Engine  Components :Layout of valves and form of combustion

chamber  : 

• cylinder head holds intake and exhaust valves or ports ,So it have  many design according to positioning valves and form of combustion chamber.

Cylinder head design  should help the improveof Swirl or Turbulence  of the air fuel mixtureand prevents fuel droplet set onto piston surfaceor cylinder wall .

• When air fuel mixture is compressed  betweenThe piston and the flat part of the cylinder head,it produces which called  squish which meansqueezing of the gases to increase it's velocityand turbulence.

Combustion is always take place withina turbulent flow field  for two reasons :

* turbulence increases the mixing process   and enhance combustion , but at the same   time* flow instability generated by  combustion released heat through gas expansion and buoyancy ,  thus enhancing  the transition to  turbulence .

• layout of valves and form of combustion chamber  : 

In petrol engines the three most combustion chamber designs are

* Hemispherical

* Bath - tub

* Wedge

Hemispherical cylinder head design  : 

• it have the Intake valve in one side of the combustion chamber and the exhaust valve at the other, and this  produces cross flow, positioning valves in this way help the engine breath.

When fuel starts to burn at the Spark plug  the flame start to travel in all directions and this called Flame propagation .

When the plug in the middle of the hemisphere start to ignite the flame have less distance to travel than other design and this give rapid and effective combustion , so this design is common in a lot of passengervehicles.

• Bath - tub cylinder head design   : 

The bath - tub combustion chamber is over-shaped like inverted bath - tub , Valves are mounted vertically and side by side which make it simple to operate.

The plug is one side and that create a short flame path, it all help to increase turbulence.

• Combustion chamber design can affect the combustion process also, and therefore the level of emissions.

• Designs that increase gas flow rate, and promote vaporization, distribute the fuel more evenly in the combustion chamber.

• Quenching of the combustion flame can occur in zones in the combustion chamber where surface temperatures are low. The flame temperature drops so low in these areas that the flame goes out, or is quenched. Fuel left unburned in these zones is then exhausted, as hydrocarbon and carbon monoxide emissions.

• If the spark plug is positioned so that the flame front travels evenly through the combustion chamber, combustion is more complete.

• Gas flow rate, and volumetric efficiency, can be improved by using 2 intake valves in each cylinder. The effective port opening is increased, and the gas flow rate increases.

• Changing valve timing also alters the combustion process. Reducing valve overlap reduces the scavenging effect. It also reduces hydrocarbon emission.

Engine Fuels

• Petrol's octane rating is a measurement of the fuel's ability to resist engine knocking. Knock occurs when the fuel-air mix in the cylinder explodes instead of burning in a controlled way. This shockwave moves within the combustion chamber, and creates a metallic 'pinging' sound.

An octane rating is often referred to as an 'anti-knock index'. If fuel has a high octane number, it will have a higher resistance to engine knocking.

Biodiesel

• A substitute for diesel fuel, made from vegetable oil (including used cooking oil). Used in boats, buses, and large trucks.