Valve Timing Diagram | Valve Timing Diagram for Four-Stroke Petrol Engine | Valve Timing Diagram for Four-Stroke Diesel Engine

            We consider that the valves open and close at the dead centre positions of the piston. But, in actual practice they do not open and close instantaneously at dead centres. The valves operate some degrees before or after the dead centres. The ignition is also timed to occur a little before the top dead centre. The timings of these sequence of events can be shown graphically in terms of crank angles from dead centre position. This diagram is known as valve timing diagram.

Valve timing diagram for four-stroke petrol engine:

diagram shows the valve timing diagram for a four-stroke cycle petrol engine. The inlet valve opens 10-30° before the top dead centre position. The air-fuel mixture is suked into the engine cylinder till the inlet valve closes. The inlet valve closes 30-40° or even 60° after the bottom dead centre position. The air-fuel mixture is compressed till the spark occurs.

01 - Valve timing diagram for four-stroke petrol engine - Valve timing diagram

The spark is produced 20-40° before the t.d.c. position. This gives sufficient time for the fuel to burn. The pressure and temperature increases. The burning gases expand and force the piston to do useful work. The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-60° before the b.d.c. position. The exhaust gases are forced out of the cylinder till the exhaust valve closes. The exhaust valve closes 8-20° after the t.d.c. position. Before it closes, again the inlet valve opens 10-30° before the t.d.c. position. The period between the inlet valve opening and exhaust valve closing is known as valve overlap period. The angle between the inlet valve opening and exhaust valve closing is known as angle of valve overlap.

Valve timing diagram for four-stroke diesel engine:

The actual valve timing diagram for four-stroke diesel engine is shown in figure The inlet valve opens 10-25° before the top dead centre position. Fresh air is sucked into the engine cylinder till the inlet valve closes. The inlet valve closes 25-50° after the bottom dead centre position. The air is compressed till the fuel is injected. The fuel injection starts 5-10° before the t.d.c. position in the compression stroke. The air fuel mixture burns. The temperature and pressure increases.

01 - Valve timing diagram for four-stroke diesel engine - Valve timing diagram

The burning gases expand till the exhaust valve opens. The exhaust valve opens 30-50° before the b.d.c. position. The exhaust gases are forced out of the engine cylinder till the exhaust valve closes. The exhaust valve closes 10-15° after the t.d.c. position. Before the exhaust valve closes, again the inlet valve opens 10-25° before the t.d.c. position. The period between the inlet valve opening the exhaust valve closing is known as valve overlap period. The angle between these two events is known as angle of valve overlap.

Parts of Tyre | Structure of a Tyre

Parts of tyre

The tyre is made of many different parts that are moulded together to form the complete structure. The parts are built up, one at a time on a collapsible drum. The components of a tyre in the order of assembly are as follows:

1. Liner                              4. Tread layer.

2. Cords and plies.           5. Side walls.

3. Beads or ribs.               6. Outer rubber covering

The liner and plies together are sometime called inner casing or carcass. The assembled parts are removed from the drum and bonded together by a process called vulcanizing. In this process, the rubber is heated under pressure to give it the required form and characteristics.

The tyre details can be seen in diagram

01 - STRUCTURE OF A TYRE - PARTS OF A TYRE

Carcass : The carcass gives body and strength to the tyre. The carcass is the tyre without either side wall rubber or tread rubber.

The carcass is made from overlapping piles. Each of the plies consists of parallel cords of rayon, nylon, polyster or fiberglass, embedded in a layer of rubber. The ends of the plies are wound around the beads and bonded to the side walls. Air pressure pushing evenly against the interior surface produces tension in the carcass. Thus the carcass resists and supports the weight of the vehicle. The outer rubber covering is moulded around the carcass.

The liner is a thin air tight layer of rubber that covers the inner surface of the carcass.

The number of piles, layers of cord varies according to the cord varies according to the use of the tyre. Motor car tyres usually have 4 to 6 piles. Heavy duty truck and bus tyres may have upto 22 piles whereas earth moving vehicles use tyres which may have upto 34 piles.

01 - PARTS OF A TYRE - STRUCTURE OF A TYRE

Bead : The bead is a bundle of separate thin steel wires. The bead in the tyre fits around the wheel rim. The wires are often wrapped with a fabric to hold them. The bead is embedded in rubber which serves to prevent the tyre from stretching and thereby leaving the wheel rim.

The greater the load on the tyre, the stronger the bead must be to keep the tyre seated on the rim. The strength of bead is increased either by using a heavier gauge wire or more loops of the wire. These are made of high tensile steel.

The bead also serves as an anchorage for the layers of carcass cords. In some designs, this anchorage is reinforced by additional layers of cord in more or less narrow strips wrapped around the bead. These additional cords extend for a short distance upto the lower side wall and thereby share the severe stresses to which this clinch area around the bead is subjected to.

Tread : The tread is that part of the tyre that is designed to run on the road surface. The tread when assembled, is a separate piece of soft uncured rubber. The tread design is formed by the mould. When vulcanized, the soft rubber is changed to a compound that is highly resistant to wear.

The tread rubber is grooved with a pattern that will provide maximum friction force,and minimum noise, when used on the vehicle application for which it was designed.

In many designs, there are some circumferential ‘belt’ going around the outside of the tyre, between the carcass and the tread layer. This gives additional stability and rigidity to the tread.

Side walls : The side walls are the two strips that connect the bead to the tread. The side wall is made of a different grade of rubber. The quality of this rubber is such as to absorb road shocks, and protect the cord plies. The side walls are relatively thinner. The rubber used to form the side walls, plies and tread are porous in nature.