Load Control 101
Brian Barnhill -
Tuner Tools, LLC
Load Control
Load is essentially a measurement of airflow since, as discussed in
our
Volumetric Efficiency article an engine is essentially a large
air pump. Since airflow determines load and is directly correlated to
volumetric efficiency, and it’s operating parameters, including fuel
and ignition requirements, it is critical that we have an
understanding and a methodology for calculating, measuring and or
programming the load of their particular engine configuration. Once
airflow is known, fueling and other operating parameter simply become
trivial scientific calculations.
There are several methods for which load can be measured, each with
their own advantages, disadvantages and applications. These methods,
however, are not nearly as trivial as the equations which follow. It
is, however, methodical and with some study, time and proper analysis
possible to determine and understand each method and how it relates to
your engine and tuning approach.
This method relies on measurement of the actual air flow (in units of
mass/time) to calculate fuel flow directly. This is the most flexible
and powerful system of load calculation, but is not without
limitations and complications. This method relies on the data from the
MAF (Mass Air Flow) sensor to calculate airflow and send this data to
the engine's control unit (usually with the application of a transfer
function.) The advantage in this method is that VE need not be known,
since it can be calculated from the mass air flow using the equation:
VE = MAF/(Displacement
x ρ(stp) x Speed)
Where:
MAF = Mass Air Flow
Displacement = SAE Displacement of the Engine
ρ(stp) = Air density (at standard temperature and
pressure)
Speed = Engine Speed
The advantage of being able to calculate
volumetric efficiency directly from measurable parameters allows
for a much more accurate fuel map, minimizing long and short term fuel
trims thus maximizing power and consistency. The MAF method also
corrects for boost and changes in throttle as the values are always
positive and seen as simply higher or lower total mass flow.
As you can see in the following equation fuel flow rate can be
directly calculated from the MAF data. Using the known static injector
flow at the system fuel pressure makes calculation of injector
pulsewidths trivial. Fuel is calculated using the equation:
Fuel Flow Rate = MAF x Target Air/Fuel Ratio
One very crucial point to the success of mass air flow systems is
ensuring that the measured output from the MAF sensor accurately
reflects the real time flow data of the engine and intake system. As
mentioned previously, MAF systems use a transfer function in the
ECM/PCM to control the variations of the signal and attempt to ensure
an accurate reading (this is where the MAF calibration takes place.)
Errors in the actual vs measured mass air flow can lead to incorrect
fuel maps and reduced and even potentially harmful engine conditions.
Placement of the MAF sensor is critical as well, as the MAF sensor
expects clean and laminar air flow to accurately measure the true
airflow of the engine. This may complicate the intake system and may
not be possible in some applications as tight bends and compact setups
may not allow for proper placement of the sensor.
It is also possible in some cases to exceed the limit of a particular
sensor. This is common in applications running very high levels of
boost or with any application requiring a very high limit (such as
very high reving engine.) Once at the limit, the MAF sensor will be
outputting it maximum value, telling the engine that airflow is
steady, where in reality airflow is increasing. This will lead to lean
conditions, and potential engine damage. In applications where a
blow-off valve is used instead of a bypass valve the release of air
which has already been metered can create issues in transient and
on/off throttle responses if too extreme or not taken into account in
the tune. In some cases these can be solved with creative tuning,
replacement with more robust sensors or other correction factors. In
some cases, however, it may warrant using a different load measuring
approach. This will vary depending on the vehicle, tuner, operating
conditions and even driver preferences.
Speed-density is one of the most common methods of load control and
airflow calculations. This method uses an equation relating the
manifold absolute pressure (MAP) and the intake air temperature with
the known
volumetric efficiency characteristics of the engine to calculate
airflow, and thus makes it possible to calculate fueling requirements.
While this method is not as robust and flexible as the MAF approach,
it is not as sensitive to placement, errors and limitations in range,
and in some applications can calculate airflow almost as accurately as
a direct measurement. Speed-Density systems also have the benefit of
not requiring an obtrusive sensor directly in the intake stream, as
the MAF sensor can often actually impede airflow. The cost of
implementation of such a system is also significantly cheaper. In
speed-density systems the
volumetric efficiency must be known and recorded in a reference
table and will be used in air flow calculations. Using the universal
gas law (PV = nRT) it is possible with the MAP, IAT and volume filled
to calculate the mass of the air. This method is simple, but accurate.
