An easy guide to engines - part 2. Piston engines instrumentation

[Lukasz]

Musica!

When I was looking through a dictionary in search for an appropriate title for this part of engine guide, I've found "instrumentation" and a description attached to it was the following: "the act of arranging a piece of music for an orchestra and assigning parts to the different musical instruments". For some of us the sound of aircraft engine is a very pleasant music, but the definition above sounds like too much, right? Well, not really. However, when we translate it into something like this : "the act of measuring a performance of an engine and assigning indications to the different panel instruments", it starts to make sense. When I add, that the gauges are not completely separated from each other and their indications are interconnected and must be examined together with the others, then comparing a pilot to a conductor, having an eye on an orchestra of engine monitoring instruments, is completely justified. "Hey, you there, on the trumpet! Your manifold pressure is a bit too high!"

Between Mt. Cylinder and Exhaust Peak

Before we move onto instruments themselves, we need to take a look at score first. Here is a chart extracted from a Power Point presentations I've found on the Internet http://www.eaa42.org/misc/gregs_show.ppt Various sources credit it to Advanced Pilot's Seminars, GAMI and William "Bill" Compton.

The chart shows, what happens inside an engine when a pilot pulls mixture lever towards lean position (read the chart from left to right). At first all parameters rise, next they peak - although at different points - and after that, they all fall. Some of them are directly reflected on respective gauges and the others are not, therefore it is double important to memorize this chart, in order to have a complete picture of engine's operation during a flight.

EGT means Exhaust Gas Temperature and is a direct indication of mixture's "quality". For an engine to run, we need a specific fuel to air ratio and EGT peaks when this ratio is perfect - fuel and air are mixed in just the right proportions (from a chemical point of view). EGT peak is the reference point, that differs so called rich-of-peak (ROP) and lean-of-peak (LOP) engine operation.

CHT means Cylinder Head Temperature and is a direct indication of how hard the engine is being pushed by the pilot. Have a look at the next curve, labelled ICP (Internal Cylinder Pressure). Both the CHT and ICP have the same shape - the more pressure (=stress inside of the engine), the more heat. CHT usually peaks around 50^OF ROP.

HP means horsepower and shows how much useful work the engine is doing at the moment. The more power, the more thrust (minus losses due to propeller's (in)efficiency), which means more aircraft performance. Higher speed, better climb, shorter take off distance, more payload on board. Power usually peaks around 80^OF ROP.

Let's stop for a moment here. How is that, the perfect fuel to air ratio (EGT peak) doesn't result in best power generated? Do you remember, when I've written, that fuel and air doesn't mix uniformly? There are always mixture pockets of various fuel-to-air ratios inside the cylinders. Some parts of the whole mixture have perfect ratio, some are too lean and others too rich. The pockets with perfect ratio burn happily at the highest (peak) temperature, which is shown by the gauge.

However, the pockets with richer or leaner ratios, burn less efficiently or don't burn at all and leave cylinders as hot air and fuel molecules, without generating as much power, as the perfectly burned pockets did. In order to compensate for that, you have to supply the engine with a little bit of extra fuel, to give that air molecules more chances to "meet" with fuel and burn, generating additional power in the process. But adding more fuel to the mix, makes the mixture more rich and moves it out of the "perfect" spot on the EGT scale. That's why the maximum power is achieved rich of peak and with lower EGTs, instead of on the EGT peak itself.

OK, but why doesn't max power match max pressures (=max CHT) inside the cylinders? Do you remember, when I've written previously, that the mixture doesn't "explode all at once", but it takes it's time (miliseconds, but still...) to "light up" and create any useful pressure on the piston? Imagine it, as a wall of flame expanding from a spark plug and moving towards piston, cylinder's walls and head. In fact, it is called a flame front and one interesting thing about it, is that a pilot has some influence over its speed. By making the mixture richer, you can slow it down and leaner mixture will speed it up (to a point).

