GR-7 Turbojet Engine Project 6/18/05

Posted on June 18, 2005

       The GR-7 Turbojet Project is finally underway! Time and funding had been in short supply for some time, keeping me from doing any work on the GR-7. With a new 2005 budget and my supportive wife Stacy’s blessing, I am now able to jump into this project with both feet. During the project I hope to further my gas turbine education by using new ideas and building techniques to forge a home- built turbojet engine.

       The basis of my research is to develop a “medium” sized, turbocharger turbojet engine that is capable of producing 65+ pounds of static thrust. This in itself is not a terrific challenge as similar engines have already been built. What makes this project a challenge for me is that I plan to build it small enough to fit into a mini bike sized frame. Compacting the engines components tightly together will enable me to fit the engine into a small, aerodynamic jet bike, built for the purpose of speed. Although a jet bike is an exciting prospect, I must first develop a successful engine small enough for the purpose.
       In addition to the size and thrust goals set above, there are a few additional goals that I have decided to attempt while building the GR-7. Here is a list of objectives I hope to accomplish while working on the project:

#1   My current GR-5A engine uses propane which has proven to be an effective fuel. Propane is convenient because it does not require a fuel pump to deliver it to the combustor, however it is not a viable option for my jet bike application. I would not be able to fit a large enough propane tank onto the small bike’s frame. To solve this problem I plan to design a combustor that will burn diesel fuel in leu of LP gas. Diesel, kerosene and jet-A all provide more BTU’s per pound of fuel than propane so a smaller fuel supply would be needed for the same amount of power. To do this I plan to develop an “evaporator” system that will be capable of burning low pressure liquid fuels.
       An evap system uses heat from the combustion process to heat an evaporator tube or tubes which “boil” the injected liquid fuel to a evaporated state. This vapor of fuel is then mixed with compressed air and burned in the combustor. Conventional liquid fuel designs use high pressure nozzles to atomize the fuel, allowing it to properly mix with the compressed air. The evaporator system will eliminate the need for a high pressure/high power fuel pump that would normally be used in a HP nozzle system. The evap system will also use a “low” pressure/low power fuel pump that should reduce the need for on board electric support power.

#2 Many commercial engine designs use stainless steel for parts that are subject to heat and corrosion. Both rotating and static gas turbine parts can be made with one or more of the stainless steel alloys. Stainless steel primarily consists of iron, manganese, silicon, nickel and chromium. The chromium component is what allows stainless to poses it’s corrosion resistant properties and the nickel is what gives it it’s strength at high temperatures.  Some stainless alloys best defend against corrosion and some against heat damage, depending on the amount of nickel and chromium in the alloy. There are stainless alloys that are designed specifically for weldability and some for the ease of machining.
       Because of it’s resilient properties, I plan to use stainless steel for my GR-7 combustor assembly. Heat and oxygen rich environments can erode carbon steel (mild steel) over time so stainless is a much better choice for a combustor assembly. I plan to use a 304 stainless steel alloy which is known for it’s weldability and will be perfect for a combustor setup where there is a lot of duct work welding. Unfortunately there are some drawbacks to working with stainless steel alloys. Stainless is generally hard to cut and machine compared to carbon steel. Stainless also requires a special welding process for it to retain it’s compositional qualities.
       Despite these issues I plan to learn what I can from fabricating with this alloy and hopefully make it work in my application.

#3   Lastly, To reduce the need for external support equipment, I plan to incorporate an electric starter capable of spooling up the turbine during startup. I hope to develop a “Bendix” style starter system that will engage and disengage automatically. This will enable the GR-7 to become a “turn key” powerplant, improving the overall ease of operation.

       I am basing the GR-7 engine on the Cummins T-50 series turbocharger which is a fairly common diesel engine turbo. Unfortunately the T-50 is a heavy turbo, weighing in at almost 47 pounds dry! This is a lot of weight to start off with so I will have to be careful not add too much additional weight to the engine. The VT-50 model that I had acquired late last year has a 3” compressor inducer and a 3.5” turbine exducer. The compressor housing is 11 inches in diameter and the turbo is 13.5 inches end to end.

       The Cummins VT-50 is an oil cooled turbo and features fixed diffuser vanes in the compressor housing. The VT-50 also has a 5” diameter compressor wheel, limiting the turbos safe top operating speed to about 66,500 RPM (tip speed of 1450 FPS). The turbo housing has an A/R ratio of .76 and has a 5” v-band exhaust flange integrated into the turbine “volute” (turbine scroll).

       Over the past few months I had been collecting the different parts needed to construct the GR-7 engine. Patient searching on eBay had paid off for me as I found a plethora of useful parts for pennies on the dollar. Pressure switches, fuel pumps, gauges, stainless steel tubing, solenoid valves, fasteners and fittings to name a few items. What I couldn’t find on eBay I was able to get from Summit Racing and McMaster-Carr which are top notch business in my book.

       I found a couple of Webster fuel oil pumps on eBay that were designed for use in residential furnaces. These eccentric gear pumps are similar in design to an automotive oil pump and can produce up to 100 PSI. The pumps have an internal pressure regulator and a pressure sensitive stop valve. The stop valve prevents fuel flow when the pump is not turning. My hopes were that I would be able to use this pump in conjunction with a 1/4 HP 12 VDC motor to produce engine fuel pressure for the GR-7. Being very curious if this pump would work, I decided to start experimenting with it first.

       I had purchased a set of “spider couplings” that would fit the electric motor and pump. After installing them on the shafts I started fabricating a pump bracket that would hold the two together.

       By using some 2-1/2” EMT and 16 gauge plate steel, I fabricated the pump bracket. I used my mill and rotary table to drill and tap the bracket to match the motor end cap. I then welded the end plate to the bracket collar.

       The final step was to weld on the bracket ears used to bolt the pump to the collar. I used 1/4” plate steel for the ears and then drilled them to accommodate the hardware.

       After bolting up the pump to the motor I conducted some performance testing to see what I could expect from the pump. I used some diesel fuel for a test fluid and installed a needle valve on the output for an adjustable back pressure load.

       The pump was operating at about 5,200 RPM and drawing about 9 amps at 12 VDC. At 100 PSI back pressure, the pump was able to flow .25 GPM (Gallons Per Minute) and at 75 PSI back pressure, the pump was able to produce .27 GPM (=16.2 gallons per hour). These readings are very close to what I estimate will be enough fuel flow to run the GR-7 at full throttle. The only way to find out for sure is to finish building the engine.
       The next step will be learning how to work with that lovely 304 stainless steel alloy!!! Be sure to check out the continuation of the GR-7 Turbojet Engine Project.

Till the next time,

Don Giandomenico


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