Technical Details of 4-Cylinder 4-Stroke OHC Petrol

Technical Details of 4-Cylinder 4-Stroke OHC Petrol

Engine ฀฀ This diagram shows the components of this engine type. Examine the names and detail of these components before moving on to the next screens where they are shown individually with further information on their basic function and constructional detail.

Cylinder Block and Crankcase The engine cylinders, when cast in a single housing, are known as the engine block. Generally, the engine

block is manufactured from cast iron or

Engine block

aluminium alloy. In the latter case, cast iron or steel liners to form the cylinder bore. The engine block forms the major component of a ‘short’ motor.

Cylinder Bores ฀฀ The cylinder bores are formed via a machining process with a boring tool to give the correct form to the cylinder within closely specified tolerances. Cast iron is

a mixture of iron with a small amount of carbon (2.5% - 4.5% of the total).

Cast Iron The carbon added to the iron gives a crystalline structure that is very strong in compression. In addition, it is slightly porous and this helps to retain a film of lubricating oil Cast iron block working surfaces. This property makes cast iron particularly suitable for cylinder bores that can

be machined directly into the casting. Crankcase The crankcase is integrated into the

cylinder block and is machined in-line to form the crankshaft main bearings. This process is

Main bearing bolts known as line boring. The main bearings are split

in two halves, one half locates in the block, the other in the bearing cap. The bearing caps are secured before the machining process and thus each cap is matched in position with its opposite half. It is important to note this when disassembling and reassembling the bearings. The caps are located via dowels and fastened via high-tensile steel bolts. It is important to follow manufacturer guidance if the bolts are removed and refitted, replacement of the bolts and tightening procedures must be followed if specified.

Water Jacket Between the cylinder walls and the outside surface of the cylinder block, voids and channels are formed

during the casting process, this is known as the water jacket and is used for engine cooling purposes. A sand former creates this space during casting and when the cast block has cooled, the sand is evacuated via holes in the side of the block. These holes are then sealed using core plugs.

Oilways In order to supply pressurised oil to the engine moving surfaces, an oil gallery is formed along the length of the cylinder block. This has drillings to supply oil directly to the bearings in the block,

crankshaft and cylinder head. Additional drillings connect the oil pump and pressure control valve to complete the oil supply system. The block is prepared, drilled and threaded in order to attach additional components like the oil sump pan and oil pump assembly.

Pistons Pistons are generally manufactured from an aluminium alloy which reduces weight and increases heat dissipation. There are numerous designs to accommodate

thermal expansion according to engine type and application.

Thermal Expansion Aluminium has greater thermal expansion than cast iron used for the block and cylinder liners. This means that the piston expands more than the block as the

engine temperature increases. When the engine is cold, the working tolerances are greater to allow for expansion. The piston has design features to allow for expansion and correct tolerances at running temperatures, for example, a cold piston is slightly oval and tapered inwards towards the crown.

Piston Pin The piston or gudgeon pin has an offset by a small amount toward the thrust face of the cylinder bore which allows the thrust forces at the piston crown to

maintain the piston against the cylinder wall. This has an effect when the engine is cold by reducing piston movement due to excessive clearance which creates a noise

known as ‘piston slap’. Note that pistons are marked so that they can

be installed correctly and this should be carefully observed.

Piston and Piston Rings Around the upper portion of the piston, grooves are cut to accommodate sealing rings, known as piston rings. Generally, there are three or four

grooves and rings, the lowest is known as the oil control ring and this is used to control the amount of lubricant remaining on the cylinder bore surface to lubricate the piston. The upper rings are known as compression rings and these provide the gas tight seal, maintaining the cylinder pressures that create force to move the piston.

Piston Pin Bore This bore is machined into the piston to accept the piston pin, also

Pin in place

Pin bore

known as gudgeon pin. The fixing mechanism of the piston

Cross section of the piston

pin to the piston and the connecting rod can vary. It can be an interference fit in the connecting rod, or a push fit in both the piston and connecting rod end. If the piston pin is clamped in the connecting rod, the piston pin bore is smooth. Circlips grooves are formed in the piston pin bore when a push fit piston pin is used

Piston Crown The piston crown forms part of the combustion chamber and experiences the full cylinder pressure applied by the expanding gases. Many different design are Various pistons available depending on engine type, complex shapes can be formed in the piston crown to allow for valve movements and to create an effective combustion chamber space, promoting the correct charge motion for efficient combustion.

Piston Rings These are used to seal the combustion chamber to prevent the escape

of combustion gases and loss of cylinder pressure, they known as ‘compression rings’.

Compression and oil

In addition, the piston rings must control the control rings oil film on the cylinder bore surface, these known as ‘oil contr ol rings’. Combustion pressure is allowed to act on the back of the cylinder sealing ‘compression’ rings to help maintain a gas tight seal of the piston assembly.

Compression Rings Compression rings are manufactured from cast iron, with a surface coating to promote fast bedding in. This means that the rings quickly wear in to give a gas

tight seal against the cylinder pressures. It is important not to damage this coating during fitting, note that rings have different cross- sections according to their mounting position on the piston.

Oil Control Rings Oil control rings can be one of two designs. A multi- part ring consists of two thin alloy rings used in conjunction with an expander between them. A cast iron

ring has a groove and slot arrangement to allow oil flow back to the sump via the ring and piston.

Piston Pin (Gudgeon Pin) The piston or gudgeon pin provides the mechanical link between the piston and connecting rod. The pin locates in the piston body

and the ‘little end’ of the connecting rod. The pin can be a clearance fit into the little end bearing or bush, and hence a corresponding interference fit, or located via circlips, in the piston