Four-Valve Beginnings

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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>. (Robert Martin/)

Today, virtually all motorcycles use engines with overhead valves, yet that wasn’t always the case. In contrast to modern high-performance bikes with their 13:1 compression ratios, the very earliest internal combustion engines had much lower compression, around 3:1, a necessity because of the Knock-O gasoline of that time. The resulting large combustion chambers allowed designers to locate the intake and exhaust valves pretty much anywhere.

Because camshafts and cam followers do best when well lubricated, there was strong motivation to locate an engine’s camshaft(s) no higher than its crankshaft, so the parts could benefit from the oil being thrown about in the crankcase.

First, the Flathead

The valve arrangement that best took advantage of this was the side-valve layout, aka the “flathead,” which placed the valves next to the cylinder, stems down, in one or more valve pockets. In this way they could be directly operated by one or two low-mounted camshafts. Side-valve engines for autos, trucks, and motorcycles continued to be produced into the 1950s.

Another important pressure on early engine design was the rapid destruction of exhaust valves by hot combustion gas. The bigger a valve, the hotter it runs. As a valve grows in diameter, it gains cooling surface in proportion to that diameter (mainly from its contact with the valve seat when closed), but it gains heat-gathering surface area much faster—in proportion to its diameter squared. Because early valve materials resisted heat poorly, big valves failed sooner and more often than smaller ones. The answer was obvious: Give each cylinder more but smaller valves, especially exhaust valves.

Related: How We Got the Motorcycle We Enjoy Today

Today a motorist or motorcyclist never sees a valve—they have become almost completely reliable. But in the early days, when pre-1910 cars were “playthings of the rich,” it was every chauffeur’s duty to reseat the valves every six weeks by grinding them against their seats with a mixture of oil and abrasive powder. This restored good sealing by removing the steady valve and seat erosion caused by exposure to hot combustion gas.

Engine designers tried a wild variety of valve arrangements in this formative period. Early intake valves (before 1904) were opened not by a camshaft but by simple piston “suction” against a light spring. These were called “atmospheric intakes,” and in 1903 Napier gave its 45 hp engine four such intakes per cylinder.

In 1905 Amedee Varlet designed a giant (62 liters!) boat engine for aperitif tycoon Marius Dubonnet with two overhead camshafts operating three vertical intake valves and three vertical exhausts per cylinder.

In 1908 Némorin Causan designed a racing single with four horizontal valves. Today this seems strange, but remember our earlier point about low-compression engines and valve layout. Delage would carry on with horizontal valves for some time, even winning the 1914 Indianapolis 500 with them.

Also in 1908, the Benz auto designed to compete in the Prinz Heinrich trial had four overhead valves per cylinder, operated by pushrods and rockers. The stems of the intakes and exhausts were tilted apart at a 60-degree included angle. Very modern looking, but without overhead camshafts.

The T-Head Appears

In fall of that same year, Swiss builder Charles Picker moved his engineering operation from Geneva to Paris, and with him came his right-hand man, Ernest Henry. When the new organization received an order from Hispano Suiza for racing cars to compete in the 1909 “voiturette” events (minicars), they gave the small four-cylinder engine a T-head.

What is a T-head engine, you ask? One with side valves located on both sides of its cylinder(s). The Picker-designed T-head on Hispano’s voiturette had bulbous side pockets in which its valves were located.

In May of 1909 engineering grad Paolo Zuccarelli raced one of these Hispanos, leading until his engine broke its crank. That summer the famous “Blitzen” Benz had two intake valves and four exhausts per cylinder, and Peugeot raced a six-valve single. Truly, every imaginable valve location was being explored at this time. A fascinating era.

Related: How Are Four-Valve Cylinders Designed?

Gaining experience, engineers and drivers noticed a pattern: Engines with valves located in side pockets sometimes encountered weird “misfiring.” The problem, in the opinion of some, was that hot exhaust gas, persisting from stroke to stroke in such pockets, sometimes ignited fresh charge prematurely.

At the end of 1910, racing drivers Zuccarelli, Georges Boillot, and Jules Goux persuaded Robert Peugeot to hire Zuccarelli and Ernest Henry to design racing engines. Peugeot already had a long history in the sport, and the latter pair knew from experience that the Picker T-head on the Hispano engine misfired above 2,000 revs.

While several other designers built engines with horizontal valves (with their easier-to-lubricate crankshaft-level cams) Peugeot’s newly hired race-engine team decided to tackle the problem of properly driving and lubricating cams located in the cylinder head itself. Such a layout allowed the valves to be placed in the head itself rather than off to the sides in the pockets believed to cause the high-rpm misfire.

The Modern Layout

The result was Henry’s 7.6-liter four-cylinder engine for Peugeot: Four valves per cylinder operated by double overhead cams, the cams driven from the crank by a vertical shaft-and-bevel arrangement. The car incorporating this engine struck the death blow to the “giant racer” era of four-cylinder engines with more than twice the displacement.

Dual overhead camshafts and four valves per cylinder: a dominant design in modern motorcycles. (Suzuki/)

In the 1912 French Auto Club GP, Boillot won with the new four-valve Peugeot, beating a 16-liter (976ci!) Fiat. He won in 1913 as well, with Goux second. Later that year, two cars were sent to Indianapolis, and Goux won the 1913 500-mile race by 13 minutes.

Henry also drew a 3-liter engine, this time with its cams driven not by a torsionally flexible shaft and bevels, but with a train of stiffer and now-familiar spur gears.

Related: Why Switch From Two Valves Per Cylinder To Four?

By 1914 the advantages of this design had become so obvious to all that most of the cars entered for the French GP that year had engines conceptually copied from it. Many alternatives had been tried, their qualities had been observed and discussed, and from them had emerged something truly superior.

The design came to the US in the form of Bob Burman’s 1913 Peugeot, whose engine had been badly wrecked. Burman went to Harry Miller, operator of a fine West Coast machine shop, and asked if he could reconstruct the engine in four months. Miller took the job, and this was how the ideas of three French racing drivers and a Swiss designer came to be refined through Harry Miller and his shop manager Fred Offenhauser. The last champ-car win by an Offenhauser engine, originally derived from the 1913 Peugeot and still featuring its DOHC four-valve head, came 65 years later, in 1978.

Who Gets the Credit?

To this day people argue over whether Ernest Henry was the source of the DOHC four-valve head or merely the draftsman, putting down on paper the ideas of three experienced racing drivers, two of whom were themselves engineers (Zucca and Goux). Is this important? I think not. I think what is important is that the experiences of these men—all of them—pushed them toward multiple valves and away from side valves located in pockets. Their experience also pushed them away from the harder-to-control extra parts (pushrods and rockers) that operated their valves “by remote control.” What was left? The four-valve DOHC head.

In 1914, Henry also designed the 500M parallel-twin motorcycle racing engine, employing the same ideas, including spur-gear cam drive. I suspect that translating the concept from a liquid-cooled auto engine to an air-cooled design needed more development than the 500M received.

The four-valve concept would leave center stage after the early 1920s, only to make a comeback in the 1960s, driven first by the new requirements of Honda’s Grand Prix racing program and then modernized and refined by Keith Duckworth at Cosworth Engineering. These concepts remain solidly dominant today.

Note: This is Part 1 of a two-part story. Part 2 will appear here next week.

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[fastbob]

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[Bender]

17 minutes ago, fastbob said:

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Thanks that saved me reading it