By Sape Mullender
Aircraft engines and car engines both have cylinders and pistons and crankshafts; they both burn a mixture of fuel and air, ignited by a spark plug, but they are still very different. Aircraft engines run at sea level as well as at 15,000 feet, a range rarely covered by any car engine.
Aircraft engines are designed to run for extended periods near their maximum rated power. Car engines never do that. Modern car engines are managed by computer, aircraft engines are, by and large, still managed by the pilot.
Given the engine troubles we’ve had in our club with the Skylane, we think it’s time to review a little engine operation theory. It’s our hope that we’ll manage to say something new to most pilots in the club and, if we don’t, that at least we’ll engender some discussion on the subject.
When 57T went into its 2005 annual, a compression test was done and three cylinders had bad compression. Two were diagnosed to have burnt exhaust valves and they were sent to Mattituck for inspection and repair. There, a crack in the cylinder was discovered as well as cracks in two pistons. This was sufficient reason to send the whole engine to Mattituck for a more thorough inspection. There were signs of overheating and more cracks were discovered. This led to an early overhaul.
The engine in 57T probably suffered some detonation which may have led to the cracks in pistons and cylinders. If we want to prevent such a thing from happening in the future, we must know how overheating and detonation happens, what causes it and how it can be prevented. And the only way of doing that is to understand what’s happening inside that engine.
Engines run on a mixture of fuel and air. We know that, for takeoff, the mixture is set to full rich, while, for cruise, we lean. Why do we do that? What happens inside the engine? If the mixture is too lean – I mean way too lean – it won’t burn: it’s all air and hardly any fuel. If it’s too rich – way too rich – it won’t burn either: it’s all fuel and there’s no oxygen left to allow it to burn. Somewhere between too rich and too lean there’s an optimum mixture; the mixture that produces optimum power.
The graph in Figure 1 illustrates this. On the left, the mixture is too lean to burn; on the right it’s too rich. The solid black curve shows the power developed by the engine. No power is developed at the extremes of the graph, because the mixture just won’t burn. At a fuel/air ratio of 0.05 by weight, the mixture becomes combustible; at a fuel/air ratio of 0.08 it produces best power and when the ratio reaches 0.125, it becomes too rich to burn.
Note that the best-power fuel/air ratio of 0.08 by weight means that, for every 100 pounds of air, we use 8 pounds of fuel. Since a pound of air occupies approximately 100 times more volume than a pound of fuel, it means that the fuel/air ratio is 0.0008 by volume: 10,000 gallons of air for every gallon of fuel.
If the engine runs at best power (fuel/air ratio of 0.08), and we lean just a tiny bit, then we reduce fuel consumption but we hardly lose any power. Best economy, therefore, is reached a little lean of best power: a little lean of peak (LOP). There’s another reason for getting better power on the lean side of peak: if we want all of the fuel to burn, we need a little excess oxygen.
Best power occurs when brake horse power (BHP) reaches its maximum. This is roughly at the mixture that produces 107 lbs/hr of fuel consumption. But, as you can see, temperatures, both exhaust gas (EGT) and cylinder head (CHT), peak well lean of best power.
The bottom graph shows ‘brake specific fuel consumption’. This is the fuel needed per brake horse power produced. It reaches its minimum (best economy!) well lean of peak, even leaner than peak temperatures.
But mixture is not the only part of the story. An engine generates heat, lots of it. And both the production of heat and the dissipation thereof must be managed. An engine is damaged by excess heat, as we have, in fact, seen happen to 57T. Let’s see what happens to the heat produced by the engine as the mixture goes from too lean to burn to too rich to burn.
Well, the heat production curve follows the power curve pretty closely: after all, the engine produces its power by burning the fuel; the better it burns, the more power is produced.
But that’s not the whole story. Let’s compare two engines, one running at 90% of best power by running rich of peak (ROP), the other running at 90% power by running LOP. The one running ROP has more fuel in its mixture and this fuel helps cool the engine: as the fuel goes through the carburetor, it evaporates and becomes quite cold (remember carburetor ice?); this causes significant cooling of the engine. Thus, an engine running ROP runs much cooler than an engine that produces the same power running LOP. This is illustrated by the graphic in Figure 2.
Now we have to ask ourselves ‘what temperatures actually do damage?’
We turn to another graphic from John Deakin’s series of AvWeb articles on engine management, shown in Figure 3. The line in the graph shows the strength of the aluminum alloy used in aircraft engines as a function of temperature. The scale it the bottom is in centigrade. At 0 °C, the aluminum is at its strongest and even at room temperature, it’s already somewhat less strong than at freezing. But a big dip in strength occurs between 232 and 260 °C (450 and 500°F). The background color of the graph indicates a green, safe, zone, a pink caution zone and a red serious-engine-damage zone.
