Fullscreen HTML5Med Res (??MB)HTML4

Pt.1: when to fire the sparkplug Automotive Ignition Timing Firing the spark at the right moment is critical when it comes to obtaining good power, emissions and economy from an engine. Here’s a rundown on the basics of ignition timing. By JULIAN EDGAR The advent of programmable engine management has meant that people outside major engine manufacturing companies are devising complete ignition advance angle maps for the first time. While the old points-and-weights system may have been modified from time to time, any changes made were always based on the starting point provided by the car’s manufacturer. However, when program­ mable engine management systems can have ignition advance curves of literally any shape programmed into them, a whole new approach needs to be taken. This month, we examine the factors which influence ignition timing and also look at the traditional spark timing mechanisms. Next month, we will examine the ignition control provided by fully programmable engine management systems and look at the timing maps devised for some specific engines. While it is obvious that more air and fuel is required at higher engine loads, the same clarity of understanding does not apply to the ignition timing requirements. However, considering the number of factors that affect ignition timing, this isn’t surprising. The requirement for adjustable ignition timing is based mainly on one factor – the finite time taken for the fuel/air charge to be ignited and burned. In practice, the period between the spark firing and the complete combustion of the fuel/air mix is very short – about two milliseconds (2ms) on average. However, while this burn time is small, it is The timing of the ignition sparks during an engine’s cycle must be continually adjusted in order to obtain correct combustion behaviour. This, in turn, is necessary for achieving peak power and obtaining low exhaust emissions. Over-advanced ignition timing can cause knocking (or detonation) in the engine. This can destroy the elec­trodes of the spark plugs (see above) and lead to major engine damage. 4  Silicon Chip TDC (TOP DEAD CENTRE) Z 90° Fig.1: when advanced ignition timing is used, the spark plug is fired before the piston reaches top dead centre (TDC). However, because the com­bustion takes a finite time, the cylinder pressure peaks after the piston has actually just passed TDC. (Bosch). still sufficiently long enough to have an impact on when the spark should occur for best performance. In practice, the ignition must be timed so that the peak pressure caused by the explosion occurs just after the piston has passed top dead centre (TDC), and so is on its way back down the cylinder bore. If ignition occurs too early, then the piston will be slowed in its upward movement. Conversely, if it occurs too late, then the piston will be well down the cylinder and so the work done on it will be reduced. The timing of the ignition is normally expressed in crank­shaft degrees before TDC. For example, if the spark is fired when the crankshaft is 15 degrees before TDC, then the spark timing is referred to as “15 degrees advanced”. The greater the ignition advance angle, the earlier the spark is fired while the piston is still heading upwards. Note that in some situations (for example, emissions control), it is beneficial to retard the timing so much that the spark is actually fired after the piston has passed TDC. PRESSURE IN COMBUSTION CHAMBER BAR CRANKSHAFT ANGLE TDC 90° 180° 270° 0° BDC 360° 40 A 20 Z 10 180° B 90° 0° 90° 180° BEFORE TDC AFTER TDC ADVANCE ANGLE Fig.2: combustion cham­ber pressure during the compression and power strokes of the engine. The “A” curve indicates the combined compression and combustion pressure, while “B” shows the compression pressure only. “Z” is the point of ignition. (Bosch). BAR COMBUSTION PRESSURE PISTON If the composition of the mixture was constant (and it isn’t), then the elapsed time between ignition and full combus­tion would remain about the same at all rpm. As a result, if the ignition advance angle was set to a fixed angle before TDC, the combustion process would be shifted further and further into the combustion stroke as the engine speed increased. This is because the piston moves faster at higher engine speeds and thus would be further down the bore by the time combustion actually occurred. To prevent this from happening, the ignition advance must be progressively increased as engine speed rises (ie, the plug must be fired earlier in the ignition cycle). The other major factor affecting the amount of advance required is the engine load. As cylinder pressures and the air/fuel ratio decline (ie, the mixture becomes richer), the speed of combustion increases, meaning that ignition should be retarded. Conversely, even more advance than that dictated solely by the engine speed is needed at low loads where lean mixtures are used. If only it were that simple! Not only does engine speed and load determine the best timing for the combustion of the mixture, but the following factors are also relevant: (1). the design and size of the combustion chamber; (2). the position of the ignition spark(s) in the chamber; (3). the fuel type; (4). the emissions levels required; (5). the engine coolant temperature; & (6). the safety margin required before knocking occurs. The latter point is of vital importance. Knocking occurs in an engine when the pressure and tem­perature rises very rapidly due to an over-advanced ignition timing (possibly exacerbated by glowing coke deposits in the cylinder head). Instead of the flame front propagating at about 34 me­tres/ second, it moves at about 10 times this pace, causing a metallic pinging noise to be emitted from the engine. The noise is of little importance; what is a cause for concern is the damage that this hammer-blow can do to the internal engine components. Broken pistons, smashed heads and shattered sparkplugs can all occur in a just a few seconds. This problem must be particularly 50 BEFORE TDC AFTER TDC 40 2 30 1 20 10 Zb 0 75° 50° Za Zc 3 25° 0° -25° -50° IGNITION ADVANCE ANGLE -75° Fig.3: if the spark occurs too early (at Zb), then combustion knock (line 2) will occur. Line 1 shows normal combustion be­haviour, the result of ignition at Za. A spark fired too late in the cycle at Zc will result in low combustion pressure as shown by line 3. (Bosch). NOX EMISSIONS SPARKPLUG 20 50° BEFORE TDC 16 40° 12 30° 8 20° 4 0 0.7 0.8 0.9 