Remember, that the engine is a precisely timed machine. Ignition must take place in just the right moment of crankshaft rotation, so that the peak pressure takes place when the engine is "prepared" to take it. By making the mixture leaner, you also make the mixture to burn faster and light up completely, before the engine had a chance to rotate far enough behind TDC, to make a good use of the pressure increase. At this point, it isn't prepared yet to take the pressure, the pressure rises rapidly, but the piston doesn't "go down" as fast as it could, if it was at the right position. So, the hot gases want to expand, but the slow moving piston holds them in place. The pressure rises even more and that is the reason behind ICP/CHT peak at the leaner mixture, than max power mixture. Obviously, when you lean the mixture even farther, the power drops, the amount of burned fuel decreases and so the pressure also starts to drop.

This sounds tricky and pretty useless in an "easy guide", but in fact it's VERY important. Understanding of these concepts is critical to avoiding serious damage to the engine. At least in real life, since most of simulators don't model at all, any of these details. However clever plugins could change that Have you ever wondered, why An-2 plugin fails the engine, when you overheat or overboost it? Right

By the way, that's more or less me, trying to write this guide, regardless of "external distractions"

http://www.youtube.com/watch?v=Zq8k8Rsd9oY

The last curve on the chart, marked 1/BSFC represents engine fuel efficiency. BSFC stands for Brake Specific Fuel Consumption and the curve normally has U shape, the lowest point on it being the lowest consumption of fuel unit per unit of generated power during a specific amount of time. For example 0.5lb of fuel used per every 1hp generated during an hour of engine's operation. Thus an imaginary 180hp engine of such BSFC, would use 90lb (14.9gal) of fuel for every hour at full power. On this chart, the BSFC curve was flipped upside-down, to rather indicate engine efficiency, which is the highest at the point, where BSFC is the lowest. Take note, how the efficiency changes, with mixture setting and power being generated.

Engine overboozed

Your grandmothers may have told you hair raising tales of aircraft engines being completely destroyed in seconds, or even violently exploding, at the most unsuitable moment, like a dramatic climbout to jump over that mountain or trees, popping out of the fog ahead. I don't know about you, but I couldn't sleep afterwards, haunted by visions of bent crankshafts, burned valves, exploded cylinders or pistons completely thrown out of the engines. And while in my grandmother's stories, the usual culprit for that was some kind of gremlins, the effects were true. Such damage is possible and did happen in real life, due to deficiences in engine operation, maintenance or design (seldom occurence, but it can happen). Even using a wrong type of fuel will ruin your day (there is even a separate failure in X-Plane for simulating that). Avgas is avgas, right? Wrong!

Piston engines, while being built basically for containing high pressure, high temperature, almost-explosive burning of fuel, do have their design limits, like any other devices or machinery out there. It will break, if you manage to heat it up too much or if you allow too high pressures inside the cylinders. Let's leave design and maintenance matters aside, as they are not much relevant to flight simulators and concentrate on deficiencies in operation, which can destroy an engine in two ways: by detonation and preignition. Remember, that both of them can develop separately or at the same time and one can act as a trigger, starting the development of the other.

Regardless of its name, detonation is the more gentle of the two, if anyone can call as such an engine damaging process. It's because it starts at low level, then moves to medium and only then turns into heavy detonation, which actually can do devastating damage. Worth noting is that, while light and medium stages don't violently damage the engine, they can easily and quickly become the heavy case and that's why it's best to avoid detonation at all, regardless of its severity.

For the detonation to develop, an engine must be operated at high power and lean mixture. Such conditions lead to increase of CHT and ICP and all four of them make a favorable environment for the detonation to start. High power means high peak pressure. Lean mixture means faster flame front and even more peak pressure. This combination of faster, more violent combustion and increased pressure, increases CHT. As the engine gets hotter, it promotes even faster mixture burning (=faster flame front) which increases peak pressure even further, with every next cycle. As you can see, it takes some time for this condition to develop, but it's self propelling snowball mechanism, which will do something bad, sooner or later.

