As well as the normal running of the engine and administering of fuel according to the map settings most EMS can perform some rather clever tricks which can help with smooth running, performance, economy and emissions.  Most of these involve a feedback loop of some kind from the various engine sensors and involve assumptions about the way in which the engine is being used.

Idle control

Typical Idle Air Control ValveWhen an engine is idling and at normal temperature, its airflow requirements are fairly constant and the ignition advance and the idle can be set at a constant rate.  If any of the environmental conditions vary, e.g. engine temperature, air density etc. then the required airflow, ignition advance and fuelling may need to vary in order to allow the engine to idle.  In a carburettor-based system there is often a fast idle that is set when the engine is cold and the choke is operating, which raises the idle speed to prevent stalling.  Most EMS systems use an idle control system for when the engine is idling.  An idle air control valve (IACV) allows the air to the engine to be metered independently of the throttle butterfly.  If the rpm falls below acceptable limits then more air is bled into the engine.  If the rpm goes beyond an upper limit then less air is bled in.  Together with fuelling and ignition variation, this system maintains a rock steady idle with acceptable emissions in all conditions whether the engine is hot or cold.

Closed loop running

In order to minimise emissions and also to ensure that the exhaust catalyst function is optimised, many EMS have special routines coded within them to exploit situations where the engine is not under full load conditions, i.e. when cruising on a partial throttle.  A large proportion of motorway driving is done under these conditions especially when cruise control is fitted to the car.  The EMS enters a state know as ‘closed-loop running’ when the throttle position and engine speed are more or less constant.  This indicates a cruising condition.  In this state the feedback from the Lambda sensor and knock sensor are used to trim the fuelling and advance to give the best possible economy and efficiency.  When running in the closed loop the EMS will progressively lean off the mixture until the feedback from the sensors indicate that it is approaching detonation and will hold the mixture just before this point until the engine telemetry tells it that the engine is no longer cruising.  This is known as ‘lean cruise’ and is only possible if the EMS has Lambda and knock sensing.  On non-catalyst cars, lean cruise can go even further with the leaning of the mixture to save more fuel.  However the mixture has to be kept near stoichiometric for the catalyst to work effectively.

Open loop

This is not really a clever mode of operation but included here for completeness.  At full throttle, the Lambda (oxygen) sensor is almost always ignored.  This is called open loop running.  In this situation, the EMS bases its decisions entirely on the information contained within the maps.  This characteristic means that self-learning cannot be used (or relied upon) to cater for the increased full-throttle fuel supply required for engine mods that increase power and therefore airflow.  However, self-learning often does help in the changed requirements occurring in part-throttle conditions.

The reason the Lambda sensor is normally ignored is that it can only indicate mixture strength through quite a narrow band of air/fuels ratios and it is likely that its feedback will be swamped by the fuelling when accelerating and at wide open throttle.  Some systems fit a wide-band Lambda sensor which can report on the mixture strength over a wider band of settings and can therefore give useful feedback even when the engine is at wide open throttle and in the acceleration fuelling band of operation.  This can allow the EMS to learn about mixture strength and monitor/adjust the fuelling even in these extreme circumstances.

Most EMS also use map information only for ignition timing in this situation.  However, a few EMS use the feedback from the knock sensor in a self-learning approach similar to that done with the lambda sensor on the injection system.

Self learning

In addition to closed loop running the lambda sensor is also used in some EMS as part of a self-learning system.  For example if the fuel pressure regulator in your car is working incorrectly and supplying less pressure than it should, the mixture will probably be a bit lean.  The Lambda sensor feeds this back to the EMS which then richens up the fuelling.  If this is happening consistently then the EMS knows that the mixtures are always a bit lean and will permanently richen up the mixture.  It has learned that the mixture is lean and that richer mixtures are needed, and will always run this correction.  If the pressure regulator is subsequently replaced or repaired, the EMS will then gradually re-learn the new requirements.  This self-learning process occurs in most manufacturers EMS but is rarer in after-market systems.  Self-learning of mixture strength is totally dependent on the Lambda sensor.

Injector cutting

In the interest of economy and low emissions some EMS can switch off the injectors completely when the engine is being overdriven, for example when you lift off the throttle totally.  The injectors resume normal service when engine revs drop to around 500rpm above idle.  If you watch the tachometer closely you can see the needle lift a bit when the injectors resume their flow.  This is more usual on manufacturers EMS than after-market ones.

Self Diagnosis

Many engine management systems also have a 'self-diagnosis' ability.  This allows you to probe the EMS using a PC and it will tell you if it has developed a problem.  For example if the engine temperature sensor wire is broken the EMS will report that there is no input from it.  Some EMS will communicate faults via fault codes or flashing lights, while others require a diagnostic computer to be attached.  Again, this is more common with OE management systems.

