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Piston Engine Management

posted Apr 17, 2011, 2:39 PM by George Finlay   [ updated Oct 8, 2011, 11:07 AM by Nathaniel Cauldwell ]

I made a sort of living for a brief time in the 60s rebuilding air-cooled normally-aspirated engines that powered the thousands of VW beatles and vans on the road then.

So when engine monitors like the JPI began to appear in the cockpit, I had a basic understanding of what I was looking at, and I was intrigued.At first, I used these monitors just as more detailed EGT gauges. I would look to see that the engine really was running a little rich of peak after I leaned by the rough method all of us learned if we learned to fly a piston engine any time in the past 50 years — pull the mixture knob out until the RPM start to drop off, then push it in a little.

Then when even more detailed engine data began to appear in the Cirruses I was training in, I promised myself I would find out more about what goes on inside an engine. If we wanted to set up for “Best Economy” cruise, the Avidyne MFD engine page was prompting me and my trainees to go where previous rote engine leaning training had warned us never to linger for long: lean of peak. In older airplanes with normally-aspirated engines and EGT gauges, the POHs uniformly instruct pilots to run at peak EGT to achieve best economy.The turbocharged engines I first encountered were the Lycomings in the single-engine Cessnas. To achieve best economy, the Cessna POHs recommend running those engines at peak TIT. Then when I took Columbia 400 factory instructor training in Bend Oregon in January 2005, I was introduced to what was then a new and controversial idea, though it was then and still is standard procedure on the Columbias. It was dubbed “The Big Pull” and it originated in the Advanced Pilot Seminar engine management seminars. The idea is to pull the mixture knob out quickly and smoothly past peak TIT and into the lean of peak zone. Originally, the POH restricted LOP operations to power settings of 65% or less, but by the time I returned to Bend the next year for certification in the G1000 Columbias, TCM was allowing LOP in cruise up to 85% power.I had been avidly reading John Deakin’s engine management articles in his Pelican’s Perch series on Avweb.com. So when I got a chance to take an early look at a proposed online version of the popular APS course, I jumped at the chance to learn some more of the underlying science. I took another long look at the online course after it had been in public release for almost a year. By now (Fall, 2008) the online course has largely replaced the live course offered over a weekend in Ada Oklahoma. The live course will be presented this month October 24 thru 26.

This article will be a review of the concepts presented in that course, specifically as they relate to the operation of a turbocharged engine. After that review, I will offer a few suggestions on improving the format of the online course.I now require completion of this online course for all my clients transitioning into the Columbia 400, unless they test out with a score above 80% on the attached turbocharged engine management quiz.

The course begins with a review of the operation of an internal combustion engine and helped to re-familiarize me with some of the terms engineers use to discuss them. We are reminded that the spark is fired before the piston reaches top dead center, typically about 20 degrees before, and that this is denoted as minus 20 Theta. Combustion in the cylinder reaches a peak sometime after the piston has started back down, and the timing of that event is critical to the efficient and safe use of these engines. A mixture of about 40 degrees F. rich of peak TIT results in the fastest burn and the highest peak pressures. Temperature of the gases inside the cylinders at peak are about 3000 degrees F. EGT is much lower because most of that heat is dissipated through the cylinder walls and fins by the time the exhaust valve opens. EGT (and TIT) is not a good measure of peak combustion temperatures, but it is a fair measure of the timing of the combustion event in the cylinder. Best power, defined as maximum horsepower for a given MP and RPM, is obtained at about 75 degrees F. rich of peak TIT. Best economy, depending on power setting, is reached between 15-40 degrees lean of peak for relatively low power settings, and 40-90 degrees lean of peak for higher power settings.Early in the course, the concept of effective timing is introduced. ThetaPP is defined as the degrees of rotation beyond top dead center at which peak cylinder pressure occurs. It can vary with the composition of the fuel introduced into the cylinder. For example, autogas has a shorter latency period than 100LL and therefore promotes quicker progress of the flame front. Latency period is the time between the firing of the spark plug and the point at which combustion is well-started. When the rising temperature and pressure in the cylinder cause pockets of the fuel-air mixture to explode before the flame front can reach them, this is defined as detonation. To use autogas without creating dangerous levels of detonation, the spark event needs to occur later to make up for the faster burn time. The presence of somewhat less than about 2 grams of tetraethyl lead in each gallon of 100LL is there only to slow and extend the latency period. Contrary to popular belief, it has nothing to do with lubricating valves. By changing the blend, oil companies may be able to produce a fuel with almost as long a latency period as 100LL, reflected in the octane rating. The urgency of that search is increasing as it is becoming more difficult for companies to justify the cost and health danger of using leaded fuel.

