Journal of the Department of Agriculture, Journal of the Department of Agriculture, Western Australia, Series 3 Western Australia, Series 3
Number 2 March- April,1954 Article 3
The farm tractor, part.1 - how the engine works The farm tractor, part.1 - how the engine works
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Recommended Citation Recommended Citation
(1954) "The farm tractor, part.1 - how the engine works," Journal of the Department of Agriculture, Western Australia, Series 3: Vol. 3: No. 2, Article 3.
Available at: https://researchlibrary.agric.wa.gov.au/journal_agriculture3/vol3/iss2/3
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Part 1.-How the Engine Works
TVTO driver can look after his tractor properly unless he knows something of the l l way it works. The modern tractor is a sturdy machine designed to stand con- tinuous h a r d work, but it contains a certain amount of complicated mechanism.
All drivers, therefore, should know enough of the working principles of a tractor to be able to undertake the "maintenance" of the one they use.
If through neglect or ignorance a tractor does not get proper maintenance—
1. It will never be capable of under- taking its full duty.
2. Repair bills will be heavy.
3. It will break down, generally at busy times.
4. Its working life will be considerably shortened.
5. Fuel will be wasted.
In the long run t h e only complete guide to the maintenance of a tractor is the instruction book issued by the makers. The instruction book is the most valuable part of the tool-kit supplied with a new tractor.
The letterpress and illustrations in this series of articles are from a handbook prepared by the National Institute of Agricultural Engineering (England) and are reproduced by courtesy of the Institute and the British Ministry for Agriculture and Fisheries.
Any owner who h a s no instruction book for his own particular model would do well to order one. But the information given
therein may be rather too detailed and too technical for the new-comer to tractor work to grasp at all readily. Moreover, there is an instruction book for every single model, and the maintenance instructions given for one are not always a safe guide to the maintenance of another. These articles have been written, not to replace the instruction book, but to give beginners in particular, and even in some cases "old hands," an insight into the construction and use of tractors in general. Thus they may more easily understand the detailed instructions given by the makers of the tractor they have to operate.
THE INTERNAL COMBUSTION ENGINE The kind of engine used in a tractor is called an internal combustion engine be- cause it converts the heat energy of fuel into mechanical work by burning the fuel inside the engine itself.
A steam engine, for example, is not an internal combustion engine because the engine part of it—cylinder, piston and so on—is quite distinct and separate from the fire-box in which the fuel is actually burnt.
The fuel burnt in an I.e. engine can be any one of the following:—Coal or pro- ducer gas, petrol or kerosene vapour, or diesel oil in the form of a finely-atomised spray.
T h e general principle of all I.e. engines is t h e same and depends on the fact t h a t air combined in the right proportion with any one of these fuels will form an ex- plosive mixture. The various types of engine differ from one another only according to t h e fuel used, the exact a r r a n g e m e n t s for introducing and explod- ing it, a n d the number of cylinders. For tractors the most common and important type is the four-stroke engine starting on petrol and r u n n i n g on a special kind of kerosene known as vapourising oil.
Fig. 1 shows a diagram of a four-stroke I.C. engine cut across the middle, and viewed from the end. All the most import- a n t p a r t s are labelled, and will be referred to t h r o u g h o u t this book by these names.
THE SINGLE CYLINDER FOUR-STROKE ENGINE
The cycle of operations is shown in Fig. 2.
1. Induction Stroke.
(a) The inlet valve opens.
(b) The piston moves down the cylinder.
(c) Fuel and air mixture is sucked into the cylinder.
(d) When the piston reaches the bottom, the cylinder is full of explosive mixture and the inlet valve closes.
2. Compression Stroke.
(a) Both valves are closed.
(b) The piston rises.
(c) The mixture in the cylinder is com- pressed into a much smaller space.