How soon? It depends on how fast everything rises up and in what condition is your engine, but don't expect more than few minutes. Maybe five, maybe one. The point is, you can't increase CHT forever. Aluminium, as every metal or alloy, has it's temperature limits, above which it looses its strength and becomes weak and plastic. Think of cold plasticine here, which is hard and stiff when you get it out of the box, but becomes easily mouldable after it was heated by the hands. Now take the increased pressure into the equation and you end up with a "plastic" engine, that has to fight more than usual stress, trying to explode it from the inside. That doesn't look any good and one can only imagine, why this condition was named "detonation".

What is worth noting, is that turbo- and supercharged engines are more prone to detonation, because turbos mean more pressure inside cylinders - even when the engine is being operated correctly. NA engines are relatively detonation-free, but that doesn't mean, that you can relax and push the engine to the max. Turbo or not - it CAN happen to everyone. Also running constant RPM propeller down to low RPM, while setting the engine up for high power, "moves" peak pressure closer to the TDC - exactly, as the lean mixture does.

What to do then, to avoid detonation? Do the opposite to the things, that cause it When at high power, like during take off and climb (or go around!) set full mixture (although be sensitive about high elevation airfields here...) and max RPM, in order to have the flame front nice and slow and the peak pressure in the right moment during crankshaft rotation. Have an eye on CHT and do not allow it to move too high. Also pay attention to such situation, when CHT still "in the green" but moving up fast, which is an indication of "CHT runaway" and a clear sign of something very very bad going on inside the engine. Do the same, if your engine starts showing signs of detonation: add fuel to the mixture, increase prop RPM, change to a lower power setting, cool the engine by opening cowl flaps or increasing airspeed (decrease climb vertical speed-increase IAS or even slightly dive for a moment).

Preignition, while may sound more politely, than "detonation", in fact is a real killer. It won't take the time to develop - as soon as the conditions are right, the engine will explode. There are seconds here, instead of minutes. Sometimes even less than seconds. Conditions for the preignition to develop are the same, as in a detonation case: higher than usual temperature and higher than usual pressure. Preignition often is a product of detonation, but it can develop on its own, as well.

What happens during preignition event, is that the mixture ignites before the spark plug fires. In other words, you've just made a diesel engine out of your powerplant, where fuel-air mixture ignites by itself, without any spark, because it is compressed and heated enough by the engine alone Now picture that: normally, the spark fires, slow flame front takes the time to light up the mixture and by the time it is starting to exercise pressure on the piston, it is past TDC and on it's way down, ready to take the pressure and make something actually useful out of it, apart from smell and noise.

During preignition however, the mixture ignites out of the increased levels of heat and pressure (and does it very violently, forget about nice 'n' slow flame front!) before the piston had a chance to move into it's usual position. It may be at TDC or even before top dead center, when the mixture ignites (or rather explodes). Piston is being brutally hammered by the pressure and the result is either bent/broken crankshaft or exploded cylinder. Or both - remember, this is a heavy piece of metal, being plastic out of the excessive heat, rotating wildly at high speed.

If you've ever worked with wood, then you know that you have to hammer the nails with just the right amount of speed, force and at the right angle (i.e. directly from above). If you do that, the nail will nicely go into the woodwork you're trying to assemble. But if you miss the "required parameters", the nail will bend, jump out or go in at angle, usually damaging the wood. Not good! So please think of the nails and wood (as well as of cold and warm plasticine), every time when abusing the engine

These two might know something about that already

"I wonder, what does this button do?"

Now once we have all the theory covered, we can move onto more practical side of engine management. There is a number of various, often exotic gauges on the panel and while it might look, as if they're put there for the cockpit to be more "professional" and busy place, all of them actually have their purpose. Remember, gauges do cost in real life and they also add to aircraft's empty weight, so they better show anything useful, if they're going to be added to the panel! Some of the info will overlap with what was already written, but I've decided to write it here again, so you don't have to jump up and down, between chart description and gauges here.

