GR-7 Turbojet Engine Project 7/5/09

Posted on July 5, 2009

       I am inching closer to the big test day for the GR-7 turbojet engine. Only a few details are left to complete before I can pack up the engine and head out to my “remote testing facility”. Some of these details include preparing the engine for certain tests I wish to perform when I am at the test site. One of the more important tests to conduct is measuring the static thrust of the GR-7. Knowing this measurement will help me determine the feasibility of using the GR-7 as a turbojet powerplant for the future GRV-2 jet bike project.
       To get the most accurate thrust reading from the GR-7 I will need to solve a common problem associated with turbocharger based turbojets; exhaust gas swirling. When a turbo-turbine is running it tends to create a swirl in the exhaust gasses as they come out of the high speed turbine wheel. This swirling action can cause a reduction in forward thrust as it spreads out the force of the exhaust gas in multiple angles as it exits the jet nozzle.
       An example of this swirling problem can be illustrated with a garden hose spray nozzle. If the nozzle is adjusted so that the water flow is in a tight stream it will produce more forward “thrust” against your hands than if adjusted to a 90* spray pattern. This is because the force of the water is axial to the nozzle and not spread out in a radial pattern counteracting it’s own force. Exhaust gas swirling is similar in that when the swirling gasses exit the jet nozzle they tend to fan out in a cone shape diluting the foreword thrust of the engine.
       Exhaust gas swirling is a common problem with turbocharger based turbojets mostly because of the high turbine speeds common to radial-inflow turbine wheels. The exhaust gasses entering a turbo-turbine do so in a circular motion which is accelerated as the gasses are forced closer to the axis of the turbine shaft (reactive centrifugal force). This accelerated “swirl” is partially diffused when the gasses exit the turbine wheel’s axial veins but not completely. This residual swirling is the problem I must deal with to prevent any loss in thrust.
       To fix this problem on the GR-7 I needed to create a set of “straightening vanes” in the jet nozzle pipe. These vanes will help gather and straighten the flow of the exhaust gasses as they leave the exducer of the turbo.

       To build the straightening vanes for the GR-7 I used some .058” thick 304 stainless steel plate. My plasma cutter was used to cut out two squares which will make up the four straightening vanes.

       Because these vane plates are made of a relatively thin material I decided to build them without using a lot of welding. Welding stainless is sometimes a tricky business and can seriously distort stainless if you are not careful. I want to keep the profile of the vanes as streamlined as possible to reduce aerodynamic (or gas dynamics) drag which warping would cause. To join the plates into the “X” shape without major welding I needed to cut two slots in the plates so they could be slid together (as seen below).
       This “cold joint” in the plates will allow them to expand at different rates and thus prolong their life on the engine.

       The plates were welded at the very ends to keep them aligned. Once the plates were solidly attached I proceeded to curve the leading edges of the vanes to hopefully match the angle at which the gasses will be striking them. This curve will hopefully help with the aerodynamics of the vanes.

       Earlier I had run the engine without the vane assembly to take note of how wide the exhaust gasses were fanning out of the jet pipe. A considerable amount of gasses were straying out in an uneven pattern which prompted me into researching a vane assembly to promote laminar flow. I now could test to see if the straightening vanes would work or not. To test the assembly I temporarily mounted the vanes into the jet pipe in preparation for a live engine test.

       I mounted the jet pipe to the engine and proceeded to fire up the engine. After several minutes of warmup time I throttled up the engine to about 50% to see if there was any improvement in the exhaust gas pattern behind the engine. It was clear that the column of gas was reduced in size which implies the vanes were working as planned. I had noticed that there was now heat discoloration on the jet pipe where each vane contacted the wall of the pipe. This was clear evidence that swirling gasses were being collected at the vanes and being straightened.
       I pointed the GR-7 towards a cherry tree in the front yard (about 35 feet away) and powered up the engine to about 80%. The pattern of leaves that were being buffeted by the exhaust clearly showed a round even flow. I was amazed at how much air was being moved over my front lawn and hitting my tree, it was sort of a super air cannon :0)

       After the engine cooled down I removed the jet pipe to inspect the vane assembly. By carefully observing the thin diesel soot film left inside the jet pipe I could see how the gasses were flowing over the straightening vanes. It was clear that swirling gas was hitting the vanes and being diverted. I was also able to tell by the soot lines that the 30* vane angle I had used was close to matching the flow of swirling gas. This was surmised by observing how the soot lines formed around the back side of the leading edges.
       After digesting this information I decided to modify the curve I put into the vane assembly to further streamline the flow of gas. I kept the same leading edge angle of 30* but broadened out the curve to improve aerodynamics.

       I welded the straightening vanes into the jet pipe just at the leading edge of two adjacent vanes. This is to allow the assembly to expand and contract without putting stress on the jet pipe itself. You will notice the small expansion gap at the top two vanes (see below).

