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km : Tech


9 May 2011

For more than a decade now, the smallest engine available to American BMW buyers has been an inline six. While sixes, eights, tens and twelves have all lived behind the twin kidneys in recent years, the company’s reputation as the Ultimate Driving Machine was actually built on the strength of its great four-cylinder cars, from the inimitable 2002 to the heart-pumping first-generation M3. If there’s any doubt about the four-banger’s importance, consider this: the company headquarters is a building in the shape of four conjoined cylindrical towers. So perhaps it shouldn’t be surprising that as big displacement and massive power begin to take a back seat to tougher fuel economy and emissions mandates, BMW is preparing to become famous again for four-cylinder vehicles, starting with the introduction of an all-new turbo two-liter.

Known internally as the N20, this newest powerplant pulls out all the tricks employed in recent years on some of BMW’s bigger engines, including some from the M division. The engineering brief was a stiff one, demanding balance on issues of weight, consumption and, importantly, performance. BMW fully expects this engine to be the bread-and-butter of its small-car lineup — it debuts this fall in the Z4 but will certainly spread to the next 1- and 3-series as well as the upcoming X1 — and as such it will be expected to perform like a proper BMW engine. On paper, the specs are mighty impressive.

The core architecture of the N20 is similar to other recent engine families within the company, but with a couple of major differences. For instance, it uses the now-familiar bedplate in place of individual bearing caps on the bottom end. Derived from motorsport applications, this design results in a much more rigid engine structure with less vibration, lending a degree of refinement usually associated with larger, heavier engines. The strength afforded by this construction also allows for fairly high compression ratios as well, especially for a turbocharged unit. Variations on the engine will run between 10:1 and 11:1 compression.

But unlike its six-cylinder counterparts, the entire block is mast in aluminum; there are no cylinder liners inserted at all. Instead, the swept area of the combustion chambers is treated with a thin layer of iron by way of electric arc spraying technology. BMW took a lesson from AMG, who first used this process on its own turbocharged engines. This method of literally spraying hot iron onto the aluminum cylinder walls not only reduces weight, it allows for cooling passages in the narrow regions between the cylinder, a critical consideration for durability and reliability on a highly-stressed turbo engine.

Reducing internal friction was one of the key objectives in attaining maximum efficiency. The pistons feature exceptionally short skirts and even the internal geometry was taken into consideration, which is why the crankshaft centerline is offset 14 mm from that of the cylinder bores. This unconventional alignment generates less friction as the pistons move through their power strokes, resulting in greater power delivery.

Other drag-abating measures include the use of an electric coolant pump and a variable-volume oil pump. The electric coolant pump has been used successfully on numerous BMW engine families recently and offers numerous advantages. It not only eliminates perpetual drag on the engine, it also allows for demand-driven engine cooling, meaning it can prevent circulation at startup to bring the engine to operating temp more quickly, or run at a higher rate during hot, low-rpm conditions such as stop-and-go traffic. The oil pump, while driven by one of the engine’s balance shafts, features a variable-volume valve to ensure the engine is pumping only as much of the thick stuff as needed for conditions.

On the induction side, the new engine takes full advantage of BMW’s technical reserves, employing variable intake and exhaust timing (Double VANOS) for nearly infinite torque and efficiency optimization and the throttle-less Valvetronic setup, which reduces the pumping losses associated with a conventional butterfly valve. The top of each combustion chamber is crowned, with two intake valves, two exhaust valves, a spark plug and a solenoid-type direct injector nozzle residing in each position within the aluminum head.

The star of the show, however, is the single, twin-scroll turbocharger. Rather than bolting on a pair of small (expensive) turbos, a single slightly larger charger is fitted. To reduce backpressure and allow for quicker spool-up, thereby producing more low-end torque and a more responsive package, two separate ducts breathe upon the input scroll. Exhaust from cylinders one and four feed one of the side, while cylinders two and three provide boost from the other. The compressed intake charge is cooled and condensed by an efficient front-mounted intercooler.

While all of the technologies employed in the N20 offer varying degrees of performance and efficiency gains, the result of their combination is a very advanced engine that makes previous BMW four-cylinders look like dinosaurs by comparison. Generating 120 horsepower per liter in Z4 tune, the new engine offers an overall performance envelope that is essentially on par with a naturally aspirated 3.0-liter six (240 hp, 260 lb-ft), but with twenty percent less fuel consumption and emissions. It’s also lighter overall and more compact, allowing for better overall vehicle dynamics. It may well be the most advanced four-cylinder gas engine in the marketplace when debuts this fall.

Ironically, this new 2.0-liter engine will wear “2.8i” nomenclature when it arrives. The Z4 will benefit first from the N20, followed later by the X1. By the time the next generation of 1- and 3-series emerge, the current base six-cylinder will have been put to rest in favor of the N20. We fully expect to see the current X3 and 5-series embrace it as well. We suspect BMW will eventually stretch this engine’s specific output beyond the 120 hp/L mark. In fact, we wouldn’t be surprised at all if the boys at M are already hard at work on a motorsport version.

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