Diesel Engine Proper 2

B. Cylinder Head

(1) Cylinder Head Structure
The cylinder head is mounted on the top of the cylinder block with head bolts. The cylinder head makes up the combustion chamber together with the cylinder and piston. Cylinder heads for water-cooled engines are manufactured by integral casting. Ordinarily they are made of cast iron.
Cylinder heads for super charged engines are made of alloy cast iron containing nickel, chrome, molybdenum etc. because of the large thermal load. (The temperature between the valves in supercharged engines is 350- 400 degree celcius while the temperature in ordinary engines is below 350"C.)

a) Intake port (duct) This connects the intake manifold and the combustion chamber as the passage for the intake air. Since the shape and inner surface finish of this passage have a large influence on intake efficiency they are important factors related to power output. The shape is designed to minimize air-flow resistance.
Swirling is specially used with direct injection type engines to promote combustion by fully mixing the fuel and intake air. The intake port plays an important role in producing swirls.

b) Exhaust port This connects the combustion chamber and the exhaust manifold as the passage for the exhaust gas. It usually has a round section on the combustion chamber side and an angular section on the manifold side.

c) Valve seat
A valve is mounted on a valve seat. A ring-shape valve insert is used for engines that operate under severe conditions, especially for diesel engines and for light alloy cylinder heads. When it wears out, it is replaced instead of replacing the cylinder head.
Valve seats are made of heat resistant steel such as special cast iron and satellite.
The shape of a valve insert is shown in the following illustration. The valve seat angle is 45 degree against the axial center in most cases, but may be 30 degree or 60 in some cases.

d) Valve gulde
A valve guide is made of special cast iron and is press fitted into the cylinder head. The valve stem slides up and down in the valve guide. The valve guide takes heat away from the valve into the cylinder head and supports the side thrust of the valve stem.

e) Water Jacket
This is the coolant passage provided to radiate the heat generated in the combustion chamber.
In addition to the above parts, a cylinder head has lubricant passages, valve spring seats, push rod holes, rocker bracket seats, cylinder head bolt holes, and rocker cover mounting.

(2) Handling of the Cylinder Head
A cylinder head will happen the following problems:

a) The bottom surface of the cylinder head does not have the correct flatness.
When mounted on the cylinder block such a cylinder head cannot maintain the necessary air tightness. This causes gas leakage and decrease the power output. Deformation of this surface must be measured using a straight edge and a thickness gauge, as with the cylinder block. In a diesel engine the bottom surface of a cast iron cylinder head is exposed to burnt gas. The Cylinder head cooling system is designed so that the temperature of the lower face (part of the combustion chamber) will be 350- 400 degree Celsius or less where it contacts the combustion frame. Since the top and the sides of the cylinder head are exposed to the outside air, there are considerable temperature differences within the cylinder head. As a result, it cannot expand uniformly and tends to deform into a concave shape in the upward direction. This produces internal stress (thermal stress) in extreme cases. Repeated occurrence of internal stress may cause fatigue and cracking. The solid lines and broken lines in the illustration on the light show how a head can be deformed when it is tightened and when it is exposed to burnt gas pressure.
The double-dot and dash lines in the illustration show how a head can be deformed by thermal expansion.

b) A cylinder head can be cracked by careless handling or by trouble in the cooling system.

c) If a valve seat is not in close contact with the cylinder head the combustion chamber will lose its air-tightness. This causes reduced the power output.

d) Solid carbon will accumulate in the combustion chambers when low quality lubricating oil is used or when the lubricating oil gets into the combustion chamber. Since solid carbon has low heat conductivity, an engines will overheat if the carbon accumulates.

C. Cylinder Head Gasket, Head Bolts

(1) Functions of Cylinder Head Gasket
A cylinder head gasket is mounted between the cylinder block and the cylinder head to prevent gas leakage between them and to prevent the entry of air from the outside.

a) It must seal openings on the cylinder block such as the cylinders, water holes and lubricant passages, and the corresponding openings on the cylinder head at the same time.

b) It should not be corroded by high temperature, High pressure burnt gas, pressurized lubricating oil, or cooling water.

c) It must have enough strength to withstand large pressure fluctuations and strong vibrations. It must withstand rapid temperature changes.

d) It must be easy to mount and dismount. It must be reusable.

(2) Handling of the Cylinder Head Gasket a) Do not carry a cylinder head gasket by both ends. It may become curved and the asbestos in the middle may be cracked.

b) Do not damage or deform the gasket. Do not store it under a heavy object. Hang gaskets on the wall in a storage area.

c) Do not immerse the gasket in liquid, because the asbestos structure will be destroyed.

d) Mount the gasket on the correct side.

