The way the EMS manages injection is quite simple - the sensors and triggers on the engine relay information to the EMS about engine speed and load. The EMS uses these to extract the appropriate injector time from the injection map and then fires the injector(s) for this length of time. If the system uses batched injection then all of the injectors are fired at the same time once per engine revolution. With grouped injection the injectors are grouped together in pairs which are fired at an optimal point in the engine's cycle which best suits those two cylinders, again once per revolution. Where the engine sensors are able to determine the engine's cycle position (usually from a cam phase sensor) it is possible to fire the injectors at the optimum time for each individual cylinder. This is known as sequential injection. Rather than firing once per revolution, each injector is fired for twice the pulse width at the optimum time in the engine's cycle, e.g. immediately before the inlet valve opens. There are minor benefits in economy and emissions to be had from using sequential or grouped injection, but power-wise there is little or no difference.
As we can see, information from these two main input sources allows the EMS to orchestrate the engine's fuelling so that the engine runs happily in normal conditions. There are times however when the engine is not running under these ideal conditions and it is at these times that other vital feedback is required to allow the EMS to run the engine properly. Generally under these conditions the EMS makes adjustments or corrections to the fuel map according to what it knows about the prevailing conditions. The main environmental conditions that are monitored by the EMS are as follows:
When an engine starts from cold it is well below its normal operating temperature. This causes some of the fuel injected into the engine to condense rather than atomising and being drawn in efficiently. Combustion chamber temperatures are also low which leads to incomplete and slow combustion. These effects cause the engine to run weak and require extra fuel to be supplied to compensate. In a conventional system the 'choke' on the carburettor performs this function. On an injection system a coolant temperature sensor provides the EMS with the engine's temperature and enables it to ‘correct’ the fuelling. This correction involves adding a percentage of extra fuel according to a predetermined correction profile by temperature, up to the normal operating temperature of the engine. The amount of extra fuel will vary from engine to engine and according to the engine's temperature and rpm, since the effects of condensing are less when air speeds are higher.
When air temperatures are high, the density of the air being inducted falls off, thereby lessening the volume of oxygen available for combustion. If the fuel that is injected remains constant then the mixture will become too rich. To compensate for this the EMS applies a correction to the base map according to a predetermined correction profile. As the air temperature rises so air density will continue to fall and hence the fuelling will be reduced. Information about air temperature is relayed to the EMS by an air temperature sensor. To some extent airflow or manifold pressure based systems can compensate for lower density air since depending on their type they may show or calculate less volume of air inducted, and this will cause the EMS to adjust the fuelling accordingly.
If the voltage of the vehicle's battery varies then the time taken to open the injectors will conseqently also vary. Since the EMS times the overall injector pulse, if the injector takes longer to open, then the time it remains open will be that much shorter and therefore the fuel introduced to the engine will be correspondingly less. Most EMS have a correction of injector times applied to the base map for variations in voltage; the corrections are usually small but during shorter injector times (idle and cruise) they can be very significant to the efficient running of the engine.
Most EMS can make use of a Lambda sensor which sits in the exhaust of an engine and measures the air/fuel ratio or strength of the mixture while the engine is running. During conditions of steady state running, the EMS is able to tell from this sensor whether the mixture is rich or lean, and can make real-time adjustments to bring the mixture back to 'chemically correct'. This generally happens only when in steady state, e.g. at idle or when cruising, and is known as ‘closed loop running’. Over a period of time the EMS can ‘learn’ whether the mixture is rich or lean and make long term adjustments.
There are some additional corrections that the EMS can apply intuitively by examining changes in state or other derived conditions. The most common are:
When the throttle is opened suddenly there is generally a weakening affect on the induction since air is lighter than fuel and is drawn in more rapidly. Weakening on throttle opening transients is also caused by the fact that the fuel has already been injected and the inlet valve is open before changes in the inlet manifold can take place due to a throttle change. This is only a transitory effect but it can cause the engine to stumble or stutter on initial acceleration. To counteract this tendency the EMS can keep track of sudden changes in throttle position or load and add a percentage of extra fuel when this happens. The extra fuel is only added for a short period and is then decayed over another short period. This is normally a number of engine revolutions rather than a period of time. This is known as ‘accelerator clamp’.
When the throttle is closed suddenly and the engine is being overdriven, the hydrocarbon levels in the exhaust can rise dramatically. It is also possible for un-burnt fuel to ignite in the exhaust system producing the characteristic popping on overrun. To overcome this some EMS will either reduce the fuel to the engine on overrun or in some cases cut it off all together.
When the engine is actually being started, the cranking speed is usually quite low (150-200rpm or thereabouts). This means that the airspeed in the inlet ports is minimal and may not be sufficient to atomise and draw in all the fuel from the injectors. It is normally necessary to add some extra fuel while cranking to overcome this drawback. The amount of extra fuel to be added can be built into the base map at speed site zero, but it is more usual to have a correction to the base map which is a percentage of extra fuel to be added when cranking. This extra fuelling can also vary with engine temperature so the correction is normally in a table for each of a range of engine temperatures. This correction normally decays quite quickly once the engine has fired since it is only required at low crank speeds. The percentage of extra fuel required will vary from engine to engine. This is often known as startup correction or cranking correction.