Power Unit Maintenance

THE internal combustion engine, power unit of modern road vehicles, depends on petrol or oil fuel for the development of motive power. The combustion of fuel, which constitutes the energy converted into a driving force, occurs inside the combustion chamber of the cylinders. It is to this principle that the internal combustion engine owes its name, as distinct from the earlier type of power plant, the steam engine, which it has supplanted; for in the steam engine the conversion of the latent energy of fuel is effected at a point external to the engine body.

Although internal combustion engines vary considerably in design all embody the same fundamental components. All make use of a cylinder in which a piston, moving up and down, transfers movement, by a connecting rod, to a crank-. shaft mounted in bearings. The cylinder, which is sealed at the head, incorporates two valves. One valve, the Inlet, controls the entry of fuel and air mixture into the cylinder; the other valve, the Exhaust, controls the exit of gases, after the fuel has been burned. A camshaft, driven from the crankshaft, operates the two valves at intervals directly co-ordinated with the movement and position of the piston. The fuel and air mixture in the cylinder combustion chamber, which is immediately above the piston head, is ignited at a predetermined point in the cycle of piston movement. The movement of the piston is converted into a reciprocating action by means of a connecting rod, attached at its lower end to the crankshaft crankpin, by which piston movement is translated into a rotary motion of the crankshaft. A flywheel, secured to the crankshaft, is embodied to absorb surplus energy, to damp out any irregular rotation of the crankshaft and, by its momentum, to carry the piston over the top dead centre position. Before dealing further with the detailed principles of operation the reader should familiarize himself with the following nomenclature. See also Figs, 1 and 2.

Cylinder Casting

The bores of I, 2, 4, 6 or 8 cylinder castings are invariably contained in one specially moulded block of cast iron. The cylinder block also generally includes the top half of the crankcase and the upper sections of the bearing housings for the crankshaft.

Cylinder Head

This is a separate casting, often in similar material, and designed to seal off the tops of the cylinder bores. The joint made by head and block is made gas-tight by means of a special washer called a gasket. Provision is made in the design of both cylinder head and cylinder block to permit the flow of cooling liquid round the walls and heads of the cylinders. This detail of design is described in the COOLING and LUBRICATION paragraphs.


The valves are located either in the cylinder head or the cylinder block according to the design of the engine manufacturer. The valves when closed are designed to make an efficient gas-tight seal.


This operates the valves through ‘tappets’ or ‘rockers’, and is carried either in bearings in the cylinder block, or at the cylinder head, depending on the location of the valves. It is driven from the crankshaft by suitable timing gears or chain drive.

Valve Operating Mechanism

This is interposed between the cams on the camshaft and the valves, and may be in the form of tappets or, if of overhead valve design, in the form of rockers.


The crankshaft rotates freely in bearings secured in the crankcase and is operated by the reciprocating movement of the connecting rod.


These are contained within the bores of the cylinders and are provided with piston rings to prevent fuel or gas leaking past them as they move up and down the cylinders.

Connecting Rods

These are attached, at the upper ends, to the pistons by gudgeon pins and small-end bearings, and the lower ends are mounted on the crankshaft, to operate on the specially machined journals of the shaft, in anti-friction big-end bearings.


The flywheel is a heavy circular mass of metal, and is usually secured to the rear end of the crankshaft. By its rotation this heavy disk absorbs energy, and its momentum tends to keep the mechanism running smoothly between successive power strokes. It is invariably utilized as a component part of the clutch. See also the paragraphs dealing with TRANSMISSION in this section.

Oil Sump

The sump consists of a suitable receptacle bolted to the bottom of the cylinder block. It encases all the lower mechanism of the engine, and at the same time acts as a reservoir for the lubricating oil.

Oil Pump

The oil pump is generally placed inside the sump, from which it draws oil. It may be driven by the camshaft or crankshaft, and delivers oil to all the working parts by suitable pipes, oil passages, ducts or galleries.

It is essential that an oil film is maintained between all working parts, to eliminate friction and reduce wear. This requirement is an essential consideration in the initial design of the engine.

How the Engine Works

Internal combustion engines operate on a four-stroke or two-stroke cycle of operations, brief outlines of which are as follows:

