100LL and O2 sensors; your thoughts

Discussion in 'General Auto Conversion Discussion' started by pfarber, Oct 13, 2019.

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  1. Oct 27, 2019 #41

    proppastie

    proppastie

    proppastie

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    My 200 hp Mooney burns 17.5 on takeoff full throttle. Leastwise that is what my fuel flow says and over 40 gal fill up it is off about 1 gal.
     
  2. Oct 27, 2019 #42

    dtnelson

    dtnelson

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    Let's do some math...

    One US gallon of gasoline contains about 114,000 BTU. That is 45.42 horsepower.

    But the rule of thumb for gasoline engine efficiency is that about 1/3 of the energy goes out as heat out the tailpipe, another 1/3 is lost in direct engine cooling (in our case that's cooling airflow, which has a drag penalty), leaving the last 1/3 to actually do work. So, you're actually getting about 15.1 HP/gallon of work, or "useful horsepower".

    So, for 200 HP, you'll need around 13.2 GPH. Bottom line, sorry, you're just not going to get 200 HP out of 7 gph, let alone 5.

    https://en.wikipedia.org/wiki/Gasoline_gallon_equivalent
    https://en.wikipedia.org/wiki/Engine_efficiency
     
  3. Oct 27, 2019 #43

    TFF

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    Engine architecture is why a 350 V8 auto engine can run 11:1 compression while a Lycoming 360 is going to be 8:1.

    One, water cooled vs aircooled. One engine is 200F and the other is 400F. The hotter engine less friendly to the air/fuel.

    Two, piston area. Bigger landscapes are more detonation prone. 5” Lycoming bore will be more detonation prone than a 4” V8 bore. Add to that combustion chamber design. Open with the Lycoming; very tight with squish area on the V8. Small piston size revolutionize sports car and motorcycle racing in the fifties and 60s. Rise of Ferrari and Honda. Same general fight if you loved Ford Cleveland engines and had a two barrel engine vs the four and they went with the two barrel heads for emissions and made a dog of an engine that was awesome.

    Knock sensors work well on a dampened water jacketed watercooled engine than a one layer of thin metal aircooled. Too much noise on an aircooled. Duel ignition on the Lycoming to have two flame fronts so the flame front never gets to detonation.

    What the Lycoming has for it is RPM matched to off the shelf propellers, wonder why? Minimum parts count; you only need four cylinders because the piston speed and weight don’t have to be light because it does not rev. It also has more efficient because of the lower parts count to around 3000 rpm. It starts capping out above that. Where the V8 picks up is where the Lycoming leaves off. It can rev and make more power. Now to use the power you have to add a transmission. Higher parts count and more points of failure that has not been tested for the application.

    200 hp is only at sea level. At 7000 ft it’s around 115. At 7000 you only have to control detonation to 115 hp. Altitude also drops the need for higher octane gas. Ever notice there is not much 93 octane in Denver?

    Variable timing is one major reason a Lycoming looses out in cruise. Big chambers have timing safe at 20-25 deg of advance that’s for full power. At cruise it could be thirty. P mags make my friends RV7 about 7 gal an hour LOP. He is not eeking out every gallon but he is confident in it to fly it over open water a couple of hundred miles. That means he is not thinking it’s going to fail. He is not worried about fetching it 500-1000 miles from home.

    You have to take in consideration if you are pulling around a two seat or four seat plane. Most homebuilt formulas are borrow a four seat airplane engine and stick it on a two seat. Gee more performance. Rare you will get more if two to two or four to four.
     
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  4. Oct 29, 2019 #44

    pfarber

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    Do you run WOT the entire flight? I don't think that's a thing. CRUISE flight, that's where you plan your fuel consumption.

    I'm shooting for ~ 10gph at cruise.

    HP = TE x Fuel flow (PPH) x 19,000 (BTU per #) / 2545 (BTU per HP per hour)

    HP = TE x Fuel flow (PPH) x 7.466

    268 HP = .3 * (6*20) * 7.466

    The closer number is

    134 HP = .3 (6*10) * 7.466 Hmmm this looks like a good cruise number... don't it?

    And for the brief WOT few minutes at takeoff climb:

    201.528 = .3 * (6*15) * 7.466 15GPH for WOT.

    So yeah, tell me why shooting for 10gph or less is way off base?

