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Electronic Engine Management Pt.4: Changing The System – by Julian Edgar There is a widespread perception that a modern engine man­aged car is not open to engine modifications; that this type of system is signed, sealed and delivered. To some extent, this is true. Manufacturers leave little adjustment capability in an electronic engine management system, with often only the idle mixture and ignition timing open to change. In some cars, even these – ostensibly, at least – are non-adjustable. In a standard car, there are good rea- sons for this ap­proach. With exhaust gas oxygen feedback loops in operation, immediate ECM recognition of sensor failure, and limp-home modes of operation, the last thing that the manufacturer wants is someone armed with a screwdriver and a hammer deciding that the car needs a tuneup! A modern car might not need the mixture adjusted even once in 150,000 kilometres, for example. For those who like to tinker with their cars – to gain more power by fitting twin carbies, for example – the old days seem to be over. However, as with previous automotive technologies, there are ways of getting an electronic system to do as you want. Basically, there are four different approaches which can be taken: (1). The engine management system can have new inputs fed into it, thus giving changed outputs. (2). The system can have mechanical, electrical or electronic additions made to it. (3). The original manufacturer’s software can be changed – ie, the chip can be rewritten. (4). The original ECM can be removed and replaced with an after-market, fully programmable engine management computer. In this feature, we’ll look at the first two methods – crude, often effective and always cheap! The need for modification The engine coolant temperature sensor is just one of several sensors that provide information to the ECM. This ECM input is one of the easiest to fool. 8  Silicon Chip But why would you want to modify the engine management system, anyway? A turbocharged car is probably easiest to under­stand in this context, because the power produced by the engine is so easily increased. A naturally aspirated engine has air pushed into it by atmospheric pressure – through the air filter, past the throttle butterfly, into the plenum chamber, down the cylinder runner, EXTERNAL CONNECTIONS SEAL WATER (COOLANT) CONTACT ZONE THERMISTOR Fig.1: basic construction of a typical coolant temperature sensor. A thermistor is used as the sensing element. past the inlet valve and then (finally) into the cylinder. As the piston sinks on its intake stroke, a partial vacuum is created within the cylinder, and one bar of atmospheric air pressure does the pushing. The amount of air that the engine inhales depends on its size, on how much flow loss is experienced by the air on its torturous path into the cylinder, and how quickly the engine is rotating (its rpm). However, if the air If data showing the sensor’s temperature/resistance relation­ship is not available, then some testing with hot water, a thermometer & a multimeter will soon reveal its characteristics. pressure is raised above atmospheric by a turbocharger or supercharger, then greater flows will occur. With extra fuel added, more power will be produced. The induction pressure above atmospheric which the turbo produces (called turbo boost pressure) greatly influences the air mass passing into COEFFICIENT OF ENRICHMENT 1.0 the engine. Manufacturers are often conserva­tive in their boost pressure, generally using around 0.5 bar (about 7 psi). However, most turbo engines will happily cope with 0.7-0.8 bar without mechanical modification. The problem comes when the volume of air passing into the engine is much greater than the manufacturer designed the EFI (electronic fuel injection) system to cope with. To some extent, the system will self-compensate for changes. The airflow meter will signal the greater air mass flowing to the ECM and this in turn will control the injector pulse width to give CONTACT TEMPERATURE Above: this close-up view shows a typical coolant temperature sensor. It is usually mounted close to the thermostat. Fig.2: the enrichment pattern as a function of engine temperature in a VL Holden Commodore. January 1994  9 A resistor or potentiometer wired in series with the coolant temperature sensor will cause the ECM to provide more fuel – a very cheap modification. the appro­ priate mixture. However, if the airflow is increased too far, the stage will be reached where the mixture starts to become lean – with not enough petrol being mixed with the air. In this situa­tion, the injectors may be held open continuously but their flow rate may be insufficient. Other causes of increased induction flow which may cause leaning-out include traditional “hotting-up” methods like larger exhaust systems, head modification by bigger valves, and so on. Fooling the ECM The ECM computes injector pulse width on the basis of its inputs and on its internal base fuel figures. If the coolant temperature sensor indicates that the engine is cold, then more fuel will be injected – the equivalent of a choke in a car with a carburet- A microswitch can be used to cause full-throttle enrichment to occur at an earlier throttle opening than normal. tor. Similarly, if the throttle position switch (TPS) indicates that your foot is hard down, then the mixture will be slightly enriched to give maximum engine power. If any conditions which would cause the ECM to enrich the mixture are artificially created, then the fuel flow into the engine will be increased, assuming that maximum fuel flow isn’t already occurring. Probably the easiest sensor input to fool is the coolant temperature sensor. This sensor consists of