|New 2.0-liter diesel with Variable Twin Turbo and 2,000-bar injection. Particulate filter is the cylinder at left rear. Click to enlarge.|
At its recent Innovation Day 2007 in Germany, BMW emphasized the role its new families of four-cylinder diesel and gasoline engines will play in increasing fuel economy while still delivering power and performance. BMW views its diesels in particular as a core technology in its strategy to reduce CO2 emissions.
The new gasoline direct injection engines and the next-generation diesel engines, already being applied in new models, all offer lower weight, more power, greater fuel economy, and optimized emissions. In addition to the various improvements and modifications within the different engines, BMW is also adding auto stop start, regenerative braking, electrical power steering and improved on-demand ancillaries control to reduce fuel consumption. (Earlier post.)
|BMW High Precision Injection cutaway. Click to enlarge.|
High Precision Injection gasoline engines. The new series of four-cylinder gasoline engines features second-generation direct fuel injection: BMW’s High Precision Injection, allowing lean burn operation of the engine throughout a wide range of engine speed thus helping to significantly reduce fuel consumption in everyday traffic despite increases in engine power.
Applied in the new 120i, the engine offers a 14% reduction in fuel consumption to 6.4 l/100km (37 mpg US) compared to its predecessor, while increasing power by 15 kW. The engine in the new 118i decreases fuel consumption by 19% to 5.9 l/100km (40 mpg US) while increasing power by 10 kW.
BMW introduced High Precision Injection for the first time in the 225 kW/306 hp straight-six power unit with Twin Turbo technology featured in the BMW 335i Coupé. (Earlier post.)
The HPI engines can operate in lean-burn mode (lambda >1) throughout a wide operating range. Piezo-injectors positioned directly next to the spark plugs support stratified charging and combustion, with the exact composition of the fuel:air mixture varying from one layer to the other.
Within the common fuel rail, the high-pressure pump generates 200 bar of pressure for the four injectors delivering fuel to the combustion chambers. The piezo-injectors allow up to six injection processes in each operating stroke.
The piezo-injectors form a stable, conical injection jet within the combustion chamber. The jet-guided process ensures a much faster and more efficient fuel/air mixing process in the direct vicinity of the spark plug, without any loss otherwise caused by fuel resting on the walls of the cylinder as in wall-guided injection.
This provides exactly the right conditions for a stratified cylinder charge characteristic of lean burn operation: various, intersecting zones of differently composed fuel-air mixtures forming within the combustion chamber. In the process the share of fuel in the mixture decreases consistently with an increasing distance from the spark plug, a rich, ignitable fuel/air mixture being maintained only in the direct vicinity of the spark plug. As soon as this richer mixture is ignited, the leaner layers further away from the spark plug will also start burning in a clean, smooth and consistent process.
This serves to maintain fuel-efficient lean burn operation throughout a very wide range of engine speeds and loads.
To support lean burn operation with a stratified cylinder charge, BMW redesigned the cylinder to support the positioning of the piezo-injectors. A highly efficient charge cycle within the cylinders is ensured by conventional valve drive with two overhead camshafts and roller-type drag arms optimized for minimum friction. Compared with engine variants featuring VALVETRONIC, this type of valve management allows a significant increase in engine speed by 800 rpm to 7,000 rpm.
To maintain a beefy torque curve throughout the entire engine speed range, both camshafts come with double-VANOS for infinite adjustment of valve opening times. In order to build up high torque as soon as possible at low engine speeds, in turn, the engine also incorporates a special intake system with variable manifold length (DISA technology).
The new lean burn engine comes with a main catalyst close to the engine itself and storage catalysts further down the line to reduce NOx emissions. BMW is initially introducing its new family of four-cylinder gasoline HPI engines only in the European markets.
Four-cylinder diesel. BMW’s new 2.0-liter, four-cylinder diesels offer an all-aluminium crankcase; variable turbine geometry or variable twin turbo technology in the most powerful variant; third-generation common rail fuel injection, and diesel particulate filters placed close to the engine.