EGT - Exhaust Gas Temperature

Mixture quality. An EGT peak means the best proportion between fuel and air, while peak EGT minus about 75^OF to 100^OF ROP means best power (for a given throttle/RPM combination). Stay more than 250-270^OF ROP and you should be safe from detonation. Or even better, take you plane to sea level, set max power and max mixture and note the EGT. That's your full power indication and do not go above it, while leaning to compensate for altitude effects, when at full power. Below 65% of power, you can set EGT (and thus mixture lever) wherever you like, as the power is too small to do any damage to the engine. Do not go "over the peak", unless your plane is LOP capable, more on which later. Some gauges have an asterisk mark on them, that's the maximum allowable exhaust temperature, so stay below it. Also EGT is the only gauge, that clearly shows which engine has failed, on multiengine piston planes. Don't try to judge by MP or RPM - instead have a look on EGT, because where is no fire, there is no EGT

FF - Fuel Flow

Indirect power and range indicator. Ah, have you ever wondered, how to set some specific power %, without need for memorizing vast power charts from POH? In reality, it's more than easy. Since you will climb at full power anyway, we have that one down. Descends are flown at reduced power, so that's also off the table. What's left, is cruise. Pick up one or two best RPM settings (2300 should be good all around default) for a cruise (for a given plane) and look into the POH for FF at that power setting for a given altitude. FFs published in POHs are usually "leaned for best power". All you need to do, is to take this plane to this altitude and set this cruise power - roughly set FF with throttle, RPM with propeller and EGT with mixture. Fine tune the indications to be perfect and note FF and EGT.

Now's the best part As long as you maintain these parameters, the engine will produce almost exactly the same amount of power, regardless of altitude (+/- 3-5hp)! I've tested this in X-Plane and it works, for NA and turbo engines alike It also works for fixed pitch propellers, although at low altitudes the power will be lower, because of lower RPM (C172 showed 10hp difference). So, after this one single test flight, you'll have your 65%, for example. After you set EGT and RPM, the FF gauge basically turns into a power gauge. Over specific FF = over 65%, under this FF = under 65%, thus safe for mixture experiments. If you don't have such detailed POH, then use X-Plane's data stripes and experiment with values. Life is good

With FF and IAS you can also calculate range that can be covered on the fuel aboard. Take fuel amount and divide it by FF - that's how long you can fly. Multiply that by TAS and you'll have your range For example FF is 10gph, fuel tank's indication after the climb is 300lb, alt is 15000 and KIAS is 120.

10gph is 60,2lb/h. 120KIAS is 156KTAS. 300/60,2 is 4,98 h of flight. So 4,98h at 156 knots gives us 776,8nm, without reserves and 659,8nm with 45 minutes reserve (how I've calculated that, is for you to determine You should have enough insight by now. I've assumed reserve FF and alt to be the same as cruise). By the way, ALWAYS fly with 45-60 minutes reserve, especially with real weather and double especially on Vatsim or similar network!

MP or MAP - Manifold Pressure

Indirect power and cylinder pressure indicator. This gauge alone won't tell you anything, short of indicating whether or not you're overboosting supercharged engine. But high MP with normal EGT will indicate normal high power, with high EGT will indicate unsafe condition, but with high EGT and low FF will indicate cruise in LOP. Too high MP with too low prop RPM is also unsafe. Too low MP may indicate high altitude, above critical altitude, turbine failure or gauge failure. So always combine MP indication with other gauges, as it will give you the picture you're looking for - that's why a complete understanding of engine operation is so important and also so satisfying for everyday flying You have to create an "engine picture" in your head, for the duration of the entire flight, instead of just "keeping the needles in green".

Also take a note, that MP shows "air component" and FF shows "fuel component" of the fuel-air mixture, while EGT shows how well these two work together. Nice, eh?

RPM - Revolutions Per Minute

Indirect power and cylinder pressure indicator (for a constant speed prop). On an engine with a direct connection between the engine and propeller, it shows the common speed of both. For an engine with a reduction gearbox between it and its propeller, this gauge may show one or the other. By the way remember, that if a gearbox is installed, then it's always making the prop to turn slower, than the engine, because engines like to run fast (because ignition timing is tuned for the max stress = full power and speed) and propellers like to run slow (because of propeller's tips going supersonic when at high RPM, which is bad).

