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In my Cessna 152 POH it is written that the horsepower rating is 110BHP at 2550RPM. My question is: is it 100% of the power of my engine or, as someone told me, if I open full throttle I will get 75% BHP? If so, then why don't I get 100% BHP at 2550 RPM? And is there any relationship between the mixture setting and BHP?

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    $\begingroup$ This is usually measured at sea level on a standard day, so if you are flying at a low altitude field on a standard day, you can expect that the engine is developing 110BHP at 2550 RPM. Mixture absolutely does affect power. Its not like a car where some of the HP is lost in the drivetrain. $\endgroup$ – Ron Beyer Mar 15 '16 at 13:59
  • $\begingroup$ Thank you for the ans. But my question is if I use recommended lean mixture setting, will it reduce the BHP? In performance chart of the POH it is given, at recommend lean mixture setting and standard temp at 2000ft at 2400RPM my engine will produce 75%BHP. At 2400RPM the engine is supposed to produce more BHP. The what is the reason behind it? $\endgroup$ – Mirajul.Pias Mar 15 '16 at 15:15
  • $\begingroup$ 2400 RPM is less than 2550 where the engine was rated, and you lean the mixture to match the amount of air (oxygen) present in the intake air. If the mixture is too rich or too lean you will lose power. Usually best power is made right around peak EGT, but I'm guessing your 152 doesn't have an EGT so usually the procedure is to lean until it gets a little rough then 2 turns back in, right? You make less power with more altitude with reduced oxygen so your engine produces less power, you need to lean to match to make the best power at that altitude. $\endgroup$ – Ron Beyer Mar 15 '16 at 15:20
  • $\begingroup$ But to put it easy, no, using the recommended lean will not reduce power, it should increase it. $\endgroup$ – Ron Beyer Mar 15 '16 at 15:21
  • $\begingroup$ I understood. But why the RPM is more and the BHP is only 75%? If I consider 2500RPM =110BHP then at 2400 it should be 105.6BHP which is 96% of total BHP, then what is reason for which I am getting only 75% BHP? $\endgroup$ – Mirajul.Pias Mar 15 '16 at 15:28
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Rated BHP is measured at sea-level on a standard day. For the Cessna 152 I'm guessing you have an O235-L2 which is rated at 110HP @ 2550 RPM.

Now to understand how mixture affects the power produced by the engine you need to understand that the mixture knob controls how much fuel enters the cylinders to be burned with the air. As a combustion engine sparks, the fuel ignites but there is the fire triangle, Air, Fuel, Heat. Too much or too little of any of those pieces means a reduction of power or no power at all.

No, depending on the fuel type there is an optimum stoichiometric ratio where the proper amount of air and fuel are burned such that there is no extra air or fuel left over. For gasoline engines this is about 15:1, so for every 15 units of air, 1 unit of fuel is required. Having a richer fuel mixture results in an incomplete burn and may cool the cylinder temperatures, reducing power.

On the flip side, having too little fuel for the air can cause hotter cylinder temperatures and hot-spots on the cylinders. This causes detonation or knocking and can damage an engine very quickly. This is especially apparent in engines that have a lot of cylinder carbon deposits where the carbon can heat up and detonate the mixture before the spark plug does. Its like hitting the piston with a sledge hammer as its still coming up.

I don't have a graph from an O-235L but I did find one from an O-235C, which shows the relationship between throttle and BHP:

O235C Power Curve

So you can see that reducing power from 2500 RPM to 2400 RPM gives a 10% decrease in BHP produced by the engine. Running at 2350 RPM is a 75% power setting (and in this graph nicely matches up with ~75 BHP). You can also see that there is a sweet-spot with relation to fuel consumption and produced power right around the 75% mark, which is usually refered to as the "performance cruise" value (best speed for fuel), and at 65% for "economy cruise" (best fuel).

So the mixture control needs to be leaned out based on the amount of air entering the engine. The levels of oxygen decrease with altitude and therefore the mixture needs to be leaned out. Its not uncommon for operations out of high altitude fields to perform take-offs with the mixture leaned, especially on hot days.

So its important to remember that the power produced by the engine is not a straight linear calculation. When you did your comment:

But why the RPM is more and the BHP is only 75%? If I consider 2500RPM =110BHP then at 2400 it should be 105.6BHP which is 96% of total BHP

You assumed that the engine can produce power at 10RPM, which just isn't true. Lets take a better example and say that at 1600 RPM it is producing 25HP and at 2600 it produces 100HP, we can do the math then.

Using a linear interpolation formula this gives us 85HP which the graph tells us is 80 HP. Remember though, the graph is curved, and linear interpolation is, well, linear.

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  • $\begingroup$ It cleared many of my doubts. You just probably missed I wanted to know, if i lean the mixture will it increase/decrease my BHP? $\endgroup$ – Mirajul.Pias Mar 15 '16 at 16:20
  • $\begingroup$ The answer is "yes". But enriching it can also decrease or increase power, the take-away is that you adjust the mixture for best power, sometimes that means leaning it, other times you need to enrich it. $\endgroup$ – Ron Beyer Mar 15 '16 at 16:37

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