The variable twin turbo technology—also referred to as multistage turbocharging—gives the top-end unit maximum output of 150 kW/204 hp, making this the first all-aluminium diesel engine in the world to develop output of more than 100 hp per liter.
The distinction between the power and torque offerings of the three variants lies in the specific modification of the injection components and the turbocharger system. Developing maximum output of 105 kW/143 hp and peak torque of 300 Nm/221 lb-ft, even the basic version of the new diesel outperforms its predecessor by 15 kW/20 hp and, respectively, 20 Nm/15 lb-ft.
The most powerful version of the new engine develops maximum output of 150 kW/204 hp, 30 kW/41 hp more than the formerly most powerful four-cylinder diesel from BMW—and at 400 Nm/295 lb-ft, the engine’s peak torque is up by 60 Nm or 44 lb-ft. The middle engine in the four-cylinder diesel range is a 130 kW/177 hp power unit developing maximum torque of 350 Nm or 258 lb-ft.
Increased fuel efficiency accompanies the increased dynamics. Fuel consumption in the entry level 118d is down by approximately 16% versus the former model to 4.7 l/100km (50 mpg US) despite an increase in power by 15 kW to 105 kW/143 hp. The new BMW 120d, in turn, comes with an increase in output by 10 to 130 kW (177 hp) and an improvement in fuel economy of the same magnitude, the engine now making do with just 4.9 l/100km (48 mpg US).
The cylinder head with its intake ducts is a new design. The intake ducts are positioned at the side and designed as a spiral and tangential manifold. To reduce emissions to an absolute minimum, the spiral duct is electronically variable in an infinite process.
With their larger diameter, the valves facilitate the gas charge cycle and are now positioned upright, facing vertically into the combustion chambers. This avoids the need for extra cavities on the piston surface, which no longer requires separate valve pockets. The turbulence duct, in turn, gives the fresh air flowing into the engine a swirl motion improving the internal mixture formation process.
While the basic engine operates at an injection pressure of 1,600 bar and solenoid valves serve to supply the fuel in appropriate doses, the two more powerful engines inject diesel fuel at a pressure of 1,800 and 2,000 bar respectively through four piezo-injectors. The most powerful version of the new diesel is the first engine ever to use piezo-injectors operating at 2,000 bar.
To make the combustion process even more efficient, both the shape of the combustion chambers and the trough at the bottom of the piston have been modified and the compression ratio reduced to 16:1. Fuel is injected in up to three doses for each operating stroke of the engine.
|The variable twin turbo unit (left). Click to enlarge.|
The Variable Twin Turbo made its debut in the six-cylinder diesel featured in the BMW 535d. The turbocharger unit in the Variable Twin Turbo comprises one small and one large exhaust gas turbocharger. The smaller turbocharger becomes active at low engine speeds just above idling. At higher speeds the larger turbocharger then also cuts in, developing extra power in the process.
This process eliminates lag, developing noticeable thrust and momentum even when the driver barely presses down the accelerator pedal. A turbine control flap distributes the flow of exhaust gases variably to the two turbochargers.
New engine electronics ensure smooth management in the transition phase between the two turbochargers and optimum interaction of the two units with one another. This sophisticated control concept coordinates the complete system of turbines, the turbine control flap, bypass and wastegate as a function of the engine’s operating conditions.
The lower-powered units each feature one exhaust gas turbocharger with variable turbine geometry. An electric step motor serves to adjust the turbine blades with supreme accuracy and minimum delay to the respective operating conditions and running requirements.
To keep the periphery of the engine as clear-cut and uncluttered as possible, the feed pipe for exhaust gas recirculation (EGR) is integrated in the cylinder head. The EGR valve is positioned on the hot side of the engine, the EGR radiator features a bypass serving to limit the emission of harmful substances while the engine is warming up. All versions of this new engine generation come with a diesel particulate filter fitted close to the engine as standard.