trailing facet of every cylinder bulged from warmth growth whereas the cooler main facet remained smaller. No easy piston ring might conform to such a heat-distorted cylinder. One of the best answer at the moment was the “obturator,” a brass cut up ring with an L-shaped cross-section. The bottom of the L was fairly slender whereas its vertical half was comparatively tall. A groove machined into the piston retained the ring’s base within the conventional style, however the resemblance ended there. It was the bending flexibility of the L’s vertical portion which allowed combustion strain to simply push it into good contact with the cylinder wall—slightly just like the leather-based washer in an outdated bicycle pump. Primarily due to brief obturator life, the time-before-overhaul (TBO) of such engines was between 10–25 hours.

We now have all examine how hot-rodders obtain actually spherical cylinders by honing them with thick plates bolted to the cylinder deck beneath customary head-bolt stress. In some circumstances, they even flow into sizzling water by the coolant passages to simulate in-service thermal distortion.

In WWII-era air-cooled radial plane engines (through which it was the crankshaft that rotated, not the crankcase), the cylinders have been “choked,” or tapered to be smaller towards the highest, within the expectation that after warmed up the warmer higher areas would increase sufficient to remove the taper.

Testing Ring Seal

What if all that isn’t sufficient? In 1981 rider Wealthy Schlachter and I by some means got here right into a hoard of piston rings for the Yamaha TZ250H we have been operating. After deburring the ends, I positioned them into a brand new cylinder, one ring at a time, and squared them up with a piston. Then I held the outcome as much as the solar. A few of the rings had no seen hole to the cylinder, however previous others I might see tiny slivers of sunshine.

Associated: Cooler Heads Prevail

Now I rigorously examined the rings that confirmed mild, imagining they could have little manufacturing burrs that have been holding the rings away from the cylinder wall. I fussed and fiddled, however lastly needed to settle for that a few of our rings weren’t going to seal, a minimum of not in that cylinder.

The strain of combustion gasoline, flowing down the highest land and coming into the clearance above the ring (ring axial clearance), can get behind the ring and push it out towards the cylinder wall. That may have made my “dangerous” rings seal up tight, however I had no manner of testing the likelihood. So we ran the “good” rings in our engine and used the others within the conventional manner: as “paddock cash,” the identical manner the incarcerated have all the time used cigarettes.

In contrast to Schlachter and myself, engine growth departments do in truth have technique of testing ring seal in operating engines. They put a flowmeter on the crankcase breather and instantly measure blowby quantity. After that, it’s simply arithmetic to work out how a lot horsepower is being misplaced to leakage. When the leakage is critical, it’s important to do one thing about it. It’s not solely a matter of shedding energy; sizzling combustion gasoline leaking previous the ring and flowing between piston and cylinder goes to have disagreeable penalties.

Honda’s Novel Ring-Seal Answer

In growing their V-10 F1 engines to peak at shut to twenty,000 rpm, Honda engineers determined that typical rings is perhaps too stiff to evolve properly to cylinder-wall profiles, in order that they diminished ring stiffness by slicing radial width. To additional enhance ring native flexibility, they milled a variety of shallow round-bottomed vertical grooves within the internal vertical face of every ring. These modifications diminished the ring’s potential to press itself towards the cylinder wall, so a compact wave-spring expander, match behind the ring, supplemented the ring’s preload. Blowby quantity was diminished.

Whereas there will be worth in persevering with to refine present and well-understood ideas, sooner or later new considering is required.

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