Watch Out, V-, the W- Might be Gaining Ground
W-shape engines, a recent twist in design, have begun to show up under the hood of several new models from the Volkswagen Group, offering a fresh approach to packing more cylinders into engine compartments, reported the New York Times on December 1.
With space made scarcer by designers staking out a greater share of the automotive real estate for passengers, the W engines could also muscle in on the V-8 engine's traditional position in the pecking order of under-hood prestige.
The W engines — there are 12- and 16-cylinder derivatives, too — are part of an array of engine designs, sizes and even shapes that power current vehicles, some of which have been tailored to meet goals not apparent at first glance.
As a result of this innovation, the choices to be considered in a new car purchase are no longer a simple matter of deciding between an in-line six cylinder or a V-8, as it was for most vehicles of a generation or two ago. Today, buyers can shop for cylinders like bologna — purchasing 3, 4, 5, 6, 8, 10 or more — to suit their driving needs.
The family of engines with their cylinders arranged in a W configuration is among the latest solutions devised to fit more cylinders in less space. The most extreme extension of this design is the 16-cylinder, eight-liter, quadruple-turbocharger engine of the 2005 Bugatti Veyron 16.4, which arrives in the United States next spring wearing an expected price tag of $1.2 million.
The Veyron's 987-horsepower W-16 is directly related to 8- and 12-cylinder versions used by three other VW brands: Audi, Bentley and Volkswagen. Interestingly, the company that earned a loyal following by building simple, economical automobiles now offers some of the most complex engines designed for road use.
Before delving into what makes this new W engine worth the engineering effort it required, here is a brief survey of the most common engine configurations produced today.
In-line engines, with cylinders arranged in a single row, are offered in three-, four-, five- and six-cylinder variations. In-line fours, which account for a quarter of the engines sold in the United States in 2002, according to WardsAuto.com, provide an excellent blend of low cost and good fuel economy.
However, as automakers expand displacement of four-cylinder engines to increase power output, vibration levels increase, too. Some automakers employ a system of balance shafts to counteract the engine's natural shaking forces; this mechanism is needed to civilize the W-8 and some V-6 engines, too.
Adding cylinders to an in-line design is an efficient way for automakers to offer variety, as the added family members can be machined on the same production lines. Volvo uses an in-line five, and produces four- and six-cylinder siblings based on the same architecture. In-line six-cylinder engines offer the benefit of being inherently vibration-free, which is part of the reason BMW has made this layout its signature power plant.
After abandoning in-line sixes in favor of V-6s, General Motors recently revived the in-line layout for truck use. Four- and five-cylinder versions of the engine — are available in 2004 Chevy Colorado and GMC Canyon pickups.
The slim proportions of an in-line four make this design ideal for sideways installation (across the length of the car, rather than front-to-back) under the hood of compact models. The benefit of a narrow engine isn't lost when the car gets larger and graduates to more cylinders. A transversely mounted in-line engine frees up more "crush space" than a V, adding a safety margin in collisions, according to Volvo, which uses transverse in-line fives and sixes in its S80 sedan and XC90 sport utility.
The V-engine configuration accounts for nearly two-thirds of all engine installations in the United States today, with V-6s being the most common. The dimensions of a V-6 engine — only three cylinders long — make for a convenient fit under the hoods of front-wheel-drive cars.
Market forces figure into automakers' plans, too. The appeal of an eight-cylinder engine — dominated by the V-8 layout since the demise of the in-line 8 a half-century ago — is an important factor for modern luxury cars, as well as for SUVs and trucks where horsepower is a top priority. Once a manufacturer perfects the internal components of a V-6 or V-8 engine, those same parts can serve in larger engines.
The opposed-cylinder design, often called a "boxer" or flat engine, is most familiar as the power behind the original VW Beetle. Following in those footsteps, Porsche and Subaru use flat engine layouts consisting of two horizontal banks of cylinders flanking a central crankshaft. Flat engines also help to lower the vehicle's center of gravity.
Volkswagen began thinking outside the box of standard cylinder layouts two decades ago. By staggering the cylinder bores in the engine block in a zigzag arrangement — like nestling the soda bottles on a refrigerator shelf, rather than lining them up side-by-side at their widest points — VW engineers produced a 2.8-liter six-cylinder engine that was about the same length as a conventional two-liter in-line four.
The nestled cylinders made it possible to close up the engine's V angle between the cylinder banks from the normal 60 (or 90) degrees to only 15 degrees. VW called this unusual engine a VR-6 (for V-reihen, or V in-line in German) because it blended characteristics of both V and in-line engines. The payoff was the ability to package six cylinders transversely under the hoods of cars originally designed for four-cylinder power plants. The space-saving VR-6 philosophy has since been incorporated into new designs. To fit eight cylinders into existing models originally designed with V-6 space under the hood, VW engineers positioned two banks of four zigzag cylinders in a 72-degree V. The new layout is called W-8 because the end view loosely resembles two conjoined V engines. The savings in length permits a four-liter, 270-horsepower, eight-cylinder engine to fit longitudinally under the hood of a midsize VW Passat without the need for a revamping of the car's structure.
While they were at it, VW engineers stretched the W concept further, yielding a 450-horsepower W-12 for the Audi A8 (coming to the United States in 2005), a twin-turbocharged 552-horsepower version of the same engine for the Bentley Continental GT and a no-complexity-barred W-16 for the Bugatti Veyron 16.4.
As might be expected, there is no free lunch in engine design. Squeeze the cylinder package in one direction and something squirts out in another dimension. The 12-cylinder W engines are significantly wider than conventional V-12s, and routing the flow of the intake and exhaust systems is a plumber's nightmare.
Do not bother to speculate over possible X-engines as the next step in the alphabetic progression: they have already been built, by Henry Ford, in the 1920s.