Targets in the design and construction process: compact, stiff, light
In developing BMWÃ¢â‚¬â„¢s new eight-cylinder power unit, the engineers and other specialists sought to reduce all moving masses to an absolute minimum, focusing above all on the crank and valve drive in their search to ensure minimum rotating and moving masses. Precisely this is why they decided to combine two rows of four cylinders at a V angle of 90Ã‚Â° and an off-centre arrangement of 17 millimetres or 0.67Ã‚Â´Ã‚Â´ to make the entire power unit extremely compact and efficient.
The decision to choose a 90Ã‚Â° angle was taken on account of the efficient compensation of mass forces provided by this geometry, serving to minimise vibrations and maximise motoring comfort. By and large, therefore, this specific geometry offers the optimum solution to the conflict of interests resulting from maximum smoothness free of vibrations, on the one hand, and maximum stiffness of all relevant components, on the other.
Engine block from BMWÃ¢â‚¬â„¢s Formula 1 foundry
The engine block featured on the new BMW M3 comes from BMWÃ¢â‚¬â„¢s light-alloy foundry in Landshut near Munich, which also builds the engine block for BMWÃ¢â‚¬â„¢s Formula 1 racing cars. The cylinder crankcase is made at the foundry in a low-pressure die-casting process from an over-eutectic aluminium-silicon alloy, with at least 17 per cent silicon. The cylinder liners, in turn, are formed by exposing the hard silicon crystals, the iron-coated pistons running directly in these uncoated honed cylinder bores and thus not requiring any additional lining. Cylinder stroke is 75.2 millimetres or 2.96Ã‚Â´Ã‚Â´, cylinder bore 92 millimetres or 3.62Ã¢â‚¬Â', adding up to provide overall capacity of 3,999 cc.
Since high engine speeds, high compression forces and high temperatures exert extreme loads on the rankcase, the crankcase is very compact in its design and dimensions and built as a very stiff bedplate structure, a concept which has already proven its qualities in motorsport. Made of die-cast aluminium, the bedplate features grey-cast-iron inlays ensuring very precise support of the crankshaft. In particular, this structure keeps main bearing play throughout the entire range of operating temperatures within close limits, the grey-cast-iron inlays reducing thermal elongation of the aluminium housing. As a result, oil flow remains almost unchanged at all times. And to form a positive engagement with the aluminium frame, the inlays have open cut-outs integrating them directly in the frame.
With the distance between cylinders measuring only 98 millimetres or 3.86Ã‚Â´Ã‚Â´, the crankshaft made of forged, high-strength steel is relatively short, making it very stiff in terms of flexural and torsional strength and reducing the weight of the crankshaft to just 20 kg or 44 lb. Running in five bearings, the crank-shaft has a main bearing measuring 60 millimetres or 2.36Ã‚Â´Ã‚Â´ in diameter, with bearing width of 28.2 millimetres or 1.11Ã¢â‚¬Â'. In each case two connecting rods act on one of the four crankpins offset from one another by 90Ã‚Â°.
Lightweight construction specifically on all moving masses
The weight-optimised box-type pistons are cast out of a high temperature-proof aluminium alloy and coated with iron. This reduces their weight to a mere 481.7 grams including the piston pins and rings. Compression height is 27.4 millimetres, with a compression ratio of 12.0 : 1.
The pistons are cooled by oil spray jets connected to the main oil pipe. Measuring 140.7 millimetres or 5.54Ã‚Â´Ã‚Â´ in length, the cracked trapezoidal connecting rods are made of a high-strength steel-magnesium alloy. Including the bearing shells, each connecting rod weighs just 623 grams, which significantly reduces the oscillating masses.
The single-piece aluminium cylinder heads feature four valves per cylinder in characteristic BMW style. The valves weighing 42 grams each are activated by ball-shaped cup tappets with hydraulic valve play compensation. Tappet diameter is only 28 millimetres or 1.10Ã‚Â´Ã‚Â´, while the intake and exhaust valves measure 35 and respectively 30.5 millimetres (1.38 and 1.20Ã‚Â´Ã‚Â´) in diameter. Measuring only 5 millimetres or 0.20Ã‚Â´Ã‚Â´ across, the valve shaft has hardly any influence on flow conditions in the intake manifold, while the hydraulic valve play compensation rules out any change in valve play, ensuring lasting reliability and at the same time reducing the cost of maintenance.
