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
|
|
0 |
1000 |
2000 |
3000 |
4000 |
5000 |
6000 |
7000 |
8000 |
|
0 |
8 |
25 |
20 |
35 |
38 |
38 |
38 |
40 |
40 |
|
1 |
8 |
15 |
20 |
32 |
34 |
35 |
35 |
38 |
38 |
|
2 |
8 |
12 |
20 |
26 |
32 |
33 |
32 |
34 |
36 |
|
3 |
8 |
12 |
19 |
26 |
30 |
31 |
32 |
32 |
34 |
|
4 |
8 |
12 |
18 |
25 |
30 |
30 |
30 |
32 |
32 |
|
5 |
8 |
12 |
18 |
25 |
30 |
30 |
30 |
30 |
31 |
|
6 |
8 |
12 |
18 |
25 |
30 |
30 |
30 |
30 |
31 |
|
7 |
8 |
12 |
18 |
25 |
30 |
30 |
30 |
30 |
31 |
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 will 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 dont 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 doesnt 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 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 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
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 having information about the engines cycle position as well as the crank position. This can simplify the implementation of the ignition system for an after-market conversion and provide feedback necessary for sequential injection.
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 air
