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Why Some Engines Smoke and Some Don’t, Top Dead Center

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One of my favourite issues to do is roll alongside in my little automotive, sipping espresso. Per week in the past I did simply that to reach at Thompson Speedway in northeast Connecticut, the place I may see a superb assortment of 350cc racebikes run. One of them, an MV Triple, was a former mount of Giacomo Agostini. The 350 class hasn’t existed since 1984, when GP racing supposedly “slimmed down for TV,” and also you’ll discover no point out of 350 in Werner Haefliger’s annual little pink guide FIM MotoGP Results.

Never thoughts. An attention-grabbing factor is that older four-stroke racebikes smoke. Ago’s 350 MV smoked on start-up, on overrun, and on gear modifications. For years, Ferraris smoked too—racing or road. And, should you watch previous films, you’ll see that prewar cars smoked. True connoisseurs of smoke know that the nice plane radial piston engines smoked worst of all. They smoked billows on start-up, all via warm-up, after which they left smoky trails behind on take-off.

In the current second, smoke from an engine is an indication of bother—most definitely cracked valve stem seals or, dearer to repair, timed-out oil scraper rings. Since the 1950s, no correct manufacturing automotive or bike engine smokes. It’s simply not completed.

The earliest oil scraper rings have been easy taper-faced rings with the taper oriented so the ring scraped downward and rode up on cylinder wall oil on the upstroke. It’s straightforward to see that though preliminary contact of such a hoop on the cylinder wall was a slim edge, regular put on rapidly widened that space of contact. That diminished the particular cylinder wall strain of the scraper, step by step degrading its scraping capability. Prewar vehicles usually had easy oil scrapers, and enormous plane engines continued with them to the tip of their manufacturing within the late 1950s.


Why not undertake automotive-type perforated two-rail scraper rings, which definitely existed from the 1930s? Aircraft engines spent their lives at between 50 and 100 p.c energy, so that they wanted sufficient oil on cylinder partitions to help heavy load. Wright Aeronautical Co. even patented what they referred to as “Uniflow” oil rings, which tended to build up oil on the piston between oppositely oriented higher and decrease oil scrapers. The similar reasoning explains why Ferrari didn’t dedicate Detroit-like effort to smokeless exhaust.

As humorist Peter Ustinov has his fictional Commendatore Fanfani say on his recording The Grand Prix of Gibraltar, “Any fool can make a car go slow, but it takes a genius to make it go fast!”

This additionally explains smoking by Ago’s MV and others from the 1950s and ’60s.

As early one-piece perforated two-rail oil scrapers wore, the width of the scraping edges bearing on the cylinder wall step by step elevated—and so did the thickness of the movie of oil they left behind on the down-stroke. John Wittner, who labored in engine growth at Moto Guzzi, famous that in extended engine testing it was all the time the oil scraper ring that misplaced its operate first. Because compression rings took hours to seat correctly, to save lots of time solely the oil scrapers could be modified, after which testing may resume rapidly.

More efficient and longer-lasting are three-piece scrapers consisting of two rails and a perforated spacer-expander. The rails, being fairly skinny, exert excessive and lasting scraping strain on cylinder partitions. Separate building permits such scrapers to higher conform to cylinder partitions the place distortion is current (as in air-cooled engines).

The large clouds of smoke which are coughed out when a giant plane radial is began outcome from the truth that such engines have a number of cylinders that time downward. On shut-down, oil drains from above and, regardless of every cylinder having an inward-projecting “spigot,” a few of this oil goes into the decrease cylinders. On a nine-cylinder radial reminiscent of powered the B-17 bomber, these are cylinders 4, 5, and 6. Because the bore of these cylinders was 6-1/eight inches they caught loads of “oil rain” at shut-down. On start-up, a few of that oil turned smoke. Owners of BMW’s “sideways” triples and fours of the 1980s see start-up smoke that originates in the identical manner—it’s oil that runs into cylinders angled downward because the bike sits on its sidestand. Some of that oil makes its well beyond the rings to grow to be start-up smoke.

Big radials smoke on take-off for 2 causes. One is similar as within the case of Ago’s 350—engines that run more often than not at excessive energy want oily cylinder partitions, particularly if they’re air-cooled (early in radial growth, cylinder wall and piston ring scoring have been frequent). The different supply of take-off smoke is rich-mixture operation. In such supercharged engines, what restricted the increase that might be used was detonation. But if combustion was cooled a bit by enriching the combination 20 p.c, extra increase may safely be used, boosting energy. That wealthy combination produced free carbon within the exhaust throughout take-off. Once take-off and climb have been safely completed, combination might be leaned for cruise and the engines now not smoked visibly.

Modern auto and motorbike engines are liquid-cooled, saved at fixed temperature by thermostats. This, with correct engineering, permits cylinders to remain rounder and straighter, creating circumstances wherein sufficient piston and ring lubrication could be achieved by thinner oil movies on cylinder partitions. Combine that with efficient anti-wear additive in oil and you can also make engines that use no oil between oil modifications and by no means smoke.

There have been a number of exceptions. In the 1980s, as extra anti-wear oil additive got here into use, the effectiveness of that additive prevented some engines from breaking in correctly, leading to persistent smoking and oil consumption. Improved cylinder wall preparation (Nikasil) and pre-lapped rings have overcome this.

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