To lean or not to lean, that is the question

I’ve recently received several emails regarding whether to operate lean of peak (LOP) or to stay on the rich side.

Let’s start with a short review of the chemistry and physics of combustion.

Liquid avgas will not burn — it must be mixed with air. The chemically correct or stoichiometric ratio of air to fuel is about 14.5 pounds of air to every one pound of avgas. In an engine this will produce the maximum temperature and pressure in the cylinder, which is called the peak. In an air-cooled aircraft engine, operation at the peak can lead to detonation and/or pre-ignition.

For maximum power, the air/fuel ratio should be about 10% rich of peak or around 13 to 1. Maximum fuel economy is about 10% LOP or around 16 to 1. (If you go more than 1 or 2 ratios leaner than that, you can get into lean misfire.)

From this we can see that for takeoff, it is necessary to operate on the rich side of peak. Likewise, with high avgas prices, many pilots want to save money and cruise on the lean side of peak. This can be done, but there are two very significant problems.

The first problem is that when you lean out the mixture, you will need to go through the peak temperature and pressure mixture zone. I believe the folks at General Aviation Modifications Inc. (GAMI) recommend that you lean out and go through the peak in 3 to 6 seconds. That is not a lot of time.

The other important part of this problem is the load factor. At takeoff power, the cylinder head temperature will be much higher at peak than at lower load factors. That is why Lycoming only permits leaning to LOP at 75% power or less and Continental permits leaning to LOP at 65% power or less.

The second problem is that in a four- or six-cylinder engine, all of the cylinders do not operate at the same air/fuel ratio. Years ago we had a test engine that had an individual exhaust system for each cylinder. I could measure the air/fuel ratio for each cylinder. What amazed me was the cylinder to cylinder variation in air/fuel ratio and the fact that the richest and leanest cylinder changed with load and rpm. I attribute this to throttle plate position and intake design, plus other factors. On this test engine, the cylinder to cylinder variation in air/fuel ratios was greater on the lean side of peak than on the rich side.

So the problem becomes, if you lean out the mixture strength to LOP, how can you be sure that all of the cylinders are LOP and there’s not one actually running at or near peak, making you susceptible to detonation and engine damage?

This is critical because many aircraft have a single point cylinder head temperature (CHT) gauge that is supposed to be on the hottest cylinder. But the hottest cylinder is usually determined during operation at maximum power, which is ROP and is influenced a lot by cylinder position and the condition of the baffles and seals.

Now when one runs LOP, the distribution may change and the cylinder that you are monitoring on your CHT gauge may be one of the lean ones and running at a lower temperature than one of the other cylinders that is running near peak.

So is LOP the dark side that we should never enter? No, one can operate LOP, but it needs to be done carefully, following a few rules.

One, never lean out to LOP at full power. I would recommend not leaning above 70% power setting.

Second, be quick about it. I think the 3 to 6 second recommendation by GAMI is a very reasonable time frame.

Three, look into a multi-point CHT setup so that you can monitor all of your cylinder head temperatures.

Four, if you plan on operating your injected engine LOP, you may want to look into a set of GAMI injectors. They not only reduce the chance of detonation at peak, but by getting the cylinders to operate closer to the same air/fuel ratio, they allow you to operate closer to the optimum fuel economy seen at 10% LOP.

We have become so used to driving automobiles that are liquid cooled and have electronic controls that manage all of this that we forget the basics. Running LOP can save you some money, but if you do not do it correctly, it can cost you a whole lot of money in broken parts and down time.