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Engine Management

Navigating around the major topics in this article may be easier by using the following links, alternatively a full Index is available which can be accessed by clicking here

Introduction
Programmable systems Vs. non programmable
Basic injection system
Injection system explained, single point/multi point
Induction systems, plenums, throttle bodies
Injection system at work
Operation of the injection system
Additional information about injection
The clever stuff, lean cruise, idle control, lambda feedback etc.
Basic ignition management ,wasted spark, distributorless
Ignition in operation, timing map adjustments
2D versus 3D systems, what it means
Conversion to mapped ignition from distributor based systems
The mapping process on a rolling road
Conversion to throttle body injection from carbs or plenum
Sample maps in digestible format
MAIN INDEX

Basics of engine management

Modern engine management systems do a fine job of ensuring that engines run cleanly and efficiently in a wide variety of conditions, they are for the most part reliable and require little or no maintenance. However they seem from the outside to be fearsomely complicated systems which defy all attempts at understanding. Amidst all this apparent hokum it is easy to lose sight of the two basic functions performed by an EMS.

To meter fuel to the engine in the right quantity

To provide a spark at the right time

What is an engine management system?

An EMS is a self contained custom built computer which controls the running of an engine by monitoring the engine speed, load and temperature and providing the ignition spark at the right time for the prevailing conditions and metering the fuel to the engine in the exact quantity required.

There are two discrete subsystems in operation within the EMS, the fuel or injection system and the ignition system. It is possible to run an engine management system which just provides one of these subsystems, for example just the ignition system. It is much more common to use the mapped ignition within an EMS in isolation than it is to use just the injection.

What is a ‘map’ ?

Most of us have heard the term ‘Mapped ignition’ and programmed or mapped injection but may not understand what this actually is. Whilst the engine is running its requirements for fuel and ignition timing will vary according to certain engine conditions, the main two being engine speed and engine load. A ‘map’ is no more than a lookup table by engine speed and load, which gives the appropriate fuel or timing setting for each possible speed and load condition. There will normally be a map for the injector timings (fuel map) and a separate map for the ignition timing settings (ignition map) within the EMS.

Each map has entries for a pre-determined range of engine speeds (called speed sites) and a predetermined range of engine load conditions (called load sites) which generally indicate how far open the throttle is. The EMS knows the engine speed (derived from the crank sensor or distributor pickup) and the engine load (from the Throttle Position Sensor or airflow meter) and will use these two values to ‘look-up’ the appropriate fuel and timing settings in each map.

If the current engine telemetry falls between the sites in the map then the value is interpolated between the nearest two sites. Normally there will be speed sites every 500 or so RPM and 8 to 16 load sites between closed and open throttle. In the example below speed sites are spaced every 1000 RPM and the 8 load sites are numbered 0 to 7.

Simple example of an ignition map

In this example the engine load increases as the load site numbers in the left column increase. If the engine were running at 3000RPM, load site 3, then the value looked up would be 26, I.E. 26 degrees of advance. If the engine were running at 3500RPM, load site 3 then the EMS would interpolate between the value for 3000RPM (26) and the value for 4000RPM (30) and calculate a value of 28 degrees.

Note how ignition advance falls as load increases, this is because cylinder filling is much better when load increases and therefore the mixture burns faster, necessitating less advance.

Programmable systems vs. non-programmable systems

Most EMS fitted to production vehicles are not programmable, that is to say that the maps within the EMS which determine the fuelling and ignition settings are fixed and cannot be varied by the owner. This makes good sense from a manufacturers point of view since the engine then runs within the permitted parameters which keeps the engine emissions and economy within known limits.

There is a burgeoning market for ‘chip tuning’ where the chip containing the maps is replaced by another which has revised map settings providing better performance from the engine, the gains to be had here are fairly small except with turbo-charged engines where the EMS controls the boost. Chip changes on these engines can yield quite large increases in engine power. Some manufacturers go to great lengths to stop after market tuners from decoding the maps within their EMS with varying degrees of success. Notable EMS which are difficult if not impossible to ‘chip’ are the Rover MEMS and the Ford EECIV system.

All after-market EMS are programmable since they have to be fitted to a variety of different engine installations in a variety of states of tune. If the map values could not be changed then the EMS would be useless for after market applications. Some manufacturers of these systems discourage home mapping and will only allow authorised dealers to undertake the mapping.

For clarities sake we will examine each of the two sub-systems within an EMS separately, in practice there is a great deal of interaction between the two, both systems utilise information from the various engine sensors.


Injection system

If we ignore for a minute the actual EMS the basic component parts of an injection system are very straightforward. Shown below is a schematic of the major parts of a multi-point injection system, single point injection systems are very similar, but they have only one injector and no fuel rail.

