Back to the Future

The Wright Aeronautical Division (WAD) Recommended Leaning Procedure

Part 2
by George W. Braly

In Part I of our trip "Back To The Future," we explored some ancient (by aviation standards) and long-forgotten wisdom about the leaning of big, round radial engines used for millions of successful hours during the airline and military piston-powered era.

We saw graphically how it is possible, using a "modern" EGT gauge, to duplicate the mixture settings used in high-powered piston engines in our own "modern" flat, horizontally opposed piston engines.

We also explored the relationship between EGT and other engine parameters. We learned that by running our engines leaner than peak, we enjoyed much lower cylinder-head temperatures, even while developing the same horsepower, and all the while burning significantly less fuel than when operating the engines in the conventional manner.

As explained last month, the catch in this little bit of engine operation magic arose because when one tries to follow the Wright Aeronautical Division Recommended Leaning Procedure using "modern" horizontally opposed engines, those engines often simply run rough. They shake and they vibrate to the point that no pilot would voluntarily operate the engine for extended periods of time without enriching the mixture.

Why do horizontally-opposed six-cylinder engines run rough on the lean side of peak?

The short answer begins with this thought: Some of our engines, a very few of them, don’t run rough on the lean side of peak EGT.

Read the following excerpt from a Lycoming engine manual for one of their 540-cubic-inch, flat, opposed piston engines:

"Lean the mixture until EGT peaks and continue to lean until the EGT drops 25 to 50 degrees on the gauge. Flying on the lean side is permissible if extended range and cooler engines are desired. Operation at peak EGT is only recommended for mixture control adjustments or when induction icing occurs. The Amount of temperature drop can be determined by resultant fuel consumption and engine smoothness.

"When operating on the lean side of the power curve, the pilot may observe that airspeed and power are less. If you desire to regain lost airspeed and continue to fly on the lean side of the curve, two steps are important. If sufficient throttle is available at the lower altitudes; first add two inches of manifold pressure to the standard cruise setting and then lean 25 to 50 degrees, (lean of peak). Occasionally, some pilots prefer to fly on the rich side of the power curve; this is permissible. Adjust the mixture control until EGT peaks and then enrich mixture until you get 25 to 50 degree drop on the EGT gauge. Acceptable continuous (cylinder) head temperature is an important reference here."

Yes! This is a direct quote. Lycoming really did recommend that these particular flat, six cylinder engines be run 25 to 50 degrees lean of peak. Lycoming made these recommendations for the same reasons that the WAD adopted the nearly identical procedure for their high powered piston engines. The flat, opposed, six-cylinder engines -- just like the big, high-powered radial engines -- run cooler, cleaner, and more efficiently, when run well over on the lean side of peak EGT.

The engine manual from which that quotation was extracted was from the 1966 Lycoming Service Training Department, and is for the "IGO and IGSO-540" Lycoming engines. These are geared, fuel-injected, engines. The IGSO is also supercharged, with a gear-driven supercharger. These engines turn at 3,400 RPM and are rated at some 380 HP. They were often used in the early Aero Commanders.

Some may note that Piper and TCM had some problems with the TSIO-520BE engines. A careful review of those problems reveals that they had nothing to do with running lean of peak. In fact, many of those problems were probably due to the operators' refusal to follow the recommended lean-of-peak leaning procedure. In the last analysis, the problems were primarily due to a limited overall cooling capability in the original Malibu cowling. The recent "RAM" nose landing gear door cooling modification has probably fully corrected the earlier cooling problems with that engine installation.

In more recent times, we have the Continental engines found in the Malibu. These engines, the TSIO-520BE, to be precise, also feature a well known recommendation to run on the lean side of peak.

These engines can be, and are, routinely run smoothly on the lean side of peak EGT because these engines have, by virtue of different engineering considerations, exceptionally well balanced fuel/air ratios among the six cylinders as part of the essential design features of these engines.

The Lycoming engine obtains this superior balance of fuel/air ratios by injecting the fuel directly into the gear-driven supercharger, which is spinning at more than 35,000 rpm and therefore thoroughly mixes the fuel with the air before it is transported to the cylinders through the induction system.

The TSIO-520BE Continental engine achieves its reasonably well-balanced fuel/air ratios by the careful design of the top down, "tuned" induction system. It is reported by one person who worked at TCM at the time that Continental’s engineers spent nearly two years tweaking the design of the induction system in the TSIO-520BE in order to get it to run smoothly at 40 to 50F lean of peak.

Fig. 2
A comparison of fuel flow spread between a GAMIejctor-equipped engine and one with stock injectors.

