GR-7 Turbojet Engine Project 6/19/09

Posted on June 19, 2009

       The Main Control Relay (MCR) board is the central hub to my ECU design and the next to be engineered. The five “satellite” boards I created earlier will all tie into this master controller. The MCR will have several functions including direct control of the oil pump and fuel pump systems. The start cycle, and cool down cycles will also be regulated by the MCR. Included in the controller are four timers; the minimum burn timer (T1), the cool down timer (T2), the fuel purge timer (T3) and the starter motor shutdown timer (T4).
       The minimum burn timers job is to insure that the pilot burner has fired for a long enough time to preheat the evaporator tubes prior to the main burners operation. The cool down timer regulates how long the engine’s starter system will spool the engine after shutdown (cooling down the turbine below 400* F to prevent bearing damage). The fuel purge timer will help release fuel pressure in the system during shutdown and also burn off any unused fuel in the evaporator tubes. Last on the list is the starter motor shutdown timer which will prevent the starter motors from being overheated during a failed start condition.

       The desired basic operation of the GR-7’s ECU can be broken down in four operational states; the run cycle, the start cycle, the cool down cycle and the emergency shutdown cycle. All of these cycles will have a predetermined order of operations to be completed and checked by the “logic” of the GR-7’s ECU. To better illustrate the desired operation of the ECU I will explain the process of all the four cycles and how I “Hope” they will function:
       When the ECU is initially turned on (the “on” or “run” cycle) it will start up the oil pump and fuel pump motors. The ECU will also arm the speed controllers, engage the starter shaft and open the automatic combustor drain solenoid (to remove any standing fuel in the combustor). Once the ECU “sees” the minimum oil pressure of 25 PSI has been met and the starter shaft is fully engaged it will allow the start cycle to be initiated from the control panel. A green light at the control panel will indicate proper oil pressure at which the start switch can be activated.
       Once the start cycle has been initiated the starter motors will spool up the turbine to 2,600 RPM. A sensor on the FDC board will sense the starter motors spinning the turbine and after 4 seconds will actuate the pilot burner circuit. The pilot burner solenoid will actuate and the ignition coil will fire igniting the fuel air mixture in the combustor. A photocell detector will have 4 seconds to sense the flame or else the pilot solenoid will shut down (too keep raw fuel from flooding the combustor).
       If the photocell reports a successful burn (indicated by a green lamp on the control panel) the ECU will wait for two things: the turbine to reach the minimum RPM of 2,600 (RPM switch) and the minimum burn timer (T1) to time out in 8 seconds. If the turbine oil is too cold the ECU will allow the pilot burner to warm the turbine and hydraulic oil (with the starter motors running) until the turbine spins freely enough to reach 2,600 RPM. Once both requirements are met the ECU will initiate a full start and begin spooling up the turbine to 24,000 RPM (a green “RPM” lamp will illuminate on the control panel at this time). The acceleration of the starter motors will be set by the servo speed reducer circuit on the SMC board.
       At this time the combustor drain solenoid will shut and FDC board will look for the throttle interlock switch to be closed before activating the main burner solenoid (If the throttle valve is manually opened before this point the FDC board will not allow fuel flow and thus preventing a “hot  start”). Once the main burner is lit the engine will continue to ramp up to 24,000 rpm. When the engine reaches the required RPM to achieve sustained operation it will activate the combustor pressure switch which in turn will shut down the starter system.
       Once the engine is idling the starer shaft will have three seconds to retract from the compressor nut or else the SSC board will shut down the engine. If the shaft retracts correctly the engine will officially be in “run” mode and indicate so with a green “combustor pressure” indicator lamp on the control panel. The engine will now be enabled to run up to full RPM while the ECU monitors oil pressure and combustor pressure. If the startup cycle “hangs” during any part of the sequence an override timer (T4) will shut down the starter motors preventing any overheating damage to the motors.
       The shutdown/cooling cycle will be activated when the engine has been running and is shut down by turning the “run” switch off. A blue indicator lamp will indicate that the ECU is in the cool down cycle of operation. During this cycle the oil pump will remain to run while the fuel solenoid to the main burner closes. As the turbine slowly spools down the fuel pump will shut down and any remaining pressure in the fuel manifold will be purged out by the pilot burner into the combustor. This will be done by the fuel purge timer (T3) on the MCR board.
       After 20 seconds the starter shaft will reengage (once the turbine has slowed down below 5,000 RPM) and the starter motors will spool up the turbine to 2,600 RPM for four minutes. This cooling cycle will help cool down the turbine volute (scroll) to below 400* F (synthetic oil only) and prevent the heat from “soaking” back into the hydrostatic bearings. If too much heat is applied to the turbine shaft after the engine has stopped (without oil flow) it could cause the oil to “coke” or carbonize on the bearings thus damaging them. Once the cool down cycle is complete the ECU will shut down all operations.
       In the event there is a malfunction or the operator needs to stop the engine immediately one can activate the Emergency Shutdown Cycle. During the “ESC” the fuel pump will shut down along with all fuel solenoids terminating any fuel flow. The oil pump will remain on to prevent bearing damage and will keep running till the operator turns the engine “off”. The ESC will override any other cycle at any point of operation making it a useful option to abort a startup cycle if needed.
       Now that I have the grand plan explained I can attempt to make it happen!

