Chapter 5 Structure
Diesel engine mountings include cylinder head, cylinder liner, engine block (crankcase and
cylinder block) and bed plate. They constitute the structure of the engine, which supporting engine
moving parts, ensuring the accurate location of moving parts, and forming the combustion
chamber, gas gallery, water gallery, oil gallery, to ensure the engine requirements for combustion,
ventilation, cooling, and lubrication. The other accessory parts are attached to mounting parts. The
follow is the typical mounting models.
Cylinder Head
1 Function and structural characteristics
(1) Function: covered the cylinder, and forms the combustion chamber together with the piston
and cylinder liner, it is one of the most complex parts in an engine.
(2) Structural characteristics:
(I) For mounting the injector, intake and exhaust valves and its’ driving parts.
(II) Layout the intake and exhaust passages, cooling water cavity, and bolts channels.
(3) Working conditions:
(I) Working under the high temperature and high pressure.
(II) Under these that bolt preload, compression stress, bending stress, thermal stress.
(4) Production requirements:
(I) To ensure the strength and stiffness.
(II) To ensure a good seal interface.
(5) Materials:
(I) Cast aluminium:
a Good thermal conductivity, light weight, good cast process.
b The high thermal expansion coefficient, easy distorted, high price, and used in the
small high-speed engine.
(II) Cast iron: high temperature resistance, good casting process, and cheaper.
(III) Cast steel: high tensile strength, high toughness, high-temperature strength, not prone
to cause fatigue cracks, poor technology.
2 The forms of the cylinder structure
There are various structure forms. Classified by the number of cylinders, there are separate
cylinder head that there is a cylinder head for each cylinder and single-cylinder head that serves
all cylinders. Classified by structure, there are combined cylinder head, welded cylinder head,
double bottom cylinder, bored-cooling cylinder, and so on. Several typical cylinders are
introduced below.
(1) The separate cylinder (animation): There is a cylinder head for each cylinder.
Advantages:
(I) Easy manufacture, easy to maintain and repair.
(II) Easy to seal the cylinder.
(III) Large heat expansion room, small thermal stress.
Therefore, the separate cylinders are commonly used in the large and medium diesel
engines.
Structure characteristics:
(I) There is a lobe on the bottom of the cylinder, matched with the annular grooves which
is on the cylinder linear. These two parts are sealed with copper gasket.
(II) The water cavity are divided into two tiers, the flowing direction of the cooling water:
the top plane of the engine block---five water guides---water cavity on the bottom of the
cylinder head (cooling the high temperature on the bottom)---injector cooling
cavity---clapboard---cooling cavity on the upper of the cylinder---exhaust valve
shell---main water outlet.
(III) Outlet of the water is in the highest: to prevent the surface from local overheating.
(Ⅳ) (figure shows separate cylinder head) (virtual model).
(2) Single-cylinder (animation): Linking several cylinder head as a whole is a single-cylinder.
Advantages: excellent rigidity, the short distance of the cylinder centres, convenient layout
for exhaust passages. It is commonly used in the high-speed diesel engine.
Structural characteristics:
(I) 4 valves per cylinder: two for intake and two for exhaust.
(II) There is intake manifold in the medial of the V-shaped cylinder, and exhaust
manifold in the lateral.
(III) Dealt the valve holes and valve guides with bronze, increase the wear resistance
and thermal resistance of the valve seats and guides.
(IV) Single-cylinder head of the 12V180 light diesel engine.
（3）Block-type cylinder head (animation)
Between the separate and single-cylinder head, one cylinder head can serves two cylinders
like 2, 4, 6, 12.
(4) Double bottom cylinder head
For the cylinder itself, it works under the high temperature combustion gas, uneven
temperature distribution, and large thermal stress. Especial on the bottom surface, it is prone
to cause cracks easily for the board between intake and exhaust gallery and the gas gallery
holes and the injector seat holes. If the substrate thickness is large, the temperature difference
between the internal and external surface is greater, and so is the thermal stress. If the
substrate thickness is small, the mechanical stress and distortion will be greater. Therefore,
the main purpose of adapting the double bottom is: resolving the contradiction between
mechanical load and thermal load.
Structural characteristics:
(I) The adding clapboard increase the thickness, which increase the rigidity.
(II) The thickness of bottom board can be thinned, which result in the reduction of
temperature difference and thermal stress.
(III) Under the condition of ensuring the quality of casting, makes the clearance between
bottom board and clapboard as small as possible.
(IV) (Figure shows the double bottom cylinder head)
(5) Large low-speed two-stroke diesel engine cylinder head (animation)
It is divided into two parts:
Upper cylinder head: Casting steel, a small thickness, reduces the thermal stress.
