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 PERFORMANCE

Diesel Engine Performance

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Diesel Engine Combustion Chambers and Their Characteristics

The combustion chambers of diesel engines can be structurally classified as follows.

Combustion Chamber 1. Open combustion chamber

a. Direct injection type

&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp&nbsp2. Divided combustion chamber

a. Pre combustion chamber

b. Swirl chamber type

c. Air chamber type

The various types of combustion chambers which give such a large influence on engine performance are explained as follows.

(1) Direct injection Type
Direct injection type combustion chambers are illustrated on the next page. They have the Simple ststructure.

Fuel is injected into the combustion chamber between the cylinder head and the piston head,as the following illustrations show. In most cases fuel is injected at high pressure (150- 300kglcm\ using a hole nozzle (multihole nozzle) in order to burn the fuel completely.

Advantages of the direct injection type
1) Since the combustion chamber has a simple shape and relatively small surface area the heat lossis small. Therefore thermal efficiency is high and the specific fuel consumption is good (150 - 200 gr/PS-Hr). This kind of engine can start smoothly and does not require a glow plug.
2) Since this structure of ths cylinder head is simple, and trouble due to thermal strain rarely occurs.

Disadvantages of the direct injection type1) Combustion starts at a higher pressure than with other types of chamber.
2) The state of atomization has a sensitive influesce on combustion.
3) There is more noise and vibration during operation than in other types of chamber, because the combustion pressure is high.
a) The NOx (nitrogen oxides) concentration in the exhaust gas is higher than with other types of injection because the combustion temperature is high.

(2) Pre-combustion chamber typePre-combustion chamber type chambers are illustrated in the following figure. Combustion chambers of this tlpe have a pre-combustion chambers in the upper part of the main combustion chamber. Fuel is injected into the pre-combustion chamber where part of the fuel is burnt, and the remaining fuel is injected into the main combustion chamber by the pressure produced from the fuel combustion in the pre-combustion chamber. Swirling created a thorough mixture of fuel and air, which is burnt in the main combustion chamber.

Generally the volume ratio (the ratio of pre-combustion chamber volume to total compression volume) of the pre-combustion chamber is between 25% and 45%.



Advantages of the pre-combustion chamber type
1) Since the fuel is burnt in the main combustion chamber after being partially burnt in the pre-combustionc hamber, combustion is not as easily influenced by fuel quality. Diesel knock is relatively small and engine operate quietly.
2) The initial fuel injection pressure is relatively low (80 - 150 kg/cm)
and combustionis not easily influenced by the state of atomization.
3) The NOx concentration of the exhaust gas is lower than with the direct injection type.

Disadvantages of the pre combustion chamber type
1) The cylinder head structure is complicated.
2) Sincet he compressed air is throttled by a nozzle hole, the pressure increase in the pre-combustion chamber is delayed. The compression temperature in the pre-combustion chamber is lower than the direct injection type because heat is dissipated for the large area of the pre-combustion chamber. Therefore, a glow plug is needed for starting.
3) The specific fuel consumption is low.

(3) Swirl Chamber Type
The characteristic of the swirl chamber types that it has a swirl chamber on the cylinder head as illustrated in the figure below. During the compression stroke a strong swirl of air is produced in the swirl chamber. Fuel is injected into that swirl and burnt. In the pre-combution type, fuel is burnt only partially in the pre-combustion chamber, but in the swirl type, most of the fuel is burnt in the swirl chamber.

Therefore, the volume of the swirl chamber accounts for 50 - 70% of the total combustion volume. There is only one passage from the swirl chamber into the cylinder.

The characteristics of the swirl type lie somewhere between those of the direct injection type and those of the pre-combustion type.

Advantages of the swirl type1) The fuel and air are thoroughly mixed by the swirl of compressed air, so the excess air factor can be relatively low and the mean effective pressure is high.
2) Since this structure is suitable for relatively high speeds, it is advantageous from the view point of maximum output and specific fuel consumption.

Disadvantages of the swirl type
1) The structure of combustion chambers of this type is complicated because a swirl chamber must be mounted on the cylinder or cylinder head.
2) It is sensitive to the ignitability of the fuel. Diesel knock occurse asily.
3) It needs a pre-heater for starting.