       To get a good idea of what I was going to build I designed a “ladder” style diagram of the MCR (as seen below in the middle). The ladder diagram is mostly what I am accustomed to in my line of work as a commercial/industrial electrician. Most of the machines I work on use this form of schematic for the control circuitry. The use of a ladder style drawing can really be helpful in diagnosing a problem with a machine in the field as it is easily understood diagrammatically.
       By drawing out the ladder diagram for the MCR I was able to “test” the board before I even built it. I did this by running different scenarios through the “logic” of the board on paper to see what the different outcomes were. Several hours of research developed the rough idea resulting in the final drawing below. Now that the hard part was complete I could just needed to connect the dots and build the board.

       Having good success with the GR-5A’a ECU led me to follow my own footsteps and use a similar component layout on a generic “experimenter’s” style circuit board. However this design will differ in that it will have to “piggyback” one of the daughter boards to conserve space. This was planned out on the MCR board before any of the relays were permanently soldered into place to allow room for the standoff screws.

       Four holes were drilled into the board to allow four 2” x 6-32 stainless steel machine screws to be mounted for the standoffs.

       I chose to use my fuel delivery control board as the piggyback board on the MCR (as seen below). The circuit board terminal blocks were placed to easily connect the daughter boards to the MCR. Knowing before had to where the daughter boards would be located helped determine where to mount the corresponding terminal blocks.

       I decided to add a set of LED indicator lights that will help enunciate what the ECU is doing while in operation. It is very helpful to know what input or output is operating in a logic controller real-time as to help troubleshoot problematic sensors and devices.

       At this point I had added the heavy current relays and fuse protection to the MCR. Back feed protection was also added to the control panel telemetry outputs by adding switching diodes to the terminal blocks.

       Before continuing on to the tedious job of installing all of the jumper wires I needed to build the four required on-board timers I had described earlier. Having very little real estate left over on the MCR I shoehorned the remaining timer transistors and capacitors in the open space as seen above. Once wired I was able to test these timers and preset the timer values to what I estimated to be correct. Notice the flagged wires below which will help me remember where they go later.

       After countless hours of stripping wires and soldering I was finished with the MCR (or at least I thought). Proper bench testing of the MCR was the next project at hand.

       At this point in the build I had installed 31 relays, 66 diodes, 78 resistors, 25 capacitors, 16 transistors, 12 potentiometers, 30 LED’s and made over 950 wiring connections! Phewwww!!!!! :oP

       Testing the ECU as a complete unit was super important at this phase of the game. This would let me know if my idea on paper would translate to a working circuit. Using 12 volt indicator lights and jumper wires I was able to simulate engine operating conditions and take note of the ECU’s reaction to them.

       A couple of undesired operations needed to be reworked after the testing process. I moved several jumpers on the MCR as well as added a few switching diodes to isolate certain timer triggers which fixed the problems. I made sure to reflect the changes to my final ladder diagram for later troubleshooting purposes in the field (if needed).
       I now had the ECU working as I had planned making it ready for installation into the enclosure I had made earlier.

       To help support the ECU boards I installed several 6-32 machine screws to the back of the box. These screws in combination with nylon spacers will support the boards in the enclosure. You may also notice the 80 amp fuse block that I added to the ECU box (just below the ESC’s). This fuse will protect the electronic speed control units and downstream wiring in the event of a short or ground fault.

       Before adding the MCR board to the enclosure I needed to mount a few of the daughter boards. The RPM board from the start really had no place to fit into the ECU box. However after looking at all of my options I finally decided to mount the RPM board to the front side of the ECU box, just in front of the MCR (as seen below). A super tight fit but it will work.

       The next item to install was the speed control relay (SCR1). This 60 amp relay is what supplies the power to both ESC’s. Without this relay the speed controls would drain the supply battery in a few hours as they draw current even when idle.

       The SMC board was next to install. The two ESC’s plugged directly into the servo style connectors on the SMC as seen below. Additional jumper wires were pre installed to the board as access in the box is limited at this point.

       The ICD board or IGN1 was installed just in front of the SMC board. No room to spare!!!

       The MCR board was now clear for installation. Nylon tubing spacers were used to space the board off of the bottom of the ECU box. Stainless “nylock” nuts were used to secure the bolts preventing loosening from vibration (if any).

       The SMC board was the last to get bolted in. It was placed right above the ICD board as seen below. By now all of the ECU’s jumper wires were in place and it was ready for engine hookup.

       To prepare the engine for ECU hookup I needed to “gut” some of the existing wiring out of the manual control panel. Most of the wires that landed on the panel will relocate to the new ECU.

       I spent some time and properly rewired all of the control and power wiring on the engine. I installed connectors at each device to allow for easy engine disassembly without the need to remove wiring from inside the ECU box. All of the connectors (not shown) were numbered as to prevent confusion when reconnecting after repairs/service is performed.

       I routed all of the control wires though the 1-1/4” chase nipple in preparation for the final termination to the ECU (below). I soldered or “tinned” each wire end before connection as to promote good contact to the terminal blocks. I also numbered all of the connections for ease of identification.

       The final layout of the control wires layed into the ECU box nicely. I would like to think this was possible due to proper planning ;0)

       Only a few more details needed to be sorted out before I could actually fire up the engine under the control of the new ECU. For now I was pleased with the layout and looking forward to spooling her up.

       Join me again in the next episode of the GR-7 engine build where I hook up the photoresistor unit and tachometer sensor. It won’t be long before the GR-7 is singing away it’s beautiful turbine symphony!!!

       Thanks for dropping by my humble little site!!!

Don R. Giandomenico


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