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:

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.

DIESEL KNOCK

One of the characteristics of combustion in a diesel engine is diesel knock.
Knocking in a gasoline engine and knocking in diesel engine are the same in that they occur due to a sudden pressure increase during the combustion process. However, there are a number of fundamental differences betwen the two in terms of the timing, cause, and state of knocking.

Diesel knock occurs when the combustible gas mixture produced during the ignition lag period burns explosively and the pressure rises suddenly.

Knocking in gasoline engine occurs when self ignition occurs too easilly, but diesel knock occurs when self ignition does not occurs easilly enough. Therefore, the causes of two types of knocking are completely opposite from one another.

In gasoline engine , there is large difference between normal combustion and knocking combustion. In a diesel engine, knocking combustion is hard to distinguish during operation. Therefore, knocking is distinguished from normal combustion according to whether or not a sudden pressure increase generates a hitting noise or brings about shock to engine parts.

Because of the nature of its cause, diesel knock can be prevented by shortening the ignition lag period. Fuel injection nozzles are generally designed to lower fuel injection during the ignition lag period.

COMBUSTION IN DIESEL ENGINE

The combustion process in a diesel engine is explained in detail as follows.
Fuel particles injected rom a nozle into the cylinder in the form of high pressure mist are heated by the high temperature and high pressure air. They ignite and burn when they begin to evaporate and are mixed with hot air.

The ilustration shows this process in terms of the pressure in reference to the crank angle. The combustion process can be divided into the following 4 periods.

1. Ignition Lag period.
During the period from A to B, fuel is injected from A in mist form, heated by compressed air in the cylinder and approaches the ignition temperature. Although this period is short, and the pressure does not increase suddenly, because the length o this period heavily influences combustion, it should be as short as possible. The length of this period is influenced by the ignitability o the fuel, the compression pressure and temperature of the air, and the injection state of the fuel.

2. Flame propagation period (Explosive combustion period).
This is the period from B to C in the ilustration. At point B in the ilustration the fuel prepared for combustion during the ignition lag period ignites at one or more locations in the gas mixture. This propagates very quickly to all parts, causing nearly simultaneous combustion. Fuel injected betwen B to C burns at the same time. As a result, the pressure increases suddenly. The increase in pressure is related to the quantity and the atomized state of the fuel injected during the ignition lag period. Most of the injected fuel is completely burnt by the end pf this period (C).

3. Direct combustion period.
This is the period from C to D. Fuel injection continues after point C. Fuel injection and combustion take place simultaneously because of the flames produced betwen B and C. Therefore, the pressure change between C and D can be regulated to some extent by controlling the rate of the fuel injection.

4. Post-combustion period.
Injection ends at point D in the illustration and the burnt gas expands. Any fuel that has not burnt completely burns during this period of expansion. The period after point D is called the post-combustion period.

If this period is too long, the exhaust temperature becomes too high and the thermal efficiency is lowered. Therefore, this period must be short. Combustion during this period is heavily influenced by the size and distribution of the fuel particles and their contact with the air.

Thus, combustion can be divided into four periods. The ignition lag period and the flame propagation period can be regarded as a preparatory period for the direct combustion period ; the quality of these periods gives a large influence on combustion.

Therefore, the initial injection pressure of the nozzle, the state of atomization, the compression pressure and the injection timing are important maintenance items for diesel engines.