    PS: my algebra would be so happy, I dun showed my math.
     
  5. Oct 29, 2019 #45

    dtnelson

    dtnelson

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    :)

    I love it when people do the math! (I'm a retired EE).

    But I think you make my point... if I'm following... When you multiply 286 HP times 0.3 (the efficiency correction?), isn't that resulting in about 86 HP? I.e., not 200 HP @ 7 GPH, which is what I think you were shooting for before?

    Shooting for 10GPH or less at 200 HP is where I'd claim the math shows a problem.

    (And yes, BTW, I'm commonly at WOT in cruise. My typical cruise power setting is WOT, 2550 RPM, whatever MP I'm pulling, lean to around 9-10 GPH, and look for 182-185 Kts true).

    Thanks for a great discussion!

    :)
     
  6. Oct 29, 2019 #46

    proppastie

    proppastie

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    well if my Mooney at full throttle is 17.5 gal/hr then..... 10 gal/hr/17.5 gal/hr x200 hp=114.28 hp.
     
  7. Oct 29, 2019 #47

    TFF

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    Fixed pitch cruise is usually pitched under max hp so there will be some semblance of load on the engine. Except aerobatic and STOL rarely do you want excess rpm availability. Fixed cruise essentially have to pick a prop that say gives you 24 square at 3000 ft and live with the other miss match or whatever else your main mission wants.

    Most flight manuals have the hp vs altitude chart. That’s why you are leaning. Strategy flying. WOT is fastest with most fuel burn. Most constant speed prop planes can run less power and gain much more range for only giving up a few minutes in the air. Definitely gain verses having to make a fuel stop for the same distance. Rare to see constant speed prop at wot in normal flight unless aerobatics is the main flight mode.
     
  8. Oct 29, 2019 #48

    dtnelson

    dtnelson

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    "Rare to see constant speed prop at wot in normal flight unless aerobatics is the main flight mode" ... well, then either you've got a turbocharged engine or you never climb very high...

    Perhaps a way to think about this is, instead of throttle and mixture settings, consider % power. I can (and do) get very high % power down low, but at 12,000, and with cruise RPM set, it's more like 50-60% - at WOT.

    So yea, not so rare to see constant speed prop at WOT. Depends on altitude. Even at middling altitudes, say 6-10K, in cruise configuration, I'm mostly at WOT and around 2500 RPM. I won't see "squared" manifold pressures at 2500 RPM until I'm below 5000 feet.

    :)
     
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  9. Oct 29, 2019 #49

    TFF

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    All my turbo flying is in helicopters and I have no need to go over 10K in a non pressure plane as I am not near mountains. I think we made the same points actually. Talking points different. Of course with the constant speed prop you can change the load and when it gets that anemic go wot if you are at those altitudes. In the Mooney and Cirrus I use to fly we alway went for range never WOT. Save five gallons and fly thirty minutes more. Speed penalty was not much. Fixed pitch you are stuck with the RPM. A friend has this problem on his 320 powered Yankee. Max pitch per STC. It will go 2850 RPM level. Technically you are supposed to pull it back to 2500 per STC. An RV 6 set up the same would have 10” more pitch.
     
  10. Oct 30, 2019 #50

    proppastie

    proppastie

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    and if your plane is as old as mine...the charts were not known for their accuracy....as they were written by the marketing department.
     
  11. Oct 30, 2019 #51

    pfarber

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    I understand the concern. You mostly cruise at 55-75% power. So you only use all available HP at T/O and climb. So I pretty much accept the high fuel flow because there is not much I can do during this phase of flight.

    But at cruise, I can clean up the AC, get a good FP cruise prop, and optimize the engine for this much longer segment of the flight.

    What optimizations? Well:

    Tuned exhaust
    With VVT I can move the torque/HP curve a little
    The LV3 is rated at 285HP at 5500. I plan on running it at 200HP@3500 and tuning for this range, So I have a LOT of headroom if I want higher performance (and the fuel numbers don't get to wacky).
    Pitch the prop for best cruise (I have yet to see an airport where my T/O numbers would be an issue)
    Dyno and map the EFI/EI to my most desired range.

    None of these options are available (some engines do have an STC for a tuned exhaust) for certified motors. Most posts/articles I read about conversions never really go into fuel efficiency, they just seem to be happy it works, and the numbers are what they are.

