CHAPTER 1 6 Engine systems 6.1 Introduction mind when carrying out diagnostic work that, with very few exceptions, all engines operate on the four- The main sections in this chapter that relate to an stroke principle. The complexity lies in the systems area of the vehicle start with an explanation of the around the engine to make it operate to its maximum particular system. The sections then conclude with efficiency or best performance. With this in mind appropriate diagnostic techniques and symptom then, back to basics: The engine components are charts.
Extra tests and methods are explained where combined to use the power of expanding gas to drive necessary. When the term ‘stroke’ is used, it means the 6.2 Engine operation movement of a piston from top dead centre 6.1 Four-stroke cycle (TDC) to bottom dead centre (BDC) or the other way round. The following table explains the spark Advanced Automotive Fault Diagnosis.1 shows a modern vehicle engine. Engines ignition (SI) and compression ignition (CI) four-stroke like this can seem very complex at first but keep in cycles – for revision purposes.2 shows the SI cycle.
Key fact Stroke means the movement of a piston from top dead centre (TDC) to bottom dead centre (BDC) or the other way round. Published by Taylor & Francis. All rights reserved.2 Cylinder layouts Great improvements can be made to the performance and balance of an engine by using more than one cylinder. Once this is agreed, the actual layout of the cylinders must be considered.
The layout can be one of three possibilities as follows: XIn-line or straight – The cylinders are in a straight line. They can be vertical, inclined or horizontal. XVee – The cylinders are in two rows at a set angle.1 Ford Focus engine (Source: Ford Media) The actual angle varies but is often 60° or 90°. 131 6 Engine systems Figure 6.2 Four-stroke cycle: induction, compression, power and exhaust Stroke Spark ignition Compression ignition Induction The fuel air mixture is forced into the cylinder through Air is forced into the cylinder through the open inlet the open inlet valve because as the piston moves valve because as the piston moves down, it makes a down, it makes a lower pressure.
It is acceptable to say lower pressure. It is acceptable to say the air is drawn the mixture is drawn into the cylinder into the cylinder Compression As the piston moves back up the cylinder, the fuel As the piston moves back up the cylinder, the fuel air air mixture is compressed to about an eighth of its mixture is compressed in some engines to about a original volume because the inlet and exhaust valves sixteenth of its original volume because the inlet and are closed. This is a compression ratio of 8:1, which is exhaust valves are closed. This is a compression ratio typical for many normal engines of 16:1, which causes a large build-up of heat Power At a suitable time before TDC, a spark at the plug At a suitable time before TDC, very high pressure ignites the compressed mixture.
The mixture now atomised diesel fuel (at approximately 180 bar) is burns very quickly and the powerful expansion pushes injected into the combustion chamber. The mixture the piston back down the cylinder. Both valves are burns very quickly and the powerful expansion pushes closed the piston back down the cylinder. The valves are closed Exhaust The final stroke occurs as the piston moves back up the The final stroke occurs as the piston moves back up the cylinder and pushes the spent gases out of the now cylinder and pushes the spent gases out of the now open exhaust valve open exhaust valve XOpposed – The cylinders are in two rows opposing of these is now the most popular, as it tends to be each other and are usually horizontal.
simpler and quieter. Note in all cases that the cam By far the most common arrangement is the straight is driven at half the engine speed. This is done by the four, and this is used by all manufacturers in their ratio of teeth between the crank and cam cogs which standard family cars. Larger cars do, however, make is 1:2, for example 20 crank teeth and 40 cam teeth.
use of the ‘Vee’ configuration. The opposed layout although still used is less popular. Engine firing order Key fact is important. This means the order in which the power strokes occur.
It is important to check in the workshop A camshaft is driven at half the speed of the manual or data book when working on a particular crankshaft. XCamshaft drive gears – Gears are not used very 6.3 Camshaft drives often on petrol engines but are used on larger The engine drives the camshaft in one of three ways: diesel engines. They ensure a good positive drive gear drive, chain drive or by a drive belt. The last from the crankshaft gear to the camshaft.
132 6 Engine systems XCamshaft chain drive – Chain drive is still BMW 5 Series 2.5 525i SE used but was even more popular a few years Timing belt ago. The problems with it are that a way must be found to tension the chain and also provide A061 lubrication. XCamshaft drive belt – Camshaft drive belts have become very popular. The main reasons for this are that they are quieter, do not need lubrication and are less complicated.
They do break now and BMW−M20 then, but this is usually due to lack of servicing. Cam belts should be renewed at set intervals. Timing marks Figure 6.3 shows an example of the data available relating to camshaft drive belt fitting. This is one of the many areas where data is essential for diagnostic checks.4 Valve mechanisms Replacement intervals: at 36 000 miles A number of methods are used to operate the valves.
