Cylinder block, cylinder head, and crankcase—these three parts form the foundation and main stationary body of the automobile engine. They serve as support and enclosure for moving parts. In modem engines, the cylinder block and crankcase form a single casting, which gives a rigid structure. Ribs are cast in the crankcase to give it extra strength and to support the main and, in some cases, the camshaft bearings. The cylinder block may also have a separate crankcase for the crankshaft, which is mainly confined to large engines, marine, and stationary engines. A separate aluminum crankcase would result in saving weight as well as enable cheaper and quicker replacement.
A cylinder block consists of three parts :
1. The cylinders in which the pistons slide up and down.
2. The ports or openings for the valves.
3. The passages for the flow of cooling water.
The round cylinder surfaces are given a precision mirror finish by accurate grinding and honing processes.
The cylinder block is usually made from gray cast iron, and sometimes, with the addition of nickel and chromium. Some blocks are cast from aluminum, in which cast-iron or steel sleeves are used. For most engines, cast iron has been found to be a satisfactory cylinder wall material, as it has better wearing qualities. In some small engines, the cylinder walls are plated with chromium, a very hard metal, to reduce wall wear and increase their service life. Tests are being carried out on high silicon-aluminum alloys to determine their possibilities as a material for cylinder blocks and crankcases. These alloys have a low coefficient of thermal expansion and high wear resistance.
The compositions of a cast iron cylinder are as follows-
- Iron- 95%
- Carbon – 2.2%
- Silicon- 1.2%
- Manganese- 0.63%
- Sulfur- 0.12%
- Phosphorous- 0.85%
A typical aluminum composition for cylinder and piston consists of –
- Aluminum- 91 %
- Tin- 2%
- Copper- 7%
Types and Functions-
The L-head engine blocks contain openings for the valves and valve ports. The bottom of the block supports the crankshaft and also the oil pan. On most engines, the camshaft is supported in the cylinder block by a bushing that fits into machined holes in the block. The intake and exhaust manifolds are attached to the side of the cylinder block on L-head in-line engines. On I-head engines the manifolds are attached to the cylinder head. Other parts attached to the block include the water pump (at the front), timing gear or timing chain cover (at the front), flywheel and clutch housing (at rear), ignition distribution, and fuel pump. The cylinder head is mounted on the top of the block. Some parts are attached to the cylinder block with sealing gaskets which provide a good seal to prevent leakage of water, oil, or gas. Some parts are attached with bolts and others with studs and nuts. In some places, lock washers are placed under the nuts or bolt heads.
Water-cooled engines have passages surrounding each cylinder, valve, and spark plug. The circulating water maintains the engine at its correct working temperature and avoids excessive expansion and hence distortion, ultimately preventing the seizure of relative moving parts.