How odd, …or not: Engines with odd-numbered cylinder configurations

Author: Dan Cooper
Published on: May 28, 2005

What is the advantage of even-numbered cylinder arrangements over one-, three-, or five-cylinder engines? Odd-numbered cylinder configurations are in the minority today. But that has not always been the case, and it appears the “odds” are making a comeback.

The first automotive engines were one-cylinder wonders. Early on, the question of whether to add cylinders to the equation of engine building, probably did not include the consideration of having an even or odd total—just an increase in power and torque.

Over the last half-century and more, the trend has been toward larger engines, with more cylinders. Today we find the multi-cylinder format to be the norm. In America six and eight cylinders have been the most popular. In Europe the inline four continues to be very popular. And even-numbered arrangements of cylinders are the unmistakable norm throughout the world.

But with “Peak Oil,” fuel shortages and skyrocketing prices looming on the very near horizon, there is a new trend today toward smaller, more economical engines. And in the process, another seemingly strange trend has emerged. The odd-numbered cylinder arrangement is making a comeback. Today the odd cylinder configurations are offered in cars and trucks, as well as commercial applications, like tractors. Kubota has a five-cylinder diesel tractor, and Lister-Wilder has several three-cylinder models.

Odd-numbered arrangements have included one, three, five, and even seven cylinders. Although seven-cylinder arrangements appear to have been reserved for aviation applications, using a radial piston arrangement. Radial engines specifically operate best with an odd number of pistons in each “row.”

Seven-cylinder layouts have been used on the Continental A-70 aircraft, and the Cameron, both around 1930. This formula is said to be obsolete in today’s world of aviation. But interestingly, a small company devoted to manufacturing alternative engines for home-built aircraft, has offered a seven-cylinder as its newest power plant. HCI introduced its new engine in 2001.

This brings us to a brief discussion of the difference between the “rotary” engine and the “radial” arrangement—two configurations quite different from the more usual reciprocating piston engines in most automobiles today.

The descriptive term, radial, refers to the arrangement of conventional cylinders situated like the petals of a daisy, around the center of the engine, that is, the rotating crankshaft. The pistons are all in the same plane, or “row” (although there may be multiple rows). Pistons are arranged in a circle around the crankshaft, their collective plane being perpendicular to the shaft. Their text explanation of this layout is weak, but a good visual animation can be seen at the Wikipedia Free Encyclopedia Website.

By contrast, in a conventional rotary arrangement, the crankshaft remains stationary while the entire cylinder block rotates around it. This may sound perhaps more “odd” an arrangement than does an odd number of cylinders, but it is a concept that has been proven to work. This engine was actually the state of the art in WWI aircraft applications. The design dates back to 1899.

Then, too, there is the Wankel. Felix Wankel designed his “rotary” engine in 1936. It did not conform to either description completely. The Wankel has an eccentrically moving crankshaft, and the internal rotor circulates and rotates. The exterior housing does not rotate, as in a true aviation-type rotary. The Wankel is classed as a “rotary combustion engine,” and has become, by far, the most successful example in automotive applications (in the German NSU, and particularly the Japanese Mazda). Here, we must talk of the number of rotors rather than the number of pistons, or cylinders, but there have been three-rotor examples. It would be a shame to discount the success of the Wankel, but for the purposes of this article, we may want to concentrate instead on the more conventional reciprocating piston engine.

For those interested, however, there is a pretty good explanation of the Wankel and 28(!) other similar patents, at this Website (including a Java demonstration).

The aviation inventor, Lawrence Hargrave, was making three-cylinder rotary engines (compressed air, and steam-driven) in the 1890’s. His earliest may have been built in 1889. But apparently, it was not until 1912 that he built a gasoline powered three-cylinder rotary.

Three-cylinder compressed air powered engines significantly pre-dated their gasoline-powered counterparts. About 1875, an existing radial three-cylinder designed by Peter Brotherhood, of Peterborough, England, was adapted and used in self-propelled underwater torpedoes.

Before we concentrate on the so-called reciprocating piston engines found in V-type and inline automotive examples, there is one more tangent to explore. In the interest of completeness, and for historical perspective, it should be noted that odd-numbered cylinder arrangements have also been used in stationary applications. As early as 1901, Westinghouse built a 90hp three-cylinder vertical engine for use in small electric generating plants. The Swedish engine builder, Scania, has recently scaled down its industrial powerplant from 16 liters, to a svelte 9.0 liter version having five cylinders.

But the automobile and its engines are our chief concerns here. And the odd-numbered cylinder arrangements have proliferated far more than most people realize today. These examples are, in fact, growing in number. And they have been with us since the very beginning.

George Selden’s famous U. S. patent on the automobile, included details of the car’s three-cylinder engine. That engine, Selden had modified from the original design by mechanical engineer, George Brayton. The engine(s) supposedly covered by the optimistic Selden patent, included all manner of liquid hydrocarbon engines, with “any convenient number” of cylinders. But the drawings in the patent included a three-cylinder version of a Brayton two-stroke cycle engine, which had been independently patented (in single-cylinder form) by Brayton in 1872, with patent #125,166. The “Brayton cycle” design became the basis for what is now the gas turbine engine. It did not hold up quite as well in automotive applications, much to the frustration of Selden and others (see related article).

Around 1901 the Standard Motor Company produced a three-cylinder engine with a hinged overhead camshaft, which permitted the changing of the engine’s valves in a matter of minutes.

From 1902 to 1906, Charles Duryea made several vehicles powered by rear-mounted three-cylinder gasoline engines.