Constituent parts

Fuel tank        Holds a reservoir of fuel for the engine, is normally baffled to prevent fuel sloshing around and the resultant fuel starvation.

Fuel filter     Since an injector pump is a positive displacement pump any foreign material ingested can stall the pump and kill it stone dead, this ‘pre-filter’ prevents rubbish from entering the pump.

Fuel pump      A high-pressure pump running at around 6 bar which supplies fuel to the injectors. The fuel pressure regulator regulates to this pressure between 3 and 4 bar (43 and 58PSI). On some installations the pump is housed inside the fuel tank with rudimentary filtration, the fuel filter then follows in the fuel line.

Fuel line           Fuel pipe that transports the fuel from the pump to the fuel rail.

Fuel rail           A small fuel gallery from which the injectors take their fuel supply.

Injectors         Electric valves which when open allow fuel to be injected into the engine under high pressure.

Pressure regulator    A device that keeps the fuel pressure at a constant rate and returns any excess fuel to the tank

Fuel return line         Fuel pipe which bleeds excess fuel back to the fuel tank


Most injection systems run at quite high fuel pressure compared to a system using carburettors, typically an injection pump will produce around 6 bar and the system will run at around 3-4 bar (43-58 PSI). This is far in excess of the pressure supplied by a typical fuel pump from a non injected system (3-10PSI). The injection system relies on a constant supply of fuel at a pre-determined pressure and generally the pump runs all the time with excess fuel being returned to the tank. The map for the engine will have been derived with the fuel supply at this pressure; variations in fuel pressure will affect the quantity of fuel injected and will seriously affect the running of the engine, sometimes terminally.

Carburettors can generally cope with a short interruption to their fuel supply since they have their own reservoir of fuel in the float chamber that can be drawn from. Injection systems on the other hand cannot cope with fuel supply interruptions so it is necessary to ensure that such interruptions don’t take place. It is standard practice to baffle the fuel tank and use one way valves to prevent fuel surge, where space allows a surge pot can be fitted to ensure that fuel surge doesn’t rob the injection system of fuel at the wrong moment.

Most fuel injection pumps are gravity fed so they need to be mounted lower than the lowest point in the fuel tank. An alternative to this is to mount the pump in the fuel tank itself, most pumps can be run completely immersed in fuel, in practice they do this anyway since inside the pump the fuel runs up and around the armature of the pump. The pumps operation is often controlled by the EMS to prevent the pump delivering fuel when the engine is not running, for example if the vehicle is involved in an accident.

The pump supplies fuel to the injectors via a fuel rail which is a small long tube with a connection for each of the injectors. The fuel supply enters the rail at one end, at the other is the fuel pressure regulator which ensures that the fuel pressure is kept constant. Since the fuel pressure can affect the amount of fuel discharged in any given injector time it is essential that this pressure is kept constant. Fuel supplied in excess of requirements is bled back to the fuel tank through the fuel return circuit that is part of the pressure regulator.

It is not uncommon for fuel pressure regulators to be tampered with to supply extra fuel pressure, this is a common dodge when an engine has been tuned and needs more fuel as a result. Since the map inside the OEM EMS cannot be varied, a certain increase in fuelling can be had by upping the fuel pressure. Rising rate fuel pressure regulators achieve the same objective, they increase fuel pressure when the engines air demands are high, often increasing the fuel pressure in response to low vacuum in the inlet manifold, E.G. when the throttle is increased. Some EMS systems are able to cope with a small increase in airflow on their own since they know when the engine is running weak due to the Lambda feedback and will increase fuelling to compensate. This can only be achieved during steady state running so there will still be glitches in the fuelling here and there.

The injectors themselves are connected to the fuel rail via a clip and ‘O’ ring which has to contain the high pressure within the fuel system. An injector is simply an electric valve or solenoid, fuel is supplied to the injector at a known and regulated pressure, the valve or solenoid is normally closed. Fuel is introduced or injected to the engine by firing (opening) the injector for a pre-determined period of time once per engine revolution or per engine cycle, the longer the injector is held open the more fuel is introduced. This injector time is known as the ‘pulse width’ and the technique of varying fuel in this manner is known as ‘pulse width modulation’ as it is the pulse width that is varied according to requirements. Since the fuel injected is per revolution or cycle, as engine RPM is increased, so is the number of times the injectors are fired, this has the effect of meeting the engines requirements for fuel regardless of RPM.


