Use of combustion in transportation began by burning fuels like wood and coal in the open to boil water to make steam to drive pistons to crank a shaft. Later combustible petroleum-derived fuels were burned directly over the piston heads. Pistons have been mostly replaced now in favor of turbines, with continuous combustion around the circumference of fine-bladed fans. Pistons continue to have advantages for some applications such as light aviation and road vehicles however. Though they are heavier per horsepower than turbines, with more moving parts, they are less expensive to manufacture and more fuel-efficient.
Old methods.
In 1983, when I learned to fly in an ancient C172, the only engine-related instruments were an oil pressure gauge and an oil temperature gauge. We were taught not to lean from full rich until we reached cruise, and never below 3000 ft. The method was to lean until RPM decreased, then enrichen a little. Imprecise. But given that this was a normally-aspirated engine, it was sufficient to keep us out of trouble. Besides, more precision needed to wait on improved instrumentation along with the improvements that fuel injection brought.
The turbocharged Cessna P210, N267LM, that Sape Mullender and I ferried from Schwäbisch Hall Germany to Waterloo Iowa in 2000 had an Economy Mixture Indicator (EGT) and a set of operating instructions P210 pg 1 and pg 2 designed to keep internal cylinder pressures at a safe level by keeping fuel flow sufficiently rich of peak.
With tuned injectors it became possible to ensure that all cylinders were receiving close to the same fuel-air charge. With improved instrumentation, it became practical to monitor key temperatures in each cylinder. Indirectly, that gave the pilot indications of internal cylinder pressures and enabled us to operate piston engine more efficiently by giving us the lean of peak option.
New methods.
I had been reading John Deakin’s Pelican’s Perch, but when Scott Marti first took hold of the mixture control in a Columbia 400 and pulled it back from 38 gph to 18 gph to demonstrate the big pull, I must admit I experienced an involuntary shudder. Old habits, especially in old dogs, take time to change. Now we all have flown thousands of hours using these new methods, and we know they work well. But they must be thoroughly understood if we want to use them most efficiently, and they are not reducible to a simple set of instructions.
Dr. Wayne Isom, who I frequently fly with, makes a comparison with new interns learning diagnosis. They come out of medical school with a set of useful numbers tatooed on their brains, like pO2 96% to indicate satisfactory respiration. He watched one miss a collapsed lung by focusing on that number and missing a rapid respiration rate.
More than once I have had pilots tell me they run their engines at 31.5 in MP, 2450 rpm, with FF at 18 gph. Like that 96, those are useful numbers, but not the whole story.
Future methods.
Low initial cost and low operating costs will keep piston engines in small aircraft for some time. We are likely to see changes in fuel. A company called Swift Enterprises for example, currently has a formulation called 702 in testing at the FAA William J. Hughes Technical Center at Atlantic City Airport in New Jersey. Preliminary tests showed the mixture, made from biomass, meets or exceeds the standards for 100LL avgas without any petroleum-derived components, and without lead. We are also likely to see variable timing on ignition systems with the advent of automated engine management systems such as the one called PRISM that GAMI is working on. The PRISM times the spark by directly referencing the internal cylinder pressure. We may see better designed fuel injection systems as well. Diesel engines like Thielert’s may become more widely used. They offer piston efficiency burning Jet A or road diesel fuel. Their Centurion 2.0, for example, is a turbocharged 4-stroke water-cooled 135 hp engine with a compression ratio of 18:1. Over 100 aircraft are equipped with it.
Animated model.
To focus discussion, a Flash animation has been developed, copyrighted by Principia Inc., 2009. Access is free to individuals for private use. Organizations or individuals who wish to offer the animation to the public will require a commercial license. To obtain a login, email Principia Inc.
Go to Principia client area to login and open the animation. It will persist in your browser’s cache, even without an internet connection, until you manually clear the cache or turn off your machine.
A Flash plug-in is available from Adobe if not already installed in your browser.
In the cylinder view, you will see a representation of the two flame fronts spreading out from the two spark plugs. Of course, that is an idealization. To see an actual video taken inside an operating cylinder, go here on YouTube.
Five red buttons on top control (from left) page, either panel or cylinder; pause crankshaft rotation; step one frame; zoom; select individual frames.
Three gauges and four sliders are active controls: MP, RPM, FF, SPARK, IAS, OAT, and OCTANE.
Fuel flow (FF) can be set independently by clicking on the FF pointer and moving it to the desired level.