The more RPM the more power, and vice versa. The more difference between high MP and low RPM, the more pressure increase inside the cylinders. You may have heard the term "oversquare", which is a common description of this difference between these two values. An engine running at 25"MP and at 2500RPM is running "square", as well as is an egine running at 30"MP and 3000RPM. Just divide RPM by 100 and that will be MP (in inches of mercury), that is required to run square. Any combination, that have "more" MP over RPM is "oversquare" and anything with less MP, than RPM is undersquare. So 30"MP @ 2500RPM would be oversquare, and 25"MP @ 3000RPM would be undersquare. How much oversquare is permitted? Always consult POH (youl'll hear that more than once yet). However as a rule of thumb to be used in a simulator, you can assume an oversquare of 4-5" to be safe for NA GA engines and of 8-9" for TC GA engines. For example, C172 at takeoff may be running 30"MP @ 2500RPM, which is 5" oversquare. Safe, provided that mixture lever is fully "in".

As usual, old & powerfull radials and "hot water twelves" might have different limits and they are very fussy about maintaining correct parameters - because of power. The more powerfull the engine, the more careful you need to be, while operating it. Why? Because high power means high stresses, but the engineering and the metallurgy behind those beasts are exactly the same, as in your low powered "ornithopter" (where in addition to engine at full power, pilot has to flap both arms and ears, for a takeoff to be successful ). So, such engines, with lots of power, are operating closer to "the edge" and the allowed operator's error margin is smaller.

I'm hijacking my own post AGAIN Back to gauges!

CHT - Cylinder Head Temperature

Engine happiness indicator Yes, that's exactly what this gauge is indicating. As long as you maintain proper engine temperature, it will be happy and it will continue to work as designed. Cool it too much or make it too hot and it may thank you for cooperation and quit - sometimes quite literally! While it's true, that too hot engine becomes plastic and can't stand stresses inside the cylinders, the same stands true for an engine that is too cold. Cold engine is very stiff and it is generally known, that stiff bodies don't take stresses well and break easily. Glass, for example. That's why wings are elastic (to a point, of course) and why you have to heat up the engine before takeoff - it has to be somehow plastic, but not too much, so it can take the stresses that come with full power operation.

There is a catch, however Manufacturers often "cheat" and move the upper redline too high, so that the pilot could push the engine further and have "more powerfull" plane. Remember, that in real life planes must sell and higher performance planes sell better? Right. Regardless of engine type and power, you should maintain CHT between 200 and 400^OF (roughly 100-200^OC, easy to remember), with cruise temperature somewhere between. Even if the green zone on the gauge tell otherwise. Will engine fail immediately, if you move outside of this range? It depends, on how far and how fast you'll move. Small excursions may only shorten the TBO, while bigger ones (like 100^OF, especially on the "hot side") may break the engine inflight (or during next takeoff... you'll never know, as this damage is cumulative). By the way, do you now understand, why during the WW2 ground crews were putting so much effort, between every flight, into the maintenance of these engines? Do you believe, that in a heat of a dogfight, ALL the limits were faithfully respected?

There is yet another catch You can exceed engine limits, even badly, but it will be cool, well inside the limits. How is that possible? Cold air. Very cold air, in fact, like during winter. Just don't push the engine too much on the cold days and you'll be fine What about high altitudes? It's also cold up there! Yes, but the air is also much more thin, thus precluding from using full power anyway. Either the engine will starve without the air or the drag on the fuselage and the propeller will be so much reduced, that the power required for a flight will be also smaller.