Single point injection

Single point injection systems use a single fuel injector that injects into the inlet manifold or plenum; the fuel injected is drawn in to the cylinders by airflow in a similar way to a carburettor. Because of the variations in length and orientation of the various branches in the inlet manifold or plenum, the fuel distribution characteristics are not ideal so economy / emissions and throttle response suffer as a result.

Although the injector position is shown in the centre of the plenum, this is just for clarity, usually the injector will be mounted on or near the throttle body where air velocity is at its highest.


Multi point injection

Multi point injection systems are much more common and generally have an injector per cylinder located in each individual manifold runner. This configuration gives much better control of fuelling and better emissions since the fuel can be metered more closely, and there is less opportunity for the fuel spray to condense or drop out of the airflow since it is introduced as four small streams rather than one large one. The closer to the inlet valve the fuel injection takes place, the better the economy and transient throttle. Most systems use one injector per cylinder but on certain engines (notably the Rover ‘A’ series) there are only two inlet ports since two cylinders share a siamesed port, in this case multi-point would mean two injectors, one per inlet port, this is still better than a single injector system.

With multi-point (or multi injector) systems there is scope for timing the injection of fuel to better suit the engines duty cycle. If the EMS knows the relative position of each cylinder within the engines cycle (usually from a cam phase sensor) then it can fire the injectors at the optimum time for that cylinder. This is known as sequential injection; sometimes the EMS will only have knowledge of the crank position rather than the duty cycle position, in this case it can optimise for a pair of cylinders, this is known as semi-sequential or grouped injection.

Some EMS systems ignore the crank and cycle position when injecting fuel, they fire all of the injectors at the same time once per revolution, this is known as batched injection. There is no penalty to pay power wise when using batched injection, however grouped and sequential injection give a slight edge on economy and transient throttle/emissions.

Induction systems

We have examined the physical hardware of the injection system itself but not actually covered the induction system, with carburettors they are one and the same thing, with injection systems they are separate.

There are two basic types of induction systems used with injection, plenum based systems with a single throttle body and multiple throttle body systems that do not use a plenum but supply the inlet ports directly.

Plenums

A plenum is a large chamber on the engine side of the throttle body that helps to even out the pulses in the inlet tract by providing a buffer of incoming air. This in turn can help economy and emissions and also provide a longer effective inlet tract which can help mid range torque, for single point injection systems it is a must, for multi-point it is optional. The plenum is a convenient place to mount airflow sensors and vacuum sensors since it is at the confluence of all the inlet runners. When the engine is running the throttle body determines how much air will flow into the plenum and therefore the engine, the plenum is generally in a condition of partial vacuum.

The EMS can maintain a good and clean idle by allowing more or less air into the plenum via a bypass valve called the Idle Air Control Valve, this together with a special idle routine in the EMS allows a perfectly controlled idle (and emissions) regardless of ambient conditions. This IACV works independently of the throttle body and bypasses its operation.


Throttle bodies

A throttle body is no more than a tube or barrel that regulates air into the engines inlet manifold, plenum or inlet port. It is normally of tubular construction with a butterfly or throttle plate that opens and closes to regulate the airstream. Some throttle bodies have provision for mounting of fuel injectors others do not; it depends entirely on the application. The type of throttle body that feeds a plenum is normally a single body and has no provision for an injector pocket. Throttle bodies are essentially like carburettors but without the float chamber or jets/venturis, their configuration is often similar to carburettor configurations in that they are generally available as individual throttle bodies or twinned as dual bodies.

Individual throttle bodies

Performance induction systems normally involve the fitment of individual throttle bodies for each inlet port/manifold runner. Individual bodies can be aligned precisely with the inlet ports and this can give advantages. A system that provides individual bodies to each of the inlet ports should maximise the airflow potential for each cylinder and therefore help to improve the engines performance. Sometimes these bodies are designed to bolt straight to the cylinder head for a particular application and can be designed to taper to an exact fit on the inlet port.

Dual throttle bodies

These perform the same function as the individual bodies but have two single bodies which are joined together with a fixed spacing between the individual barrels which may not be absolutely in line with the inlet ports. These are not unlike Weber DCOE or IDA carburettors in appearance. Often the difference in alignment between barrels and ports is negligible and so does not affect the performance of the engine; a set of dual throttle bodies is normally substantially cheaper than a set of individual throttle bodies. Dual bodies can often be fitted directly in the place of existing carburettors utilising the same manifold, air filters etc., which can bring down the costs considerably.

The injection system at work

The EMS needs to know a number of things about the engines condition in order for the fuelling to be metered correctly. During normal running these boil down to the engine speed and the throttle or load position. Generally this information is relayed to the EMS by sensors or triggers on the engine, the engine speed is determined by either a crank position sensor (which gives crank position from which speed can be derived) or a trigger of some kind in the distributor (if fitted). Engine load can be determined using a number of different methods.