CYLINDER HEAD VALVE
Compression Rings INLET or EXHAUST PORT
" ' XCrankshafr Timing Gear
THE 4-STROKE ENGINE
Journal of agriculture Vol. 3 1954
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Journal of agriculture Vol. 3 1954
I N D U C T I O N STROKE COMPRESSION STROKE
FIG. 2 EXHAUST STROKE
3. Firing Stroke.
(a) At the end of the compression stroke both valves are still closed.
(b) As the piston reaches the top of this stroke a spark at the sparking plug ignites the compressed mixture.
(c) The force of the explosion of t h a t mixture drives the piston down the cylinder.
4. Exhaust Stroke.
(a) The exhaust valve opens as the piston reaches the bottom of the firing stroke.
(b) The piston moves up t h e cylinder again.
(c) The burnt gases are driven out past the exhaust valve.
(d) The exhaust valve closes. \ 137
Then once more t h e inlet valve opens—
fresh mixture is drawn into t h e cylinder and t h e whole cycle starts over again.
An engine of this kind is called a four- stroke engine because, as indicated above there a r e four strokes of t h e piston for each explosion. T h e explosion stroke is the only working stroke, i.e., t h e only one in which t h e piston drives t h e crankshaft.
During t h e remaining three strokes of t h e cycle t h e crankshaft continues to turn round because of t h e momentum of t h e flywheel.
THE FOUR-CYLINDER ENGINE Smoother r u n n i n g a n d a more uniform output of power will obviously result from using several cylinders with t h e corres- ponding pistons all coupled to the same crankshaft a s indicated in Fig. 3.
T h e vast majority of tractor engines have four cylinders working on t h e above prin- ciple, i.e., t h e y have w h a t are called four- cylinder four-stroke engines. I n such a n engine one cylinder fires for each stroke:
t h a t is, two cylinders fire during each com- plete revolution of t h e crankshaft and fly- wheel. The order of firing in a four- cylinder four-stroke engine is either 1-3-4-2
or 1-2-4-3 depending on t h e make of t h e tractor. The more common is 1-2-4-3: but the firing order will usually be found marked somewhere on t h e engine. A four- cylinder engine runs smoothly and gives a fairly uniform output of power because, whatever t h e position of t h e crankshaft, one or other of t h e pistons is actually on the firing or "working" stroke.
PETROL STARTING FOR TRACTORS RUNNING ON VAPOURISING OIL Most tractors in this country use vapour- ising oil (kerosene) as fuel because it is relatively cheap. But a tractor cannot be started from cold on vapourising oil be- cause this fuel will not t u r n into vapour until it h a s been warmed up. For this reason all vapourising oil tractors are started on petrol a n d should be run on petrol until their engines a r e hot enough to vapourise t h e normal fuel properly.
OTHER KINDS OF TRACTOR ENGINE I n America petrol is cheaper t h a n it is here so it is more often used as a tractor fuel. A petrol-air mixture can be more highly compressed t h a n a vapourising oil- air mixture without giving trouble on t h e
THE 4-CYJJNDER ENGINE
Journal of agriculture Vol. 3 1954
INDUCTION STROKE COMPRESSION STROKE
FIRING STROKE FIG. 4 EXHAUST STROKE
THE DIESEL CYCLE
firing stroke: or, as we generally say, an THE DIESEL ENGINE
engine running on petrol can have a higher In some parts of the world vapourising compression ratio than an engine running oil is either expensive or difficult to obtain, on vapourising oil. With a higher com- and the only cheap fuel is diesel oil. In pression ratio more power can be obtained those places tractors with diesel engines from the same size of engine, and so, if it are popular; some tracklaying tractors of has been specially designed for the purpose, this type are also to be found in this a petrol engine will generally be more country. The diesel engine works on the powerful than a vapourising oil engine of same principle as the vapourising oil the same size. engine, but there is no sparking plug and
fuel is not introduced until t h e end of t h e compression stroke (see Fig. 4). The cycle is as follows:—
1. Induction Stroke.—The inlet valve opens—the piston goes down, drawing in pure air—the inlet valve closes.
2. Compression Stroke.—Both valves are closed—the piston comes up and com- presses t h e air in t h e cylinder. The engine h a s a high compression ratio and on this stroke t h e air is compressed so much t h a t it get hot—as it does in a bicycle pump when a tyre is being inflated.
