Automotive
Engine Cylinder Heads

• Your cars cylinder head is made of cast iron or aluminum. On overhead valve engines, the cylinder head contains the valves, valve seats, valve guides, valve springs, rocker arm supports and a recessed area that makes up the top portion of the combustion chamber.

• On overhead cam engines, the cylinder head contains these items plus the supports for the camshaft and camshaft bearings.

• Cylinder head design is one of the most influential factors that affects the overall performance of an engine. The size and shape of the intake and exhaust ports affects velocity and volume of the mixture entering and leaving the cylinders. Cylinder head design are carefully tested and calibrated by the manufacturers to ensure optimal performance and fuel economy for the intended application.

• Large opening in the cylinder head allow coolant to pass through the head. Coolant must circulate through-out the cylinder head to remove excess heat. The coolant flows from passages in the cylinder block through the head gasket and into the cylinder head. The coolant then passes back to other parts of the cooling system.

Combustion Chamber

• Performance of an engine, its fuel efficiency, and the level of pollutants in the exhaust all depend to a large extent on the shape of the combustion chamber.

• Manufacturers have designed several different shapes of combustion chambers. To understand these you must know the definition of two terms.
• • Turbulence: Turbulence is a very rapid movement of gases. Turbulence causes better combustion because the air and fuel are mixed better.
  • Quenching: Quenching is the mixing of gases by pressing them into a thin area. The area in which gases are thinned is called the quenching area.

Wedge Chamber

• In the wedge-type, chamber, the spark plug is located at the wide part of the wedge. As the piston comes up on the compression stroke, the air/fuel mixture is squashed in the quench area.

• The quench area causes the air and fuel to be mixed thoroughly before combustion, which helps to improve the combustion efficiency of the engine.

• When the spark occurs, a flame front moves from the spark plug outward.

 

 

 

 

 

Hemispherical Chamber

• The hemispherical combustion chamber gets it name from its basic shape.      
• Hemi is defined as half, and spherical means circle.
• It is also called a Hemi-head or Hemi.
• This design has several advantages.
• • The flame path from the spark plug to the piston is short, which gives efficient burning.
  • The cross-flow arrangement of the inlet and exhaust valves allow for a relatively free flow of gases in and out of the chamber.
  • The result is that the engine can breath deeply, meaning that it can draw a large volume of mixture for the space available and give a high power output.
  • One of the more important advantages of the hemispherical combustion chamber is that air and fuel can enter the chamber very easily.

• The hemispherical combustion chamber is considered a nonturbulent-type combustion chamber. Little turbulence is produced in this type chamber. The air/fuel mixture is compressed evenly on the compression stroke. The spark plug is located directly between the valves. Combustion radiates evenly from the spark plug, completely burning the air/fuel mixture.

• The wedge combustion chamber restricts the flow of air and fuel to a certain extent. This is called shrouding. Because the valve is very close to the side of the combustion chamber, this causes the flow of air and fuel to be restricted, which reduces volumetric efficiency. Hemispherical combustion chambers do not have this restriction.  

• Some hemispherical engines use domed pistons.  This type of piston has a quench area to improve turbulence.  Several variation of this design are used today. 

Intake and Exhaust Valves

• Every cylinder of a four-stroke cycle engine contains at least one intake valve to let the air/fuel mixture enter the cylinder and one exhaust valve to let the burned exhaust gases to escape.

• The intake and exhaust valves, along with the spark plug gasket and the cylinder head gasket, must also seal the combustion chamber.

• The type of valve used in automotive engines is called a poppet valve. This is derived from the popping action of the valve as it opens and closes.

• A poppet valve has a round head with a tapered face, a stem that is used to guide the valve, and a slot that is machined at the top of the stem fore the valve spring retainer.

• The area between the valve face and the head of the valve is called the margin. The thickness of the margin helps keep the head of the valve cool.

• Most valves are make from a special hardened steel or stainless steel. The metal must be able to withstand high temperatures.

• The head of the valve is the large diameter end and is used to seal the intake or exhaust port.

• The seal is made by the valve face contacting the valve seat. The valve face is the tapered area machined on the head of the valve. To prevent damage to the valve excessive heat is tranfered from the valve face to the valve seat.

• • The valves of today's engines are highly heat resistant. Heat resistance is very critical for exhaust valves because they must withstand working temperatures of between 1,500 and 4,000o F (815 and 2,204o C).

• There are two ways for the exhaust valve to cool. First when the valve face in contact with is seat, the heat from the valve will be transferred to the cylinder head, which is liquid cooled. The second is through the valve stem to the valve guide and again to the cylinder head.

• The valve seat area must be hard enough to withstand the constant closing of the valve and supply good heat transfer.

Valve Components of Four-Stroke Engines

Valve Guides

•  Valve guides are the parts that support the valves in the head.  They are machined to a fit of a few thousandths of and inch clearance with the valve stem.

• This close clearance is important for the following reasons:

• Valve guides can be cast integrally with the head or they can be removable.

Valve Springs, Retainer, and Seals

• The valve assembly is completed by the spring, retainer, and seal.  Before the spring and the refiner fit into place a seal is placed over the valve stem.  The seal keeps oil from running down the valve stem and into the combustion chamber.

• The spring, which keeps the valve in a normallly closed position, is held in place by the retainer.  The retainer locks into the valve stem with two wedge-shaped parts that are called valve keepers.

• Some engines are equipped with mechanisms that cause the exhaust valves to rotate.  Their purpose is to keep carbon from building up between the valve face and seat.

• Rocker arms are designed to do two things: They change the direction of the cam's lift force and they provide a certain mechanical advantage during valve lifting.

• On some engines, the valve will open more than the actual lift of the cam lobe. This is done by changing the distance from the pivot point to the end of the rocker arm.  Usually rocker arm ratios rang from 1:1 to 1:1.75.  A ratio larger than 1:1 results in the valve opening farther than the actual lift of the cam lobe.

• The camshaft is part of the cylinder head assembly in all OHC-type engines.  The most common design is similar to that of the crankshaft and main bearings.  These cylinder heads are machined to accept one or two camshafts about the valves and have caps that secure the camshaft.

 Other readers found these articles interesting as well.

Operation of exhaust system components.

Automotive Engines.

Automotive Engine Lubricating Systems.

Description of the tools found in an automotive shop.