Tdc

Why overhead valves?

TDC

Kevin Cameron

IN THE EARLY DAYS OF THE INTERNALcombustion engine, fuel economy was a selling point, so everyone measured his competitors’ fuel consumption. From such measurements it became cleareven in 1900—that good fuel economy depended on three large issues.

First was compression ratio. The higher the ratio, the more energy the engine extracted from fuel. Then, as now, compression ratio was limited by detonation. A hundred years ago, with fuel octane fluttering in the 55 range, a 3:1 compression ratio was the limit. Today, with octane numbers in the mid-to-high 80s, the maximum for small-bore cylinders is more like 12:1.

Second concern was combustion chamber surface area. The larger the combustion chamber, the more surface it had from which to lose heat, and therefore the less energy actually reached the rear wheel. It was known in 1900 that a sidevalve, or Flathead engine, with its two valves in a single pocket side-byside next to the cylinder, gave better fuel economy than a T-head engine, which had two such pockets-one valve on each side of the cylinder.

The third issue was cooling. It was known that a water-cooled engine could make its power with a leaner mixture than could an air-cooler of the same size. Air-cooled engines of that time had little finning and cooled badly, so they ran hot. Hot running leads to detonation, which triggers a vicious circle of overheating. The remedy in that time was to enrich the fuel mixture, thereby reducing the flame temperature and keeping the engine cool enough to stay out of detonation.

It didn’t take much thought to realize that combustion chamber surface area could be reduced a lot by inverting the valves and placing them in the cylinder head, directly above the piston. This would eliminate the extra surface area of side-mounted valve pockets.

It’s risky to attribute an invention to one person in a time when so many were working hard on the problem it solvednew scholarship has a way of changing the names. However, it appears that one of the first to propose putting the valves in the head was Boris Loutzky, a liaison engineer between the Daimler Company in Germany and engine users in Russia.

Much is made of the hemispherical combustion chamber, but its value is easy to understand. A sphere is the shape that has the least surface area in relation to its volume. At low compression ratios, making the combustion chamber in the form of a hemisphere reduced its surface area-and the associated heat loss-to the least possible value.

The J.A. Prestwich company in England, engine suppliers to countless motorcycle firms, built its first ohv powerplant in 1904, with both valve stems vertical. The natural valve arrangement in a hemi chamber, of course, is with the stems inclined to each other at right angles so that each valve head blends into and forms a part of the hemispherical shape. Fiat took a step in this direction with its 1905 GP car engine, a fourcylinder 16-liter monster. Such inclined valves could be operated by a forest of pushrods and rockers from cams down in the crankcase, but who could resist putting the camshaft where the action was, atop the cylinder head? This is what Hans Ledwinka did in 1905-06. His two rows of valve stems were at 90 degrees to each other, their heads in hemi chambers, all operated by a central single overhead cam and roller rockers. The racing Peugeot auto engine of 1912 had one cam for each row of valves-dohc!

These changes were not made for the reasons we embrace today. Today, to make production 750cc Fours rev to 13,000 rpm, all possible weight must be removed from the valvetrain so that springs of practical size can make the valves follow the cam contour without float or bounce. But in 1905, engines turned only one-tenth as fast, so their reasons were different. Valves then were made of simple steels that slowly yielded to exhaust heat. The stronger the valve spring, the faster the hot valve stem stretched or broke, and the more deeply the glowing valve head cupped under its pull. The lighter the spring, the longer the valve lived. In those days, valve springs were so soft that engines often kept running even when an intake pushrod jumped out-intake suction was enough to pull the valves open! Often, both cars and bikes with pushrod ohv used helper springs on the pushrods to relieve the valve spring of the job of moving the pushrod and tappet.

Car engines could afford ohv and ohc sooner than bike engines did because cars cost 20 to 40 times what motor-bicycles did. Sports or racing engines with sidevalves had special valvegear problems because it was hard to get cooling air to valve and spring, where they nestled under the hot exhaust port, close to the cylinder. Springs would lose their strength and sag, so racers placed perforated metal “insulators” between exhaust springs and their seats to save them from the port’s heat. The more power their engines made, the greater the heat-until it looked like ohv was the best way to cool hard-worked valves and springs. Fully exposed on top of the cylinder, cooling air could reach them from any direction.

Designers realized that as compression ratio increased beyond 6:1, a full hemisphere was no longer the combustion chamber of minimum surface area. Flatter, shallower chambers with the two valves set at narrower included angles like 60-degrees took their place, with flatter piston crowns to match, as in Norton Twins or Kawasaki’s classic Z-l. This process continues in our own fourvalve era. To enable engines to breathe at ever-higher revs, they need bigger valves, set into bigger bores. To keep chamber and piston surface area at minimum, both have become progressively flatter. Valve included angles are now in the 25-degree range-barely enough to leave room for access to the central sparkplug. What comes next? □