3. Firing Stroke.—As t h e piston reaches t h e top of t h e compression stroke a spray of diesel fuel is shot into t h e cylinder t h r o u g h a nozzle called a n injector. The h e a t of t h e compressed air is sufficient to ignite t h e fuel, causing a n explosion which drives t h e piston down.
4. Exhaust Stroke.—The exhaust valve opens a n d as t h e piston comes up t h e burnt gases are driven out of t h e
cylinder. T h e exhaust valve t h e n closes a n d t h e cycle s t a r t s again by drawing in fresh air.
Most four-stroke diesel en- gines c a n be s t a r t e d from cold but their h i g h compression r a t i o makes t h e m difficult to t u r n by h a n d , a n d they often h a v e auxiliary s t a r t i n g de- vices. Thus some diesel- engined t r a c t o r s have a small petrol engine for starting t h e m , while others a r e a r - r a n g e d so t h a t they c a n start on petrol, a n d operate a s ord- inary petrol engines until they w a r m u p .
tend to spring outwards and so project slightly from t h e piston a n d press against the cylinder wall.
The connecting rod is connected to t h e piston by t h e gudgeon pin, which passes through t h e little-end bearing in t h e con- necting rod a n d is supported a t each end in t h e wall of t h e piston. The end of the connecting rod attached to t h e crankshaft is called t h e big-end bearing and is in two parts bolted together round t h e crankshaft.
The crankshaft, like t h e pedal and crank of a bicycle, t u r n s t h e up-and-down motion of t h e piston into rotary motion, which can be used to drive t h e tractor wheels. The crankshaft itself is carried in bearings known as main bearings—in a four- cylinder engine there can be two, three or five main bearings, three being most com- mon.
The valves are generally of t h e poppet or mushroom type consisting of a mushroom- shaped head on a central stem. They move
Crankshaft Timing Gear
Fan Belt Pulley
FIG. 5 THE CRANKSHAFT
This section aims a t show-
ing how t h e principles of t h e internal combustion engine are p u t to practical use in a tractor.
THE TRACTOR ENGINE
T h e piston must make a gas-tight joint with t h e inside wall of t h e cylinder and yet be free to move up and down. This gas-tight fit is obtained by three or more cast-iron rings (piston rings) which are let i n t o grooves around t h e piston. The rings
up a n d down in "sleeves" called valve guides and are held in t h e closed position by valve springs. There are two common arrangements—side valves and overhead valves as shown in Fig. 6. Practically all tractors, except t h e Fordson, have over- head valve engines. In tractors, overhead valves are operated by push rods from a camshaft a t t h e side of t h e engine.
The Camshaft.—The valves are opened and closed a t t h e correct times by means 140
Journal of agriculture Vol. 3 1954
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FIG. 6 OVERHEAD VALVE
of cams, which are merely projections from the camshaft. The setting of t h e camshaft is of great importance in the
running of the engine. On this will depend the valve timing, t h a t is, the exact moments a t which the valves open and close.
The Carburettor.—As the piston descends, a stream of air is drawn over a small hole called the jet from which it picks up some fuel and carries it on into t h e cylinder. The jet is fed from the fuel t a n k through a fuel chamber con- taining a float. If more fuel enters the float chamber t h a n the engine needs, the float will rise until the valve at the top closes the pipe from the tank. When the engine needs more fuel, the float will fall and allow more to enter the float chamber. The quantity of fuel reaching the jet is controlled by the setting of a needle valve (see Fig. 8).
ourises, i.e., t u r n s into gas.
up from the jet rapidly vap- Such a mixture of air and petrol vapour explodes easily.
Vapourising oil does not t u r n into gas unless it is heated. It leaves the jet as a very fine spray made up of small drops of fuel, which must then be heated in a vapouriser to turn them into gas.