How to maintain CHT within the parameters? You have to maintain equilibrium between heating from the inside and cooling form the outside. When one side prevails, decrease it or increase the other. At first try cowl or radiator flaps, if you have them. The next step is to increase/decrease airspeed, in order to do the same to the cooling airflow running around the engine itself or external cooler, found on most warbirds. The slower the flow, the less cooling effect. For long climbs at high gross, try to use step climb technique - if you could couple it with the reduction in plane's weight, due to fuel burn, it would be even better and pro (remember about that aerodynamic efficiency stuff, with regards to weight and altitude?) In the end, you could also reduce power, but that should be considered as a last resort move, because less power means less performance - which is bad for a situation requiring it, as in climb. But as a rule, always climb at full power. If you can't - decrease VS in order to increase IAS an cooling airflow. Also try not to load the aircraft up to gross weight on hot, sunny days or at high elevation airports. The supercharged engine might have the power required to overcome the effects of hot (=thinner) air, but also you have to think about the performance of propellers, wings and control surfaces.

P & T - oil pressure and temperature

Secondary engine happiness indicators. Peas and teas (I must be half-engine, because a good tea really makes me happy ) They need to be looked upon together. Both high - engine is working too hard (and may be very unhappy about that soon...), high P and low T - oil too cold and too dense (not good, increased engine wear, do not push the engine), low P and high T - oil level too low (an oil leak! a serious emergency! yikes!), one high or low and the other normal - monitor carefully, as it may develop into something nasty, also it may indicate a faulty gauge.

Oil is mainly used for lubricating the moving parts inside the engine, for changing prop pitch and for lubricating turbine bearings. You need the sufficient pressure so it can be injected into all the small places, to lubricate metal there and prevent it from touching while moving at some crazy RPM. You need the right temperature, so it is fluid enough, not too much and not too little - the same as you want the engine to be a little plastic, but not too much. Oil temperature rises, when there is metal to metal contact already (too little oil to lubricate every part) or when the airflow around the oil cooler is too small (too low airspeed or the engine is working too hard and heats the oil too much, so the oil cooler is overwhelmed and throws the towel).

Cold oil is more dense, hence the increase in pressure. This is normal for engine start and both of them should "normalize" after the engine and oil are heated up to their working temperatures. Do not exceed 900-1000RPM until that happens! You risk damaging or shortening life of a "dry" and cold engine. Abnormal oil pressure may also indicate other mechanical failure, like low pressure - oil pump failure.

TIT - Turbine Inlet Temperature

Mixture quality. Found on turbine-equipped engines, same rules as for EGT apply. Well, maybe with some small twists, to make our life even better and more interesting There is always upper limit for TIT, for turbonormalized engines it is usually in the lower 1300s and for the "truly supercharged" the common limit is set at 1650^OF; the POH will have the right values. Try not to exceed these values, really. Turbine works in even hotter environment than egine's parts and also it spins MUCH faster. So the design limits are even tighter here. Even few seconds in "the danger zone" above the redline and that's it. Remember that rule, because turboprops are even more sensitive to rough handling. But what else would you expect from an engine basically build around a turbocharger?

If your engine has a turbo, then TIT gauge "replaces" EGT gauge in funcionality and you should always lean mixture with the help of TIT indications. EGT may still be installed, but only as a secondary and backup instrument. Take a note of a slight difference in indications between the two.

That's it, as far as gauges go. Some engines on some planes may use other gauges, but you won't see them on "normal" modern GA planes. Also, these exotic gauges are usually self-explanatory.

Rich is not always good

So far we've deliberately stayed on the "rich" side of EGT peak, because that's how most of the engines are operated. That doesn't mean however, that lean of peak or LOP engine operation isn't possible. It is, and also it's a better way to use the piston engines. Engine is more fuel efficient, because there is no excessive unburned fuel, leaving the exhaust pipes - only excessive air, which is for free. There is no risk of fouling the sparkplugs with the deposits of rich mixture burning. The engine runs cooler, for the same power setting. The concept isn't new, as Lindbergh used LOP principles to stretch the maximum range of his plane, during his famous flight over the Atlantic Ocean and later he has passed his knowledge to the crews of US bombers, taking their part in WW2. Old piston powered airliners, from a more elegent era also utilised LOP, for increased range and minimized fuel burn. With the beginning of a jet era, the concept was forgotten, only to be "reinvented" recently, with the help from modern engine instrumentation and design details.