The engine always keeps a cool Ã¢â‚¬â€œ cylinder Ã¢â‚¬â€œ head
Compared with conventional systems, the cross-flow cooling concept featured on the new V8 power unit significantly reduces pressure losses in the cooling process, spreading out temperatures smoothly and consistently throughout the cylinder head and thus reducing temperature peaks at all critical points. To ensure a perfect flow of cooling around each cylinder, the coolant flows from the crankcase via the exhaust side crosswise through the cylinder head and the collector rail on the intake side to the thermostat and, respectively, to the radiator.
Double-VANOS Ã¢â‚¬â€œ but with low instead of high pressure
Focusing on the engine concept, the engineers at BMW M sought from the start to increase engine output through an optimum charge cycle at high engine speeds. Quite simply, this is because reduced charged cycle losses offer not only more power, but also an improved torque curve and optimum engine response as well as a further reduction of fuel consumption and lower emissions. And precisely these are the benefits offered by variable double-VANOS camshaft control introduced in the BMW M3 for the first time worldwide back in 1995.
With its extremely short adjustment times, double-VANOS now also perfects the cylinder charge cycle in the eight-cylinder power unit of the new BMW M3. Under low loads and at low engine speeds, for example, double-VANOS ensures a higher valve overlap and, as a result, better internal recirculation of exhaust gasses. This, in turn, reduces charge cycle losses and helps to minimise fuel consumption.
The level of power delivered by the engine depends on the position of the gas pedal and engine speed. So double-VANOS adjusts the precision and angle spread on the camshafts infinitely to these two parameters with precise management by a control map. Unlike the ten-cylinder power unit featured on the BMW M5 and BMW M6, the eight-cylinder uses a double (and not a single) chain to connect the crankshaft and sprocket. The sprocket, in turn, is connected to the camshaft by a step motor/actuator, and not by a helical gearing.
The advantage is that the low-pressure M double-VANOS developed for the eight-cylinder is able, unlike the high-pressure VANOS featured on the V10, to run with regular engine oil pressure acting on the step motor. In other words, there is no need for a separate high-pressure system of pipes to turn the crankshaft versus the sprocket in a relative motion at maximum speed and with utmost precision. This means that the angle on the intake camshaft may be varied by up to 58Ã‚Â°, the angle on the exhaust camshaft by up to 48Ã‚Â°. Maximum angle adjustment speed, in turn, is 360Ã‚Â° per second crank angle, low-pressure adjustment thus ensuring very short adjustment times and providing the optimum adjustment angle synchronised to ignition timing and injection volume as a function of load and engine speed.
Reliable oil supply even under extremely dynamic conditions
The high standard of driving dynamics offered by the BMW M3 obviously calls for a sophisticated supply of oil to and within the engine. Indeed, the engine oil supply is conceived for longitudinal and lateral acceleration of up to 1.4 g Ã¢â‚¬â€œ far more than the forces acting on a passengerÃ¢â‚¬â„¢s body when taking off and landing in a jet aircraft.
The eight-cylinder is reliably supplied with lubricant in all driving situations by two volume flow-controlled pendulum shift cell pumps consistently providing exactly the right amount of oil required by the engine. This is ensured by a change in eccentricity (off-centre arrangement) of the inner rotor in the pump versus the pump housing as a function of oil pressure in the main oil duct.
In consideration of the physical forces and loads encountered in an extremely dynamic driving situation, it is conceivable that when the driver applies the brakes particularly hard, for example, the supply of oil to the oil sump serving as an interim storage reservoir would not be sufficient, particularly as the oil sump is positioned behind the front axle subframe for reasons of space.
So if the worst came to the worst, lubrication would be interrupted completely. This is however prevented by the concept of Ã¢â‚¬Å“optimum-dynamic wet sump lubricationÃ¢â‚¬Â', a system with two oil sumps: a small oil sump in front of the front axle subframe and a large oil sump behind the first one. A separate recirculation pump draws oil from the small oil sump at the front under all conditions and feeds the oil to the larger sump at the rear. The larger sump, in turn, is carefully sealed off in order to avoid any splash losses and the formation of foam.
The new eight-cylinder power unit from BMW M also comes with electronic oil level control determining the level of oil by means of a sensor fitted in the oil sump. The data thus measured is transmitted by a serial databus to the engine management evaluating this data by means of various algorithms. The value thus obtained, corrected by the carÃ¢â‚¬â„¢s lateral and longitudinal acceleration, is then presented to the driver in the instrument cluster.
