Simanaitis Says

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AT 2400 RPM, a conventional engine has an orchestration of its intake and exhaust valves, each operating 10 times every second. Multiply this by a factor of six for a current Formula One engine as it nears its 15,000-rpm rev limit. Over the 140 years of the Otto Cycle engine, there has been a variety of valve actuation means, all devised to open and close valves precisely in concert with the rotation of the engine’s crankshaft. Here are four of my favorites. (I plan to share three less classic ones in a later item.)

Otto’s Silent [though exposed] Engine, 1876. Otto’s original four-stroke Silent Engine had a single cylinder aligned horizontally, with valve actuation largely in the open for all to see and hear.


My GMax rendering of the Otto Silent Engine, 1876.

The Otto’s spinning crankshaft also drives a lengthy shaft through a bevel gear. At the shaft’s other end, one cam and rocker arm bring about the opening and closing of a conventional poppet valve for the exhaust. A smaller cam on the same shaft and another rocker arm actuate a slide-valve that serves double duty: It controls the Otto’s intake of air and fuel as well as the timing of its flame ignition. These activities all occur at an Otto Silent Engine speed of no more than 160 rpm amid, despite its name, considerable whirs, clanks and clatters.

Bugatti’s bevel gears, 1910. Ettore Bugatti was an artist as well as an innovative engineer. Like other high-performance engines of that (and our current) era, Bugatti’s had overhead valves actuated by one or more camshafts also located above the combustion chambers. Such overhead camshafts, single or double, are often abbreviated sohc or dohc, respectively.

Bugatti was one of the first to engineer a multiplicity of valves per cylinder, either with pairs for intake and exhaust for a “four-valve” design, or in a “three-valve” configuration of two intake valves and one exhaust. The Type 23 Brescia Tourer, 1920, was the first four-valve engine in series production. Engine designations often cite the totality of valves, the Bugatti four-cylinder equivalently called the “sixteen-valve.”


Bugatti Type 13 Brescia four-cylinder powerplant, with single overhead camshaft, four valves per cylinder, 1921. Image from Hugh Conway’s
Bugatti Magnum.

The Bugatti camshaft is driven by a vertical shaft with bevel gears at either extremity at the front of the engine. Part of the wonderful sound of a Bugatti engine is the whir of these bevel gears.

Bentley’s mini cranks, 1926. W.O. Bentley’s 6 1/2 Litre and Speed Six six-cylinder engines also had single overhead camshafts and four valves per cylinder. However, Bentley devised a highly unorthodox means of driving this overhead camshaft: a pair of mini-crankshafts linked by auxiliary connecting rods.


Bentley 6 1/2 Litre valve configuration, four valves per cylinder, its single overhead camshaft driven by a pair of mini crankshafts, 1926. Image from Classic Cars in Profile 1-24

Spur gears at the crankshaft rear rotate a small three-throw crankshaft. Thin albeit tall connecting rods translate this crank motion to a similar mini crankshaft spinning the overhead camshaft. W.O. chose this complex hardware for reasons of refinement. It largely eliminated the noisy whir of gears used by others to impart camshaft spin.

It was a Speed Six Bentley with H.J. Milliner saloon coachwork that Wolf Barnato, one of the race-driving Bentley Boys, used in winning his race against France’s fabled Blue Train in 1930. He and a colleague got from Cannes, in the south of France, to his Conservative Club in London four minutes faster than the Blue Train made its Cannes/Calais run.

BMW’s cross rods, 1936. The BMW 328 sports car, built between 1936 and 1940, had an inline-6 featuring overhead valves inclined to form a hemispherical combustion chamber. However, its camshaft actuating these valves resided within the engine block, not overhead, with its drive from the engine’s crankshaft via a short duplex chain.


BMW 328, camshaft in block, overhead valves, some operated in unorthodox fashion via horizontal pushrods, 1936. Image by Leo Bestgen.

Conventional overhead-valve practice had (and continues to use) vertical pushrods actuating rocker arms operating the valves. And, indeed, BMW operated its intake valves in this way. However, exhaust valve actuation depended upon pairs of pushrods, one vertical and the other operating horizontally across the cylinder head.


BMW 328 engine. Image by Lothar Spurzen.

BMW encased the overhead hardware in castings that resembled those of a double-overhead-camshaft design. The giveaway that it’s not a dohc is the presence of those unorthodoxly horizontal pushrods encased in tubes between the two castings.

The design survived World War II when English visitors from Bristol Aeroplane and sports car maker Frazer Nash commandeered BMW technical data and a 328. A Brit version of the cross-rod engine powered a variety of cars, including Bristols, Frazer Nashs, Wacky Arnolt’s Arnolt-Bristols and a variant of the AC before Carroll Shelby stuffed in a Ford V-8 to form the Cobra. ds

© Dennis Simanaitis,, 2015

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