Engine and Lubrication
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Engine and lubrication
Internal combustion
Most cars are powered by a piston engine that burns a mixture of petrol and air. The mixture is burned in a combustion chamber within a cylinder above a piston.
When it burns, the mixture expands rapidly and the pressure that this exerts on the top of the piston forces it down the cylinder.
The underside of the piston is connected by a rod (the connecting rod) to a cranked shaft (crankshaft) and this arrangement allows the downward path of the piston to be transformed into rotary movement of the shaft. From the crankshaft the power is transmitted to the wheels that drive the car through the clutch (or torque convertor), gearbox and final drive.
Inlet and exhaust valves at the top of the cylinder control the entry of the petrol/air mixture and the exit of burned gases into the exhaust system. The valves are operated by eccentric lobes on a camshaft, driven from the crankshaft.
The mixture is ignited in the combustion chamber by a spark plug. The high voltage or high tension current necessary to produce a spark is generated within a separate ignition system and usually fed to each cylinder as required by a distributor, normally driven by the camshaft.
To start the engine it must be rotated. This is done electrically by a starter motor which rotates the crankshaft, usually by engaging a small pinion (or gear wheel) with the gear teeth round the outer edge of a flywheel, bolted to the end of the crankshaft. Besides providing a means of starting the engine, the flywheel smoothes out the power pulses from the pistons and allows the crank- shaft to turn relatively smoothly.
Once the starter is rotating the crankshaft, the up and down movement of the pistons sucks mixture into the cylinders, and when the ignition is switched on, combustion begins and the engine starts. In addition, the internal combustion engine will have a water- or air-cooling system, and its own lubrication system.
Basic principles: the four-stroke cycle
The internal combustion engine in most cars works on the four-stroke principle. This means that to produce one pulse of power the piston must travel up and down the cylinder four times.
Each stroke of the piston performs a separate function in the cycle as follows:
Induction stroke Begins the process. The inlet valve is open and rotation of the crankshaft is moving the piston down the cylinder, sucking in a mixture of fuel and air which travels from the carburettor, along the inlet manifold and past the open valve.
Compression stroke Both valves are shut and the rotating crankshaft now raises the piston, compressing the mixture above it into the combustion area.
Power stroke Both valves remain shut, and a spark jumping across the electrodes of the spark plug has set the mixture alight. It burns rapidly, expanding very quickly just as the piston begins its downward movement. The energy released rams the piston to the bottom of the cylinder, driving round the crankshaft half a turn.
Exhaust stroke Spent gases from the power stroke leave the combustion chamber through the open exhaust valve, helped out by the pressure created by the rising piston.
When the piston reaches the top of the cylinder at the end of this stroke, the exhaust valve will close, the inlet valve will open and the cycle begin again with another induction stroke.
During the four strokes, the crankshaft rotates twice, but since the valves only need to operate once during each cycle, the camshaft that opens them is driven at half crank- shaft speed and rotates only once every four strokes.
Compression ratio
The power that an internal combustion engine develops depends on how much energy can be released above the piston at each power stroke. This in turn depends on the quantity of fuel/air mixture in the cylinder and the efficiency with which it is compressed.
The amount that the mixture is squeezed up is referred to as the compression ratio. This is the difference between the volume of the mixture in the cylinder when the piston is at the bottom of its stroke, and the volume when the piston is at its highest position. If the upward movement of the piston reduces the mixture to one eighth its original volume, the compression ratio is 8:1.
In theory the more the mixture is compressed, the more energy it releases when it burns. In practice, however, very high compression ratios result in knocking or pinking in which some of the mixture furthest away from the spark plug explodes or detonates causing uneven burning, over- heating and loss of power. For maximum efficiency, burning of the mixture should occur rapidly but smoothly.
Valve overlap
So far we have assumed that the incoming mixture rushes past the inlet valve as soon as it opens. In practice, the mixture is slow to accelerate, and in order to fill the cylinder as completely as possible, the inlet valve is opened a little early, when the piston is near the end of the exhaust stroke, and while the exhaust valve is still open. This is called valve overlap.
It might seem that opening the inlet valve early would offer an alternative exit for the exhaust gas, but provided the amount of overlap is carefully chosen, the opposite happens and the last wisps of exhaust leaving the cylinder help drag the fresh mixture in past the inlet valve.
Once it is moving, the inlet mixture does not stop automatically when the piston reaches the bottom of the cylinder, and if the closing of the inlet valve is delayed, the cylinder fills more completely, even though by now the piston has started to rise on the compression stroke.
In practice, in order to make the most of the momentum of fresh mixture and exhaust gas flowing in and out of the cylinder, the exhaust valve opens before the piston reaches the bottom of the cylinder and closes after it has reached the top. Similarly, the inlet valve opens before the piston reaches the top of the cylinder and closes after the piston reaches the bottom.
Valve adjustment
Most four-stroke engines use two poppet valves to allow the mixture into the cylinder and let the exhaust out.
As they are in direct contact with the combustion process, both valves are made from heat resistant material.
Of the two, the inlet valve, which is cooled by the inrush of petrol/air mixture on each induction stroke, runs cooler. The exhaust valve, which in normal use runs at a red-hot 80000, is usually made of a higher temperature alloy steel than the inlet, and transfers much of its heat to the cylinder head when it is closed.
Both valves are shut by a powerful spring, usually in the form of a coil round the outside of the valve stem. The bottom of the spring rests on the cylinder head casting and the top presses against a retainer fitted to the end of the valve stem. Some valves have two coil springs, fitted one inside the other, to shut them.
A valve is opened simply by being pushed down against spring pressure, and there are several methods of doing this.