The temperatures in this graph are cylinder-head temperatures (CHT). Although some engine manuals allow temperatures to enter the pink zone for brief periods of time (i.e., minutes), it is much safer to err on the side of caution and never to let CHTs get above 450°F. In fact, the Pratt&Whitney book actually says:
The lower limiting temperature (450° F) is the maximum for continuous operation. It should never be exceeded except under the restricted operating conditions mentioned in the previous paragraph.
It is sound practice to hold the cylinder head temperature 50° F (30° C) below this limit to keep the cylinder head materials at high operating strength.
So P&W advises us to keep temperatures at least 50°F below 450°F; that is 400 °F and, as John Deakin says, that’s a nice, round, easy to remember number.
Now, if we go back to Figure 2 (or you can immediately go to Figure 4) and we decide not to use mixtures that produce a CHT of 400 °F or higher, that eliminates all fuel flows between 93 and 120 lbs/hr. Looks like we’re forced to run seriously LOP to avoid engine damage.
As it turns out, that’s not what we recommend (for reasons we’ll go into later). The POH on all our aircraft specify full rich mixtures for take-off. In fact, when the throttle is firewalled, it triggers a mechanism to make the mixture even richer. Thus, our engines run at a fuel-flow off the right hand side of Figure 2, where CHTs will be below 400 °F (but not very much below).
Note that Figure 2 represents an engine running at 25″ manifold pressure and 2500 RPM: pretty much full power. When power is reduced, the graph changes, as illustrated in Figure 4. At full power, the mixture must either be very rich or very lean; at an intermediate power setting, there’s just a small area that must be avoided; and, at an economic cruise power setting (60% power), it becomes impossible to exceed maximum CHT.
Now, we’ll take a look at what happens in a typical four-stroke aircraft engine. Figure 5 shows the sequence of events during combustion. The fuel/air mixture is ignited before the piston reaches its highest point. It takes time for the fire to reach all corners of the combustion chamber, so pressure in the combustion chamber rises gradually. Pressure is still relatively low when the piston reaches top dead center (TDC); pressure peaks when the crankshaft has progressed another 15 to 20 degrees, in spite of the fact that the volume of the combustion chamber is already increasing. The burning gas/air mixture pushes the piston down all the way to the bottom.
There are several observations to make. The first is that the mixture is ignited before the piston has reached the top. The burning mixture thus has a tendency to push the crankshaft in the wrong direction. However, under normal circumstances, the mixture burns slowly enough that this push back effect is minimal. And it is offset by the fact that maximum pressure is reached where it can transfer the most energy to the crankshaft.
The second is that the ignition event takes a fairly constant amount of time (given a particular mixture). Thus, if the engine runs at low RPM, a larger fraction of the burning process takes place before TDC is reached. Ignition typically takes place 20 to 25° before TDC. This setting is chosen to give best takeoff power: high RPM, full rich mixture, full throttle. This setting is a compromise: in fact, most cars these days, measure a number of engine parameters to adjust the ignition timing dynamically. In our engines, we do not have this luxury.
The third is that, leaner mixtures burn faster. Thus, if we are flying at full power and we lean, the combustion event goes faster and maximum pressure occurs earlier. It therefore also occurs when the combustion chamber is smaller and this causes much higher pressures and thus, much higher temperatures.
If maximum pressure is reached when the piston is at or near TDC, that pressure cannot be converted into motion. It is like hitting the piston with a hammer.
This is detonation.
Detonation is combustion that goes too fast: the crankshaft hasn’t turned enough to allow the combustion event to properly push the piston down and rotate the crankshaft. So, why do we allow this to happen? Why do we ignite the mixture well before the piston reaches TDC, why not adapt the moment of ignition to engine RPM and maybe mixture too? Well, that can be done. An A&P mechanic wrote:
Under normal circumstances the fixed timing in an aircraft engine suits it quite well. Many of the large radial piston engines have variable timing and many of the early small aircraft engines had it also. It was discovered that it didn’t have much effect at the high power settings where most flying is done. The complex variable timing systems were dropped in favor of the more reliable fixed timing. Slick makes electronic magnetos with variable timing.
I flew a 172 with the system. It makes it much easer to start, but not much else. If you carefully read their literature, the timing profile is exactly the same to 3000 feet. At 10,000 feet the engine gains 1% in fuel efficiency. The system cost about $10,000 several years ago.
Not really worth it, I guess.
As we have seen, two things can cause detonation: mixture too lean and RPM too low. Lean mixtures cause faster burning and low RPM causes the crankshaft to make less progress during the combustion event. The combination of lean mixture and low RPM is the deadliest when it comes to detonation.