With the mass airflow now known fuel requirements can be calculated
using the same equation used with the MAF method.
Speed-Density systems are very sensitive to temperature changes. As
such, it is critical to pay close attention to temperature correction
factors. This is due to changes in air density at differing ambient
air temperatures (recall the VE and MAF equations use density at
standard temperature and pressure.) Speed-Density systems also require
more time to calibrate, as the entire V.E. table must be programmed
into the ECM/PCM, and will be affected by engine component changes,
especially parts that drastically change airflow behavior such as
forced induction, cams and intake manifolds. Care should also be taken
to ensure the VE map is as smooth as possible while maintaining
adequate air/fuel ratios. An additional drawback with Speed-Density
system can be a decrease in airflow resolution (sample rate) due to
the calculation in lieu of direct measurement. Note that installation
of forced induction or large increases in boost may also require the
MAP sensor to be upgraded to a unit with a higher range.
Alpha-n tuning is often referred to TPS referenced load control, as
this method uses just the data from the throttle position sensor with
relation to engine RPM and correction factors to control fuel
delivery. This method is popular in many race cars, notably those with
independent throttle bodies, where consistent and repeatable mass air
flow and manifold pressures are not able to be measured. In this
method empirical data is used to determine airflow at a given throttle
position vs rpm, and is subsequently used to calculate fuel delivery.
This method is the simplest, using only the TPS and RPM data. The
drawbacks can be many though. On street cars, it often lacks the
resolution to provide proper drivability and emissions controls.
Additionally, since there is no direct correlation to airflow and
throttle position, any changes and tuning require significant time and
testing to recalibrate the tune. The addition of boost will create
issues, since a steady state throttle position may see changes of
airflow due to the additional air provided by the turbo or
supercharger. Additionally, Alpha-N tunes are sensitive to changes in
barometric pressure, since the air density changes with altitude, and
as such require a barometric calibration, in addition to temperature
and other correction factors. This approach can often be used in a
hybrid system however, such as a partial speed-density/Alpha-n tune,
as can be seen in forced induction cars with independent throttle
bodies. The alpha-N tune is used in “off-boost” condition, where
consistent MAP signals may be difficult to obtain. As boost increases,
the system will blend, and often switch entirely to a MAP based
system, thus creating a more robust and accurate system.
In all systems, certain correction factors will be necessary to ensure
consistent and reliable operation in all conditions. Cold starts,
changes in ambient temperature and air density, and even changes in
driving conditions can all impact the fueling requirements of an
engine. Cold starts will use the engine coolant temperature can
determine how much fuel to add to aid in starting and running of the
engine before it is at proper operating temperatures. Battery voltage
compensations will ensure that fuel mass flow will remain constant,
even with a drop in voltage (which may impact how long the injector
actually opens.) Acceleration enrichment can often be important
(especially in speed density systems) to ensure that fuel is added
during initial throttle tip in so lean conditions are not encountered
during acceleration These are just a few, and which correction factors
you use, AND the order in which they are applied to your calculations
will vary depending on vehicle setup, driving requirements/conditions
and even your engine management system. It is best to determine which
corrections are most important to your tuning method and how they
change your required fueling, then apply these in the proper location
in your engine management algorithms.
So which one do I use?
All this may leave you asking, “So which method should I use?”
As you may have already figured out, there is no set answer to this.
Each system has its advantages and disadvantages and can work on
virtually any setup, providing similar final results. The decision is
ultimately, one that should be decided by yourself and your tuner.
Choose the system which fits your budget and requirements, but is
flexible and accurate enough to safely tune your vehicle, and most
important of all, the method which you and/or any one else tuning your
car is most experience and comfortable tuning with. The best results
will always come from the system which is best understood by the user,
regardless of the data and methods used to achieve the results.