Prop pitch directly limits engine RPM at high power settings, and higher RPM is conducive to later ThetaPP because when the pistons are moving faster, they stay further ahead of the advancing flame fronts. Another way of looking at it is that higher RPM leaves more room for the combustion event and therefore leads to lower peak pressures in the cylinders. The ideal point for the peak pressure has been found to be about +16 Theta. With a turbocharged engine, the pilot has the option of reducing RPM while maintaining quite high manifold pressure settings. Since there is excess air in the mixture when operating LOP, it follows that the mixture control also controls horsepower in this zone. So it is easy on the engine to reduce RPM when desired to extend range, provided mixture is kept lean enough to ensure that TITs remain in a safe range (see above) when expressed as degrees Fahrenheit LOP.  Additionally, the CHT should not be allowed above about 380°F, and the TIT should be kept at or below the published limit.The maximum certified continuous power setting in the Columbia 400 is 85% of 310, or 264. When operating LOP in an engine with a compression ratio of 7.5:1, you get 13.7 hp/gal. That means your fuel flow limit LOP is about 19 gal/hr to remain within that 85% MCP.

For simplicity, I introduce transitioning pilots to two cruise settings in the 400, Go Fast, and Go Far. Obviously, if you are really in a hurry, then you will be willing to live with high fuel flows and Go Really Fast. Then you will stay 150 degrees or more ROP. But going LOP only costs a few knots of airspeed, and it is much easier on the engine, so both of my introductory cruise settings are LOP. TIT is the guide, with a watchful eye on CHT. TIT redlines at 1700 on the 400, so 1650 is as hot as you want to run continuously, and I teach 1600 to 1625 to allow a little buffer on that. Want to go fast? Most of us do. To get to my Go Fast setup, provided the engine is set up reasonably accurately, leaning to a TIT of about 1625 will give you a fuel flow of about 18.5 to 19.0 gal/hr, with CHTs running below 380. I leave RPM about 2500 to 2550, just below the redline of 2600, and I back the MP off a little too, to about 32 inches, which puts it below the 33.5 inch maximum cruise setting. If any temperatures trend up, I fine-tune the mixture by leaning a bit, which will immediately cool the TITs and eventually the CHTs as well. At this setting, I see airspeeds on the 400 ranging from 180 TAS at low density altitudes all the way up to 220 TAS up in the flight levels.

Got a nice tailwind and you are thinking you can make that 1200 nm trip without a fuel stop if you save a little fuel? Then Go Far. As long as you are LOP, there are plenty of variations on this, and depending on just how strong that tailwind is and how far you would like to extend your range, you might decide to back off even more than this. I like to keep the RPM fairly high, so I recommend about 2400 rpm with about 30 in MP, and mixture leaned to about 15 gph, which will only cost you about 7-10% of your TAS.

