The engine in all models is a water cooled inline 4-cylinder 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 oil pump is mounted below the crankshaft and driven by the intermediate shaft. The 16-valve engine has additional oil passages and spray nozzles for cooling the under side of the pistons. On 8-valve engines, the oil pump drive shaft includes an extension that engages the drive lugs on the distributor.
The cylinder head is lightweight aluminum alloy. The intake and exhaust manifolds are mounted on the same side of the cylinder head. The valves are opened and closed by the camshaft lobes operating on cupped cam followers which fit over the valves and springs. This design results in lighter valve train weight and fewer moving parts. The intermediate shaft and the camshaft are driven by a steel-reinforced toothed belt. 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 16-valve engines, there are two overhead camshafts that operate two intake valves and two 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.
The Quantum 5-cylinder engine is an inline engine with a single overhead camshaft and fuel injection. The engine is installed in the straight ahead manner and tilted to the right. The crankshaft runs in six main bearings, the cylinder block is made of cast iron and the cylinder head is light weight aluminum alloy. The oil pump is driven by the crankshaft, while the distributor is camshaft driven.
VW introduced the diesel engine option on 1977 Rabbit models. The key difference between the gasoline and diesel engine is that the diesel does not use a carburetor or electrical ignition system. There are no plugs, points or coil to replace. Combustion occurs when a fine mist of diesel fuel is sprayed into hot compressed air (1,650°F/899°C) under high pressure (850 psi). The air is heated by the compression as the piston moves up on the compression stroke. The diesel engine has a compression ratio of 23.5:1 compared to an average gasoline engine's compression ratio of 8.2:1.
VW's diesel block, flywheel, bearings and crankshaft are identical to those in the gasoline engine. The connecting rod wrist pins were strengthened and new pistons and cylinder head, made of aluminum for lightness, were designed.
The cylinder head has an overhead camshaft to actuate the valves and the cam is driven by a flexible toothed belt which also operates the fuel injection pump. This engine has a spherical pre-combustion chamber in which combustion begins. The burning fuel/air mixture is given a swirl pattern by the chamber's shape. The swirl promotes more complete combustion as the combustion 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.
The turbo diesel engine shares the basic design and principals of the normally aspirated diesel, however various modifications have been made to suit the special requirements of turbocharging.
Modifications include a new cylinder head alloy, as well as new materials in the valves, valve seats and swirl chambers-all of which improve heat resistance. A new cylinder head gasket is used to provide better heat resistance and sealing. The engine block has been reinforced to accept 12mm stretch type cylinder head bolts. Piston cooling jets have been installed in the block to provide a spray of oil to help cool the pistons and internal temperatures. The pistons have been modified and strengthened, while the piston rings have been redesigned to provide better sealing and wear characteristics. The surface of the crankshaft connecting rod journals have been hardened to increase torsional rigidity and the front crank pulley size has been increased to help reduce vibration.