A four-cylinder engine crankshaft with connecting rods and pistons 1 Tractor engines are usually four-cylinder engines but it is not uncommon for them to have six cylinders; one, two an
Trang 1UNIT 7 : TRACTOR ENGINES
Fig 1 A four-cylinder engine crankshaft with connecting rods and pistons
1 Tractor engines are usually four-cylinder engines but it is not uncommon for
them to have six cylinders; one, two and three cylinders are also used Figure 1 shows a crankshaft with pistons and connecting rods for a four-cylinder engine
2 Whether an engine is vertical, horizontal, or is single- or multi-cylinder, does not
alter the method in which it operates The basic cycle still takes place independently in each cylinder of the engine; which means that in a multi-cylinder engine, each cylinder with its piston, connecting rod and valves, etc ,should be considered as an independent mechanical unit
3 From Fig.2 it will be seen that if the flywheel is turned, the crankshaft will also
turn and this will cause the piston, which is free to move in the cylinder, to travel up and down Because the gear wheel on the crankshaft is in mesh with the gear on the camshaft, the valves will also move up and down If a thrust is applied to the top of the piston, the flywheel and crankshaft will again turn and in doing so will also operate the camshaft and valves
4 This in fact is just what happens in the engine Fuel / air mixture comes into the
cylinder when the inlet valve is open, and when the valve closes, the mixture is compressed by the piston moving up The mixture is then ignited and expands rapidly, and the expansion produces a downward thrust to the piston which in turn causes the crankshaft to rotate The thrust is sufficient to keep the crankshaft turning so that the piston also continues to move up and down The movement of these parts in relation to each other makes possible the operating cycle
The Four-stroke Cycle (Spark Ignition)
5 Engines that are designed to run on petrol or propane require an electric spark to
ignite the fuel / air mixture These are known as Spark Ignition engines
6 No 1 The induction stroke The piston moves down , inlet valve open, exhaust
valve closed This downward movement of the piston creates a partial vacuum inside the cylinder causing fuel/air mixture to rush in through the inlet valve passage When the
Trang 2piston reaches the bottom of its stroke the inlet valve closes to prevent any escape of the fuel, therefore the fuel is trapped inside the cylinder
Fig 2 A section through a single-cylinder engine showing the relative position of the
working parts.
7 No 2 The compression stroke The piston moves up, the inlet valve is closed, the
exhaust valve is closed This upward movement compresses the fuel/air mixture into a small space at the top of the cylinder This space is usually provided by having the underside of the cylinder head suitably shaped, e.g slightly concave
8 No 3 The power stroke When the piston is at the top of the compression stroke
with both valves closed, a spark is arranged to take place across the points of a spark plug which protrudes partly inside the combustion space This spark immediately sets fire to the mixture which burns and expands, and in doing so forces down the piston; this provides the power
9 No 4 The exhaust stroke The piston moves up the cylinder again, the inlet valve
is closed, the exhaust valve is open This upward movement pushes the burnt gases out through the exhaust valve outlet and to the atmosphere
10 At the end of the exhaust stroke, the cycle starts again with a fresh charge of
fuel / air mixture being taken into the cylinder
11 Of the three strokes in operating cycle, only one does the work This is the third
stroke which is the power stroke The other three strokes make the power stroke
possible, and the thrust of the power stroke keeps the crankshaft turning so that
Trang 3these strokes can take place A stroke is one movement from top dead centre to bottom dead centre or vice versa
I COMPREHENSION CHECK :
1 Look at paragraph 1 again Are these statements correct or incorrect ?
• Tractor engines never have six cylinders
• Tractor engines can have 1, 2, 3, 4, or 6 cylinders
2 Look at par 2 again Is this statement correct or incorrect ?
• The method in which an engine works depends on the number of cylinders
3 Look at par 3 again Is this statement correct or incorrect ?
• Both the pistons and the valves move up and down
4 Look at par 4 again Are these statements correct or incorrect ?
• A fuel/air mixture enters the cylinder when the valves closed
• The piston moves up and down because the crankshaft keeps turning
5 Look at par 5 again Is this statement correct or incorrect ?
• Engines which use petrol or propane are called Spark Ignition Engines
6 Look at par 6 again Is this statement correct or incorrect ?
• The engine part which stops fuel escaping is the piston
7 Look at par 7 again Is this statement correct or incorrect ?
Trang 4• The fuel/air mixture is compressed into the piston.
8 Look at par.8 again Is this statement correct or incorrect ?
• The mixture is ignited when both valves are closed
9 Look at par.9 again Is this statement correct or incorrect ?
• Burnt gases escapes because of an upward movement
10 Look at par.10 - 11 again Is this statement correct or incorrect ?
• The crankshaft is turned by all four strokes in the operating cycle
II VOCABULARY;
Listen to the lecturer He is going to tell you about the main parts of an internal combustion engine Then look at the parts below and write down the letters that correspond to the different parts
- Cylinder block - - Cylinder head
- Cylinder liner - - Piston
- Crankshaft - - Connecting rod
- Gudgeon pin - - Valves
- Camshaft - - Valve springs
-III GRAMMAR:
1 Using compound adjectives with participles for describing.
A Shorten the following sentences without changing their meaning, like this :
Weeding is a task that consumes a great deal of time.
Weeding is a time-consuming task.
1 You use a tractor which lays tracks in market gardens
Trang 52 Plants which produce fruits can be classified in many ways.
3 The pineapple is a perennial which grows at a low level
4 Hawaii is an area that produces pineapples
5 The mango is a tree which bears fruits
6 The tsetse fly is an insect which carries disease
7 The banana is a herbaceous plant which grows fast
8 Some kinds of celery branch themselves
9 Some plants are plants which pollinate themselves
10 Farmers must feed a world population which is increasing all the time
B Shorten the following sentences without changing their meaning, like this:
Lettuce is a crop which is harvested by hand.