LOP principles impact other modes of flight, starting with taxi and runup. If mixture is leaned aggressively, the gross mag and ignition check is more likely to reveal a weak spark plug, because with the leaner mixture, a stronger spark is needed. For this reason, APS recommends doing a mag check in the air at cruise LOP to get early warnings of weak ignition. On one mag, all EGTs should rise together and stay up. The flame front does not spread as fast or as far when ignited by just one plug. So when the exhaust valve opens at the bottom of the power stroke, the gases released are hotter, since the combustion event has ended more recently than it would have had it been set off by two sparks.

In the initial climb in the Columbia 400, I stay with the factory recommendation to climb all the way to cruise at full power, with mixture set full rich. If you are already LOP and fuel flow becomes restricted in the climb, temperatures and pressures drop, posing no threat to the cylinder. If the altitude change is minor, and the engine is already set up for LOP cruise, I do the climb LOP, carefully monitoring TIT and CHTs.

In descent from LOP cruise in a turbo like the Columbia 400 it makes sense to stay LOP and resist the urge to push the mixture in to full rich. Except for the increase in OAT and air pressure, the effect of which is minor at lower power settings typical in the descent, the engine is not aware of the decreasing altitude, so there is no need to add fuel to compensate. Worry about shock cooling appears to have no basis in fact. APS points to the long engine life typical in tow planes, which repeatedly climb at full power, then descend immediately at idle, day in and day out, with no evidence of damage from thermal changes.  Or Bob Hoover, who used to go from full power to full feather and back to full power repeatedly during his act.  His engines usually made TBO with no cylinder work.On final in the 400, I follow the APS recommendation to leave the prop alone, but I move the mixture ROP, leaned a little for the upcoming taxi. I have found that if I leave the mixture LOP, the engine is prone to quit on the rollout, which can be inconvenient.

In the event of a missed approach or go around, there is plenty of time to check that mixture and prop are full forward and the backup pump is on before adding power for the climbout.  On the other hand, I do not attempt to change a trainee’s habits if he/she pushes prop and mixture full forward on final, since there is no harm done to the engine at that point.

After landing, I follow APS and ignore the 5 minute cooldown waiting period that Columbia calls for. The thinking is that as much cooling as will ever happen has already happened in the low power approach and glide to the runway. A quick look at the engine page before shutdown will confirm that the 1-2-3 shutdown criteria will have been met in the taxi to the ramp. That 1-2-3 is an acronym for TIT below 1000 degrees, oil temperature below 200, and CHT below 300.  In fact, the data shows that temperatures will start increasing again while turning off the runway.

Finally some comments on the look and feel of the online APS seminar:

Overall, the online version is a very nice piece of work, and I commend APS for being pioneers in the use of the web for aviation training. The layout has a scrollable text box on the left side of the screen for each slide, most of which are static, but a fair number of which are animations, including some video footage taken in flight. At the end of each section, there is a brief quiz. There is a glossary you can refer to at any time to refresh your understanding of the technical terms. When I went through the course, I spent a total of about 10-15 hours on it, spread over maybe a dozen sessions. It is easy to log back in and pick up where you left off last time. The cost of access to the course for 60 days is $395.

What could be improved in future versions of the course? Some of the slides are a little difficult to see in their full detail, and adding a zoom capability would solve that. The animations could also benefit from zoom capability as well as the usual stop/start rewind/fast forward capability found on most video players. After you successfully pass a section quiz, you cannot go back to review information in that section. Access to the entire course via a table of contents would be an improvement. There is footage showing how readings change on a test engine’s instrumentation as throttle and mixture are changed, and as autogas is introduced. This would be more effective online if it were an interactive simulation, rather than a video recording of a test engine. And finally, there are video recordings showing changes on instruments during flight. Online, this information would be more clearly presented via an interactive simulation built from engine data dumped after the flight from the Avidyne MFD in a Cirrus, for example.But all of this will take time and money of course.

When and if the Prism electronic ignition is approved, and GAMI starts making serious money, maybe we will see these improvements in the course, along with others I haven’t thought of.

Links:

Pelican’s Perch

Advanced Pilot Seminars

Engine Management Quiz