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 fueling
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.
Ignition management
There are two types of ignition management system, those
triggered by a distributor and those triggered from a crank position sensor, often
called distributorless. The adoption of the term distributorless can be misleading
since many crank triggered systems still use a distributor cap and rotor arm to
dispatch the spark to the appropriate cylinder. With these systems a crank sensor
and not the distributor does the triggering to the EMS.
Distributor based systems
Distributor based systems use a conventional distributor to
trigger the EMS but the distributor will have no in-built advance mechanism.
Typically the trigger will come well before the ignition point and the EMS will
work out when to fire the ignition coil. The spark is then carried to the
appropriate cylinder in the conventional way via the rotor arm and HT leads.
Crank trigger based
Since crank triggered systems only know the engine position and
not the cycle position they need a way of ensuring that the correct cylinder
receives the spark. There are three common ways of achieving this.
The first is to use a conventional distributor cap and rotor
arm that is normally attached to the end of one of the camshafts and routes the
spark to the appropriate cylinder.
The second method is to use two coils that are paired to fire
cylinders 1 & 4 and 2 & 3 respectively. When one of the coils fires it
sends the spark to both of its cylinders. One of these will be on the firing
stroke and will fire normally, the other will be on the scavenge part of the cycle
(exhaust stroke) where the spark will be wasted, for this reason these systems are
known generically as ‘wasted spark’.
The third method is to use an additional sensor on one of the
camshafts so that the EMS is aware of the engines cycle position and can fire the
appropriate cylinder at the correct time using individual coils for each cylinder.
©1987-2010
a Triumph Spitfire