The vapouriser is a narrow box, one side of which is h e a t - ed by the hot gases leaving the engine on each exhaust stroke. When t h e engine h a s been running for a short time on petrol the vapouriser becomes hot. If the engine is then turned on to vapourising oil, the spray of fuel from the jet will be heated and turned into a gas, which forms with air an easily exploded mix- ture (see Fig. 8).
FUEL FROM TANK
The Vapouriser. — When
petrol is used, the fuel picked
FIG. 7 THE CARBURETTOR
CHOKE VALVE EXHAUST GASES VAPORISED
FUEL A N D AIR
FIG. 8 THE VAPORISER
The Throttle is shown in Figs, 7 and 8. It is a flat disc of metal which can be set so t h a t it partially blocks the induction pipe, i.e., t h e pipe along which the fuel-air mix- t u r e flows to t h e engine. The speed and power of t h e engine are controlled by the a m o u n t of mixture t h a t the throttle allows to enter t h e cylinders. In Fig. 7 the throttle is open, in Fig. 8 it is closed.
The Choke.—The engine requires a "rich m i x t u r e " for starting, i.e., more petrol vapour in t h e mixture t h a n is needed for n o r m a l r u n n i n g . For this reason another disc, similar to t h e throttle, is placed in t h e air i n t a k e so t h a t the air has to flow past it before reaching t h e jet. Normally this disc—called the choke—is wide open, but for s t a r t i n g it is partially closed to restrict t h e flow of air and make the mix- t u r e rich in petrol. I t is shown in the two diagrams—in t h e top one it is open for n o r m a l r u n n i n g and in the bottom one it is partially closed for starting, i.e., the engine is "choked."
T h e Governor is a n automatic device which keeps t h e engine running at the same speed whether the load is heavy or light. The governor control allows the driver to alter t h e speed a t which the governor keeps t h e engine running. If t h e load on a tractor increases so t h a t the engine speed tends to fall, the governor opens t h e t h r o t t l e a n d gives the engine more fuel. When t h e load decreases so
t h a t the engine tends to speed up, the governor closes the throttle and cuts down the fuel supply.
The Magneto produces the spark a t the sparking plug which fires the mixture in the cylinders. I t is a small dynamo and induction (or
"shocking") coil combined.
When t h e end cover is taken off, the inside will look some- t h i n g like the diagram in Fig.
9. This shows the moment at which a spark is produced at the sparking plug in No. 1 cylinder. A spark is produced whenever the points of the contact breaker open, which occurs each time the end of t h e rocker a r m passes the cams on the stationary cam ring. The spark appears a t No. 1 plug because the rotor arm of the distributor is passing the end of No. 1 plug-lead. In the magneto shown, this rotor a r m is geared to revolve a t half the speed of the contact breaker, so t h a t when the points next open the rotor arm of the distributor will be opposite No. 2 pluglead. The magneto must be "timed"
so t h a t t h e points open, i.e., the spark occurs, in the right cylinder and a t the right time in relation to the position of the piston a t the end of each compression stroke.
A Sparking Plug is the means whereby the mixture is ignited in the cylinder by a spark appearing at the gap between its points. Sparking plugs must be kept clean and in good order or the spark they give will be weak. In t h a t case the mixture will not be ignited properly: starting will be difficult and power will be lost.
It is most important, therefore, t h a t plugs should be taken out periodically and cleaned thoroughly.
Not all sparking plugs are alike, and, whenever possible, the particular type of plug recommended by the makers of a tractor should be fitted.
The Impulse Starter is a device which makes tractors easier to start. When turned slowly a magneto gives a weak spark t h a t makes starting difficult, so while the engine is being turned slowly some means has to be found to increase the magneto 144
Journal of agriculture Vol. 3 1954
C O N T A C T BREAKER
— ROCKER ARM
FIG. 9 THE MAGNETO
speed. When the engine is turned the im- pulse s t a r t e r prevents the magneto from t u r n i n g with it until the piston reaches t h e top of the compression stroke. At this m o m e n t the magneto is released, and is driven round a t high speed by a spring so t h a t it delivers a "fat" spark. The spark t h u s produced by h a n d - t u r n i n g is as good as t h e spark delivered in normal running a t normal engine speed. The impulse s t a r t e r engages automatically when the engine stops and can be heard to "click"
as t h e engine is turned over by hand. As soon as t h e engine s t a r t s it disengages itself automatically. Any modern tractor can therefore be started simply by pulling up t h e s t a r t i n g handle.
The Lubrication System can be one of t h e following:—
1. Splash.—The big-ends dip into trays carrying oil in the sump, and so cover the inside of the engine with oil.
2. Pressure Feed.—Oil drawn from the sump through a screen is pumped through pipes to every part of the engine (see Fig.
3. Force Feed and Splash is a combina- tion of the above. Usually the big-ends, little-ends and piston are lubricated by splash and the rest of the engine by a pump feed.
Few tractors nowadays have splash lub- rication systems. Most of them are wholly or partly pressure fed and a gauge is fitted to show t h a t oil is circulating. This should be watched during work and the engine stopped at once if no pressure is showing on the dial. In addition, an oil Alter may be fitted to keep the oil free from con- tamination. This filter should be cleaned or replaced periodically as the makers advise in the instruction book.
The diagram (Fig. 10) shows a typical pressure-fed lubrication system. After
FEED TO OVERHEAD VALVE GEAR OIL PRESSURE GAUGE
VALVE V _ / TIMING GEARS
OILWAY DRILLED IN CRANKSHAFT
SCREEN DRAIN PLUG
FIG. 10 A FORCE FEED LUBRICATION SYSTEM
Journal of agriculture Vol. 3 1954
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Journal of agriculture Vol. 3 1954
being pumped through a filter the oil is carried by pipes to t h e various bearings, reaching the big-ends through oil-ways drilled in the crankshaft. In the drip feed to the overhead valve gear, the oil finds its way down the push rods and tappets to lubricate the cams before returning to the
H O T WATER
A I R
C O O L WATER
FIG. I I RADIATOR SECTION
sump. Lubrication of the cylinder walls and little-ends is effected by splash from the crankshaft and by oil-mist.
The Cooling System.—An internal combustion engine, with its large number of ex- plosions per minute in each cylinder, needs some cooling device to prevent damage by excessive heat. Water is used, and the usual method of cooling is by t h e t h e r m o - syphon system. In this sys- tem the water circulates be- cause hot water is lighter t h a n cold. Hot water from the parts of the engine n e a r - est to the cylinders rises and passes through a pipe to the
top of the radiator. The radiator is a series of tubes with fins, through which air is drawn by the fan. As the water is cooled in the radiator it becomes heavier, falls to the bottom and passes to the lower part of the cylinder jacket, to r e - place the warm water leaving at the top.
It is usual on tractors to have a pump or impeller at some point in the circuit to assist the circulation and improve the efficiency of the cooling.
In practice, tractors r u n - ning on vapourising oil tend to keep cool, especially on light loads. In a cool engine the fuel is not completely vapourised, with the result t h a t maximum power is not developed and u n b u r n t fuel passes the pistons to the lub- ricating oil in the sump with harmful effect. There are three main methods of con- trolling the engine temper- ature:—
1. Radiator blinds.
2. Radiator shutters.
3. A thermostat control- ling the pump circulation.
1 and 2 are most satisfac- tory when used in conjunc- tion with a thermometer, which can be read from the
C O O L I N C f l N S
THE COOLING SYSTEM 149
drivers seat. The thermometer is best raised to keep the engine warm for fitted near the top of the radiator, where starting or during cold weather^ and the water should be kept just under boil- periods of light foaSng, 7 n d l o w e r e d mg pomt. Radiator blinds can be for warmer weather or heavy Toads
P T O A N D PULLEY DRIVE
REAR AXLE HALF SHAFT DIFFERENTIAL
FIG. 13 THE TRANSMISSION SYSTEM
Journal of agriculture Vol. 3 1954
Radiator shutters are opened and closed by a hand lever from the operator's seat.
Some tractors use a valve called a t h e r - mostat which is fitted between the top of the cylinder and the radiator. When the water round the cylinders is hot enough, this valve opens and allows the water to flow to the radiator for cooling.
THE TRANSMISSION SYSTEM The transmission system of a tractor is divided into three p a r t s : —
1. The Clutch may be one of two types:—
(a) Single plate.
(b) Multiple plate.
The single plate clutch is the more com- mon in modern tractors. It consists of one plate held tightly between two surfaces by powerful springs. One of these surfaces is usually on the engine flywheel, the other is attached to the shaft which drives the gearbox and back axle. Pressing on the clutch pedal, parts these two surfaces so t h a t the plate is free, and the gearbox is no longer driven by t h e engine. Releasing the pedal allows t h e clutch springs to clamp the plate between the two surfaces, and so to connect t h e engine to the gear- box. Single plate clutches are fitted in a separate compartment in the transmission system where no oil can reach them and
are known as dry clutches. The multiple plate clutch consists of a number of plates, half which are driven by the engine and half fixed to the shaft driving the gearbox.
These plates are arranged alternately—one engine driven, then one t h a t drives the gearbox and so on. When the clutch pedal is depressed the plates are allowed to spread, so t h a t they do not touch and drive each other. In the same way when the pedal is released the clutch springs clamp the plates together, and the engine drives the gearbox. Multiple plate clutches are usually fitted in a part of the transmission to which the engine oil can penetrate and thus, running in oil, are known as wet clutches.
2. The Gearbox gives a choice of tractor speeds so t h a t the power of the engine may be used to the best advantage, and the speed chosen for each farming opera- tion. The gearbox is designed to allow gear changes only when the tractor is station- ary, and the gears should not be changed while the tractor is moving.
3. The Back Axle is the final drive from the gearbox to the rear wheels. It con- sists of two half shafts and a differential gear which enables the driving wheels to turn a t different speeds when going round corners.
(To be continued)
THE Director of Agriculture (Mr. G. K.
Baron Hay) wishes to remind all bee- keepers t h a t under the Bees Act, 1930- 1950, any person keeping bees must be registered for the current year, the period of registration ending on December 31, annually.
Persons owning hives of bees who have not previously registered are required to pay a registration fee of 2s. 6d. for any number of hives up to 25. A further fee of Is. should be paid for each additional 25 hives or lesser numbers in excess of the first 25. Persons registering for the first time as beekeepers are also required to take out a registered brand for which a further fee of 7/6 is payable.
Beekeepers who have previously regist- ered must re-register and are reminded
t h a t re-registration fell due on J a n u a r y 1, 1954.
In applying for registration or r e - registration, beekeepers should forward the full name, address, and particulars of location. Country applicants should add exchange to cheques drawn on country bank branches.
BRAND OF PHENOTHIAZINE
THE MOST EFFICIENT WORM DRENCH
INFESTATION OF YOUR SHEEP WITH EVEN SMALL NUMBERS OF PARASITES CAN REDUCE THEIR APPETITE BY 5 0 % AND CAUSE LOSS OF WOOL PRODUCTION AND GROWTH.
Eats o n l y 3 Grass d a i l y .
Eats 5 lbs.
Grass d a i l y .
Don't let worms starve y o u r sheep. Obtain your highest return from worm free sheep.
Drench w i t h ' P H E N O V I S ' .
•JL O n e of a series of advertisements d e s i g n e d to convey as a simple sfory the results of many years of research. A c k n o w l e d g - ment is made to many research workers whose results have been p u b l i s h e d in t h e Australian Veterinary Journal and elsewhere.
IMPERIAL CHEMICAL INDUSTRIES
OF AUSTRALIA & NEW ZEALAND LIMITED
ALL CAPITAL CITIES
^ B B a a M M — — ^ — P H E 16X. 4110
Please mention the "Journal of Agriculture, W.A.," when writing to advertisers
Journal of agriculture Vol. 3 1954