The Encyclopedia of American Automobiles cites the first Elmore three-cylinder engine as being made in 1907. But the Luray Antique Car Museumin Virginia reportedly has on display an Elmore engine of this type, dating from 1906. The engine was touted as having only three moving parts per cylinder.

According to one source, which quotes a Cosmopolitan magazine article, in 1906 there were five gasoline engine models available in the U.S. with three-cylinder engines, 13 with one cylinder and 54 with two cylinders. That statistic for three-cylinder engines, of course, would not include those made outside the United States, such as the Attila, made by the Hunslet Engine Company of Leeds, England. At the time, Rolls-Royce had also been cranking out three-cylinder engines for several years (as well as those with two, four, and six cylinders).

In 1931 an Englishman named Foden employed an existing Gardener five-cylinder diesel in a prototype flatbed truck (that is, flatbed lorry). DKW had a prototype three-cylinder engine in 1939. And the Spanish SEAT company still offers a three-cylinder in their strange little car.

As “odd” as it may sound, there have also been V-type engines with an odd number of cylinders. This is particularly true in the world of motorcycles. That venue has been one of the most used applications for three- and five-cylinder engines.

In 1912, J.N. Williams built a prototype 53 cubic inch radial three-cylinder engine, which was mounted in an unusual fashion. It was positioned inside the rear wheel of the motorcycle.

Honda motorcycles have enjoyed tremendous success with the five-cylinder format. Their 125cc road racer won more than a hundred Rider's and Manufacturer's World Championships. Kawasaki introduced a five-cylinder in 1996. And Triumph currently makes its Speed Triple.

If we flatten the V-type engine into a configuration where the two banks of cylinders are 180 degrees opposed (instead of the more typical 60, or 90), we have a flat, or “boxer” type engine. Typically, these are flat-four, flat-six, flat-12, or other similar even-numbered arrangements. But yes, there have been flat-five engines, the Audi S2, for example. A flat-three configuration was unveiled in the 1996 Mini Spiritual concept car.

Why is it that even-numbered arrangements have become the norm in today’s automobile? Quite simply, the necessity to achieve balance in the operation of the moving parts of an engine, is the primary reason for even-numbered configurations.

It has been speculated that any engine with an odd number of cylinders per bank will have an undamped vibration pattern emerging from the lack of inherent balance. Vibration and balance are the key issues in any discussion of the potential drawbacks of an odd number of cylinders. But interestingly, there is some disagreement on this point.

In an article by Kevin Clemens, the author quotes Dr. Wilfried Bockelmann, the member of the Volkswagen board of management responsible for technical development, concerning five-cylinder engines, such as the one in the new Volkswagen. Bockelmann is quoted as saying, “Even numbers of cylinders … result in an unpleasant harmonic peak in engine vibration (for a four-cylinder engine, that peak is at around 3000 rpm). But with an odd number of cylinders, like the VW R5, the harmonic does not exist and the noise energy from vibration increases at a constant rate with increasing engine speed.” How this opinion is reconciled with the obvious differences noted in the vibrations of other odd-cylinder arrangements is left unexplained.

Referring to the perceived differences in a tested V-five engine, another source specifically terms the observed effects as ‘harmonics.’

“The five-cylinder engine layout, meanwhile, makes for some interesting harmonics at various points in the rev range,” says David Morely in a used car review article published at The Age Website. “But it's never intrusive and merely serves to remind you that you're not driving a [more] conventional four- or six-cylinder car.”

And a third source, a review and road test from New Car Test Drive, identifies a strangely different sound emanating from an inline five-cylinder during heavy acceleration. “The five-cylinder engine idles and cruises quietly, but the uncommon number of cylinders makes a peculiar siren-like sound when accelerating.”

Whatever balance and vibration problems exist in the odd-cylinder arrangements, it appears they are not insurmountable. The number of odd-cylinder offerings continues to increase. In these times of fuel-consciousness and foreseeable economic recession, the value of a car with fuel consumption like that of a four-cylinder, and performance approaching that of a six, is a pretty hard combination to ignore.

VW introduced a five-cylinder engine in 1984, in the Quantum. They had another in the EuroVan in the early 90’s, and in 2005 have introduced a new 2.5 liter inline five for their Jetta, built specifically for the U.S. market. VW’s five-cylinder diesel, manufactured in South Africa, is expected to top 440,000 exports by the year 2010.

Daimler-Chrysler introduced its Sprinter Van for European use in 1985, with a five-cylinder diesel. Volvo introduced a five-cylinder in 1993. They brought out another one in 2001, a turbo diesel, and have substantially updated it in 2005.

Other examples of the proliferation of five-cylinder engines in the modern marketplace include Mercedes (1974), Audi (since 1977), Lancia (1994), and Fiat Stilo Abarth (2001). Since 2003, Land Rover has offered one in its European models.

The 2004 General Motors “mid-sized” pick-ups, Chevy Colorado and GMC Canyon, share the company’s new 3.5-liter Vortec inline five-cylinder engine. And the Sprinter engine from Daimler-Chrysler is now offered in Dodge vans in the U.S.

With the total number of available automotive models destined to shrink in the near future, and the number of models boasting five-cylinder power plants on the rise, their proportionate influence in the market should increase. Who knows? We may someday fairly soon see the “odds” vying with the “evens” for domination of the marketplace.

Copyright Dan Cooper, 2005

A hearty thank you, and recognition, to Dan Ellsworth, who prompted me with the excellent idea for this article.