Por Byran Joslin, 26/02/2006
BMW’s headquarters may be contained within the symbolic four-cylinder tower that rises above Munich, but these days it’s the six-cylinder engine that drives the brand. The inline six is so important in fact that it can be found in every current BMW model series, at least in Europe.
Over the years there have been numerous variations of the modern BMW six, starting with the M30 engine family. The M30 was derived from the company’s M10 four-cylinder engine and featured a chain-driven single overhead camshaft. BMW’s first “big” cars, such as the original 5, 6 and 7 Series, were powered by the M30, which remained in production until the early ‘90’s.
In the late Seventies, BMW developed a smaller six-cylinder engine, the M20, which filled the gap between the four-cylinders and big sixes of the day. Though still a SOHC engine, this one was driven by toothed belt rather than chain. More than 2.3 million M20’s were produced between 1977 and 1992, thanks in large part to the popularity of the E30 325.
The M20 gave way to the M50/M52/M54 series, a much-improved modern power plant with double overhead cams and 24 valves. This engine family was the first to employ VANOS variable valve-timing technology and was the first all-aluminum six-cylinder from BMW.
In addition to the standard-issue six cylinders, BMW’s M division has cooked up its own derivatives over time. The M88 engine debuted in the E28 M5, eventually becoming the S38 for the E34 M5. But the most significant M powerplant family must be the S50/S52/S54 found in E36 and E46 M3’s, as well as derivatives like the M Coupe and M Roadster.
As advanced as the company’s engines had become, the existing six-cylinders faced insurmountable obstacles in light of changing emissions requirements. At the same time, the market demanded even higher fuel economy and greater performance. Rather than continuing to develop its existing engines, BMW started with a clean sheet of vellum to address these modern challenges. The result is the new N52 engine, and it achieves these lofty goals through three main methods- lightweight materials, groundbreaking manufacturing processes, and reduction in mechanical drag and friction
Starting from scratch allowed BMW’s engineers to consider new concepts in terms of components, materials and processes. Perhaps the greatest new development is the composite block construction utilizing both magnesium and aluminum.
Armed with the mission of creating the lightest six-cylinder engine possible, BMW engineers examined the largest single engine component, the block. The outgoing M54 engine was already one of the lightest engines in the market thanks in part to its all-aluminum construction. Reducing the weight of the block through conventional methods would have undermined the new engine’s structural integrity. A completely new approach was considered, and the resulting design was nothing short of an engineering breakthrough.
The N52 engine block uses magnesium alloy for the shell, with an aluminum “inner block” for the cylinder liners. The aluminum structure, which also includes coolant ducts, is cast first. This “insert” provides the necessary structure required to maintain stability under extreme thermal and mechanical strain, while at the same time providing mounting for both the cylinder head and crankshaft.
The magnesium “jacket” is placed in position over the aluminum insert, and the two are fused together as the magnesium component cools. The block’s jacket contains the oil ductwork and accessory brackets, and the timing chain case is integrated into this housing as well. When fused, this composite construction is extremely strong.
Magnesium finds its way into other engine components as well, including the valve cover, but more important is its use in the bedplate. Derived from BMW’s motorsports efforts, the bedplate does away with the individual bearing caps used in conventional construction in favor of one solid piece that houses the sintered steel bearing surfaces. No conventional gasket is used; instead, a liquid sealant is injected under high pressure into a groove on the contact surface between the bedplate and the crankcase.
Employing magnesium in a production automobile engine was no easy task for BMW engineers. Despite its many advantages, it poses numerous challenges .One of the biggest concerns over the use of magnesium is its incompatibility with water. To avoid potential corrosion issues, the coolant jacket is contained fully within the aluminum insert cast into the crankcase.
As magnesium and aluminum expand and contract at similar rates to each other, but significantly different than that of steel, special bolts were also required. The bolts used to attach parts to the magnesium components are made of aluminum, further reducing the total weight of the engine.
Inside the ultra-light crankcase reside extremely light and robust connecting rods. The trapezoidal design of the N52’s con-rods saves a few grams per part, but those few grams are precious at higher engine speeds. Like all BMW connecting rods, those in the N52 are “cracked” into two pieces at the bottom rather than machined. The resulting uneven mating surfaces of the two halves makes for a stronger assembly when bolted together, again allowing for greater stability under the strenuous conditions the engine is expected to withstand.
Lightweight components find their way into the cylinder head as well. Aside from the composite construction of the engine block, the most significant reduction in weight comes from the use of hollow camshafts. Typically camshafts are cast or forged to their rough shape, then machined to their final form. For the N52 engine, BMW takes advantage of hydroforming technology to shave more than a pound out of each camshaft, a 25% reduction.
The starting point for these new cams is a steel tube, over which the individual cam lobes are installed. The parts are placed together in a forming die that accurately orients the lobes and the bearing surfaces before the tube is filled with a high-pressure burst of water. At over 58,000 psi, the pressure from the water expands the tube, forcing it together with the lobes to create the complete camshaft.
The N52 engine family is BMW’s first six-cylinder to employ the company’s successful VALVETRONIC technology. Based in large part on the tried and tested bi-VANOS variable valve timing system, VALVETRONIC goes a step further, eliminating the need for throttle bodies altogether by controlling air intake entirely by the opening and closing of the intake valves.
The N52 is also the first engine to make use of the second-generation of this BMW-exclusive technology. VALVETRONIC originally debuted in 2001 on a four-cylinder engine and has thus far been used on 8- and 12-cylinder engines as well. This latest iteration allows for higher engine speeds without effecting drivability, further enhancing the high-performance driving characteristics BMW owners have come to expect.
In total, the use of extremely light materials and breakthrough production processes has reduced the weight of the N52 engine by 22 pounds compared to its predecessor. Weight savings are only part of the formula for the N52. Reducing unnecessary drag and friction on the engine frees up power and improves economy as well. Moving fluids through the engine takes its toll on efficiency, so BMW engineers took a new look at the coolant and oil pumps for a solution.
With the N52 family, BMW also becomes the first manufacturer to replace the belt-driven water pump with an electric water pump. This eliminates a nearly 3-horsepower drag from the engine, returning better performance and economy. It also provides the cooling system with complete independence from engine speed as a factor. The electric pump allows for faster warm-ups by not pumping coolant at low engine temperatures, and also allows the coolant to be circulated even after the engine has been shut down.
The N52’s capacity-controlled oil pump works in a similar manner. By determining the engine’s actual lubrication needs based on current temperature and pressure, the pump provides only as much oil as is required by conditions. In contrast, a typical oil pump delivers a high quantity of oil and bypasses the unneeded oil back to the sump. This unneeded amount can be as much as 80% of what is pumped. The N52’s oil pump thereby consumes nearly 3 horsepower less than a conventional oil pump.
BMW’s newest six-cylinder debuted last year in the all-new 3 Series (E90) in both standard- and high-output forms, producing 215 and 255 horsepower respectively. The same arrangement now powers the Z4 Roadster and Coupe for 2006. In Europe, the high-output version produces 265 horsepower in BMW’s smallest six-cylinder sedan, the 130i.
The N52 has just begun its development cycle; BMW’s M Division hasn’t even gotten its hands on it yet. Certainly future variations will produce more power and will likely adopt even newer technology as it emerges. More importantly, the technological breakthroughs achieved in the development of this engine will most definitely find their way into other BMW engine families.