Seven graphs display results of changes in variables, from top left, turbine inlet temperature (TIT), exhaust gas temperature (EGT), cylinder head temperature (CHT), internal cylinder pressure (ICP), brake horsepower (HP), air to fuel ratio (A:F), and brake specific fuel consumption (inverted), a measure of fuel efficiency (1/BSFC); colors are used to depict temperature changes, ranging from yellow (cooler) to red (hotter); colors are used to indicate air/fuel ratio ranging from light blue (leaner) to dark blue (richer); redlines on TIT and CHT are indicated.
Failures are selected with toggle keys along the bottom: INJECT/VAPOR for effect of reduced fuel flow from either plugged injector or vapor lock; PLUG for one failed spark plug; CHT-IND for a failed CHT probe; DETON for detonation; PREIGN for preignition; VALVE for an exhaust valve stuck partially open; and CRACK for a crack in the exhaust manifold
Variations.
MP (throttle) increase results in higher potential internal cylinder pressures as indicated indirectly by TIT, EGT, and CHT data.
RPM (engine and prop revolutions) increase tends to increase fuel flow due to increase in engine-driven fuel pump speed.
FF (fuel flow) adjust to show impact of air:fuel mixture changes.
Spark timing can be advanced or retarded to illustrate the impact of earlier or later ignition.
OAT (outside air temperature) increase tends to cause internal cylinder pressures (ICP) to rise by decreasing air-cooling efficiency; also tend to limit maximum LOP fuel flow by limiting available oxygen mass in the mixture.
IAS (indicated air speed) increase tends to decrease ICP by improving air-cooling efficiency.
Octane increase decreases the rate of flame front progress.
Examples.
Hot and high cruise: MP 32 IN RPM 2500 IAS 150 OAT -5 C (ISA +30 at FL250).
Already only about half as dense at this altitude, cooling air is also warmer than standard. Result is less efficient cooling, resulting in higher pressures and temperatures in the combustion chamber, reflected in higher TIT, EGT, and CHT indications. Induction air, being warmer, is less dense than standard, even after being compressed by turbochargers. It therefore contains fewer oxygen molecules, and is able to oxidize fewer fuel molecules. In light of those two factors, LOP fuel flow will need to be lower than standard to keep cylinder pressure (as indirectly indicated by TIT, EGT and CHT) at an acceptable level.
Cessna’s PIM for the 400, RC0500005HIM, Revision level H, pg 5-32, suggests a fuel flow of 15 gph might work under these extremely hot conditions, to keep TIT at a value at least 50 dF below peak. It might. But rather than setting that fuel flow, and accepting whatever TIT results, the prudent pilot will reduce fuel flow until TIT cools to 1625 dF or below, and monitor CHTs to ensure they do not rise much above 380 dF. More prudent still to set fuel flow to keep CHTs below 380, even though it might mean sacrificing airspeed. Suppose that 15 gph turns out to work. The 7.5:1 compression ratio in for the TSIO-550-C produces 13.7 BHP per gph LOP, yielding 205 of 310 maximum, or about 66%.
That same page in the same PIM predicts 231 KTAS under these same conditions, which yields 147 KIAS. Interestingly, that airspeed prediction is marginally better than what is predicted at standard conditions, despite the predicted lower fuel flow and lower BHP. What gives? Less dense air at warmer than standard temperatures has one nice positive outcome: higher airspeed due to lower parasitic drag.
Autogas: hypothetical but educational. Substituting regular US auto fuel for 100LL avgas.
FAA-approved STCs are available for most lower compression aircraft engine installations, but no turbocharged engines and not for Part 135.
Why just lower compression engines? The lower octane rating of auto fuels signifies among other characteristics, their lower auto-ignition temperatures. That is the temperature (pressure) at which a gaseous fuel mixed with air will supply the energy to support combustion. In a diesel engine, we want a relatively low auto-ignition temperature. That is because a spark plug is not used to trigger ignition. Instead, liquid fuel is injected directly into the cylinder once the piston is starting down after compressing air in the combustion chamber and thereby raised the temperature above diesel fuel’s auto-ignition temperature by compression. The injected diesel fuel immediately vaporizes and combusts, creating additional heat and pressure to accelerate the piston down.
However, in engines designed to burn avgas or autogas, a mixture of fuel vapor and air is pulled into the combustion chamber on an intake stroke, then compressed on the next stroke, during which process, it must resist the urge to spontaneously combust before ignited by the spark plug at the end of the compression process. Otherwise, the cylinder could be damaged. This is known as pre-ignition. For 100LL avgas, that auto-ignition temperature is about 440 dC. For auto fuels, it is much lower, about 260 dC. Hence the limitation to lower-compression aircraft engines.
