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Clean Snowmobile
Piggyback

The object of the Piggyback group is to reverse engineer the existing stock engine control unit (ECU) on a 2003 Arctic Cat four stroke 660 cc snowmobile engine.  This reverse engineering will allow improved control of emissions after the addition of a catalyst.  Particularly, hydrocarbon (HC) and nitrogen oxides (NOx) emissions will be monitored for optimum performance in the SAE Clean Snowmobile Challenge (CSC).  Since HC and NOx emissions are the most important to control in an engine, the CSC has based the emissions part of the competition on measurement of HC and NOx in each of teams’ sleds.  One of the goals in this project is to minimize the HC and NOx emissions for a total minimum percentage of emissions required for the greatest amount of design points.  (CO emissions are also considered in the test, but are weighted less than the HC and NOx emissions levels).  Table 1 lists the operating conditions of engine emissions testing.   Scoring for the emissions event is listed in Equation 1.  The control on emissions will be attained by modifying the existing engine control unit.

 

Table 1: Operating Conditions for Emissions Testing

Operating Conditions

        

Equation 1: Sled Emissions Number, must be greater then 100
Equation1 

  Table 1 and Equation 1 courtesy of SAE CSC 20056Rules

Equation 2: Scoring for CSC Emissions Test

Snowmobile scoring

There are several options in modifying an ECU.  A “stand alone” system allows a completely new engine control unit to be employed on the engine.  This requires all new sensors, wiring and computer to allow the engine to operate.  Another option is to use a “piggyback” or override for the existing ECU.  The idea behind the piggyback is to allow a separate control unit or computer programmed micro controller to bypass the existing engine control unit when certain operating conditions are met by the engine. 

 Sensor Block Diagram

Background information on the operation of ECUs is a major part of our project and a key element in decision between a piggy back system and a stand alone system.  The main purpose of the ECU is to determine the pulse width of the fuel injectors.  Pulse width is defined as how long the injectors stay on.  The pulse width is determined by sensors which act as inputs to the ECU.  The ECU looks at the incoming signals, and through programming logic and data tables determines the appropriate pulse width for the situation.  Below in Figure 1 a block diagram is shown with the inputs and outputs from the ECU.

 

Figure 1: Input/Output Diagram for snowmobile Engine Control Unit
 

Input-output
Last Years Piggyback
           
            The piggyback design from last year used the wideband O2 sensor to find out what the Air/Fuel ratio was on the 2003 Arctic Car.  With the A/F ratio being controlled on the sled the exhaust gas temperature could be increased in the catalytic converter.  This would help the converter to burn hotter, reducing the HC, NOx and CO.  Two graphs depicting the emission reductions with the piggyback are shown below in Figure 2 and Figure 3.

                                       
                               Figure 2: Reduction from Catalytic Converter w/Piggyback                          Figure 3: Reduction from Stock emissions w/Piggyback
                                       reduction previous                                            reduction stock

            Although the emissions were greatly reduced there was one part of the sleds testing that emissions could still be improved.  While the sled was at idle both  HC and CO emissions were still high. This can be seen below  in Figures 4 and 5. From time 0-30 seconds the sled was being held at the idle RPM range, which is about 1200 RPM.

                                                               Figure 4: HC vs. RPM                                                                                                 Figure 5: CO vs. RPM
                                       HC                                          CO
            To better control the sled at idle we add an additional input to the design.  By reading the output of the Throttle Position Sensor we have another method to control the sled, in addition to the O2 sensor.  This will tell us the difference in the sled at idle, then we can write a computer program to control this and lower the emissions further. A diagram of the piggyback can be design below.

Micro Skematic