Engine speed and position

Engine speed and position is normally monitored by one of the following two methods

Crank Sensor

This is now the most common method of determining engine speed on a modern engine. It comprises a disk mounted on or machined into the flywheel/front pulley that turns with the engine. The disk has a certain number of teeth around its circumference and a fixed closely mounted induction sensor that pulses when it encounters a tooth. There is generally a pattern of missing teeth so that the EMS can tell exactly the crank position as well as speed. Although the EMS knows the engines crank position from this sensor, it does not know the engines cycle position. In a four-stroke engine the engine cycle involves two complete revolutions of the engine with the piston at TDC twice during the cycle. One of these times the cylinder is ready to fire, the other time is at the end of the exhaust stroke, a crank sensor alone can only indicate that the piston is at TDC, it cannot know which of the two cycles positions the engine is at.

Distributor pickup

Some older systems and many after-market systems use a distributor pickup to determine engine speed. The type of distributor used is normally Hall effect, magnetic reluctor or Optronic and has no in-built advance mechanism. A distributor-based system has the advantage of mechanical awareness of the engines cycle position as well as the crank position. By attaching an inductive pickup to spark plug lead number one the EMS can be made aware of the engines cycle position This can simplify the implementation of the ignition system for an after-market conversion and provide feedback necessary for sequential injection.


Engine load

Engine load is normally determined by one of the following methods

Throttle Position Sensor

The most common engine load sensor especially on after market systems. A TPS is a small potentiometer (or ‘throttle pot’) which is connected directly to the throttle shaft and turns with it. It returns a value to the EMS depending on the throttle position. TPS sensors are normally used on performance engines where airflow sensors might become confused because of pulses in the inlet tract, because they do not measure airflow but simply give a throttle position, airflow is assumed to be constant for any given engine speed and throttle position. If the engine is further modified the airflow characteristics may change and the engine may need re-mapping. EMS systems that use direct airflow measurement can often cope with changes more effectively and can alter the fuelling to suit without a re-mapping session.

Air metering flap

Another way of determining the engine load is to measure the airflow into the engine and this can be done using a flap which is deflected by incoming air, this is commonly known as an airflow meter. These are common on older injection systems, but can be confused by reverse pulses in the inlet tract when more extreme cams are used and can be restrictive to the inlet airflow.

Manifold Air Pressure sensor

These measure the vacuum or air pressure in the inlet manifold that in turn gives an indication of load, more commonly used on turbocharged engines to give an indication of boost level. This is often referred to as a MAP sensor, although not to be confused with a map.

Hot wire

This approach uses a heated platinum wire and measures the current required to keep it at a particular temperature. As air passes over the wire it cools it down, the more air that passes, the greater the cooling effect and therefore the greater the current. The hot wire system can be also be confused by reverse pulses when more extreme cams are used.

Operation of the system

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 engines cycle which best suits those two cylinders, again once per revolution. Where the engine sensors are able to determine the engines 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 engines 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 engines 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.

Engine temperature

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 affects cause the engine to run weak and require that extra fuel be supplied to the engine 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 engines temperature and enables it to ‘correct’ the fuelling. This correction involves adding a percentage of extra fuel according to a pre-determined 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 engines temperature and RPM since the affects of condensing are less when airspeeds are higher.

Air temperature

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 an extent airflow meters can compensate for lower density air since depending on their type they may show less volume of air inducted and this will cause the EMS to adjust the fuelling accordingly.

Battery voltage

If the voltage of the vehicles battery varies then it is likely that the time taken to open the injectors will 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. Some EMSs have a correction applied to the base map of injector times 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.

Mixture strength

Some EMSs make use of a Lambda sensor that 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.

Knock sensing

A knock sensor is an acoustic sensor that listens for pre-ignition more commonly known as knocking or pinking/pinging. It is most likely eradicated by adjusting the timing but there are circumstances where the mixture needs trimming as well. When this is detected the EMS is able to adjust the fuelling if required in order to help eradicate the problem.


Other Corrections

There are some additional corrections that the EMS can apply intuitively by examining changes in state or other derived conditions, the most common are:-

Acceleration fuelling

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 affect 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’.

Deceleration fuelling

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 unburned fuel to ignite in the exhaust system producing the characteristic popping on overrun. To overcome this some EMSs will either reduce the fuel to the engine on overrun or in some cases cut it off all together.

Cranking fuelling

When the engine is actually being started the cranking speed is quite low (150-200RPM or so) 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.


Additional information

There is some additional information about injection systems which does not fit neatly into any particular category but is nonetheless useful information. This is detailed below.

