GR-7 Turbojet Engine Project 11/26/05

Posted on November 26, 2005

       My goal of building an all stainless steel combustor was about to come to fruition. The GR-7’s combustor had been bench tested and was now waiting for the last touches before final assembly. I wanted to complete the stainless fabrication so I could bead blast the entire combustor and then install it on the engine frame. To do this I had to fabricate and install one last system into the combustor.

       I had spent some time to outline the basic layout for the ECU (Electronic Control Unit) and discovered that I did not make any provisions for a burner control circuit. To properly control the starter and fuel flow solenoids, the ECU would need to have a flame sensor in the combustor. The flame sensor will tell the ECU if the burner has ignition or is just dumping fuel in the combustor.
       In a commercial oil burning furnace, a photoresistor sensor is used to notify the burner circuit that the furnace has ignition. If the “photocell” reports that there is no ignition light the burner circuit will either stop the fuel pump or try to re-ignite the burner. I plan to use a similar setup where the ECU will only allow fuel flow when the flame sensor sees ignition.
       I needed to install a window that could be used with a photoresistor cell to sense when the combustor has ignition. To do this I would need to devise a sight glass assembly that could be welded into the side of the combustor. I had picked up some parts from
McMaster-Carr including a 1” 304 stainless threaded cap, 1” hex head plug and a piece of heat resistant borosilicate Pyrex glass.
       My plan was to modify the plug and cap to hold the 1” X 1/4” piece of Pyrex glass thus creating a sight glass assembly. To start off I cut the back side of the 1” cap off so that there was only about 3/4” left of threads. I then cut out a 10 gauge disk of 304 to be used as a back plate for the assembly. A hole was then bored into the 1” hex plug and then modified to receive the 1” Pyrex disk. Notice the stepped face of the plug, this will help center the Pyrex glass disk in the assembly.

       I welded the 10 gauge plate onto the back of the cap threads to create the sight glass receptacle. The plate was then bored on the lathe to match the hole in the 1” hex plug. I cut out some high temp gasket material to seal the Pyrex glass disk and cushion it from the stainless parts on either side. The assembly was now ready to be welded into the side of the combustor.

       I decided to locate the sight glass on the inside top of the combustor, exactly opposite of the spark plug hole. This will be a good spot for the photoelectric sensor to be located. I plan to hold the photocell against the Pyrex glass window with a rubber plug. I hole-sawed the hole for the assembly and then TIG welded the sight glass receptacle into place.

       I positioned the sight glass inverted and on an angle so it would not collect dirt and oil in the window port. With the same idea in mind I purposefully did not place the sight glass over a flame tube hole. This is because I wanted to reduce the chance of fuel spray from contaminating the glass. With any luck the sight glass will stay clean in it’s location and not require frequent cleaning.
       The sight glass was the last addition to the combustion chamber. I was now ready to bead blast the combustor. I prepared the combustor by lightly sand blasting it with silica sand to take off the rough spots. This will prepare the stainless for the “gentle” bead blasting process and help produce an even finish. I loaded up the combustor into the very small blasting cabinet and started my work.

       Unfortunately my blasting cabinet was a little too small for the combustor and fought me the whole way. I broke the lamp inside of the cabinet several times just flipping the combustor around. :0( Despite the difficulties I was able to complete the job in about an hour. The combustor was looking good and ready for the next step.

       The next step was to weld on the combustor elbow. This would be the last weld I would have to do on the combustor. After all of the time and money spent on this unit I wanted to be sure I would do this one right. I tack welded the combustor to the elbow and then taped up all of the holes. I then installed the back purging stinger into the turbo flange side of the combustor and taped it into place.

       I purged the combustor out for a few minutes and then proceeded to weld the elbow joint. I tried my best to penetrate the joint but not undercut or sink the bead. I “practiced” on the underside joint first and then moved up to the other three sides. All in all the joints looked good and the back purging system kept the back side of the weld pretty clean.

       I blasted the combustor before I welded on the elbow because of the anticipated blasting difficulties I encurred earlier. I figured that I would weld the two after the process and then touch up the weld joint later. This is exactly what I did. I sand blasted the weld joint and then bead blasted it in the cabinet. I barely got the combustor to fit into the cabinet diagonally :0)
       It was finally done!!! After 200+ man hours over five months I was done!!! The GR-7 combustor assembly was complete and ready for mounting. I was rewarded with a great sense of accomplishment for what I was able to build in my little household garage.

