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2 Answers

Metering fuel

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Aircraft Systems, Commercial Pilot, General Aviation

When in cruise once the mixture has been leaned for best peak in a C172, I execute a climb from let's say 2500ft to 5000ft for some maneuvers. In a fuel injected plane without adjusting the mixture how does the engine meter the fuel flow to allow for best mixture if the mixture control is not being used?

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2 Answers



  1. John D. Collins on Mar 06, 2013

    You will normally have to adjust your mixture to enrichen it for climb. That having been said, many fuel injected engines have a throttle – fuel controller linkage that increases the fuel flow a greater amount at the full throttle position, thus enrichening the mixture for high power settings. I don’t have information on the C172 so I don’t know if it applies, but you can test it by being in level flight, full rich mixture, and retard the throttle slowly. If the RPM stays at the same RPM for a fair amount of throttle travel before it starts to drop, then I would surmise that the initial movement of the throttle reduction is just leaning the engine and it is still developing full power until it reaches a point where additional reductions start to reduce the power. On my Bonanza, I can probably reduce the throttle 1/4 of its full power range without noticing a change in power, in other words the movement of the linkage in my engine is primarily leaning the engine for the first part of the throttle reduction movement.

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  2. David Brown on Mar 07, 2013

    Perhaps you could clarify the question a bit better. For a start there is no such thing as best peak. In fact the whole concept of “best” anything is suggestive of one thing being better than another. We should refer to it as most efficient, most power or perhaps peak power and peak efficiency.

    Also you refer to a C172, after 1996 they had injected IO engines but most are carburettor. The Precission fuel servo is a mass airflow device so assuming that you have a mixture set according to your needs, as you change the throttle setting the engine will suck more air, and as it does so it will maintain a F/A ratio some what consistently. It is not linear though.

    The best method to climb from say 3000′ to say 6000′ is to use the target EGT method taught by APS. This means that the EGT value you have at takeoff, full power full rich at sea level is the same EGT value you want to maintain in the climb. This is simply done every two thousand feet.

    Assuming that you have an IO engine it may also have at least a single probe EGT, so take not of the EGT at takeoff and if you use this value during the climb, or subsequent climbs you will be doing the right thing.

    As John has mentioned above many engine have an “enrichment valve” in the circuit, but there are differences in different engines, but no matter what the system, target EGT works for all of them.

    Cheers

    David Brown
    Advanced Pilot Seminars Australia
    http://www.advancedpilot.com

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