Traction-control, cruise-control and drive-by-wire

There are areas of an EMS that can interact with other systems on the vehicle such as traction-control and cruise-control.  In the more sophisticated systems a separate traction-control unit can communicate with the EMS to invoke a variable rev limit that cuts engine torque if it senses that traction is being lost.  Normally this is done by using a soft-cut rev limiter which is invoked at will.  On other systems the EMS is actually able to back off the throttle.

Some recent EMS that are installed alongside intelligent or adaptive transmissions are designed to co-operate with the transmission.  A common practice is ‘drive-by-wire’ where there is no direct connection between the accelerator and the throttle butterfly.  Instead, a stepper motor controlled by the EMS applies the throttle.  This makes it easy for the cruise-control or adaptive transmission to orchestrate the engine as it sees fit.  A traction-control system might back off the throttle in response to lost traction.  A cruise-control system will both apply and back-off the throttle to maintain its programmed speed.

Rev limiting

Most EMS systems implement a rev limiter, some allow a soft-cut where the engine selectively misfires followed by a hard-cut a little higher up where the engine simply stonewalls.  Some limiters cut off all fuel at the prescribed engine speed, withholding it until you're 500rpm below the limit.  Other rev limiters cut off the spark (or injectors) of individual cylinders one after the other, progressively cutting more and more until the hard-cut limit is reached so that you can barely feel that you have reached the maximum allowable rpm.  These soft limiters mean that the car can be used right to the rev limit without a worry.  Normally the EMS will maintain the tacho signal consistently to ensure that it doesn’t go crazy.  Often the rev limiting is coupled with a shift light that warns the driver that the rev limiter is about to operate and he should change up a gear.  With batched and grouped injection systems, selective cutting of fuel can be dangerous since the fuel is not injected at the optimum time for each cylinder and it is quite possible for a cylinder to induct only a partial charge of fuel which could result in detonation and resulting damage.

Tacho and tell-tale

Most EMS systems drive the tachometer (rev counter) directly which allows them to maintain the tacho reading even when the rev limiter is invoked.  Some after market EMS also provide a tell-tale facility that will flick the tacho needle to the highest rpm attained during its previous use.

Fan control

EMS systems as fitted to production cars can also control other aspects of the engine's systems.  It is very common for the EMS to control the cooling fan, switching it on and off as required.

Water injection

Some aftermarket EMS systems can control a secondary water injection system that is used in forced induction engines to cool the incoming charge and to prevent detonation.  They may also be capable of controlling water-cooling sprays onto charge coolers that help to cool the air inducted into the engine.

Nitrous oxide injection

Nitrous Oxide (NO2) is a gas that contains much more oxygen than air does on a weight by weight basis; NO2 is often used to boost the power of an engine.  It is injected with extra fuel and effectively increases the amount of fuel and oxygen inducted into the engine with similar affects to turbocharging or supercharging.  Some aftermarket EMS systems have provision for controlling the nitrous injection and the extra fuel requirements.

Turbo Anti-lag

One of the problems associated with turbocharged engines is the time taken for the turbocharger to spin up to speed and provide boost.  When the engine is accelerating the turbocharger is spinning rapidly and making boost, but when the gear change takes place or when the throttle is lifted, the turbo will slow down and boost will drop off.  The boost takes some time to get going again which means that the engine will drop off the power band.  This time between planting the accelerator and boost becoming available is called ‘turbo-lag’ because the turbo lags behind the accelerator.  Some EMS systems are able to minimise this when the engine is backing off by firing the mixture in the cylinder when the exhaust valve is open.  The burning gases expand rapidly and exit the exhaust valve at high speed instead of trying to push the piston down.  The ‘kick’ from the exhaust keeps the turbo speed up and minimises lag.  Generally this is only done when the engine is being backed off, so although the cylinder doesn’t fire properly, the net effect on the vehicle's performance is marginal.  However the effect on the turbo spin speed is quite marked.  Firing the cylinder when the exhaust valve is open also provides those spectacular backfiring, banging and exhaust-flaming antics seen so frequently on the WRC turbo cars.

Auxiliary device outputs and control

Since the EMS knows so much about engine conditions, it is often useful to be able to harness the information to drive or run other systems associated with the engine.  Many EMS do provide outputs or feeds which enable the more enterprising to use the EMS information to make improvements to other aspects of the car.  For example EMS information can be used to switch an alternator off at high rpm and thereby minimise the parasitic losses associated when the power is needed most, or to modulate the cooling fan at times when the engine's power is needed.

Feature disclaimer

There are many other features and options within aftermarket EMS which may or may not be used with a particular installation.  Some are obscure and are designed to meet the particular requirements of a certain piece of injection hardware or another co-operating device.  It would be madness to attempt to list all of this rich cornucopia of functionality for the many and varied EMS available. Suffice it to say that the features listed above cope with the vast majority of what is required from a management system and in the interests of keeping it simple I will elaborate no further.