Fullscreen HTML5Med Res (??MB)HTML4

Electronic Engine Management Pt.10: Ignition Systems – by Julian Edgar The conventional automotive ignition system comprising points, a combination of centrifugal and vacuum advance mechan­isms, a coil and spark plugs has been largely replaced in modern engine managed cars. Multiple coils and electronic timing control are often matched with platinum plugs which may require changing only once every 50,000km. Ignition timing While it is obvious that an engine working at full throttle requires more fuel than at idle, the changes needed in the timing of the spark plug firing are not as easy to understand. On average, it takes about two milliseconds from the time of ignition until the end of the fuel burn. The optimum time for this process to occur is slightly after the piston has reached Top Dead Centre (TDC) – ie, when it has started on its way down again. If the spark occurs too early – ie, when the piston is moving upwards - then the combustion process will slow the piston and detonation (an uncontrolled burning) may occur. Conversely, if ignition occurs too late, then the pressure developed in the combustion chamber will be lessened as the piston will already have descended too far down the cylinder. Fig.1 shows the different cylinder pressures experienced with different ignition timing. The timing of the ignition is described in degrees of crankshaft rotation before or after TDC; ie, BTDC or ATDC. If the spark plug is fired late in the crankshaft’s rotation (ie, ATDC), the spark is said to be retarded. If it’s fired early (BTDC), then the spark is said to be advanced. Combustion time This photo shows a typical small moulded coil from a current ignition system utilising a distributor. 22  Silicon Chip Because the combustion time varies little over the rev range, a fixed ignition timing ATDC would mean that combustion would extend further and further into the power stroke as the engine rpm increased. Thus, in order to maintain maximum combus­ tion pressure, the ignition point must be advanced as rpm in­creases. If it were this simple, then that would be the end of the story – but it’s not! The optimal ignition timing is also in­fluenced by engine design factors, such as spark plug position and combustion chamber shape, and transient factors like mixture richness, engine load and engine temperature. In practice, the correct ignition advance is a compromise based on the criteria of: • • • • maximum engine power; economical fuel consumption; no engine knock; and clean exhaust emissions. Traditional systems The conventional system of ignition timing advances the spark by means of centrifugal weights mounted within the dis­tributor. This produces an advance curve which is solely depend­ent on rpm and so a vacuum canister connected to the intake manifold is used to additionally advance the ignition point as a function of load. The typical resulting ignition advance curves are shown in Fig.2. The high voltage (25-30kV) required to generate the spark for ignition is obtained from the ignition coil. During the dwell period (when the points are closed), current flows through the primary side of the ignition coil which stores energy. When the points open, a high-voltage pulse is generated in the secondary side of the coil and this is applied to one of the spark plugs. The “correct” plug is selected by the rotor arm inside the dis­tributor. Fig.1 (left): the ignition timing must be correct for the combustion pressure to be at its peak immediately after the piston passes top dead centre (ATDC). However, if the timing is over-advanced, knocking may result. (Bosch). Engine managed systems With input sensors in place to control the fuel injection, extending the influence of these to control the ignition timing was a logical next step. Fig.4 shows a typical electronic ignition system as used in some Fig.2: a conventional weights-and-vacuum ignition advance system can produce only a relatively simple advance map. (Bosch). Fig.3: by using the input data from various sensors, an electron­ically-managed ignition system can provide a far more comprehen­sive advance map than the old weights and vacuum system. This ensures optimal spark timing over a much wider range of load and rpm conditions. (Bosch). July 1994  23 Fig.5: unlike a conventional ignition system, an ECM system can have a special softwarecontrolled ignition advance map for very cold staring. Note the complex shape of the ignition advance curve when this engine’s coolant is below 0°C. Fig.4: this diagram shows the ignition timing inputs to the ECM in a recent Nissan system. Nissan engines. It comprises the ECM, an ignitor (or power transistor) ignition module, and the traditional distributor, coil and plugs. The ignition timing is provided by “maps” (such as shown in Fig.3) built into the ECM software, with ignition angles selected on the basis of inputs from the crankshaft posi­tion sensor, airflow meter, coolant temperature sensor and knock sensor. Nissan timing system The Nissan electronic ignition timing control can be clas­sified into three different phases: ordinary operation, engine starting, and idling and decelerating. During ordinary operation (sensed when the throttle position sensor or TPS is in its off-idle position), the ignition timing advance is selected from the maps stored within the ECM. During starting, the coolant Fig.6: the Subaru Liberty Turbo ignition system uses a coil mounted on each spark-plug. The ‘ignitor’ module is external to the ECM. (Subaru). Fig.7: this Daihatsu Mira system uses a power transistor within the ECM to control a single ignition coil which then feeds a distributor. (Daihatsu). 24  Silicon Chip tempera­ture has a major input into timing, especially if the temperature is below 0°C – see Fig.5. If the battery is nearly flat during starting, combustion might occur before the piston reached TDC – with reverse rotation a possibility. To prevent this, the ignition is further delayed when the cranking speed is below 100 rpm. Finally, when the TPS indicates that the car is decelerating, the ignition angle se­lected is retarded at engine speeds over about 2000 rpm, probably to benefit exhaust emissions. The external ignition module – containing the power tran­sistor to switch the primary side of the coil – may also contain its own inbuilt timing. Usually, this is bypassed and the ECM controls ignition timing, but should a problem develop in the ECM the ignition module will run the engine with the small amount of ignition advance built into it. This limp-home advance is rpm dependent. Multiple coil systems While the Nissan system discussed above uses full electron­ic timing control, it is slightly old-fashioned in that a single coil and a distributor are used. More sophisticated systems use multiple coils and power transistors, and avoid the use of a distributor totally. One such system is used by Subaru on their Liberty Turbo, with some Saab, Nissan and BMW engines using similar systems. Other manufacturers (like Holden on their V6) use multiple coils and a waste-spark system. Subaru mount four coils directly on top of The Subaru Liberty Turbo uses four individual coils, each mount­ed on top of its corresponding plug. The platinum spark plugs only need changing at 50,000km intervals. the spark plugs, meaning that no high tension leads are used at all. The ECM switches four power transistors (which are exter­nally mounted in an ignitor module) and determines the correct spark timing based on the inputs from seven sensors. Fig.6 shows the layout of the Subaru system. Fully programmable aftermarket ECMs like this Autronic unit, shown here installed on a 260kW turbocharged rotary engine, can have full ignition maps programmed into them. These maps give the appropriate ignition timing for a variety of engine conditions. July 1994  25 Fig.8: the Mazda RX-7 Turbo ignition system uses two coils for the rotary engine. Turbocharged engines require very good knock-sensing if advanced timing is to be run without engine damage being caused through detonation. (Mazda). Knock sensing is used, with a self-learning algorithm incorporated into the ECM. Knock sensing is particularly import­ant in turbocharged engines like the Subaru, because best power will be gained by advancing the ignition timing almost to the point of detonation. Detonation (knocking) can severely damage a high-performance engine within a few seconds, 26  Silicon Chip however. In some cars, the knock sensor input is used to immediately retard the timing by up to 7°, with the timing then progressively advanced back to stan­dard. In Saab’s Automatic Performance Control (APC) system, the turbo­ charged cars will run on fuels varying in octane from 91 to 98. (Note: the octane rating of a fuel is an indica- tion of its anti-knock properties. The higher the octane number, the lower its propensity to detonate). The APC system uses the input from a knock sensor to regulate turbo boost pressure, meaning that the engine can extract more power from the fuel than an engine with conventional ignition timing (which must always have a SC large safety margin).