Being Called Out By Readers

[Hugh Janus]

Sometimes readers call Kevin Cameron out in the comments. Here he responds. (Robert Martin/)

Readers should complain if something I have written on _Cycle World_ seems incorrect, contradictory, or unclear. The recent story “Basics of Power” drew several such complaints.

NoahKatz wants to know why I said in that piece that jetting leaner than a chemically correct mixture reduces combustion temperature. It’s true that anyone with experience with air-cooled two-stroke engines soon learns that going leaner can lead to seizure. But the proper question here is, leaner than what?

RELATED: Basics of Power

Internal combustion engines are driven by heat, and so peak power is achieved at the mixture that gives maximum heat release. As it turns out, that is slightly rich from chemically correct—which means that every hydrogen atom in the hydrocarbon fuel is reacted to water and every carbon atom to carbon dioxide, leaving behind no uncombined hydrogen, carbon, or oxygen. Why slightly rich? Because some extra power is given by the increase in the number of molecules produced (some CO, for example), but this is a limited effect.

The air-cooled two-strokes we worked with back when had quite limited cooling ability, so as we jetted down from quite rich, moving toward a best-power mixture, we often reached a point at which the increasing heat released by combustion became too much for the engine’s cooling ability, and a partial or full seizure resulted. Yet if we’d had equipment to measure our mixture, we’d have seen we were still on the rich side of best-power.

Less than maximum heat is released when we burn mixtures richer or leaner than the best-power mixture because in either case, the heat produced has to be shared with either excess fuel molecules (rich) or excess air (lean). The effect of either is to produce less power.

Air-cooled engines have always been touchy with respect to mixture. Early four-stroke air-cooled radial piston engines (1920s, early ’30s) had not yet achieved best-practice design for cooling, so to make up for it they were intentionally jetted as much as 30-percent rich. This caused their critics (who generally dismissed such engines as “radial rockcrushers”) to call them “fuel-cooled.”

The air-cooled Yamaha TD1 250 twins on which I cut my teeth also had to be fuel-cooled, but to a lesser degree. We found that best-power jetting would give us one or two fast laps, after which the engine became so hot that significant power was lost from the reduction in air density as mixture passed through the hot crankcase and then into the very hot cylinder. We found that by jetting about 10-percent rich (looking like maybe two jet sizes on the spark plugs) we got less power on the first lap or two, but more average power over a 10-lap race.

For many people, the takeaway from this was that going leaner increases heat and makes seizure more likely. But this was not jetting leaner from best-power—it was approaching best-power mixture from the rich side. In that situation, yes, more heat was released with every reduction in jet size.

At the end of the two-stroke era in GP roadracing (1975–2001) the cooling system design of 250 twins making more than 100 hp had been improved to the point that they did not seize if you jetted down to best-power, or even went leaner than that. They just slowed down, because they had been jetted down past the point of maximum heat release, which is also the best-power mixture.

NoahKatz and NotFred point out that aerodynamic drag increases as the square of vehicle speed, so fuel consumption should always be less at lower speeds.

Aero drag does increase just as they point out, but the specific fuel consumption of four-stroke IC engines is not constant, but varies with rpm and load. Brake-specific fuel consumption (BSFC) is the amount of fuel necessary to produce 1 hp for one hour, as measured on a dynamometer, or brake. The BSFC curve of an engine is “bucket-shaped” (over the fairly broad middle the curve is low, but it rises at its extremes) at low load and at higher rpm.

BSFC rises at low load because pumping loss is strongly related to intake vacuum, which rises as the throttle is closed. BSFC rises at high rpm because engine mechanical and windage/oil churning friction loss goes up steeply there—in race engines it can take 25 percent of engine power. This gives us minimum BSFC somewhere in the middle—above the high pumping loss region of low throttle angle, and below the range of steeply rising friction loss at higher revs.

Ten or so years ago, in a conversation with Ducati’s present CEO Claudio Domenicali, he pointed out that a motorcycle engine’s maximum BSFC can easily be _two-and-one-half times_ greater than its minimum BSFC. This means that even though aero drag is very small at 30 mph, it presses noticeably against a rider’s head and chest at freeway speeds, this BSFC effect can actually cause mpg to be higher at lower speeds than at higher.

Reader Ferd takes issue with my statement that production bike BMEP has until recently risen steadily, only now to fade a bit as the effect of Euro 5 emissions limits narrows engine tuning options. (BMEP, or brake mean effective pressure, is that pressure which, if it were to act on the piston through its entire power stroke, would give the same power as the actual constantly changing in-cylinder combustion pressure.)  Ferd quotes an earlier story of mine in which I observed that Norton’s classic Manx single-cylinder racer achieved a 200-psi BMEP a lifetime ago. So what’s this about steadily rising BMEPs?

Yes, it’s true that race engine BMEP reached a practical maximum a very long time ago—possibly as early as 1938! But when I was a tiny boy, production bike BMEPs were around 100 psi, and when the Japanese factories began to crank out large numbers of big four-strokes in the 1970s, BMEP had risen to around 140 psi owing to better cylinder filling and higher safely usable compression ratios. And then when sales competition in the 600 sportbike category got really hot in the 1990s, the use of intake, exhaust, and airbox resonances, plus rigorous programs of friction reduction ran production bike BMEP up to or even slightly above 190 psi.

What I should have said is that while racebike (four-stroke) BMEP neared its limits long ago, that of production bikes has risen steadily.

At the end of the comments section, reader Corny complains that “Basics of Power” lacks precision. Here I am, trying to fix that.

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