       Proper testing of the GR-7 is an important safety issue for me as a possible passenger on the future GRV-2 jet bike. Since the engine will be in close proximity to my backside I want to make sure it is bulletproof and reliable. On test day I am going to subject the engine to sort of a torture test and see if it will safely run with no failures. Although I am pretty confident that this turbo’s heavy design will aid in it’s resilience I still want to make absolutely sure it can handle some reasonable abuse for a period of time.
       The most common turbo-turbine failure is too much heat put to the turbine wheel causing the turbine blades to warp and finally fail. Most times the turbine’s heavy cast iron “volute” or scroll will contain the molten shrapnel. However the volute can (rarely) separate from the CHRA (Center Housing Rotating Assembly) and spray metal everywhere. This is why it is paramount to keep the engine’s temperature, oil pressure, RPM and balance in check at all times.
       To help simulate the harsh condition of overheating (for testing purposes) I plan to use a “choke” on the GR-7’s exhaust nozzle. This choke will constrict the flow of exhaust gas and consequently increase the EGT of the engine. This increased EGT will help me determine if the engine can handle the extra torture of an overheating condition. I decided to choke down the jet nozzle about 5% for starters.
       I built a choke out of a small 20 gauge stainless exhaust transition which was modified to match the existing angle of my megaphone jet pipe. The diameter of the end was cut to 68 mm which is about a 4 mm difference from the existing jet pipe.

       I mounted the choke on the jet pipe with a single 8-32 screw on the underside of the pipe.

       With the choke in place I decided to run the engine to see if the back pressure had any effect on temperature. I was very surprised to see a 50* Fahr. increase in temperature at idle and nearly 100* at 80% (52,000 RPM). This would definitely be in the ballpark to create a stressful condition for the GR-7!

       At this point I was fairly prepared for testing the GR-7. Other than packing up the test equipment and loading the engine in the trailer I was ready. However I had one problem to fix and that had not encountered till now. As the GR-7 has been built up over the last four years it has gotten a little heavy. In fact the engine is clearly too heavy for one person to lift anymore.
       In order for me to test the thrust of the GR-7 I will need to remove it from it’s engine stand which is where it has been all it’s life. To do that it will need to be supported while the hardware is remove on the engine stand. The easiest way to accomplish this would be to lift the engine with a hoist but up until now I had not tried to do so. Luckily I have a 800 lb electric hoist in my garage that will do the job.
         To help aid in lifting the GR-7 now and in the future I decided to add “lifting eyes” to the engine frame. These eyes will allow me to attach a lifting bridle to the engine frame and hoist cable. I fabricated a set of eyes out of 1/4 mild steel and prepared the engine frame for welding.

       My plan was to weld on two eyes at the Center of Gravity (CG) on either side of the engine. I located the CG by lifting the engine with the hoist using a rope bridal placed on either side of the engine. In addition I had to remove the control panel temporarily to see where the true CG was. Once I located the CG of the engine I remounted it to the engine stand and replaced the control panel.
       I cleaned away the primer paint from the engine frame and aligned the lifting eyes for welding to the frame as seen below. Notice the damp towels I used to prevent the welding slag from sticking to the engine. Welding slag will stick ferociously to any exposed stainless :0P

       Once both eyes were welded in place I was able to repaint the frame.

       To attach the frame to the hoist hook I would need a lifting bridle. I went to the local Home Depot and picked up some 3/16 stainless steel wire rope and fittings to make a bridle. A set of clevis shackles would do the job of attaching the eyes to the bridle.

       Using my electric hoist in my garage I removed the GR-7 from its engine stand. At the same time I decided to get a rough weight measurement of the engine by adding a 110 lb scale to the cable running through the pulley block. Because the pulley block (hook) reduces the load on the cable by 50% I will need to double the reading of the scale. In this case it was a necessity as the engine is clearly over 100 lbs.

       Once the engine was suspended I added a weight bag to the jet pipe to counterbalance the control panel on the front of the engine. After deducting the weight of the bag, control panel and lube oil I estimate the engine to weigh just over 155 pounds dry.

Note from the author:

       Publishing my work on the web is an intense effort. Most folks don’t realize how much work is involved until they try to publish a site themselves. I now have several years of experience which helps to streamline the process but it still is a ton of work. The one thing that really helps keeps me motivated is the support of my readers. Every week I get a handful of very cool e-mails from people all over the world showing their appreciation for me posting my work to the web.
       Most of these folks are just like me, technically minded people who love machines and what makes them tick. Well, I can say that it is a pleasure sharing my work with those who appreciate it and I always look forward to hearing from folks who find my site useful. I will do my best to keep the information flowing and the projects interesting granted there is someone out there reading it. Keep those e-mails coming!!!    

       The GR-7 is now ready for it’s big test day and I am starting to get nervous, I mean excited :0) A lot is riding on how the engine will perform so wish me luck and please come back for the continuation of the GR-7 turbojet engine project!!!

Don R. Giandomenico


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