PISTON
The piston has a homogenous aluminum alloy with graphite coating on the sides. This coating gives reduced friction and noise. The piston rings are of different materials depending on where they are located on the piston. The upper compression ring is a nitrated steel ring. The lower compression ring is manufactured from cast iron and the oil scraper ring is a three piece design, all made of nitrated steel (does not apply to B5xx4Sx). For B5244S7 engines the oil scraper ring is two piece instead of three piece. It is more wear resistant than the three piece ring. The ring height is 2.5 mm compared to 2.0 mm for the three piece ring.
The piston pin is hardened steel and is held together using steel snap rings. The piston is oval and conical and flat at the top. By reducing the area from the upper piston ring to the top of the piston, the release of hydro-carbons from the engine to the three-way catalytic converter (TWC) is reduced.
The pistons are oil cooled so that they can have a lower compression height. Oil is led through a valve in a longitudinal channel in the lower section of the cylinder block on the exhaust side. There is a nozzle screwed to the channel at each cylinder. This leads the oil towards the underside of the piston.
Scheme 744
The camshafts are cast iron. The cam lobes which press against the valve lifters are hardened to tolerate the contact pressure. The valve lifters are steel and mechanical, not hydraulic.
There is a certain amount of valve clearance between the valve lifters and cam lobes. The valve clearance is adapted to compensate for differences in length between the valve and cylinder head due to expansion when warming up. Mechanical valve lifters ensure more precise valve timing, reduced friction, more stable combustion and reduced mass.
The material in the lifters is case-hardened steel. The surfaces between the camshaft and valve are slightly convex to guarantee centered contact.
Scheme 745
The timing belt which drives both the camshafts and the coolant pump is a conventional single tooth belt. The camshaft's pulley wheels are secured by three screws in each camshaft. The holes for the screws are oval to allow the correct setting of the camshaft positions.
The timing belt tension is retained by a mechanical belt tensioner. The belt tensioner presses the belt via a tensioner roll installed on a lever. The idler wheel on the other side of the belt prevents the belt from swinging.
Scheme 746
The auxiliary equipment consists of two belts and two auxiliary units. The auxiliary units (generator (GEN), Air conditioning (A/C) compressor) are driven by two Poly-V drive belts tensioned by a mechanical tensioner. The belt tensioners are all located on a bracket that connects the auxiliary units (generator (GEN), Air conditioning (A/C) compressor).
The generator (GEN) receives its power via the air conditioning (A/C) compressor and the air conditioning (A/C) compressor receives its power through the vibration damper which is located on the crankshaft. The power steering pump is driven by an electrical hydraulic pump.
Scheme 747
The tensioner consists or a spring and a friction element. The friction element provides the required damping to absorb small oscillations and speed variations. The spring ensures correct belt tension, irrespective of wear and temperature. The belts are made of cord reinforced rubber.
Scheme 748
The oil is led from the oil pan via a suction piece to the oil pump. The oil pump is located on the cylinder block. The oil is then pumped onwards to the oil cooling system and then to the oil filter. The oil flows from the filter through a cast oil duct in the intermediate section to the main bearings. The oil then flows through drilled channels in the crankshaft to the connecting rod bearings.
The camshafts are supplied with oil by a bored channel in the cylinder block. The channel runs through the cylinder head, where it flows out at the bottom of the upper half of the cylinder head. There is a cross duct in the channel to the cylinder head which carries oil to the pistons via a piston cooling valve.
The oil flows on via an oil duct to the bearing for the left-hand camshaft and the valve lifters (intake side). The bearings for the right-hand side camshaft (exhaust side) are supplied by a cast cross duct at the front edge of the upper half. This cast cross duct also supplies pressurized oil to the solenoids for the VVT unit. Drain holes in the cylinder block release the oil from the cylinder head and crankshaft bearing back to the oil pan.
Scheme 749
The oil filter holder is made of die cast aluminum. It is on the cold side of the engine. The oil filter cover is plastic. There is a by-pass valve in the cover. A O-ring in the cover ensures that it is sealed.
Scheme 750
The piston cooling valve is manufactured in steel with a hardened piston, a spring and a spring stop. A copper washer ensures that oil does not leak onto the cylinder block.