Lower cylinder head: Casting iron, a large thickness, reduces the mechanical stress.
(6) L6 cylinder head (virtual model) and cylinder cover (virtual model)
Cylinder Liner
1 function:
(1) Constitute the work cycle space.
(2) As the guiding hand for the piston, and there are slider plate and guide board for the
crosshead.
(3) Transfer heat to environment.
(4) For the two-stoke engine, there is a scavenging port.
2 Working conditions (very bad)
(1) The inner surface stand up the repeatedly high temperature and high pressure of the
combustion gas (about 2273K).
(2) It is blow by the inlet air which about 373K in the intake stroke.
(3) The outer surface is corroded and eroded by coolant.
(4) Stands up the gas pressure, bolt preload, mechanical stress, and temperature difference
between inner and outer face (thermal fluctuation, fatigue damage).
(5) Stands up the lateral thrust, moves with a relatively high speed to piston, which cause
frication and wear.
3 Requirements:
(1) Strength: Mechanical strength and thermal strength, mechanical load and thermal load.
(2) Stiffness: Do not have great distortion when installation and work.
(3) The inner surface has a high accuracy and finish, good resistant to abrasion and corrosion.
(4) The outer surface has good resistance to corrosion and anti-cavitations of coolant.
(5) Have reliable seal to the cylinder gas and outer surface cooling water.
4 Structure of the cylinder liner: (virtual model)
(1) The wet and dry cylinder liners: Installed in difference ways.
In comparison:
Wet: (I) Good for cooling.
(II) Thick board, convenient to manufacture and replacement (for marine diesel).
(III) Having the corrosion and cavitations of coolant.
Dry: (I) No corrosion and cavitations of coolant.
(II) Thin board, distance between cylinders centre be reduced, compact structure, low
stiffness.
(III) The cylinder cylindrical must be grinded and the hole in the cylinder body must
be honed.
(2) Location (figure)
Axial position: The low end is not fixed, and it can elongate under heating.
Radical position: Flange prevents it from lateral moving.
(3) Seal: Place a flexible gasket between the cylinder head and cylinder liner (legend).
(I) Gas-tight seal (legend)
The material: Aluminium, asbestos covered with copper, mild steel.
Corresponding cylinder head, there are main two types of cylinder gasket: en-bloc type
and single-ring type.
(II) Water-tight seal (legend)
The temperature is low in the lower of the cylinder liner; it can be sealed with elastic
ring.
(4) Bored cooling cylinder liner (animation)
(I)
The upper shoulder is high and thick, the hole forms an angle with centreline, and
have a good cooling effect.
(II)
The strength and the stiffness are increased, but the processing cost also increased.
(5) The large low-speed two-stroke diesel engine cylinder (animation) [Virtual model]
(I)
The top ring
Good flexibility, so it not have a big thermal stress when outward expansion.
Greatly reduce the height of the cylinder, and reduce weight.
(II)
Protected ring
It is made of heat-resistant steel material. The cylinder liner don’t contact with
high temperature gas directly, which increases the heat-corrosion resistance, and
enhance the strength.
Engine Block and Bed Plate
Classification: According to the structure of the main bearing.
（a） normal-seat main bearing type;
（b） Upside-down-seat main bearing type;
（c） Tunnel-seat main bearing type.
(1) Home-seat main bearing type
The Bed plate withstands the gas pressure transferred from crank link mechanism, and has a
great strength and stiffness. Between the two sides of the seat and the seat and main bearing,
solid skeletons and rebar are set up. The great weight and size is suitable to the
medium-speed and low-speed diesel engine that size and weight requirements are not strict
and long life required. It is well that the crankshaft centreline under the subdivision surface.
(Legend TM410 engine block and seat) and (Legend GMTA500 engine block and seat)
(2) Saving-seat main bearing type
No bed plate, only block, having a oil pan, used to medium speed and high speed diesel
engine. (Legend light 12V180 diesel crankcase) and (Legend MWM360 engine block and
seat)
(3) Tunnel-seat main bearing type
Casting the cylinder block and crankcase as whole; there is no subdivision surface in main
bearing; disk roller bearing. Greater magnitude of interference and weight; Complicated
technology; inconvenient in maintenance and assemblage. (Legend 12V180 tunnel engine)
and (Physical photo)
(4) Structure of crosshead diesel engine
Main bearing bolt upward support the frame, downward suppresses the bearing cap and
bearing shell, suppressing the bearing cap on main bearing seat tightly. (Legend)
(5) PA6B engine block (Virtual)