Four-Stroke Cycle

The cycle or complete sequence of operations consists of tivo revolutions of the crankshaft and four strokes of the piston. This will be obvious for if the piston starts from its highest position, and the crankshaft is turned, when the crankshaft has completed one-half revolution or 1800, the piston, moving with the connecting rod, will have moved down to its lowest position, known as Bottom Dead Centre. As the crankshaft completes the first revolution of 3600 the piston will have risen to its highest position again, known as Top Dead Centre. When the crankshaft completes the second half revolution, I.e. a total of 5400, the piston will have again descended to its lowest position, and when the second revolution completes a total of 720° the piston will again have risen to its highest position. During the first downward, or Induction stroke of the piston, the camshaft mechanism opens the inlet valve so that a fuel and air mixture is drawn into the cylinder from the carburettor. As the piston begins to rise on the second, or Compression stroke, the inlet valve closes and the mixture is compressed in the cylinder by the rising piston. As the piston reaches T.D.C., and the fuel is fully compressed, an electric spark at the sparking plug, provided by either a magneto or coil and distributor, ignites the mixture of air and fuel. The sudden expansion of the burning fuel constitutes the explosion which forces the piston down on its third, or Power stroke. When the piston rises on its fourth or Exhaust stroke, the exhaust valve is opened and the expended gases are expelled from the cylinder combustion chamber. The exact phases at which the inlet and exhaust valves open and close in the cycle of operations and the point at which ignition occurs are determined in the initial stages of engine design. The four strokes of the piston are respectively referred to in the sequence in which they occur as the Induction, Compression, Power and Exhaust strokes. These four strokes of the piston complete the cycle of operations, and the sequence is repeated as long as the engine runs.

It will be seen that out of the four strokes, only on one stroke, the Power stroke, is force imparted to the crankshaft. Enough energy must be generated on this stroke, imparted to the crankshaft and stored by the flywheel, to carry the mechanism through the other three strokes of the cycle and to do any external work. This sudden delivery of power on every fourth stroke would produce heavy vibration and irregular running if the flywheel were not provided. Often, when an internal combustion engine is revolving slowly, or a load is applied too suddenly, the engine will then stop abruptly. Most motorists have experienced this occurrence when letting in the clutch without first speeding up the engine sufficiently. This is a characteristic inherent in internal combustion engines and occurs when the load imposed on the engine exceeds the energy developed by the power stroke. But it is less noticeable as the number of cylinders is increased. This will be readily understood, for with the single cylinder only one power stroke occurs in a crankshaft rotation of 7200, but in an eight cylinder engine a power stroke occurs every 900; consequently a more continuous impulse is given to the crankshaft even at low engine speeds.

Two-Stroke Cycle

Some power units, chiefly those installed in motor cycles or other small machines, operate on the two-stroke cycle. This cycle of operations is completed in one revolution of the crankshaft and two strokes of the piston, every down stroke of the piston being a power stroke. This is made possible by placing the induction and exhaust ports in such a way that the piston, during its travel up and down the cylinder, is itself employed to open and close them without the assistance of independent valves. The crankcase, which is gas-tight, is utilized as a pump for the fuel and air mixture from which it is fed into the cylinder by a transfer port. The action of the engine takes place in the following manner. As the piston moves upwards compressing the fuel and air mixture, it also creates a partial vacuum in the crankcase into which fresh fuel is drawn. When the piston has reached its highest position the fully compressed fuel and air mixture is ignited and the piston is forced downwards on its power stroke. As the piston descends the fuel and air mixture in the crank-case is slightly compressed as soon as the induction port is covered, in readiness for its transfer to the cylinder. Further descent of the piston uncovers the exhaust port allowing the burnt mixture to escape, and immediately afterwards the transfer port is uncovered. This allows the fresh fuel under partial compression in the crankcase, to pass into the cylinder and assist in the expulsion of the expended gases, the shape of the piston head being specially designed to facilitate the expulsion of exhaust gases. The rising piston then commences the compression of the fresh fuel and air mixture, and a new cycle of operations.

With two engines of equal capacity, one working on the four-stroke cycle and the other on the two-stroke cycle, one might be led to expect double power from the two-stroke considering that it has one power stroke in every two compared with the four-stroke engine’s one power stroke in every four. This, however, is not the case as the two-stroke cycle is wasteful of fuel and its efficiency is lower for reasons inherent in the design.

In the matter of cost, the two-stroke engine holds first place, for the elimination of expensive valve-operating mechanism considerably cheapens manufacture.

Of course, in addition to the bare details described, there are numerous other refinements in a modern car engine, but if the reader fully grasps the action of the four- and two-stroke cycles he will be able to follow the working of any internal combustion engine.

Compression ignition engines, or Diesels, as they are probably more widely known, also work on either the four-stroke or two-stroke cycle. On the induction phase however, air alone is drawn into the cylinder. The fuel is injected into the cylinder, when the air is under high compression, through an atomizer in the head of the combustion chamber. The high compression of air in the combustion chamber creates a high temperature sufficient to ignite the injected fuel and produce the power stroke. A fairly low grade non-volatile fuel oil is used for the running of Diesel engines. In comparison with petrol-driven engines its power output is subject to certain disadvantages from the aspect of engine weight to the horse power developed.

Power developed by an engine increases as its speed of rotation increases, but the speed range varies according to the design within which it operates most satisfactorily and economically.

The following charts which give engine data and typical causes of engine running defects are appended for quick reference in addition to the general information contained in this section.


THE frame of a vehicle is generally rectangular in shape, comprising two main side members braced together by suitable cross members. Attached to this framework are the power unit, gearbox, rear axle, front axle and steering gear, the whole assembly being referred to as the chassis. Mounted on the frame, and attached thereto by suitable fittings, is the body.