Is detonation bad? You betcher! Detonation causes very high temperatures that lead to burned valves and weakened metal (remember Figure 3?). And detonation causes excessive forces on piston, connecting rod and crankshaft. As you know, we saw all of these in 57T’s engine recently.
So when can detonation happen? I think the likeliest scenario is a go around (or a missed approach). You’re descending from altitude: low power setting, mixture lean for altitude, low cruise RPM and you forget the GUMP check. Then you have to do a go around, or, on a practice instrument approach, you miss: you firewall the throttle and now you’re flying lean and at low RPM, the deadly combination. Less than a minute of flying like this can cause damage to the engine.
We have Lycoming engines in our fleet. They’re all carbureted and this presents some specific conditions when it comes to engine management.
Figure 6 is a typical graph (for a Continental engine — I couldn’t find a chart for Lycoming engines) showing the EGT for all six cylinders as the mixture is changed from rich to lean. Note the vertical spread between EGTs at any particular mixture setting and notice the difference in mixture settings required to keep all cylinders at or lean of peak and at or rich of peak. This difference is indicated by the two vertical blue lines.
What does this mean? If you lean using data from one cylinder, or the average of (some of) the cylinders, (see Figure 7), it is unclear what happens to the other cylinders, or the cylinders farthest away from the average. If you try to run LOP, some of the cylinders will be too lean to produce power, while others may overheat: You’ll have a very rough running engine that is in danger of being damaged by excess heat in one or several cylinders.
The Continental engine shown in the picture is a fuel-injected one. By and large, fuel-injected engines show better consistency across cylinders that carbureted ones. Our engines are carbureted, so, for them, the situation must be expected to be worse than Figure 6 shows!
57T’s retractable gear causes the air-intake manifolds to be longer and more asymmetrical than in many other airplanes. The asymmetry makes the difference between cylinders worse.
If you look back at Figure 2, take a look at the horse-power curve on the rich (right-hand) side of the graph. Observe that power does not change much with mixture. On the other (lean) side of the graph, however, power decreases sharply with mixture. This implies that, if we’re flying with all cylinders ROP, some will, of course, be richer than others, but the difference in power output is not too great. The engine will run smoothly. Now image we run with all cylinders LOP. Some will be leaner than others and the power output of those cylinders will be much less than those closer to peak: the engine will be rough.
This explains the leaning technique used for 8SQ (and 54E and 57T as well): lean the mixture until the engine starts running rough — this indicates that one or more cylinders have gone past peak — then enrich until the engine runs smooth again — this indicates that all cylinders are at or below peak — and finally enrich by one or two turns of the mixture knob, making sure that all cylinders are running ROP.
In 54E, there’s an EGT gage that could be used for leaning. But look at Figure 2 again: EGT doesn’t peak until after CHT has peaked. Looking for peak EGT and then enriching a bit can put you right at peak CHT.
The Skylane has an EDM-700 engine monitor from J.P.Instruments, see Figure 8. Pilots flying 57T, it’s a good idea for you to download the manual. Go to:
It displays EGT and CHT (among other things). The display is shown in Figure 8. Each bar represents the EGT of one of the cylinders. The gap in the bar represents the CHT. The numbers under the bars also display EGT (left) and CHT (right), cycling through the cylinders.
The button on the right is the ‘Lean Find’ button. When it’s pressed and the engine is gradually leaned, it will start flashing when the first cylinder’s EGT peaks. It’s essential that leaning happens very, very slowly (maybe half a turn every fifteen seconds); otherwise the probes will lag too far behind. After reaching peak, the mixture should be enriched by two or three turns.
The instrument is useful for keeping an eye on CHT too. Never let it go past 400 and preferable not past 380. Safety margins are very useful.
Use 57T’s cowl flaps. Don’t forget to open them when full, or nearly full, power is applied. Keep them closed in cruise and during descents. But the important item is never to fly at full power with cowl flaps closed, not even in winter.
The Skylane also has a propeller control. Although the operating handbook allows running the engine with throttle and RPM anywhere in the green, running the engine at maximum throttle and minimum RPM, especially when the engine is leaned too, means you’re running it hotter and closer to its operational margins. Adding power (without adjusting RPM and mixture first) may lead to detonation and could result in engine damage.
In 57T, it can be a good idea to lean during taxi. The spark plugs foul easily. If you lean with the engine running 900 RPM (you’ll have to turn that knob for quite a while), you’ll observe an increase in RPM; very soon after that the engine will want to quit. Find the increased RPM sweet spot. The `Before Takeoff’ checklist reminds you to go back to full rich for the run up. Normally, I don’t lean again after the run up – you don’t want to take off with a leaned engine. If, however, I have to wait a long time for a clearance (or for lots of other planes to take off before me), I lean again and do a miniature run up when it’s my turn to go to the runway.
