Controls System

1.  DC Heat-Exchanger Pump Control System

2.  AC Heat Rope Control System

DC Heat-Exchanger Pump Control System

      The number of coils in the pre-heating system is fixed. Therefore, a level of control must be implemented so that one can adjust heat input to the fuel during engine operation.  A J-type Omega thermocouple reads the temperature of the SVO as it exits the heat exchanger.  This temperature is reported back to a microcomputer (PSoC CY8C29466-24PXI), which checks to see if it is in the optimal range (100-120°C). 

      Hysterisis is used to allow the microcomputer to turn the pump off when it reaches 110°C but turn back on only when it drops down to 100°C.   If the temperature is below the optimal range, the micromputer sends a signal to turn on the pump.  If the temperature of the SVO rises above the optimal range, the pump is turned off.  When the pump is turned off, the propylene glycol no longer circulates within the copper tubing drastically reducing the heat transferred to the SVO from the exhaust. 

      Because of its extremely high boiling and burning temperature, the propylene glycol can actually sit in the exhaust heat exchanger without burning or bursting the copper pipes.  Figure 13 below is the circuit diagram implemented to control the pump.  The output of the microcomputer has either a high (5V) or a low (0V) signal.  This signal operates a Fairchild Semiconductor 4N25 FS phototransistor octocoupler.  This part is used to separate the low power part of the circuit from the higher power part of the circuit.  It is important to know that because of the thermocouple temperature inputs (and their microchip requirements), the same ground could not be used for the high power and low power parts of the circuit.

DC On/Off Pump Control Schematic

      Inside the octocoupler there is an LED, which is connected to the low power circuit.  A phototransistor- which is turned on and off by the light of the LED within the DIP package- connects to the high power circuit.  In this way, as the low power circuit turns the LED on, the high power circuit is switched on as well by the phototransistor.  A 5V power supply is used in conjunction with a 330Ω resistor to ensure that the LED gets enough power to brighten enough to induce the phototransistor to turn on and off. 

      The phototransistor octocoupler has a built-in inverter to account for the fact that current flows in the opposite direction than expected (from the 5V supply, not from the microcomputer) meaning that the LED would turn on when the signal from the microcomputer is low rather than high.  An unfortunate side effect of this is that if the 5V source is turned off (thus sending a low signal to the octocoupler) this is interpreted as a high signal from the microcomputer (via the inverter) and the pump is turned on. 

      To deal with this, a relay controls the 12V from the battery such that it can only be connected to the pump when the 5V source is actually on.  A buffer is connected between the octocoupler and the microcomputer to protect the microcomputer from the 40mA current that would be headed its way. The phototransistor controls a higher power IRFIZ 14G transistor (60V, 8A).  This transistor is what turns on and off the connection between the pump and the 12V battery.  Any transistor that can handle above 2.58A (the current required by the pump) will work, but by keeping the amperage requirements of the transistor as low as possible, the capacitance of the transistor remains as low as possible thus requiring less power to turn it on and off.  A diode (FES16JT) is used to ensure current flows in the correct direction. 
 

     The circuit also shows a voltage divider for the temperature input (from the thermocouple circuit) that protects the microcontroller from large currents.  Te program within the microcomputer accounts for this division of voltage.  The 330Ω resistor and LED are tied to the same high/low output as the transistor, thus creating an easily visible marker letting the user know whether or not the transistor (and thus the pump) is on.

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AC Heat Rope Control System

      The high-pressure fuel line that leads from the pump to the fuel injector is replaced by a 7/16th in. copper fuel line to allow better heat conduction into the fuel.  The fuel line is wrapped with 180W heat rope model 3641K23 from McMaster-Carr.  Heat rope is a highly resistive wire that gets very hot when a current runs through it due to electrical resistance.  The heat rope and the fuel line are then insulated with ceramic fiber insulation in order to maximize the heating of the fuel.  The custom copper fuel line has thermocouple (Omega Type – J) fitting placed right before the fuel enters the fuel injector.  This thermocouple reports the temperature of the fuel to both a Labview program for analysis and to a microcomputer.        

AC On/Off Heat Rope Control Schematic

      The microcomputer (PSoC CY8C29466-24PXI) checks the temperature of the fuel to see if it is in the optimal range (100-120°C).  Using the two stages of heating monitored by two thermocouples and hysterisis the microcomputer turns the pump off when it reaches 110°C and back on when it drops down to 100°C.   If the fuel is too cold at the engine input, the microcomputer sends a signal to turn on the heat rope.  Similarly, if the fuel is within the optimal range or too hot, the microcomputer sends a signal to turn off the heat rope. 

      The circuit used to translate the high/low signal of the microcomputer is seen in Figure 14 above.  The 100Ω resistor limits the current if it were to enter the microcomputer, thus protecting it.  The 10kΩ resistor allows extra current to drain to the ground.  The NPN 2N2222A transistor is turned on and off by the high/low signal from the microcontroller.  The transistor, when energized, in turn completes the circuit allowing 12V to be supplied to a relay.  The IN4003 diode allows a current to run back through the loop.  When the relay is energized, it completes a secondary circuit connecting the 110V AC power source to the heat rope.  The circuit also shows a voltage divider for the temperature input that protects the microcontroller from large currents.  The 330Ω resistor and LED are tied to the same high/low output as the transistor, thus creating an easily visible marker letting the user know whether or not the transistor (and thus the heat rope) is on.