Lettuce is a hand-harvested crop.
1 Snap bean is a crop which has large seeds
2 Celery is a crop which is propagated from seed
3 The pineapple has leaf bases which are shaped like funnels
4 The Muturu is a breed of cattle that has short horns
5 The papaya is a plant which has a single stem
6 Cucurbita is a squash which has a fine grain
7 Lettuce is an annual plant which has a swallow root
8 Some crops must be transported in cars which are cooled by ice
9 Most watermelon are of the type which has a red flesh
10 A spade is an example of a tool which is operated by hand
2 Expressions of degree: too and enough
A Rewrite the following sentences using too and make any other changes that are
necessary The first two have been done for you
(a) The soil was dry so the seed could not germinate
Or The soil was too dry for the seed to germinate.
(b) The soil was heavy and clayey and, as a result, it was unsuitable for root crops
Or The soil was too heavy and clayey to be suitable for root crops.
1 The soil particles are fine so the water cannot percolate easily through the soil
2 Because the soil was compact, it was not suitable for root crops
3 Soil aeration was inadequate and consequently the plant could not receive a proper supply of oxygen
4 As a result of the land being waterlogged, it was not possible to produce a healthy crop
5 The soil profile was so shallow that it could not give the roots sufficient anchorage
6 As the root system was poorly developed, the plant could not produce a good top growth
Trang 67 It was dark and as a result photosynthesis could not take place.
8 The current was swift so that the silt would not be deposited
B Now look at the sentences you have written and rewrite them using not +
adjective + enough, using the adjective given for each sentence.
Examples: - The soil was too dry for the seed to germinate (wet)
= The soil was not wet enough for the seed to germinate.
- The soil was too heavy and clayey to be suitable for root crops (light and
sandy)
= The soil was not light and sandy enough to be suitable for root crops.
1 coarse; 2 Loose; 3 Adequate; 4 Well-drained;
5 Deep; 6 Well-developed; 7 Bright; 8 Slow
New words and expressions:
- four-cylinder engine:
- connecting rods:
- alter (n.):
- an independent mechanical unit:
- flywheel (n.):
- be in mesh with:
- a downward thrush:
- fuel/air mixture:
- compressed (adj.):
- crankshaft (n.):
- spark ignition engine:
- create a partial vacuum:
- the induction stroke:
- the compression stroke:
- the power stroke:
- the exhaust stroke:
- spark plug:
- protrude (v.):
- combustion space:
- top/bottom dead centre:
- cylinder head:
- gudgeon pin:
- valve springs:
động cơ bốn xy-lanh thanh truyền động thay đổi
đơn vị cơ học độc lập bánh đà
ăn khớp với lực đẩy xuống hỗn hợp nhiên liệu/khí nén
trục khuỷu động cơ đánh lửa tạo ra một khoảng chân không bộ phận thì nạp nhiên liệu
thì nén thì sinh công thì xả
bu-gi đánh lửa nhô ra
buồng đốt điểm chết trên/dưới nắp xy-lanh
chốt nối van lò xo o0o
-Further reading:
Trang 7Connecting Rods and Crankshaft
The connecting rods link the pistons with the crankshaft and transmit to it the loads arising from the combustion gas pressure taken by the pistons In operation, the connecting rod is subjected to both gas pressure and inertia loads, and therefore, it must
be adequately strong and rigid and light in weight as well Connecting rods are generally fabricated from a high-quality steel in the form of a bar with ring-shaped heads at its ends, the heads being known as the connecting rod big end and small end and serving to attach the rod to the crankpin and the gudgeon pin of the piston respectively
Shank, or blade 3 of the connecting rod is provided with an I-cross section to give the rod maximum rigidity with the minimum of weight The small end of the connecting rod is made in the form of a continuous eye into which bronze bush 2 is pressed so as to provide an interference fit, whereas the big end of the rod is split into two halves with the upper half integral with the rod shank and the lower half in the form of detachable cap The crankshaft takes the downward thrusts of the pistons and connecting rods when the fuel-air mixture is burned in the cylinders and changes these thrusts into torque which
is transferred to the drive line of the automobile; it also drive various engine mechanisms and components The periodic gas pressure and inertia forces taken by the crankshaft may cause it to suffer wear and bending and torsional strains The crankshaft therefore must
be adequately strong and wear-resistant
The crankshaft is either forged from a high-quality steel or cast in a high-strength iron It consists of main bearing journals, crankpin, webs, or cheeks that connect the journals and crankpins together, a nose(front end), and a shank (rear end) Counterbalance weights 12 necessary for balancing the crankshaft are either formed integrally with, or attached separately to, the crank webs The main bearing journals and crankpins are induction hardened to improve their wear resistance Drilled diagonally through the crank webs are oil holes to supply oil to the crankpins The crankpins are bored hollow in order to reduce the crankshaft inertia The open end (or end where angular blind holes are necessary to clear counterbalance weights) are sealed by screw plug, since the hollow interior of each crankpin also acts as an oil supply duct for big end lubrication and as a centrifugal oil cleaner With the crankshaft rotating, mechanical impurities (wear products) contained in the oil inside the hollow crankpins settle in the crankpin interior walls under the action of centrifugal forces In V-type engines, each crankpin has two connecting rods assembled on it, and therefore the crankpin here are longer than in in-line cylinder engines The crankshaft front end carries one or two gears for driving the valve mechanism and also other engine mechanisms, fan drive-pulley, and
a starting crank jaw (ratchet) or bolt Mounted between the crankshaft pulley and gear is oil slinger that throw oil away from the crankshaft front bearing seal In some engines, the crankshaft gear is carried on the rear end of the shaft
o0o