Proper Mixture Control

[Yenn]

The Jabiru adjustable mixture carbide was I think either a hoax or just stupidity.

 

I tried to see how it worked and it seemed to be able to lean out a rich mixture, so you had to start with a rich mix. 

 

I phoned Jabiru and the personI talked to had no knowledge of it.

 

 

[Rexx]

G'day Folks,

 

Thought I might chime in on this one all be it a little late, I got the feeling that there might be a need for a bit more detail with regard to what different EGT values represent.

 

From the outset, either too lean or too rich and combustion ceases, it's that simple. If someone tells you you are running your engine too lean, just smile and agree.

 

Starting from the rich side and working back, most conventional aircraft engines are set to deliver a value ~ 175 to 250 degrees ROP when full rich at T/O power, this provides extra cooling and slow burning and hence a great margin from detonation.

 

In the cruise with <75% power set when leaning from full rich the first landmark is at ~125 ROP, this value represents max power from the engine/fuel due to the extra fuel combining with some remnant O2 after the initial burn. This also equates to max rpm and the mixture for high density altitude T/O's.

 

Leaning further from here and power starts to fall, the next minor landmark is 50 ROP, i say minor as it has no chemical value but is a number you have all heard of, promulgated by manufacturers as a compromise value to cruise at. 

 

The next significant point is ~25 ROP, this represents the closest margin in the mixture range to detonation, it is also the point at which the CHT value will peak.

 

Leaning a bit further and you arrive at Peak EGT, this is the chemically correct air/fuel ratio, stiochiometric, and a reference point between ROP and LOP. In reality more exciting to chemical engineering types than pilots. Depending on which side of this point you operate at may determine which side of the aeroclub bar you should drink at. 

 

The last milestone is at ~32 LOP and this represents the best Brake Specific Fuel Consumption, BSFC this is the point that your engine will achieve the most horsepower from each unit of fuel, the best economy. Not to be confused with the 125 ROP point which is the most power full stop from the engine which ignores the ratio of fuel use to horsepower.

 

Running LOP is inherently and principally good for and engine and your wallet, However if there is competition between a smooth running engine and a LOP running engine, smoothness wins every time. there are some minor down sides to LOP excess NOx being one, however it is barely worth consideration.

 

A typical EGT spread on an injected engine might be around 75 degrees reference the EGT peak point for each cylinder, not the raw numerical value you see displayed and for a carby engine 200 degrees, it takes some amount of work to either compensate the induction air flow anomalies with tuned injectors or redesign the whole system to deliver even airflows. However if the former is carried out the spread can be all but eliminated then LOP really does pay dividends.

 

Hope that added something to the conversation.