You may be wondering what we mean when we talk about blending… Is fuel mixed with something? Isn’t it just fuel and that’s it? Well, yes and no; we could say that yes, it mixes with something, and no, it is not just fuel.
The fuel mixture is the ratio of oxygen to fuel delivered to the engine. The stoichiometric setting, i.e. where the chemical reaction is perfect, is 1 part fuel to 15 parts oxygen.
This value is represented by the Greek letter lambda, λ. We say that the mixture is rich when λ>1; if λ<1, the mixture is lean.
The mixture must not be allowed to reach very lean values, as this significantly reduces the power output and can cause damage to the cylinder bores.
Some believe that the intention behind adjusting the mixture is to save fuel, but the reality is completely different.
In piston engines, the spark required for combustion is generated at the spark plugs, so if we continuously fly with the mixture at its most advanced position, there is a risk that the spark plugs will be flooded and their effectiveness reduced.
During take-off and landing, the mixture should be in a rich position, i.e. the lever should be advanced so that less air enters. In this way, we ensure that the maximum amount of fuel reaches the engine.
Once we have reached our cruising altitude, we must pull back the lever to adjust the amount of fuel reaching the engine.
To adjust the mixture correctly, we have to consider several engine parameters: the RPM, the CHT and the EGT. CHT being the cylinder head temperature and EGT being the exhaust gas temperature.
When we are at our final altitude and start to trim the mixture, the RPM will be increased, as the engine will start to run more efficiently. CHT and EGT will also increase, as the fuel also serves as a coolant in the engine.
These values will continue to rise until the peak of the mixture is reached, i.e. the point at which the engine is running as efficiently as possible. If we continue to trim the mixture, the parameters we have already seen will start to drop.
Adjusting the mixture correctly allows our fuel consumption to provide us with maximum power, maximum range or maximum autonomy.
The checklist for each aircraft specifies the settings that must be made to achieve the different fuel consumptions. Let us look at a practical example:
In our Cirrus SR20, to achieve maximum power, the EGT temperature must be 75ºF above peak.
That is to say, when you start to make the adjustment, the EGT will start to rise, there will come to a point where it stops rising and starts to fall. This point is known as the peak, at which we must advance the lever again, until the exhaust temperature is 75ºF above the peak.
To have the best economy setting, i.e. minimum fuel economy, the EGT must be 50ºF below peak.
Some believe that the intention behind adjusting the mixture is to save fuel, but the reality is completely different.
In piston engines, the spark required for combustion is generated at the spark plugs, so if we continuously fly with the mixture at its most advanced position, there is a risk that the spark plugs will be flooded and their effectiveness reduced.
In some aircraft, the aircraft software can help us to trim the mixture; a clear example of this can be found in our Cirrus SR20.
Once the cruising altitude is reached, there is a lean assist option on the multifunction display, which tells us when we have reached the peak. It also tells us whether we are flying at maximum power, best range or maximum range.
These three terms may seem quite similar to you, but there are differences between them. Do not miss our next posts, where we will talk about them in more depth.
Other aircraft in our fleet are equipped with ECU, Engine Control Unit, i.e. computers that are responsible for controlling the different engine parameters.
In ECU-equipped aircraft, the pilot cannot adjust the mixture, as the computer will do it automatically.
All our Diamond DA42, as well as the Diamond DA40 NG, are equipped with ECU.
We hope this article has helped you learn a little more about what fuel mixture adjustment is and why it is important to do it in flight.
If you want to continue learning about other curiosities related to aeroplanes, we recommend you to take a look at our aviation blog, where you can find explanations about the emergency location transmitter or the route of the suitcases at the airport, among many other topics.
