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Application Overview
accounted for. The end result in this case is the top dead center must be defined as tooth 7, creating an
offset of 180 degrees.
Once this is compensated for in the application, adjustment of the timing for fuel and spark is handled by
the application maps. It may be necessary to create a test map for fuel and spark to verify timing is being
properly controlled. Small test maps help provide useful validation tools. Test maps with all the same data
are good for basic timing validation. Progressive value maps provide a way to validated table look up is
performed as expected.
In the example application, one of the most important calibration variables is the MSTART. This is a fuel
modifier variable that adds additional time to the fuel pulse when the application is in the START state.
Proper setting of this variable will depend on the engine and fuel injector used, as well as atmospheric
conditions. With the debugger open and the application running, it is possible to adjust the MSTART value
between starting attempts to adjust the amount of extra fuel is added during startup. This is a delicate
process and leads to the art of engine calibration. As calibration is performed, it is important to get back
to a steady state before testing a new value. Specifically for MSTART, it is difficult as the failure to start
and run the engine can leave the engine cylinder with raw fuel and easily foul the spark plug.
As development of your application progresses, it will need to incorporate features for performance and
safety. It is advised to balance hard testing with simulation exercises. This is a necessary step in the
development process that should account for worst case code execution. Independent exercises for RAM
utilization and execution timing should be considered so that bandwidth of the system is not at 100%,
which can easily result in poor engine operation or runaway code due to stack overflow. This leaves room
for addition unforeseen complexity that can only come code execution on the real system.
Small Engine Reference Design User Manual, Rev. 3.0
Freescale Semiconductor
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