GASOLINE (EXCEPT VR6)
The engine in all the models covered in this section are water cooled inline 4-cylinders with a cast iron block and an aluminum alloy cylinder head. The crankshaft is supported in five plain main bearings and the center bearing includes a 4-piece thrust bearing. The forged steel connecting rods are equipped with plain bearings at the big end and coated steel bushings at the small end. The full floating wrist pins are held in place in the pistons with circlips. The pistons are fitted with two compression rings and a 1-piece oil control ring. The oil pump is mounted below the crankshaft and driven by the intermediate shaft. The 16 valve 2.0 liter block has additional oil passages and spray nozzles for cooling the under side of the pistons. On 8 valve 1.8 liter engines, the oil pump drive shaft includes an extension that engages the drive lugs on the distributor.
The intermediate shaft and the camshaft are driven by a steel-reinforced toothed belt. The overhead camshaft acts directly on the valves through hydraulic lifters for quiet, maintenance free operation. The valves move in alloy guides that can be replaced when worn. The bearing surfaces for the camshafts and lifters are machined directly into the cylinder head and cannot be serviced.
On 16V engines, there are two overhead camshafts that operate 2 intake valves and 2 exhaust valves per cylinder. The exhaust camshaft is driven by the belt and the intake camshaft is driven by a chain connecting the two camshafts. The 4-valve per cylinder design allows a central spark plug location for a more controlled and symmetrical combustion. This allows a higher compression ratio for more power and cleaner combustion with the same fuel consumption. The intake and exhaust manifolds are on opposite sides of the cylinder head to improve the engine's ability to ``breathe" over the entire rpm range.
Volkswagen's VR6 engine has a unique 15° V-angle between its cylinder banks. Traditional 60 or 90° engines require a great deal of engine compartment space. VW's compact VR6 was designed to enable it to fit into a chassis that was originally designed to accommodate a longitudinal inline 4 cylinder engine. The VR6 consists of a cast iron block with a one piece aluminum alloy cylinder head.
Aside from the obvious external features, there are several internal differences between the ECODiesel engine block and the standard 8 valve engine block it is based on. The cylinders have a smaller bore providing a total piston displacement of only 1.6 to 1.9 liters. There are also additional oil passages and spray nozzles for cooling the under side of the pistons. Even though this engine operates at a compression ratio of about 23:1, the special pistons are designed to do this using only two compression rings for reduced friction losses.
The cylinder head has spherical pre-combustion chambers made of steel set into the lower surface of the head. The fuel injector projects into the pre-chamber and combustion begins there almost at the very beginning of the injection cycle. The burning fuel/air mixture is given a swirl pattern by the chamber's shape. The swirl promotes more complete combustion as the process continues in the main combustion chamber. Using the swirl chamber has other advantages: it reduces the peak load which the force of combustion would normally exert on pistons, rods, bearings and crankshaft, enabling VW to use many standard components. Aside from the pre-chambers, the cylinder head is quite similar to the gasoline engine. The valve train is the same single overhead camshaft with hydraulic lifters