(3) Cylinder Head Bolts
Cylinder head bolts are used for tightening the cylinder head and cylinder block. Generally, the cylinder head will be deformed as shown the illustration due to tightening to the block through the gasket. This is because the top end of the liners protrude above the top of the block. This raises the pressure on the sealed part around the combustion chamber. Be sure to adjust the valve clearance after re-tightening the cylinder head bolts.

Tightening and dismounting the cylinder head
When mounting a cylinder head on a cylinder block or when dismounting it, be sure to tighten (or loosen) the bolts in the correct sequence. The cylinder head can be deformed by tightening (or loosening) the bolts in the wrong sequence.
To tighten the bolts, start at the center and move out in a radial direction. To loosen the bolts, start from the outside.
Do not tighten the bolts to the specified torque all at once. Tighten them gradually step by step. Be sure to follow any instructions about tightening the cylinder head in a cold state or warm state.

Diesel Engine Proper 1

A. Cylinder, Cylinder Block
(1) Cylinder
A combustion chamber is made up of a cylinder, a cylinder head and a piston. A cylinder has a cylindrical shape and its inner surface is perfectly finished.

The piston slides up and down between top dead center and bottom dead center within the cylinder. The cylinder receives the most complicated forces in the entire engine because of the influence of the pressure and heat generated by the burnt gas.

Cylinder classification
Cylinders can be structurally classified into the following types:
1.In block cylinder : The cylinder and the cylinder block have singgle integrated structure
2.Liner type cylinder : The cylinder is inserted into separately manufactured cylinder block
(Dry liner type & Wet liner type )

In block cylinder type
The cylinder and the cylinder block are manufactured as a solid unit. Since no cylinder liner is used with an in-block cylinder, it has fewer parts than a liner type cylinder. For this reason, this type is suitable for mass production. Currently in-block cylinders are used most widely for gasoline engines with cast iron cylinder blocks.

Dry liner type
The cylinder liner housing of the cylinder block is finished into a cylindrical diameter with a fitting tolerance from the finished dimensions of the cylinder. A separately manufactured cylinder liner is inserted into this. The cylinder liner is surrounded by the walls of cylinder block, so it never comes into direct contact with the engine cooling water.

Wet liner type
The cylindrical part of the cylinder is made up entirely of the cylinder liner. The outer surface of the wet liner comes directly in contact with the cooling water.
For this reason, this type of cylinder liner can be cooled efficiently. Wet liners are easier to manufacture and assemble than dry liners.
The upper part of the cylinder liner has a flange which is used for positioning during assembly and which prevents water from leaking from the top. The lower part of the cylinder liner has a "rubber ring" to prevent water leakage. Ordinarily the liner thickness is 6 – 8% of the inner diameter of the liner.

(2) Cylinder Block

a) Cylinder block structure
A cylinder block has the following structural parts in addition to the cylinders that generate the power:
* Water jacket : The passage for the cooling water used to cool off the heat generated by the engine ( Not needed in an air-cooled engine).
* Oil gallery : Passage for oil sucked in by the oil pump.
* Crankshaft bearings : These hold the crankshaft by the bearings.
* Camshaft bearings : These hold the camshaft (Not needed for an overhead cam type cylinder block).
* Oil pan mountings, gear train mountings, etc,

The cylinder block must have enough strength to withstand the forces generated by the
explosions (combustion) within the engine and the inertia related to high speed rotation of the crankshaft. For this reason, the upper part of the cylinder block and the crankcase usually have a mono-block structure.

b) Cylinder and cylinder block materials
Cylinders must have the following properties because they are constantly exposed to the high temperature and high pressure generated by repeated combustion explosions:
a) They must have superior abrasion resistance in order to endure the reciprocating motion of the pistons.
b) They must have high melting temperature in order to withstand the hot burnt gas.
c) They must have high strength and hardness at high temperatures.
d) They must have large oil film retention strength.
Special cast iron is the most widely used material for cylinders at present. This is because cast iron has large abrasion resistance. Special cast iron contains phosphorus, nickel, chrome or molybdenum to achieve even higher abrasion resistance or copper to raise corrosion resistance.

(3) Cylinder Characteristics
a) Cylinder capacity
The highest piston position in a cylinder is called top dead center, while the lowest position is called bottom dead center. The distance that the piston moves between these two points is called the stroke and the capacity is called the cylinder capacity (displacement). This is the maximum volume of air that can be sucked in by the descent of the piston. The engine displacement of a multi cylinder engine is obtained by multiplying the cylinder capacity by the number of cylinders. It can be calculated by the following formula:

b) Compression ratio
If the air taken into a cylinder is burnt without being compressed, not enough force will be generated to operate the engine. In order to obtain sufficient rotational power, the air must be compressed to some fraction of its original volume before it is burnt, causing explosive combustion.
Air is sucked in by the descent of the piston to bottom dead center, and the air is then compressed by the ascent of the piston to top dead center. The ratio of the volume after compression to the original volume is called
the compression ratio. The compression ratio
can be obtained by the following formula: