2020-10-11
237
D. Newbold, A. Bonnick, “Automobiles”, N3, 2012
Camshafts
As we have seen, the function of the camshaft is to open the valves at the correct time in the cycle of operations of the engine. It is also used as a drive for various auxiliary units such as the distributor, fuel pump and oil pump.
The position of the camshaft can be in the cylinder block (often termed as side-mounted).
The main advantage of this arrangement is that the timing is not disturbed when the cylinder head is removed. An alternative position is on the top of the cylinder head (termed the over head cam or OHC). This has the advantage of there being a considerable reduction in components that are required to transmit the movement of the camshaft to open the valves.
The camshaft driving gear or sprocket is located on the shaft by means of a woodruff key or dowel peg to ensure correct fitting, and therefore correct timing, and to give a positive location of the driving gear.
Camshaft drives
Several methods are employed to transmit the drive from the crankshaft to the camshaft, these are chain, gear and toothed belt. The most common one in use on modern OHC engines is the toothed belt drive. This has the advantages of being silent in operation, requiring no lubrication and being fairly easy to remove and replace.
Inlet valve
This is made from high tensile alloy steels, e.g. those containing nickel, chromium and molybdenum.
Exhaust valve
This is also made from high tensile alloy steels, for example, those containing alloys of cobalt chromium and silicon chromium, silicon chromium austenitic steel, all of which resist oxidation, corrosion and wear. Under full power it can reach temperatures of around 650 °C. For extreme operating conditions the valve stem is made hollow and partly filled with sodium, which is a very soft metal having a melting point of approximately 98° C. Under running conditions it is molten, and in splashing from end to end of the valve stem it assists the transfer of heat from the hot valve head to the valve stem.
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Valve stem seals
Because a clearance is necessary between the valve stem and the guide, valve stem seals are fitted to prevent excessive oil from passing down the stem and into the combustion chamber or exhaust manifold. They are most commonly fitted on the inlet valves as this is on the suction side of the combustion chamber and the oil is more readily drawn into the cylinder. As it is burnt it causes blue smoke to be passed out of the exhaust into the atmosphere.
Valve springs
The purpose of the valve spring is to close the valve. They also prevent the valve from bouncing open at the wrong time in the engine cycle. Always fit close coils towards the valve head. The springs are made from either plain high-carbon steel or a low alloy chromium-vanadium steel.
Valve guides
These are usually made of cast iron and are a press fit in the cylinder head, although bronze is sometimes used, particularly for exhaust valves, because of its better heat conducting properties. If the cylinder head is made of cast iron then the guide will be part of the same casting.
Вариант 5
Piston
D. Newbold, A. Bonnick, “Automobiles”, N3, 2012
The main function of the piston is to provide the movable end of the cylinder, so as to convert the expansion of the burning gases on the power stroke into mechanical movement of the piston, connecting rod and crankshaft. On some types of engines the piston crown is designed to a specific shape instead of being flat. This allows for the shape of the combustion chamber to be included in the piston crown instead of in the cylinder head, and may also have an effect on the flow of gases into and out of the cylinder.
Several shapes are used in the manufacture of the lower part of the piston, called the piston skirt, e.g.
· Solid skirt used in both CI and SI high speed engines, where heavy loadings may be placed on the piston;
· Split skirt where small clearances are used to reduce piston slap when the engine is cold;
· Slipper type which is used to reduce the weight of the piston by cutting away the bottom of the non-thrust sides of the piston skirt; at the same time it reduces the area in contact with the cylinder wall, and also allows for a reduction in the overall height of the engine as BDC is now closer to the crankshaft.
When cold, the piston head is smaller in diameter than the skirt. When the engine is operating at its normal temperature the piston head expands more than the skirt due to its being closer to the very hot gases and also the fact that there is a greater volume of metal at this point.
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The piston ring seals the gap left between the piston and the cylinder wall. Made from high-grade centrifugally cast iron, it is split to enable the ring to be assembled onto the piston. Some rings may be coated on their outer edge with chromium to give better wear characteristics and longer life. Normally three rings are fitted. The top compression ring takes most of the compression pressure and forms the first defence against the heat and escaping gases. It may be stepped so that it misses the ridge that tends to form in the cylinder bore at TDC. The second is also a compression ring that completes the sealing against compression loss. The third ring is the oil control ring. It is this ring that removes the excess oil from the cylinder wall, passing it back to the sump through holes drilled in the oil control ring groove of the piston. This ring may be made up from a number of steel rails that have radiused chromium plated edges. A crimped spring fitted in the ring groove next to the piston expands the rails against the cylinder wall. This type are commonly fitted where the piston comes very close to the oil in the sump, i.e. short stroke engines, or where some wear has taken place in the cylinder bore but not enough to warrant reboring the engine and fitting new pistons and rings.
The con-rod connects the piston to the crankshaft. Its action converts the linear (straight line) movement of the piston into the rotary movement of the crankshaft. It is attached to the piston by the gudgeon pin via the little-end and to the crankshaft journal by the big-end.
They are manufactured in the shape of an ‘H’ as this gives the greatest resistance to the stresses under which it operates whilst at the same time being as light as possible.
Вариант 6
Lubrication
D. Newbold, A. Bonnick, “Automobiles”, N3, 2012
To understand how the oil does its work in the operation of the engine and other parts of the motor vehicle, we first need to understand what is meant by friction. The term friction is defined as a resistance to movement between any two surfaces in contact with each other.
In some cases friction on a vehicle is useful. The type of friction which keeps our feet from slipping when we are walking also provides the frictional grip that is required between the tyres and the surface of the road, the brake pads and the brake disc, the drive belt and the pulleys of the fan and crankshaft.
If friction occurs in the engine it can cause serious problems as it destroys the effectiveness of the engine components due to the heat generated.
This in turn causes wear and early failure of components such as bearings and their journals. It follows then that this type of friction must be reduced to a minimum to allow the engine to operate satisfactorily.
Many years ago it was found that considerable effort was required to drag or push a heavy stone along the ground. It was found to be much easier to roll the stone. It was later discovered that when the stone was put onto a raft and floated on water it was easier still to transport the same stone.
It was almost impossible to move the stone by sliding it along the ground because dry sliding friction creates a lot of resistance. This type of friction is used in the brakes and so is useful. When the stone was rolled it was found to be easier to move. Rolling friction creates a lot less resistance and therefore far less heat. This type of friction exists in the ball-androller-type wheel bearing. When the stone was placed on a raft and floated on the water it made the work lighter still. This is called fluid friction and exists in the sliding bearings under certain conditions as it does in the crankshaft bearings.
The fact that the raft floats on the water is not the most important factor. When the water comes between the raft and the bed of the river the only force resisting the movement of the raft carrying the stone is the resistance caused by one particle of water sliding over another. This resistance is a lot less than the resistance of dry friction when an object is in direct contact with the solid ground.
To summarize, less force is required to overcome rolling friction than sliding friction. However, when no lubricant is present, the same wear, heat or eventual seizure of the surfaces in contact will occur, but to a lesser degree in the case of rolling friction.
Вариант 7
Oil pump
D. Newbold, A. Bonnick, “Automobiles”, N3, 2012
The most common types of pumps used in the motor vehicle engines are the gear, rotary or vane.
Gear pump consists of two gears in a compact housing with an inlet and outlet. The gears can be either spur or helical in shape (the helical being quieter in operation). The pump drive shaft is mounted in the housing and fixed to this is the driving gear. Oil is drawn via the inlet into the pump. It passes through the pump in the spaces between the gear teeth and pump casing and out through the outlet at a faster rate than is used by the system. In this way pressure is created in the system until the maximum pressure is reached at which time the pressure-relief valve will open and release the excess pressure into the sump.
The main parts of rotary type of pump are the inner rotor, the outer rotor and the housing containing the inlet and outlet ports. The inner rotor, which has four lobes, is fixed to the end of a shaft; the shaft is mounted off-centre in the outer rotor which has five recesses corresponding to the lobes. When the inner rotor turns, its lobes slide over the corresponding recesses in the outer rotor turning it in the pump housing. At the inlet side the recess is small; as the rotor turns the recess increases in size drawing oil up from the sump into the pump. When the recess is at its largest the inlet port finishes, further movement of the rotor reveals the outlet port and the recess begins to decrease in size forcing the oil under pressure through the outlet port.
Vane-type pump takes the form of a driven rotor that is eccentrically mounted (mounted offset) inside a circular housing. The rotor is slotted and the eccentric vanes are free to slide within the slots, a pair of thrust rings ensuring that the vanes maintain a close clearance with the housing. When in operation the vanes are pressurized outwards by the centrifugal action of the rotor rotating at high speed. As the pump rotates the volume between the vanes at the inlet increases, thus drawing oil from the sump into the pump; this volume decreases as the oil reaches the outlet, pressurizing the oil and delivering it to the oil gallery. This type has the advantage of giving a continuous oil flow rather than the pulsating flow that is rather characteristic of the gear-type pump.
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As engine speed increases the oil pump produces a higher pressure than is required by the engine lubrication system. A pressure-relief valve is therefore fitted in the system to take away the excess pressure and maintain it at a level appropriate for the bearings and seals used. It will be seen then that the relief valve performs two important functions: first, it acts as a pressure regulator; and second, it acts as a safety device in the lubrication system. The main types in use are the ball valve, the plate and the plunger or poppet valve. Each is held in the closed position by a spring. As the oil pressure in the oil gallery rises above the setting for the relief valve, the valve opens against spring pressure allowing the oil to bypass the system and return back to the sump via the return outlet.
Вариант 8
Oil filter
D. Newbold, A. Bonnick, “Automobiles”, N3, 2012
When the oil passes through the engine it becomes contaminated with carbon (the byproduct of the combustion process), dust (drawn in from the atmosphere), small metal particles (from components rubbing together), water and sludge (a combination of all these impurities mixed together). All these will cause engine wear if they remain in the oil, so the engine must be equipped with a filtering system that will remove them and keep the oil as clean as possible. Most modern engines are equipped with a filtering system where all the oil is filtered before it reaches the bearings. This arrangement is called the full-flow system. There is another system also in use where only a portion of the oil passes through the filter, called the bypass filter system.
The importance of filtering the oil is shown by the results of an investigation into the wear on the cylinder and piston, using the two filtering systems. It was found that maximum wear (100 %) occurs in engines working without an oil filter.
When a bypass filter is used, wear is reduced to about 43 % on the cylinder and 73 % on the piston, which means that the life of the piston and cylinder are almost doubled. Minimum wear occurs when a full-flow filter is used, wear is again reduced by a further 15 % on the cylinder and 22 % on the piston. This means that the life of the piston and cylinder is four to five times longer than in an engine working without a filter. A good oil filter must be capable of stopping the flow of very small particles without restricting the flow of oil through the filter. To meet this requirement different materials are used as the filtering medium. Resin-impregnated paper is widely used, the paper being folded in order to make a large surface area available for the oil to flow through; particles are left on the paper and clean oil is passed to the lubrication system. In this way when the filter is changed the impurities are removed at the same time. In other types of oil filters different kinds of fibrous materials are used. The filtering material is enclosed in perforated cylinders, one outer and one inner to form a filter element.
The oil enters through the perforations in the cylinder, passes through the filtering element and leaves through the central tube outlet.
Many modern filters are now the cartridge-type which is removed complete. The advantages of this disposable type are that it cleans the oil very efficiently, it is relatively easy to change and it is less messy to remove. The filter element can also be located in a removable metal container. With the replaceable-element-type it is only the element itself that is changed, the container is thoroughly cleaned and the ‘O’ ring replaced.
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The most widely used filtering system is the full-flow filter. The construction of the filter is very efficient because all the oil is passed through the filter before it flows to the bearings.
Вариант 9
Oil filter types
D. Newbold, A. Bonnick, “Automobiles”, N3, 2012
When the oil passes through the engine it becomes contaminated with carbon (the byproduct of the combustion process), dust (drawn in from the atmosphere), small metal particles (from components rubbing together), water and sludge (a combination of all these impurities mixed together). All these will cause engine wear if they remain in the oil, so the engine must be equipped with a filtering system that will remove them and keep the oil as clean as possible.
The most widely used filtering system is the full-flow filter. The construction of the filter is very efficient because all the oil is passed through the filter before it flows to the bearings. After a certain length of time the element becomes dirty and less efficient and must therefore be changed. If the element is not changed regularly the impurities will accumulate and the element will become clogged, restricting or preventing the oil from passing through the filter. For this reason a relief valve is fitted into the filter which opens and allows the oil to bypass the clogged filter element and flow directly to the bearings unrestricted. If the condition is allowed to continue, unfiltered oil will carry abrasive particles to the bearings causing rapid wear.
In the cartridge filter the relief valve is in the filter. Many of the filters now contain a valve underneath the inlet hole which opens when oil pressure forces oil into the filter. When the engine stops and the oil flow ceases, the valve closes and the oil is kept within the filter. This prevents it from draining back into the sump. It also has the advantage of enabling the engine to develop the oil pressure more quickly when starting from cold.
Disc filter is a type of full-flow filter. It is used in large diesel engines. The oil is filtered by being forced through very narrow gaps (0.05mm) between thin steel discs which form an assembly which can be rotated. The narrow gap between the discs prevents impurities in the oil from passing through. The deposits accumulate on the outside of the discs, which are kept clean by scrapers which scrape off the deposits as the disc assembly rotates. In most cases the assembly is connected to the clutch pedal; each time the pedal is operated the disc assembly is rotated a small amount. The filter must be drained as per manufacturers’ recommendations, this being done by removing the drain plug allowing dirt and some oil to be flushed out.
Again mainly found on larger engines, centrifugal filter consists of a housing with a shaft and rotor inside. The oil is forced through the inlet ports by the pump and fills the rotor through the inlet holes in the rotor shaft, passing down the pipes to the jets. Due to the force of the oil passing through the jets the rotor rotates at very high speed. Owing to the centrifugal force, the impurities (which are heavier than the oil) accumulate on the walls of the rotor. The filter must be periodically cleaned by dismantling the filter and washing with a suitable cleaning fluid.
Вариант 10
Cooling systems
D. Newbold, A. Bonnick, “Automobiles”, N3, 2012
During combustion, when the engine is operating at full throttle, the maximum temperature reached by the burning gases may be as high as 1500–2000° C. The expansion of the gases during the power stroke lowers their temperature considerably, but during the exhaust stroke the gas temperature may still be approximately 800° C. All the engine components with which these hot gases come into contact will absorb heat from them in proportion to:
. the gas temperature;
. the area of surface exposed to the gas;
. the duration of the exposure.
For all these reasons the heat will raise the temperature of the engine components. If the temperature of the exhaust gas is above red heat it will be above the melting point of metals such as aluminium from which the pistons are made.
Unless steps are taken to reduce these temperatures a number of serious problems could arise.
4) The combustion chamber walls, piston crown, the upper end of the cylinder and the region of the exhaust port are exposed to the hottest gases and will therefore reach the highest temperatures. This will create distortion causing a leakage of gas, water or oil. It may even cause the valve to burn or the cylinder head to crack and as a consequence there will be a loss of power output.
5) The oil film will be burnt causing excessive carbon to form. The loss of lubrication of the piston and rings will cause excessive wear or the piston to seize in the cylinder.
6) Power output will be reduced because the incoming mixture will become heated so reducing its density. It may also cause detonation (this is an uncontrolled explosion in the cylinder) making it necessary to reduce the compression ratio.
7) Some part of the surface of the combustion chamber could become hot enough to ignite the incoming charge before the spark occurs (called pre-ignition) which could cause serious damage to the engine if allowed to continue.
For these reasons the engine must be provided with a system of cooling, so that it can be maintained at its most efficient practicable operating temperature. This means that the average temperature of the cylinder walls should not exceed about 250° C, whereas the actual temperature of the gases in the cylinder during combustion may reach ten times this figure. One of the other things to remember is that the engine should not be run too cool as this would reduce thermal efficiency (this is how good the engine is at converting heat into mechanical power), increase fuel consumption and oil dilution and cause wear and corrosion of the engine.
The cooling system works on the principles of heat transfer. Heat will always travel from emitted by all substances and may be reflected or absorbed by others. This ability will depend upon the colour and nature of the surface of the objects, for example, black rough ones are best for absorption of heat and light polished ones best for reflection of heat.