Valve adjustment
Metal expands when it is heated, and to make sure the valves are able to shut fully when the engine is hot, a small amount of play or clearance is whether the clearance should be checked with the engine hot or cold. It is important that the recommended clearances and checking systems are used, since incorrect adjustment can damage the valves.
On pushrod and rocker layouts where the camshaft is a long way from the valve, a larger clearance is needed than on a direct acting overhead camshaft system. The valve clearance is measured when the valve is fully shut by checking the slack in the operating linkage with a feeler gauge. The manufacturer will specify the clearance.
A few engines have hydraulic tappets which have two parts, one sliding within the other. Oil under pressure expands these tappets and takes up the clearances when the engine is running. On these engines, no valve adjustment is needed.
Driving the camshaft
Until the development of the internally-toothed rubber belt, most overhead camshafts were chain-driven. Because of the length of chain involved, a tensioner was needed to prevent whipping. The tensioner was in the form of either a synthetic rubber pad, spring loaded or hydraulically pressed against the side of the chain, or a spring blade or rubber-faced steel strip bearing on one side of the chain.
A toothed-belt camshaft drive is quieter than a chain and, since it needs no lubrication, it can be mounted externally. The oil resistant rubber is moulded on to non stretch cords and on the inside has a series of square section teeth that accurately match cut outs in the crankshaft and camshaft pulley wheels. The belt is usually tensioned by a jockey wheel which bears on its smooth side.
Multi-cylinder engines
A single cylinder four stroke piston engine spends three quarters of its running time exhausting burned gas, drawing in fresh mixture and compressing it.
On only one of the four strokes—the power stroke—is any energy produced and this makes the output of a single cylinder four-stroke engine very uneven.
This can be smoothed out if more cylinders, with their pistons driving a common crankshaft, are used. A twin-cylinder four-stroke, for instance, will produce one power stroke for each revolution of the crankshaft, instead of every other revolution as on a single-cylinder engine.
If the engine has four cylinders it produces one power stroke for each half-turn of the crankshaft and at no time is the crankshaft free-wheeling’ on one of the three passive strokes.
Even better results can be obtained using six cylinders, as the power strokes can be made to overlap, so that the crankshaft receives a fresh impulse before the previous power stroke has died away—on an in-line six-cylinder engine the crankshaft receives three power impulses each revolution.
In theory, the more cylinders you can use to drive the crankshaft, the smoother the power output, and 8-and 12-cylinder engines are used on some of the more expensive cars.
A large number of cylinders can pose practical problems. An engine with eight cylinders in a straight line for instance would have a very long crankshaft which would tend to twist and be more likely to break at higher engine speeds. The car would also need a long bonnet to enclose the engine.
So in the interests of crank- shaft rigidity and compactness, 8-and 12-cylinder engines have their cylinders arranged in a V, with two cylinder heads and a common crankshaft.
There are also V-6 and V-4 cylinder engines.
The other layout in popular use is where the cylinders are horizontally opposed in two flat banks, with the crankshaft between them. Its low build makes the flat engine particularly suitable for rear installation. In 4-or 6-cylinder form, the flat engine has excellent mechanical balance as movement of a piston assembly in one direction is perfectly balanced by movement of similar components operating in the opposite direction.
Checking and adjusting valve clearances
For maximum economy and performance it is important that the valve clearances are correct; most manufacturers recommend that they are checked at regular service intervals, usually every 5,000—6,000 miles. Some engines have hydraulic tap- pets which automatically pro- vide the correct setting, and these do not need checking. Some engines need special tools to check the valve clearances, and in this instance valve checking and adjustment should be left to a garage.
Hot and cold adjustment
The car manufacturer will indicate in the handbook or workshop manual whether the valve clearances are to be checked and adjusted with the engine hot or cold. For practical purposes, a cold engine is one that has stood idle overnight, whereas a hot engine is one at normal operating temperature, which is achieved after driving at least four miles. On a few engines the manufacturer will recommend a specific engine temperature for checking and adjusting valve clearances. If this is the case the job is best left to professionals.
Making adjustments
The direct-acting overhead camshaft system is the easiest to check, but can be the hardest to adjust, because on some engines the camshaft must come out and the bucket tappets lifted so that adjusting shims can be fitted underneath —a major dismantling job. To keep servicing costs down, manufacturers of these engines may specify a minimum valve clearance. Provided the gap does not go below this figure, the engine will operate satisfactorily. On most other valve mechanisms, adjustment of the clearances is made by turning an adjusting screw or nut and is straight- forward.
Inlet and exhaust clearances
Many engines have a different clearance figure quoted for inlet and exhaust valves. Since all valves look the same when viewed from the stem end, they can be distinguished by tracing the line of the two manifolds.
The inlet manifold branches will aim at the inlet valves, and the branches of the exhaust manifold will point towards the exhaust valves.
Turning the engine
Because each valve clearance must be checked with the valve in the fully closed position, it will be necessary to turn the engine to check them all. On inline engines it is possible to ‘pair’ the valves and save a lot of engine turning. On four- cylinder inline engines the ‘rule of nine’ is used. Number the valves from the generator end. Turn the engine until one valve is fully open. Subtract its number from nine, and the answer is the valve to check. For instance, when valve no.2 is fully open, 9—2=7, so valve no. 7 is the one to check. The same principle applies to six-cylinder in-line engines, using the ‘rule of 13’. The crankshaft can be turned by using a spanner on the crankshaft pulley nut.
Using a feeler gauge
If the clearances are incorrect, on most engines it is only a matter of turning an adjusting screw to put each one right. The clearance is correct when a clean feeler will slide into the gap with only moderate end pressure. If the gauge is a loose fit the gap is too wide. If the blade buckles under pressure, it is too small.