If you forget to lean before take off, and you push in the throttle, the mixture becomes so lean that the engine will stop running. But for this to work, you’ve got to make sure to lean all the way: On the ground, lean all the way or not at all.
<![if !supportLists]>1. <![endif]>Follow the recommendations in the POH.
<![if !supportLists]>2. <![endif]>At full throttle and below 3000 feet, always run the mixture full rich
<![if !supportLists]>3. <![endif]>At lower power settings, or above 3000 feet, leaning is allowed.
<![if !supportLists]>4. <![endif]>Lean by turning the mixture knob, one half turn per 15 seconds or so. The RPM will increase slightly and then decrease again. The POH recommends leaning until the RPM peaks and then drops by 25 to 50 RPM. But at power above 75%, the POH recommends not leaning beyond peak RPM. At low power settings, such leaning may cause the engine to run rough. When this happens, the mixture should be enriched until the engine runs smoothly again.
<![if !supportLists]>5. <![endif]>Add power right to left: first enrich the mixture, and then increase power. When reducing power, go left to right: first reduce power, and then re-lean.
<![if !supportLists]>6. <![endif]>When entering the pattern, or at the FAF, enrich the mixture, make sure the fuel selector valve is on both (GUMP check).
<![if !supportLists]>7. <![endif]>When a go around or missed approach is initiated, make doubly sure of GUMP by pushing mixture and then throttle in (push all controls in starting on the right — it’s a good idea to make this a habit, this is good practice for 57T). Finally deal with flaps.
<![if !supportLists]>1. <![endif]>Follow the recommendations in the POH.
<![if !supportLists]>2. <![endif]>At power settings where MP is above 23″ (top of the green), always run mixture and RPM full forward.
<![if !supportLists]>3. <![endif]>Reduce power into the green before reaching 1000 ft AGL.
<![if !supportLists]>4. <![endif]>During climb mixture should be full rich until reaching 3000 feet (POH). Climbing at higher airspeeds improves engine cooling and forward visibility – and you could even get to your destination more quickly.
<![if !supportLists]>5. <![endif]>Cowl flaps must be open for taxi, take-off and climb.
<![if !supportLists]>6. <![endif]>Add power ‘right to left’: first enrich the mixture, then increase propeller speed, finally add power (and open cowl flaps when going to climb power).
<![if !supportLists]>7. <![endif]>Reduce power ‘left to right’: first reduce power, then reduce propeller speed, finally adjust the mixture (and close cowl flaps when transitioning from climb to cruise).
<![if !supportLists]>8. <![endif]>Only lean when MP and RPM are in the green. Lean by pushing the lean-find (right-hand) button on the engine monitor. Then lean at a rate of a half turn per 15 seconds. When the first cylinder peaks, its bar starts flashing and ‘LEANEST’ is displayed. Enrich by at least 50°F EGT.
<![if !supportLists]>8. <![endif]>Before entering the pattern, or upon reaching the FAF, enrich the mixture, lower the gear and make sure the fuel selector in on ‘Both’. It is permissible to postpone setting the RPM to high until power is further reduced (see the next item)
<![if !supportLists]>1. <![endif]>To reduce power below the green MP range, pull carb heat; pull the throttle and push the propeller control all the way in (remember `left-to-right’?).
<![if !supportLists]>2. <![endif]>On base and again on short final (or upon reaching MDA or before reaching DH), do a GUMP check to make sure gear is down and everything is set for a possible go around.
<![if !supportLists]>3. <![endif]>When a go around or missed approach is initiated, make doubly sure of GUMP by pushing mixture, prop control, and throttle in (`right-to-left’! — it’s a good idea to make this a habit, even in 8SQ and 54E); when a climb has been established, raise gear and open cowl flaps. Finally deal with flaps.
<![if !supportLists]>4. <![endif]>Don’t ever lean just a bit when taxiing: you may forget to enrich before takeoff and that can ruin your day. Don’t lean at all or lean aggressively — on the ground.
<![if !supportLists]>5. <![endif]>Never allow CHT to get above 400 °F
<![if !supportLists]>6. <![endif]>Avoid running at low RPM and high power. Never, ever lower RPM when the manifold pressure is above the green arc.
I found a wealth of good stuff about engine operations in ‘Flying High Performance Singles and Twins,’ by John C. Eckalbar, published in 1994 by SkyRoad Projects.
On the Web, there’s a series of very informative articles about engines (with a big emphasis on balanced fuel-injected engines) by John Deakin. Start with http://avweb.com/news/columns/182085-1.html and follow the pointers to his earlier articles in the first paragraph. You can also study http://www.avweb.com/other/Leaning.pps
February 2008, Sape Mullender