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Application Overview
6
Application Overview
The scooter demo application uses the same simple state machine that described above. Complexity of
the application is limited to an Alpha N engine management strategy with system modifier parameters.
Engine management strategy uses table look up for fuel and spark parameters based on throttle position
and engine speed. Although table look up is restricted to exact values on the table, interpolation between
points is can be easily implemented through custom code. Once the table look up value has been
obtained for fuel and spark, various modifier values can used to obtain the final value used by the fuel
and spark controllers.
These fuel and spark modifiers are determined and managed by the User Management task. The
example application only uses two modifiers in the application but provides the framework for an
advanced algorithm. These variables are maintained in the User Management task but are added to the
table look up base value in a low level function. This action is performed in u16Calc_Fuel_Pulse_Width(),
which is used by the Engine Management task. Additionally, two fuel variables are available for tuning
that let you directly control the fuel pulse output when a fuel event is scheduled. These two variables are
Fuel_Cut and Fuel_Add and have companion variables for spark, referenced as Spark_Advance and
Spark_Retard. These direct fuel and spark modifiers are extremely useful for coarse and fine tuning of
the engine without the complexity of multiple modifiers.
For starting conditions, a fuel modifier value, FM_MSTART, adds additional fuel to the base look up table
value. When in the START state, a calibration value of MSTART is used. When the engine speed
increases to the RUN state, the FM_MSTART fuel modifier is decayed at a specific rate,
MSTART_Decay_Timeout, based on execution of the User Management task. The amount of the decay
is set by MSTART_Decay. Both values are sent in the User Management header file. This working
example shows how others can be used in a more advanced implementation.
For transient operation, the fuel modifier FM_MACCEL is used. Parameters associated with the transient
fuel detection are found in the user management header file. Tip in and tip out can be detected and
individually handled. Thresholds based on ADC count changes between new TPS_Filtered data values
can be configured along with decay size and rates. Transient fuel operation is intended to emulate the
accelerator pump functionality of a carburetor.
This application also demonstrates active load control through time-outs. The example that is used is the
fuel pump. When the application state machine is in the STOP state, the application must be ready to
begin starting the engine at anytime. As a strategy, pressurizing the fuel system is best done before any
attempt to fuel the engine is made. As a result, the fuel pump is turned on in the STOP state. However, if
the fuel pump was left on unconditionally, the battery would be discharged quickly. To prevent this, a
time-out is used so that the fuel pump is turned off after 3 seconds in the STOP state. This parameter is
defined in the User Management header file as FUEL_PUMP_TIMEOUT and the fuel pump activity is
controlled by the Fuel_Pump_Controller(). This simple routine is provided to show how time based
management of the loads can be done using engine operating states.
For a four-stroke engine, the process is a 720 degree cycle. Additional synchronization techniques must
be performed to transfer the low level processes from a two-stroke to a four-stroke operating mode. For
a one cylinder engine, a simple technique can be done to eliminate the need of an additional camshaft
sensor. If the system uses manifold absolute pressure (MAP), measurements can be performed to
determine if the engine is in a compression or exhaust stroke. This requires looking for a specific
signature of the MAP signal. This must be done at the low level. To do this, a toothed based MAP data
collection process is selected in the application header file. Specific teeth for collecting data are defined.
Signature detection is implemented in the Crank_State_ISR() in the SYNCHRONIZED state in
Crank_Sensing.c file. This signature detection algorithm can be easily customized. Implementation using
a cam sensor is also easily accommodated for as a simple flag enables the transition from the two-stroke
mode to four-stroke mode.
Small Engine Reference Design User Manual , Rev. 3.0
Freescale Semiconductor
27
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