Eight individual throttle butterflies with electronic management
In motorsport this is standard technology, in Ã¢â‚¬Å“regularÃ¢â‚¬Â' cars it is quite rare Ã¢â‚¬â€œ the use of a separate, individual throttle butterfly for each cylinder. But precisely this mechanically very sophisticated system is unsurpassed in giving the engine a spontaneous response and supreme performance. And precisely that is what counts in a BMW M Car.
The power unit of the BMW M3 so closely related to motorsport comes with eight individual throttle butterflies, four butterflies on each row of cylinders being driven by one actuator in an electronically controlled process. To provide this precise electronic management, the position of the gas pedal is scanned by a touch-free hall potentiometer 200 times a second and evaluated immediately, the engine management registering any change in the position of the pedal and adjusting the individual throttle butterflies accordingly via the two actuators in an instantaneous, split-second process.
As a result, it takes the throttle butterflies only 120 milliseconds to reach their maximum opening point Ã¢â‚¬â€œ about as long as a routined driver needs to press down the gas pedal. The result, first, is a sensitive and smooth response of the engine at low engine speeds and, second, an immediate, direct reaction of the car whenever the driver calls up more power from the engine.
Flow-optimised air intake
To give the engine an instantaneous, immediate response, the air volume on the intake side of the throttle butterfly must be reduced to an absolute minimum. The problem in this case, however, is the large intake cross-section and air collector volume required by a high-performance power unit of this calibre. So to meet both of these requirements, the throttle butterflies in the intake manifolds are positioned right in front of the intake valves.
From front to rear, the entire flow of intake air in the new eight-cylinder power unit does not require the usual hot-film air mass flow meter with its obligatory sensors. Instead of determining engine load by means of such elaborate sensors, therefore, which would also create disadvantages in air guidance due to the geometry of the components involved, the V8 power unit of the new BMW M3 uses the engine control unit to perform this function. To do this, the system determines engine load under current driving conditions by taking the position of the throttle butterfly and idle adjuster, the position of the VANOS control unit, engine speed, air temperature and air pressure into account. This, in turn, gives the engineers at BMW M GmbH new freedom in the configuration and optimisation of the engine air intake process. And at the same time this management concept operates with maximum reliability.
The length and diameter of the eight intake funnels also helps to ensure an optimum charge effect in the oscillating tube. Like the single-piece, extra-large air collector, the funnels are made of a light composite material with a 30 per cent share of glass fibre. The air filter cartridge in the air collector, in turn, uses the maximum filter area possible, the air collector being supplied with air by an extra-large intake air silencer with three intake air openings.
Innovative exhaust system
Though its design and configuration, the exhaust system also serves to optimise the cylinder charge cycle. To give the eight-cylinder power unit optimum power and torque behaviour, the focus is on keeping counter- pressure as low as possible, which is why exhaust gas flows through two chambers into the rear-end silencer.
The development engineers have likewise given their full attention to consistent lightweight engineering also in developing the exhaust. To achieve these and other targets, the dimensions of the exhaust manifold, the entire exhaust system, as well as all suspension and fastening elements were calculated by means of the CATIA CAD computer system, the 3D data obtained in the process being used consistently also in production and quality assurance.
High-thrust innovation for ultra-thin pipes
BMW M GmbHÃ¢â‚¬â„¢s particular innovative strength in engine construction is also borne out by the production methods used. Indeed, one example dates back all the way to 1992, when BMW M GmbH became the first company to use the inner high-pressure moulding process for the then BMW M3 Ã¢â‚¬â€œ and since then this process has been consistently refined.
Inner high-pressure moulding serves to shape the seamless stainless-steel exhaust pipes from inside under a pressure of up to 800 bar. The result is pipes with extremely thin walls between 0.65 and 1.0 millimetres (0.0256 and 0.0394Ã‚Â´Ã‚Â´). Clearly, this helps to optimise both the weight of the exhaust system and the response of the catalytic converters.
At the same time this sophisticated technology allows moulding and shaping processes previously not conceivable, as well as even better geometric tolerances. And since all primary and secondary pipes are made of one single piece despite their complex shape and dimensions, many of the former connectors and welding seams are no longer required. Similarly, there are no folding points or tight bends changing the cross-section of the pipes. Instead, the pipes retain their maximum cross-section at all points, minimising any flow resistance in the process.
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