Injector position

The position of the injector in the inlet tract has a noticeable affect on the way the engine runs, it can affect economy, transient throttle and power output. It is generally accepted that injector positioning close to the inlet port gives good economy, transient throttle and idle together with good emissions and that injector positions further back in the inlet tract improve power at the expense of these criteria. Ultimately for the best power output the injector should be sited as far back as possible, I.E. in the trumpet or air-horn. Siting the injectors here does give a big problem at low throttle openings and low RPM since the fuel hits the butterfly; it can also cause fuel to be bounced out of the trumpet by the shock waves in the inlet.

Dual injector systems

Dual injector systems attempt to exploit the benefits of the close to port injector while also gaining from the power increase to be had from having the injector in the trumpet. The way this is done is to fit two injectors, one close to the inlet port and one in the trumpet. The EMS controls these two injectors using the near injector for part throttle, low RPM and transient and switching to the second trumpet mounted injector when the engine is at WOT (Wide Open Throttle). Some systems switch from one injector to the other immediately a certain set of conditions is reached, other system go 50/50 between the injectors or grade one injectors usage down while ramping the others up. This system if implemented properly gives the best of both worlds.

Twin injector systems

Twin injector systems are normally used when the size of injector required would be very large and might affect the metering and atomisation capabilities at low RPM and idle, typically on a turbocharged engine where fuelling requirements vary enormously from transient to wide open throttle. The fuel can be metered through one injector when requirements are low, and through both when requirements grow exponentially, or it can be metered through both at all times. Often a second set of injectors are fitted by after market tuners whose modifications may require fuelling beyond the capacity of the current injectors, this is most likely to happen in turbo or supercharged installations.

Injector duty cycle

In order to inject a fuel into the engine the injector is opened for a period of time, known as the pulse width, this time is always the same for a given quantity of fuel, regardless of engine speed. As engine RPM increases the time available per revolution to fire the injector is less, at 6000RPM the time available is exactly half the time at available at 3000RPM. As this injection opportunity gets progressively smaller the injectors are required to fire much more frequently; this can result in the injector being open almost all the time. When the injection system used is sequential the requirement is to be able to deliver the fuel at a time when the inlet valve is closed; this further reduces the injectors opportunity to fire.

The percentage of time that the injector is open is known as the ‘duty cycle’ and this represents the relationship between the time the injector is closed measured against the time it is open. If the duty cycle goes above 90% anywhere in the rev band (I.E. the injector is open for more 90 percent of the time) then the injector capacity is being reached and the engine may require larger injectors. These will discharge more fuel in a given period of time which means the injector times can be decreased bringing the duty cycle into acceptable limits. Unfortunately this also means that the engine will need re-mapping to suit the new larger injectors or the mixture will be hopelessly rich.

Some EMSs have a scaling factor which represents the relationship between the map figure units and the pulse width, by varying this the whole map can be scaled up or down for different sized injectors. This is not a perfect way of coping with a change of injector size because the time taken to open the injector is the same and the scale factor affects this too, however it will get 95% of the way there when changing injector sizes.

Injector sizing

In order to size injectors for a given engine it is important to know their discharge rate, from this and an approximation of the engines potential RPM and potential peak power and torque an estimate can be made and an appropriately sized injector chosen. It is better to err on the large side just in case you reach the injector capacity while mapping and have to start from scratch. Larger injectors have a couple of disadvantages in that the granularity of adjustment is larger and the atomisation of fuel is poorer with a larger orifice.


The clever stuff

As well as the normal running of the engine and administering of fuel according to the map settings some EMSs 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

When 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 which is set when the engine is cold and the choke is operating that 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 EMSs 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 and save more fuel, however the mixture has to be kept near stoichiometric for the catalyst to work effectively.

Open loop

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 EMSs also use map information only for ignition timing in this situation. However, a few EMSs 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 EMSs 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 EMSs 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 EMSs 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 EMSs 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 EMSs will communicate faults via fault codes or flashing lights, 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 systems 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 500 rpm 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 EMSs also provide a telltale 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 engines systems, it is very common for the EMS to control the cooling fan, switching it on and off as required.

Water injection

Some 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 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 gearchange 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 affect on the vehicles performance is marginal, however the affect 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 in 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 systems do provide outputs or feeds which enable the more enterprising to use the EMS information to make improvements to other aspects of the car. EMS information can be used for example 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 engines power is needed.


Feature disclaimer

There are many other features and options within after market EMSs 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 systems available. Suffice to say that the features listed above cope with 99.99% of what is required from a management system and in the interests of keeping it simple I will elaborate no further.