a thermistor located in the engine cooling system, usually close to the thermostat. Fig.1 shows an example of a coolant temperature sensor. In Fig.2, the pattern of enrichment which the ECM carries out in response to low engine temperature is shown for a VL Holden Commodore. As the coolant temperature rises, the FULL THROTTLE CONTACT THROTTLE SHAFT CONTACT PATH (CAM) IDLE CONTACT (MICROSWITCH) ELECTRICAL HARNESS PLUG 10  Silicon Chip Fig.3: basic layout of a typical throttle position switch (TPS). The idle contact microswitch is normally closed at idle & opens as the throttle moves off its stop. The full throttle contacts are normally open but close at full throttle settings to provide extra fuel enrichment. resistance of the sensor decreases. A typical coolant temperature sensor has the follow­ing characteristics:    0°C 6000 ohms 20°C 2500 ohms 30°C 1800 ohms 40°C 1200 ohms 70°C   450 ohms 90°C   250 ohms 100°C   190 ohms 110°C   110 ohms If a 5kΩ pot is placed in series with the sensor, then the ECM can be easily persuaded that the engine coolant temperature is anything from 0°C to its real value! Feeding information to the ECM which understates the actual temperature of the coolant will cause the mixture to become richer than it otherwise would be. More fuel will be injected as the ECM program tries to overcome the expected cold-engine affects of poorer fuel atomization, thicker oil, and so on. However, while enrichment may be quite substantial at some rpm settings, it’s unlikely that the ECM was designed with the idea that the engine will be revved at 6000rpm with the coolant temperature at 5°C! Cold-start enrichment usually declines with increasing load and/or rpm. On the other hand, if the engine runs slightly lean throughout its rev range (because of engine modifi­cations), then a potent­ iometer in series with the cold-start sensor can be a very good starting point in overcoming it. If full throttle enrichment is wanted earlier in the throt­ tle opening, then a microswitch operated by the rotation of the throttle shaft can be used to trigger this input – a func- tion usually provided by the throttle position switch (TPS). Fig.3 shows a typical TPS. The correction coefficient used with the base fuel figures increases with increasing rpm – and the final correction step is inducted by the throttle position switch. Other sensors with the potential for deliberate misuse include the knock sensor (to retard timing), the airflow sensor (to change mixtures), the vehicle speed sensor (remove speed limiter), the MAP sensor (remove turbo over-boost fuel cutoff), and the induction air temperature sensor (change mixtures). Extra Injectors If the injection system provides insufficient fuel flow at full load, then extra fuel injectors can be added. The most so­phisticated way of doing this is to control the extra injector by the use of a commercially-made supplementary injection computer, which has various inputs to monitor load and rpm. However, be­cause full load usually coincides with maximum airflow, the accuracy with which fuel mixtures must be held for good performance is fairly low. An extra injector can be mounted prior to the plenum cham­ber to promote good fuel mixing and can be wired in series with one of the normal injectors. To prevent it from enriching the mixture constantly, it needs to be switched on and off. In a turbocharged car, the simplest way of achieving this is to use a pressure switch which is mounted on the plenum chamber. Adjust­ able pressure switches – under the Hobbs brand name – are avail­able from automotive instrument suppliers. However, switch-on of the injector can be triggered in a more sophisticated manner by monitoring one or more of the stan­ dard engine management sensors. By using voltage comparators, the airflow meter and throttle position switch could be monitored, with the extra injection occurring only at high gas flows and wide throttle openings. Of course, the extra injector does not come on stream gradually with this system. Instead, the mixture undergoes a sudden enrich­ ment by 10-20% (depending on the supple­ ment­ary injector size). To overcome this, a circuit can be made up which duplicates the commercially availa- Ancillary injectors can be used to provide more fuel if the original injectors prove to be inadequate after engine modifica­tions. The injector on the left is a cold-start injector, while at right is an injector from a 4-cylinder Nissan engine. The typical cost from a wrecker would be $10 each. This Holden VL Turbo Nissan engine has been fitted with extra fuel injectors which, along with other modifications, provide a 50% power boost. The additional injectors are triggered by mani­fold pressure switches & are pulsed by the standard computer. ble injection computer by increasing supplementary injector pulse width in response to greater gas flow, etc. In prtactice though, this is not always needed. Another approach is to use two sequentially-operated low-capacity supplementary injectors. The extra injector load placed on the output transistors of the ECM doesn’t appear to cause problems, although the supplemen­tary injectors should be of the same resistance as the original injectors. The power capability of the ECM output tran- sistors may also vary from computer to computer. With extra injectors available cheap­ ly from wreckers of Japanese engines (about $10 each), a supplemen­ tary in­jection system can be added for very low cost. The final mixtures should always be checked. The best way to do this is to use a chassis dyn­a­mometer in conjunction with a four-gas exhaust analyser. Another (cheaper) method is to build an oxygen sensor output meter and closely monitor the mixtures in SC real driving conditions. January 1994  11