Load Control and Calculation
Methods of calculating, measuring and determining Load: Speed-Density, Mass Air Flow and Alpha-N
By: Brian Barnhill - Tuner Tools, LLC
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 = MAFmeasured/MAFtheoretical
-MAF measured= Mass Air Flow measure by the MAF sensor
-MAF theortical = calculated MAF at 100% VE (as shown in VE 101 article)
Remember that our Mass Air (Maf Theortical) equation (also used in the speed density equation) is:
MAF (lb/min) = [(Displacement *RPM/2)/123] * 2.7 (P/T)
- Displacement = engine size in cubic inches
- RPM = Engine Speed (the division by 2 is because an engine cycle takes 2 rpms)
- Division by 123 if for units, as displacement is in cubic inches
- P = MAP in PSI absolute
- T = IAT in temperature Rankine
The advantage of being able to calculate VE 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.
OEM engineers may spend 100s of hours or more just properly calibrating a MAF system, placement of the sensor, transfer functions, etc - this is not always fesible in an aftermarket application. So thought should be given to when and where a MAF is appropriate and when you should opt for a Speed-Density or other load and fuel stratagy. I often see many new tuners automatically assume a MAF will solve all their problems and simplfy the tuning case - and this is often not the case!
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. The equation for calculating mass air flow with the speed density method is shown in the VE 101 article and shows earlier in this article.
Speed-Density systems are very sensitive to temperature changes and altitude changes. As such, it is critical to pay close attention to temperature and barometric correction factors and how your ECU uses temperature in it's Mass air calculation. Speed-Density systems also MAY 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.
- Need sensors for your Speed Density setup? Check out Tuner Tools options HERE!
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” or certain regions and 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.