Bore And Stroke

BORE AND STROKE

IGNITION

TDC

IT'S ALL ABOUT COMPROMISES

KEVIN CAMERON

Where is the sense in bore and stroke? Recent auto engines, Harley-Davidsons and Honda's NC700X all have bores slightly smaller than their strokes. Yet the bores of Formula 1 engines may be outrageously bigger than their strokes—as much as 2.5 times bigger! Step back to recent sportbike engines and you find bores near 1.5 times their strokes.

Around 1900, British engineer Frederick Lanchester observed that since the piston was the internal-combustion engine's most highly stressed part, it made best sense to make it small. Back then, pistons were made of cast iron a metal that conducts heat poorly. The larger you made the piston, the farther its hot center was from the cooler cylinder wall and the hotter it ran. So hot, in fact, that iron piston crowns in racing engines ran hot enough to act as ignition sources. They also sagged. Therefore, it was just good sense to make pistons smaller until they could be adequately cooled by contact with the cylinder. Some of those early-days engines had strokes that were three times their bore!

World War I proved the durability of aluminum pistons. Although aluminum has a much lower melting point than iron, it conducts heat so much better that aluminum pistons naturally run cooler than iron. But established ideas die hard. With such small bores, the only way to stuff adequate valve area into heads was to use four or more valves. That's just what Peugeot did with their game-changing 191213 Grand Prix car engine.

Fiat in 1922 went another way, making two large valves fit into small bores by tilting their stems away from each other at 90 or 100 degrees, creating a deep hemispherical combustion chamber. This was sensible at the low 5:1 compression ratio of the time because, with a flat-topped piston, there was plenty of room for the fuel/air mixture to swirl about.

Trouble began when chemists raised fuel octane numbers steeply through the 1930s, allowing makers who raced their bikes in the Isle of Man TT races to boost engine torque by increasing compression. The quick way to do this was to make pistons with ever-taller domes, but the taller the dome, the greater the piston crown area and the more heat that flowed into the piston. This was intensified by the development of airflow science, which, by improving cylinder-filling, increased power and its dark partner, heat flow. Fortunately, the circulation of oil through engines had also increased about three-fold in the 10 years since 1925, so the extra oil that happened to hit the piston had significant cooling effect. Lubrication is not its only job.

Designers fretted about slow combustion, caused by intrusive taller piston domes interfering with flame-speeding turbulence. A subtle movement toward flatter, moreopen combustion chambers resulted, as valve-included angle was reduced from 100 to 90 then to 80 degrees. But these reductions left less room for adequately sized valves, so progressive thinkers also moved slowly toward bigger bores with more room for valves.

Would those bigger pistons live? By the 1960s, this is where design was stuck. Advanced British Twins, like the Norton, dropped valve angle as low as 60 degrees. Honda, needing power to win Grands Prix in the 1960s, stayed at bores only 1.1 times greater than their strokes but reached for higher revs by using larger numbers of smaller cylinders.

Why not just rev up what you have? Piston acceleration is proportional to stroke, so piston cracking and rodand mainbearing overload limit how high a given stroke can be revved. The shorter the stroke, the higher the potential maximum rpm.

Then, in the mid-1960s, English designer Keith Duckworth changed everything at once instead of making cautious, one-at-atime modifications. By switching back from two valves to four, he provided adequate valve area without resorting to the classic large-valve-angle combustion chamber. Four valves also put the sparkplug in the ideal location: at the center of the chamber. With this shallower, flatter head, Duckworth could make the piston crown flat, as well, eliminating the extra piston heating that went with tall domes.

BY THE NUMBERS

3

NUMBER OF 28-CYLINDER PRATT & WHITNEY R-4380 RADIAL ENGINES PLUGGING UP MY SHOP. (THEY NEED A PLACE OF LOVE AND PRESERVATION.)

NOTEBOOI(S IN MY OFFICE. (l(EPT AT BAY BY A PRIMITIVE FILING SYSTEM.)

Relieved of this extra heat, his pistons could survive an increase in bore. His ratio of bore to stroke jumped to i.~yi, increasing valve area (more room in the head) and rpm capability (via the shorter stroke) together. Since the instant success in 1967 of this concept in Duckworth's Cosworth DFV Formula 1 engine, most makers of auto and motorcycle engines in the world have adopted his combination.

Fl went further every year, pursuing higher revs through shorter strokes and bigger bores all the way to 2.46:1. What can keep such large-area pistons from overheating, sagging and failing? Oil jets are aimed up under their domes, carrying away the heat. But for emissions-limited and economy-driven production auto engines, a smaller bore reduces heatloss area and increases fuel efficiency. The smaller bore reduces total piston-ring length, so less unburned mixture is pushed by combustion pressure into ring-crevice volumes during each cycle to come squirting back out a moment later as unburned hydrocarbons. Such engines moved toward a compromise of about a 0.9:1 bore/stroke ratio, which is found in Honda's car-derived NC700X economy Twin.

Between these extremes, sportbike engines moved toward a middle ground around a 1.5:1 bore-stroke ratio. That provides enough valve area to fill cylinders at high but not dreamland revs, without resulting in paper-thin combustion chambers that burn sluggishly. The modern Formula 1 world accepts the slow combustion of such wide, tight chambers as the price of ultra-short strokes that achieve a net power gain from their sheer ability to rev. For production-bike engines, hitting the price point, as well as the need to honor warranty and emissions laws, requires something a bit more realistic.

IN THE 1960s, HONDA REACHED FOR HIGHER REVS BY USING LARGER NUMBERS OF SMALLER CYLINDERS.

Don't long strokes and small bores boost torque? As you make the stroke longer, leverage does increase, but at the same time, piston area decreases in strict proportion, so torque doesn't change. What small bore/long stroke actually does is force the use of smaller valves. Because small valves deliver their best flow at lower revs, long-stroke engines feel wonderfully torquey because they pull when sportbike engines would be chain-snatching and coughing.

Pick your compromise.