1.0 1.1 EXCESS AIR FACTOR 1.2 1.3 Fig.4: not only is power output affected by ignition timing but also exhaust emissions and fuel economy. The affect of timing on the emission of oxides of nitrogen with different air/ fuel ratios is shown here. (Bosch). guarded against in en­ gines using forced induction (ie, supercharging or turbocharging). In this type of engine, September 1995  5 MANUAL LINKAGE CAM SLOT CENTRIFUGAL FORCE RETARD ADVANCE NO ADVANCE AT IDLE CENTRIFUGAL FORCE CAM ROTATION CLOCKWISE FULL ADVANCE AT HIGH ENGINE SPEED Fig.5: early vehicles used a fully-manual advance/retard mechan­ism, in which the breaker plate was rotated by the driver by means of a dash-mounted lever. This type of system could provide good control but only if the driver was interested! the burn occurs very quickly and so knocking can easily occur. To counter this, the ignition advance angle is retarded during boost periods. Traditional mechanisms The first adjustable ignition timing mechanism was a fully-manual advance system. In this system, a dash-mounted (or steering wheel-mounted) lever was used to rotate the dis­tributor plate. By moving Fig.7: the centrifugal timing mechanism increases spark advance as the engine speed increases. This is achieved by the action of weights and springs attached to the breaker shaft. As the engine speed increases the weights swing out, causing the shaft to shift position. this lever back and forth, the points could be made to open earlier or later in the cycle. A lever scale allowed the driver to gauge the degree of adjustment. In practice, the timing was normally retarded for starting and then advanced for running. However, although this approach meant that the timing could be fully controlled (with a sensitive driver), its efficiency depended so much on the indi­vidual that it was BREAKER POINTS CAM ROTATION VACUUM DIAPHRAGM LEVER BREAKER PLATE ROTATION Fig.6: the mechanical spark timing system used until quite re­cently combines both vacuum and centrifugal advance mechanisms. At times of low load, the manifold vacuum is high and a vacuum diaphragm is used to advance the spark. Conversely, high engine loads result in low vacuum and a relatively retarded spark. 6  Silicon Chip soon abandoned. The next step saw the introduction of an ignition timing mechanism which was to last for the next 80-odd years. This system took into account both the engine speed and load using centrifugal advance and vacuum advance mechanisms. Centrifugal advance was used to change the ignition timing on the basis of the speed of the engine, via a series of spring-loaded weights attached to the breaker shaft within the distribu­ tor. As the engine speed increases the weights swing out, causing the shaft to shift position. In turn, this causes the points to open sooner. Conversely, when the engine slows, the spring-loaded weights retract, allowing the breaker shaft to shift back in the other direction and thus retard the spark. At the same time, the vacuum advance mechanism is used to adjust the spark timing to suit the engine load. Vacuum advance takes advantage of the fact that when the throttle is only just open, a low pressure (high vacuum) is created in the manifold after the throttle butterfly valve. As the throttle opening increases, the vacuum decreases (a MAP sensor in an EFI system makes use of this parameter to determine engine load). In vacuum advance ignition timing mechanisms, one side of a diaphragm is linked to the distributor plate by a rod, while the other side is linked pneumatically (ie, via a hose) to a source of manifold vacuum. At times of high vacuum (low load), the distrib- 40° ADVANCE ANGLE BEFORE TDC B 30° A 20° 10° 0° TORR MBAR 600 This photo shows the workings of a traditional distributor that used points and relied on centrifugal and vacuum advance mechanisms. Fig.8 (right) shows the final timing curve achieved by this type of system. The full-load advance curve (A) is achieved through the centrifugal action of the weights, while the partial load advance curve (B) additionally advances the timing when loads are light (and vacuum is therefore high). “Road load” means that the engine is only partially loaded, while “full load” means that the throttle is wide open. (Bosch). utor plate is turned so that the spark is advanced. Con­ versely, when the vacuum drops, the spark is retarded in response to the increased load. Although this primitive system work­ed well for many years, it did not take into account many of the factors required for optimum ignition timing. Consequently, the safety-margin to detonation (knocking) was generally left high, thereby limiting power and reducing efficiency. Electronic systems The use of electronic spark advance systems revolutionised ignition tim- VACUUM 400 500 ROAD LOAD 400 300 A2 300 200 200 100 0 A1 100 0 1000 ing. First, the traditional parameters of engine speed and load are now catered for by a detailed timing map stored in an EPROM (electrically programmable read-only memory). An electronic timing map is far more accurate than the timing parameters produced by the traditional mechanical systems and is far more reliable. Many modern cars also now have antiknock sensors and these allow the engine management computer to set the ignition timing for best performance while still maintaining safe operation. In addition, other factors which affect optimum timing can now be IGNITION ADVANCE LOA D FULL LOAD 2000 3000 4000 5000 REV/MIN taken into consideration. Typically, a modern com­puter-controlled engine management system accepts data from sensors which monitor engine speed, manifold vacuum, throttle position, engine temperature, air temperature and battery vol­ tage, and sets the ignition timing accordingly. Such systems also usually provide full fuel management as well, by controlling the fuel injectors. As a result, electronic engine management systems provide improved starting and idle speed control, better fuel economy, increased performance SC and lower engine emissions. IGNITION ADVANCE ED E SPE ENGIN Fig.9: electronic spark advance and engine management systems allow complex ignition maps which provide optimal timing for a large range of load and engine speed conditions. (Bosch). LOA D EED E SP ENGIN Fig.10: the traditional mechanical advance mechanism produces a map which is far simpler than that achieved by electronic means. As a result, the ignition timing is far from optimum in many operating conditions. (Bosch). September 1995  7