    This is why I stated 10GPH, but I want lower. End even if I stay at 10GPH, for a 200HP motor that's still not bad.
     
  12. Oct 30, 2019 #52

    blane.c

    blane.c

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    Ditch the leaded fuel. Leaded fuel is the problem. Duh! Obviously.

    Use unleaded fuel, high octane unleaded. Problem solved.
     
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  13. Oct 31, 2019 #53

    pfarber

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    Sure, now I will only land at airports with MOGAS on site. There are DOZENS of them!

    The problem is the FAA won't force the removal to lead. Airplane motors are still trapped in the 50's.

    The certified engine people can engineer the solution, but they simply don't want to, because the FAA has granted a duopoly to Lyc/Cont. Its not like MOGAS isn't STC'd into 100's of engines. Because it is.
     
  14. Oct 31, 2019 #54

    dtnelson

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    Lead is an octane booster. Octane is a measure of how much compression a fuel can take before detonating.

    Octane is NOT a measure of how much energy the fuel contains.

    Lead doesn't add or subtract energy (or, more accurately, ability to do work).

    Removing (or adding) lead has nothing to do with the energy content of the fuel...

    :)
     
  15. Oct 31, 2019 #55

    blane.c

    blane.c

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    Leaded fuel is still the problem. Whatever else is going on. How to get more airports to have unleaded fuel option may be a better question/problem to solve.
     
  16. Oct 31, 2019 #56

    dtnelson

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    From an environmental standpoint, yes, for sure! But, unfortunately, Lycoming and Continental engines are made for leaded fuel, mostly because the lead acts as a lubricant for the top of the cylinder. Only a few of them (Lycoming O-320s, I think), have been STC'd for non-leaded fuels.

    :)
     
  17. Oct 31, 2019 #57

    pfarber

    pfarber

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    At 7.5:1 you don't see much need for 100octane.

    The Rolls Royce Merlin engine made 1000HP with 87 octane Avgas that had .14 g/L TEL (lead). Today an puny O-320 needs 100LL at .56g/L with only a 1PSI boost in compression?

    This isn't about the energy of the fuel (although the additives to control the lead would lesson the available BTUs).. no one said that at all, ever.

    The problem is that Lyc/Cont simply don't want to fix the issue. Cars have run without lead or high sulpher for DECADES with a few simple changes.

    At with magneto's at a fixed 20deg advance, its not like timing is an issue, either.

    Convince me there is a need for 100LL other than some outdated 'detonation' theory.
     
  18. Oct 31, 2019 #58

    pfarber

    pfarber

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    "Auto Fuel STCs are approved on 48 different engine types and 100+ airframes including nearly all 80/87 octane engines and the majority of airplanes in which these engines were installed. Several high compression engines are also approved for the use of 91AKI auto fuel including the 180 horsepower 0- 360 and the 115hp (or less) 0-235-L2C."
     
  19. Oct 31, 2019 #59

    pfarber

    pfarber

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    This.

    MOGAS at airports is rare, and I don't see FBO's paying the money to install tanks and pumps unless the gas sells. But few engines can burn car gas, so chicken, meet egg.

    Finding a compromise that allows both fuel types would open a floodgate of possibilities. Even if the O2 sensor is a 100 hour consumable, that's 2, maybe 3 years of 'normal' flying for most E/ABs
     
  20. Oct 31, 2019 #60

    TFF

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    Actually most engines made in the last ten years from Continental and Lycoming can be run lead free. Valves and seats are different than of old and the roller cams are easier on valve train and can accept more epa acceptable oils. Those engines still might need high octane, but those fuels can be unleaded. Legacy engines are the hold up. The FAA can’t protect the safety of the throngs minions if you put unleaded in an engine last rebuilt in 1975. When they made 100ll it was to get rid of multiple fuels. Now there is a need. No money to implement. The problem arises on how do you keep people from pumping the cheaper unleaded airplane fuel when it’s supposed to be the more expensive 100ll? The FAA is waiting for some attrition number until the unleaded will be released. This is not an industry on the upswing so there is no money to push forward. That’s why unleaded airplane fuel is market stagnant. Auto fuel STCs allow unleaded just no alcohol.
     

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