Repurposing fuel injectors as exhaust water injectors for valve life

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Doggzilla, May 18, 2019.

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  1. May 18, 2019 #1

    Doggzilla

    Doggzilla

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    Anyone ever seen any kind of exhaust injection to cool down exhaust valves for higher performance?

    Was considering putting a motorcycle turbo on an industrial liquid cooled 18hp engine, but the exhaust valve temps concern me the most.

    It would not be timed, it would just have a gallon reservoir and spray continuously at about 2-3gph during full power climbs, so it would last several flights between fills.

    As the exhaust would clearly blow the opposite direction it would need to spray when closed in order to get on the valves at all.
     
  2. May 18, 2019 #2

    PMD

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    Water, and aqueous solutions of ethanol or methanol have been used for many years to limit detonation in highly boosted engines. The result is that exhaust valves run cooler. The spray is injected into the intake, not the exhaust to achieve results (that are far more than just cooling exhaust). There are many commercially available kits that can be found on the net around turbocharger and other high perf auto supplier sites. Also very easy to make you own boost powered injection system.
     
  3. May 18, 2019 #3

    poormansairforce

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    Do it. The water/steam expands 1600:1 so it should drive a turbo well while lowering temps through it. I don't know why bigger setups haven't tried this. Material selection will be important to control corrosion. Maybe it would use the exhaust heat better and therefore raise the efficiency.
     
  4. May 18, 2019 #4

    pictsidhe

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    Fuel injectors will need to be cleaned and dried immediately after every use or they will sieze up. People have used them for intake water injection, but it tends to be limited to record attempts due the PITA factor needed to stop them dying after use. I'm also somewhat dubious that they will take the heat. You'd be much better off just using a fixed stainless spray nozzle. Do you have any idea how to measure exhaust valve temperature?
     
  5. May 19, 2019 #5

    Doggzilla

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    Great advice, guys. Since it doesn't need precise timing I might as well just use the nozzle with a simple valve to avoid locking up.

    I'll have to fit a check valve as well to avoid exhaust being driven back into the tank in case of a pump failure. And use a metal tank and lines just to make sure.

    I think the most difficult part will be power control. I believe a constant speed prop will overcome the control issues with such a setup, in combination with a turbo pressure control valve.
     
  6. May 19, 2019 #6

    mm4440

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    sotaro likes this.
  7. May 19, 2019 #7

    CharlieN

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    You might consider doing an online search about water injection, there is allot of information about this as it has been used in industry for decades as others are guiding you. Injecting in the exhaust does nothing to aid the engines combustion and reduce detonation and you will be quite amazed how quickly the exhaust valves will fail from shock cooling should you spray water directly on them.
     
  8. May 19, 2019 #8

    Doggzilla

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    I've considered it.

    Detonation really shouldn't be an issue. Manifold pressure can run up to 50 inches with 95 octane. That's what the Germans ran during the war, and used alcohol water injection for any higher pressure.

    Detonation is mainly an issue with air cooled engines due to uneven cooling.
     
  9. May 19, 2019 #9

    TFF

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    I would think if you could actually get the spray to hit the valve, shock cooling would have it broken in no time. I’m assuming you are already planning on machining some high quality valves to put in there first along with high quality guides and seats. Cost wise, a high pressure pump and injectors would probably cost more than the engine.
     
  10. May 20, 2019 #10

    pictsidhe

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    Which engine is it? I don't know of many smallish 18hp liquid cooled engines.
    I did buy a Honda mower with a water cooled twin two years ago as possible aircraft engine, but it turned out to have not been sold to me by the owner :/ Those are pretty uncommon.
     

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