The side members are usually of channel section steel, which has been found most suitable to withstand the stresses and strains peculiar to road transport vehicles. Where necessary, the side members are strengthened by deepening the dimensions of the section. It is now becoming common practice to use a box section for the side members, as this imparts a much greater degree of rigidity to the frame.

The cross members do not conform to any standard, and are placed wherever it is most convenient in the general design for positioning the various units. They are of steel and of a cross-section that will lend itself to the mounting of the units. To resist any tendency to ‘lozenging,’ I.e. one side member being set back relative to the other side member by impact, the ‘crucification’ frame, in which two large diagonal cross members brace the two side members together, was at one time very popular. Today, however, there is a tendency to utilize the structure of the body for this purpose, the principle being referred to as ‘frameless construction’ or ‘monoshell.’

To obtain a reasonable turning circle on the steering, the distance between the side members at the front is smaller than at the rear so that the front wheels will not foul the frame when on the turn. It is also usual to upsweep the side members at the rear so as to clear the rear axle and make a low body line possible.

It is important that the frame should retain its true shape, otherwise the front axle will probably get out of alignment with the rear axle, and the steering stability and efficiency of transmission will be impaired. If the vehicle has been involved in an accident, or the truth of the frame is suspected for any other reason, tests should be made.

The orthodox frame for the motor cycle is a diamond shape constructed of tubular steel, and was originally a development of the pedal cycle frame. But so many improvements and modifications have taken place that the modern product bears little resemblance to a bicycle. In most designs, the top tube runs roughly parallel with the ground from the steering head to the saddle pillar. The front down tube running from the steering head to the bottom tube is inclined rearwards to miss the front wheel. The bottom tube, roughly parallel with the top tube, runs from the front down tube to the rear hub, where the rear down tube meets it, the bottom and rear down tubes being split into forks to accommodate the rear wheel. Another down tube from the saddle pillar runs roughly diagonally across the diamond shape to the junction where the front down and bottom tube join. This braces the whole diamond shape. Some manufacturers favour a ‘duplex’ frame, in which all the main tubes are doubled, hence the name. Pressed steel shapes have also been tried, but in all cases they conform roughly to the triangular diamond shape.

Here again the truth of the frame is essential to stability on the road, for a twisted frame will throw the axes of the two wheels and the steering head out of alinement. When a sidecar is attached, the wheels should all be :’n truly vertical and parallel planes. This should be regularly checked and rectified, if necessary.

Furthermore, when the driving chains have to be adjusted, usually by sliding the gearbox to adjust the primary chain, and by sliding the rear wheel for the drive chain, the alignment of the sprockets must be maintained. This is best done be removing the chain and lining up the faces of the sprockets with a straightedge.


Apart from contributing to the comfort of the passengers, the springing of a vehicle protects frame and body from shocks due to road irregularities. All springs have a distinct vibration rate defined by the term frequency, according to their structure, and this frequency (or periodicity) varies between 90 and 145 vibrations per minute. A vibration frequency of about 90 would give a much more smooth and comfortable ride than a short harsh spring with a frequency of 140. Unfortunately, the sensitive spring, while being very satisfactory at moderate speeds, allows too much freedom of movement at high speeds, when the car would bounce and roll and possess poor road-holding qualities. Also, with many vehicles there is a very great difference between the unladen and laden weights, and if the springs were made strong enough to carry the full load satisfactorily they would be too stiff when riding light, and would then transmit even the smallest shocks. It is therefore usual to effect a compromise between these two extremes, and to introduce further control by fitting additional devices in the form of dampers or shock absorbers. Some designers favour the use of ‘helpers’, springs which only come into effect when the main springs have deflected a certain degree under load. With this arrangement it is possible to have a sensitive light spring giving maximum riding comfort when the vehicle is running light, but when the load is applied the helper comes into play, and the required result of a strong spring is effected.

The most common type of spring fitted to both cars and lorries is the curved, laminated or leaf spring. This is built up of a number of leaves of gradually decreasing length piled on top of each other and secured to the largest or master leaf by a centre bolt. When the spring flexes—I.e. Tends to straighten out—the leaves slide over each other at their outer ends; consequently, if they are not regularly lubricated, the sliding action of the springs will become progressively harsh and heavy wear will occur. There are many forms of the leaf spring under various designs from full elliptical to quarter elliptical and cantilever types, but it is unnecessary here to go into details as they all function similarly and all require lubrication. The shackle pins or spring eye bolts, which attach the springs to the frame, also require lubrication. The full weight comes on these components and there is continual movement at these points as the springs flex.

Today, with the advent of independent suspension, each wheel is allowed independence of movement, free from any influence of the other three road wheels. Coil springs, helical springs, and torque bars are becoming more popular. These forms of spring are not subject to friction and do not therefore require lubrication.

The springing of motor cycles is usually confined to the front forks and the rider’s saddle. Coil springs, with suitable linkage, are invariably employed to spring the front wheel, and the pins in the linkage must be regularly lubri- cated. The rate of flexing is regulated by shock absorbers and these may be adjusted very efficiently to riding conditions to suit the rider, by a hand adjuster.

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