The Lycoming GSIO engine rarely is seen in general aviation at the present time. Thus, the TSIO-520BE, TSIO-550B, and the IO-550G are the only big bore engines currently in production that have reasonably good balance in the fuel/air ratios. The induction and fuel injection systems in the TSIO-550 and the IO-550G, like the Malibu TSIO-520BE, while good, still do not provide as good a balance in fuel/air ratios among the cylinders as is desirable.

The rest of the available engines for general aviation are really pretty dismal, when it comes to achieving good balance of the fuel/air ratios among the cylinders.

The top half of figure 2 is a graph of a rather well-balanced TCM IO-550 engine. But it is well balanced in its cylinder-to-cylinder fuel/air ratios only in comparison to the typical TCM engine. This particular engine had all of its six fuel injectors carefully re-calibrated to the center of the TCM nozzle flow specification range, just prior to the recording of the data from which the graph is generated. For that reason alone, it has significantly more uniform fuel/air ratios than the average TCM engine found in the field.

Compare the graph in the top half of figure 2 to the graph in the bottom half. The same engine, on the same day, under identical conditions was used to obtain the data depicted in the bottom half of figure 2. However, in the mean time, this engine had the fuel/air ratios of the individual cylinders corrected or balanced by the use of specially calibrated and STC'd fuel injectors. (Note: The nozzles are sold under STC from General Aviation Modifications, Inc., Ada, Oklahoma.)

As you can see, there is a significant improvement in the spread in fuel flows as measured from the point at which the first cylinder reaches its peak exhaust gas temperature to the point when the last cylinder reaches its peak exhaust gas temperature. Note, the difference in the fuel flows is the measure of uniformity, not the differences in the absolute value of the individual EGT’s. Many pilots misunderstand this important concept.

Why do variations in fuel/air ratios cause engines to run rough on the lean side of peak but not on the rich side of peak?

The answer is really pretty simple. But it has nothing to do with what you probably have been taught. Most pilots are taught that engines run rough on the lean side of peak due to some mysterious "lean misfire" witchcraft. Nonsense.

The engines run rough because on the lean side of peak, most engines do not have uniform cylinder-to-cylinder horsepower output. This cylinder-to-cylinder horsepower imbalance causes vibration, which has been often and wrongly characterized as lean misfire. We know this is pure nonsense because if one does precisely balance the fuel/air ratios of all of the cylinders, one can lean the engine to more than 100 degrees lean of peak - even 120 to 140F lean of peak - and the engine will just smoothly lose horsepower until it finally, and gently, dies.

If you refer to the graph from part 1 of this series [show popup], you can see the reason most engines run rough on the lean side of peak. On the rich side of peak EGT, there is a large surplus of fuel over available oxygen to react with the fuel. The curve that depicts horsepower is relatively flat on the rich side of peak, and doesn't vary significantly with fuel flow.

If, while running at 100F rich of peak, you were to install a device that allowed one to vary the fuel/air ratio of, for example, the No. 3 cylinder, by adding 5 percent more fuel to that one cylinder, you will not increase the horsepower in that cylinder, since there is no available excess oxygen to burn the additional 5 percent of the fuel.

Likewise, if you reduce the fuel to the No. 3 cylinder by five percent, it will not significantly reduce the horsepower in the No. 3 cylinder, since the available oxygen (at a mixture setting of 100F rich of peak) is already much less than adequate to burn even 95 percent of the fuel already passing through that cylinder.

On the other hand, if you were to conduct the same experiment while running 50 F on the lean side of peak EGT, you would find that adding an additional 5 percent to the fuel injected into the No. 3 cylinder would increase the individual horsepower output of the No. 3 cylinder by about 5 percent, and vice versa.

You can see this by looking at the horsepower curves on the lean side of peak, and noticing that those curves are not flat as on the rich side of peak; but on the lean side of peak the curves slope steeply downward with decreasing fuel flow.

If you have a digital EGT instrument, it is pretty easy. Figure 3 shows a completed form, with data obtained in the following manner:

  1. Establish the airplane in uncongested airspace at your normal cruise altitude, MP, RPM and a mixture at least 125F rich of peak EGT or peak TIT, as appropriate. Use the autopilot, if you can. Take someone along to help watch for traffic or record the data.
  2. Write down the fuel flow, all six of the EGT's, the TIT (if any) and the IAS. CHT's are optional.
  3. Lean the mixture, on one engine, in very small increments - the smaller the better, especially near peak EGT. Three-tenths of a gallon/hour (2 lbs/hour) is plenty.
  4. Repeat steps 2 and 3, above, until the engine runs noticeably rough. This should occur on the lean side of peak. Some engines have such poor balance of their fuel/air ratios, they will start to run rough at or near peak EGT.
  5. After you have gathered the data, go through each column of the EGT’S, and circle or highlight the highest number in each column. That number represents the peak EGT or the so-called perfect stoichiometric ratio of fuel-to-air for that cylinder. The fuel flow at this point is a key reference point, that can be used to compare the fuel/air ratios of each cylinder to the others.
  6. Note the fuel flow on line No. 4. The NO. 1 EGT, using TCM standard injectors, reached peak at 1442 F at 14.2 gph. That will be the leanest cylinder. Now, look at line No. 9. Both EGT 5 & 6 reached peak at 12.5 gph. These are the richest cylinders. The "difference" in these two fuel flows (14.2 less 12.5, or 1.7 gph) represents the "spread" in fuel flow from the leanest to the richest cylinder.

Note: Do not become confused. The cylinder that reaches peak first during a normal lean cycle is the leanest cylinder, even though the indicated total fuel flow continues to decrease as the engine is further leaned.

The matrix of numbers at the bottom of Figure 3 represent the same engine, after the fuel/air ratios had been balanced. Notice EGT 3 reaches peak first and is the leanest cylinder at 14.0 gph. The other cylinders all peak at the same time, at 13.5 gph. Notice also that the total "spread" from the richest to the leanest cylinder is now only 0.5 gph.

What do the results mean?

A typical TCM big bore engine, will have a spread of from 1.2 to 1.7 gph. We have seen that spread go as high as 2.5 gph, when the injectors were worse than normal.

Other than the noted TSIO-520BEs and IO-550G models, the best stock, factory-new or remanufactured, or shop-overhauled engine we have ever measured had a spread of 0.9 gph. The next best was 1.2 gph. The others were all worse. This includes a review of data from well over one hundred engines during the last 12 months. These values represent cylinder-to-cylinder variations in fuel flows of between 8 and 12 percent.

A mixture spread of more than 1.0 to 1.2 gph will virtually guarantee that your engine will run rough well before you are able to lean the engine to 50F lean of peak.

How good can my engine be?

As the graph in the lower half of figure 2 shows, the variation, or "mixture spread", in cylinder-to-cylinder fuel flows at peak EGT can be readily reduced to a level of 0.5 gph. In some cases, we have seen this spread virtually disappear, when the fuel/air ratios were carefully "tweaked". Any value under 0.6 gph is generally considered to be acceptable. The 0.5 gph value represents a cylinder-to-cylinder variation in fuel flow of less than 4 percent. A 4 percent, or smaller, variation in fuel/air ratios, will ensure that your engine will run smoothly to more than 50 F lean of peak.

Fig. 3
Flight test data forms showing results from stock TCM injectors (upper) and GAMIjectors (lower).

Why am I only hearing about this now?

Excellent question! Obviously, the engine manufacturers have known about this issue for a long time. A product, made by General Aviation Modifications, Inc., of Ada, Oklahoma, that completely solves the problem for almost all of the normally aspirated and turbocharged big- bore Continental engines, and recently, all of the Lycoming engines.

Probably the primary reason that the major engine manufacturers have never tackled this problem head on, is because, in order to balance the cylinder-to-cylinder fuel/air ratios in current production models of continuous flow, port injection engines, takes a great deal of time- consuming and precision tweaking of fuel injectors and/or induction systems. It also requires a modification to the engine type certificate, either directly by the manufacturer or by an STC.

Even during the most recent energy crisis, no one at TCM or Lycoming thought it worthwhile to tackle the issue. To TCM’s credit, they did create an engineering fix, albeit, with only a limited set of engines, installed in only a few hundred airplanes. Even those engines seldom end up in the field with spreads as good as is shown in the bottom half of figure 2 and the bottom half of the number matrix in figure 3.

In their defense, it is not a trivial task. The calibration of fuel injectors to the tolerances required to obtain smooth engine operation on the lean side of peak requires extremely specialized equipment, capable of measuring pressure and fuel flow with precision, accuracy, and repeatability at least 10 times better than is customarily found in fuel-system repair stations around the country.

There is no readily available commercial equipment, that can be used to construct a flow bench with the necessary stability and precision to calibrate nozzles to the tolerances required to reliably produce balanced fuel/air ratios as good as those depicted in the bottom half of Figure 2.

In part three of this article, we discuss why balancing the fuel/air ratios of your engines cylinders will save you fuel, and possibly save you a costly premature top overhaul, even if you continue to run your engine at 50 to 100°F rich of peak, just like you always have.

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