Three common types are shown in Figure 6.4 and a Engine setting position: basic explanation of each follows: TDC at No. 1 cylinder XOverhead valve with push rods and rockers – Special tools: The method has been used for many years and Viscous coupling tool 11.040 Pulley retaining tool 11.030 although it is not used as much now, many vehicles still on the road are described as overhead valve (OHV). As the cam turns, it moves the follower, which in turn pushes the push rod. The Torque settings: Crank pulley Tensioner 23 Nm 23 Nm 6 Fan coupling 43 Nm push rod moves the rocker, which pivots on the rocker shaft and pushes the valve open.
As the Special notes: cam moves further, it allows the spring to close the valve. The fan coupling has left hand thread. Set timing for camshaft XOverhead cam with followers – Using an and crankshaft and check TDC mark on the flywheel. Check for overhead cam (OHC) reduces the number of rotor arm alignment to distributor casing.
After fitting belt, rotate engine two full turns and recheck the timing marks. In the system shown here, the lobe of the cam acts directly on the follower Figure 6.3 Timing belt data which pivots on its adjuster and pushes the valve open. When an engine is running, the valves become XOverhead cam, direct acting and automatic very hot and therefore expand. The exhaust valve adjusters – Most new engines now use an clearance is usually larger than the inlet, because OHC with automatic adjustment.
This saves it gets hotter. Regular servicing is vital for all on repair and service time and keeps the cost components but, in particular, the valve operating to the customer lower. Systems vary between mechanism needs a good supply of clean oil at all manufacturers, some use followers and some have times. the cam acting directly on to the valve.
In each case, though, the adjustment is by oil pressure.5 Valve and ignition timing A type of plunger, which has a chamber where oil can be pumped under pressure, operates the Valve timing is important. The diagram in Figure valve. This expands the plunger and takes up any 6.5 shows accurately the degrees of rotation of the unwanted clearance. crankshaft where the inlet and exhaust valves open and close during the four-stroke cycle.
The actual Valve clearance adjustment is very important. If it position in the cycle of operation when valves open is too large, the valves will not open fully and will and close depends on many factors and will vary be noisy. If the clearance is too small, the valves slightly with different designs of engine. Some cars will not close and no compression will be possible.
now control valve timing by electronics. The diagram 133 6 Engine systems 2 3 1 2 2 1 1 Figure 6.4 Valve operating mechanisms. Left: 1 – Cam; 2 – adjusting screw in direct acting rocker. Centre: 1 – Cam; 2 – hydraulic follower.
Right: 1 – Cam; 2 – pivot and adjuster; 3 – finger follower 0° ivo evc Induction 3 Compression 1 Combustion 360° Exhaust 2 720° 1 2 540° ivo – Inlet value opens ivc – Inlet value closes ivc 180° evo evo – Exhaust value opens evc – Exhaust value closes Value lead, lag and overlap (roll over numbers) Figure 6.5 Valve timing diagrams (1 – lead; 2 – lag; 3 – overlap) is marked to show what is meant by valve lead, piston is nearly at the top of the exhaust stroke lag and overlap. Ignition timing is marked on the when the inlet valve opens (IVO). The piston diagram. Note how this changes as engine speed reaches the top and then moves down on the changes.
Just after starting the compression The valve timing diagram shows that the valves stroke, the inlet valve closes (IVC). The piston of a four-stroke engine open just before and close continues upwards and, at a point several degrees just after the particular stroke. Looking at the before TDC, the spark occurs and starts the mixture timing diagram, if you start at position IVO, the burning. 134 6 Engine systems The maximum expansion is ‘timed’ to occur after 6.2 Test equipment TDC; therefore, the piston is pushed down on its power stroke.
Before the end of this stroke, the Compression tester exhaust valve opens (EVO). Most of the exhaust With this device the spark plugs are removed and the gases now leave because of their very high pressure. tester screwed or held in to each spark plug hole in The piston pushes the rest of the spent gases out turn. The engine is cranked over by the starter and the as it moves back up the cylinder.
The exhaust valve gauge will read the compression or pressure of each closes (EVC) just after the end of this stroke and the cylinder. inlet has already opened, ready to start the cycle once Cylinder leakage tester again. A leakage tester uses compressed air to pressurise each cylinder in turn by a fitting to the spark Key fact plug hole. The cylinder under test is set to TDC Maximum expansion is ‘timed’ to occur after compression.
The percentage of air leaking out TDC on the power stroke. and where it is leaking from helps you determine the engine condition. For example, if air is leaking through the exhaust pipe, then the exhaust valves The reason for the valves opening and closing like are not sealing. If air leaks into the cooling system, this is that it makes the engine more efficient by then a leak from the cylinder to the water jacket may giving more time for the mixture to enter and the be the problem (blown head gasket is possible).
Figure spent gases to leave. The outgoing exhaust gases 6.6 shows a selection of Snap-on diagnostic gauges – in fact help to draw in the fuel air mixture from the vacuum, compression and leakage. Overall, this makes the engine have a better ‘volumetric efficiency’.