Why not to use it then, if it's that good? It's because most of the modern engines can't handle LOP well. They start to run roughly and quit, when you lean the mixture too much. All of it is caused by the fact, that each cylinder is receiving a little different fuel-air mixture. Some are lean, some are rich and the others may be somewhere in between or even farther in either way. When you pull that mixture lever, some cylinders "move" behing the peak and run at various levels of lean of peak, while the others are still on the rich side, some more rich and some less rich. Again have a look at the power chart:

While the power curve on the rich side is more or less on the same level, on the lean side it drops more visibly. Which means, that you can get away with some minor differences in mixture ratio between cylinders on the rich side and still have nicely running engine, while at the same time it is a no-go on the lean side, as even small differences in mixture will produce big differences in power output between cylinders. The uneven power distribution between cylinders means, that some want to turn the crankshaft faster and harder, while the others are more laid back. And since all of them are on the same crankshaft, this causes uneven stress distribution, as well as over- and underrevving of the cylinders, with regards of the speeds commanded by their respective mixture settings. That is the real cause behing engine rough running when too lean. It's like having two different personalities under one roof, one waking up and going to sleep early, and the other partying until daylight and sleeping till midday. There will be roughness between them, for sure!

So in order to be able to run the engine on the lean side of EGT peak, mixture ratio must be equalized between the cylinders, so that all of them "peak" at the same moment and develop the same amount of power - or at least to make the differences of such small magnitude, that they won't cause engine roughness, as was demonstrated on the rich side. It's relatively easy on the fuel injected engines. All you need to do, is to measure the exact amount of air, that the each cylinder is receiving and then to produce custom fuel injectors, that will supply right amount of fuel, so that every cylinder is running on the same fuel to air ratio. Unfortunately, carbureted engines usually have so bad fuel/air distribution between cylinders, that nothing can be done. They're "stucked" on the rich side. By the way, radial engines are in general more "willing" to run LOP, because of their geometry. All pipes, that supply fuel and air to the cylinders, have identical lengths and shapes, which allows for very similar fuel to air ratios between cylinders.

Another thing is that you need all the cylinders either on rich or on the lean side. You can't operate the engine "halfway". It's always either rich or lean. But how do we know, what are the cylinders doing, with one EGT gauge? That's why anyone wanting to use LOP absolutely have to install a kind of all-cylinder engine monitoring system. It can be either modern multichannel engine monitoring gauge or a built-in element of an engine page in a glass cockpit installation. It's fine, as long as it can provide at least separate EGT and CHT readings for every cylinder. Six cylinders - six "stripes", but usually these systems have a lot of other useful functions and with that in mind, they are also recommended for engines that can't run LOP. Why? Imagine that you have four cylinder engine, with cylinders #1 and #2 being the hottest ones. CHT probe was installed on #1, but it's #2 that has experienced CHT runaway... Now #2 is going haywire, but you still see perfect readout for #1 and you think, that the rest of the three cylinders are performing in a similar way. First sign of trouble you'll ever receive, is well beyond last point, at which any corrective action was possible. Obiously, this is no good!

It doesn't really matter for simulators, but if you want to faithfully replicate proper engine operation techniques, then use LOP only on fuel-injected engines, with appropriate monitoring installation. Either stick to planes, that are equipped as such or try Lean Assist addon found in the x-plane.org download manager. It's free and fun to use and it opens new possibilities for flight planning One thing to remember, though. When running LOP, you have less power available, in general. You can make up for it to some degree, by running approximately 3" higher MP. Fuel flow and fuel economy will be from the lean side, but this additional MP raises pressures inside the cylinders a bit, which allows for an increase in power output. Flying in such manner, allows for about 20% increase in fuel efficiency, at the cost of about 5% decrease in performance. A fair trade, if you ask me Again, have a look at the Power Point presentation mentioned at the beginning of this post.

That's all for today Now, repeat after me: piston engines are fun, piston engines are fun