       I wasted no time and prepared the combustor for engine assembly. First I needed a end plate gasket made to seal up the combustor so I decided to cut one out. I used a light grease to coat the combustor end plate flange so I could use the flange as a stamp to leave an impression on some gasket material. I then cut the gasket using the stamp impression as a reference.

       I installed the Pyrex glass into the sight glass receptacle. I then installed the burner assembly and end plate onto the combustor using the newly cut gasket.

       I installed the completed combustor assembly into the engine frame. I attached the two fuel lines to the fuel valves and plugged up the drain holes on the bottom side of the combustor. I coated the spark plug threads with an anti-sieze agent and then tightened it into place. I then attached the plug wire and tested it for spark.

       I followed up the assembly by installing the VT-50. I hooked up the oil lines and clamped on the compressor coupling.

       It occurred to me that I was subconsciously preparing the engine for a test run. In the feverish desire to complete the combustor I had developed a need to test the engine right now. I was really not prepared for a test at all as I had not properly plumbed the fuel system or pressure sensors yet. Everything was kinda temporarily strapped into place.
       I had no EGT gauge, no combustor pressure gauge, no RPM gauge  just a fuel pressure gauge and an oil pressure gauge to work with. It would be a gamble to attempt to start the engine without proper instrumentation but I had to know if it was going to work. Hopefully my previous turbine experiences would help me to get the engine started safely.

       I rolled out the engine stand out and chocked up the wheels to prevent it from rolling away. I then wired up the fuel and oil pumps to my lawn tractor battery and tested them for proper pressure. The ignition system was also wired to the battery and tested once more. Everything seemed to be in the right place and working properly.
       My nerves were kinda shot as I prepared for the start. This attempt would be the culmination of months of work and literally thousands of dollars. If I burned up the turbine or combustor it would probably kill my desire to continue (well, for a little while :0) It was kind of one of those things I did not want to think about too much. Kinda like jumping off a 50 foot cliff into a lake, the longer you hesitate, the less you are sure you can do it.
       I turned on the engines support systems and fired up the leaf blower. I was not sure if the leaf blower would have enough power to spool up the VT-50. I would just have to feel it out and listen very carefully to how the turbine responded to the blower. I ignited the pilot flame while the blower was attached and running at idle. This allowed me to warm up the turbine a bit and “free” up the bearings.
       I spooled up the engine and cracked the evaporator fuel valve to hear a distinct growling noise. The valve was very sensitive so I had to be careful with it. I shut down the pilot flame and allowed the engine to warm up some more. I then slowly opened up the fuel valve and listened for the “ramp up” of the turbine. I bumped the valve a bit and a 12” flame shot out of the exhaust. I quickly closed it to quench the flame which in turn shut down the combustor.

       I re-lit the pilot flame and started the evap system once again. I slowly increased fuel pressure while at full blower throttle. I heard a hissing noise coming from the exhaust that was increasing in volume. The noise subsided to a turbine whine which was increasing in pitch. I attempted to pull the blower out of the inducer and sure enough, the GR-7 was running!!!
       I was hesitant to increase the throttle much because of the lack of instrumentation. However I did mess around a bit to check throttle response and to see how low it would idle. I was checking to see if there was any visible fire coming out of the turbine wheel and couldn’t see any. I did notice a  little smoke coming from the turbo center section but is was probably just a bit of oil burning off.
       I checked out the sight glass to see what I could and noticed the color of the flame was bluish white. This was a far cry from the sooty yellow flames that were coming out of the combustor during the bench test. It seems that the higher atmospheric pressure did in fact change the diesel’s burn characteristics.          

       After running the turbine for 15 minutes I shut her down. I used the leaf blower to cool off the massive turbine volute so I did not cook the bearing oil to the turbine shaft. I let the engine rest for a bit and then set up my video camera to get the next start attempt on film (See the video here). The second try went a little smoother but once again I almost had a hot start and had to close the fuel valve to contain the flames.
       Wow! It works!!! I couldn’t be more ecstatic. This is just what I needed to get me motivated to finish the build. With so many systems left to develop, I am going to need the extra encouragement this test has given me. I can now rest assured that the project is a partial success and that I am not working toward a lost cause :0)
       I still have two major goals left to complete: #1 To achieve 65+ pounds of thrust and #2 to build an electric starter system for the GR-7. Both of these goals will not be easy but if there is a will there is a way. Be sure to check back soon as I plan to work on the throttle valve assembly and plumb up the engine’s pressure sensors.

Till Then............

Don Giandomenico        

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