MIKE BUSCH COMMENTARY / SAVVY AVIATOR

Energy and Effi ciency Why are our piston aircraft engines so @#$%*! ineffi cient?

OUR PISTON AIRCRAFT engines convert chemical energy into (1) At takeoff power, the engine is turn- mechanical work, but they don’t do it very efficiently. It turns out ing at 2700 rpm. Since it’s a four-stroke that only about one-third of the energy contained in the 100LL engine, each power cycle requires two we burn winds up getting to the propeller and doing useful work crankshaft revolutions. Therefore, the to propel us through the air. The remaining two-thirds winds up engine is operating at 1,350 power cycles getting lost between the fuel truck and the prop hub. At today’s per minute. stratospheric avgas prices, that’s pretty depressing. (2) The displacement of the engine is 550 cubic inches, or 0.32 cubic feet. Due to LET’S DO THE MATH induction system losses, however, the Consider a Continental IO-550 engine rated at 300 hp. If the fuel engine’s “volumetric efficiency” is only system is set up properly per Continental Service Bulletin about 85 percent, so it “inhales” only about SID97-3F, fuel flow at maximum takeoff power is about 26.6 gal- 0.27 cubic feet of air per power cycle. lons/hour or 156 pounds/hour. How much chemical energy does (3) Multiplying 1,350 power cycles per that fuel provide? second times 0.27 cubic feet of air per cycle, We can calculate that. 100LL is rated at a “minimum lower we calculate that the engine should inhale heat value” of 18,700 BTUs per pound. Let’s convert that figure 365 cubic feet of air per minute. into something more meaningful to pilots like you and me. (4) Sea level air under standard atmo- (1) Divide 156 pounds per hour by 3,600 seconds per hour to get spheric conditions weighs 0.0765 pounds 0.0433 pounds per second. per cubic foot. Therefore, the engine (2) Multiply by 18,700 (the thermal content of 100LL in BTUs breathes 27.9 pounds of air per minute. per pound) to get 810 BTUs per second. (5) Best power mixture requires an air- (3) Multiply by 1.414 (the horsepower equivalent to 1 BTU per fuel ratio of about 12.5 to 1 by weight. second) to get 1,146 hp. Dividing 27.9 by 12.5, we get a fuel burn of Does this mean that your IO-550 consumes 100LL with ther- 2.23 pounds of fuel per minute—or multiply- mal energy equivalent to 1,146 hp, and yet produces only 300 hp ing by 60, we get 134 pounds per hour or of output power? Unfortunately, that’s exactly what it means— 22.3 gallons per hour calculated fuel flow at and that works out to a miserable thermal efficiency of 26 best power mixture. percent. Good grief! The actual book fuel flow figure of 26.6 Should an IO-550 really be drinking this much fuel? Well, we gallons/hour or 156 pounds/hour is higher can calculate that, too. than this calculated value because of the

30 Sport Aviation October 2014

30-32_savvyOCT.indd 30 9/10/14 11:11 AM unusually rich mixture required to provide efficiency down to 57.5 adequate detonation margins at full take- percent times 85 per- off power. cent, or 49 percent. Mixture losses— WHAT ABOUT LOP? Optimum fuel Surely engine efficiency is much better at efficiency occurs at cruise power settings with aggressively very lean mixture set- lean mixtures, right? Let’s take a look. An tings (so-called “best IO-550 engine running at 65 percent economy mixture”) power and operating LOP uses approxi- with an air-fuel ratio mately 13 gallons/hour or 78 pounds/hour. in the vicinity of What kind of thermal efficiency does that 18-to-1 by weight. Best represent? Repeating the calculations: economy mixture (1) Divide 78 pounds per hour by 3,600 occurs very LOP, how- seconds per hour to get 0.0217 pounds ever, and most pilots Figure 1: Functional breakdown of effi ciency losses by John T. Lowry, Ph.D. 1) Otto Cycle, 2) per second. don’t operate that lean. Volumetric, 3) Mixture, 4) Mechanical, 5) Accessory, 6) Other, 7) Net Power Output (2) Multiply by 18,700 (the thermal con- (Not to mention that tent of avgas in BTUs per pound) to get 405 many engines won’t run BTUs per second. smoothly that lean.) Many pilots operate loss, bringing total efficiency down to (3) Multiply by 1.414 (the horsepower rich of peak EGT in the vicinity of best- 34 percent (which agrees with our equivalent to 1 BTU per second) to get power mixture, at an air-fuel ratio around earlier figure for an IO-550-B at 65 573 hp. 12.5-to-1, which provides a fuel efficiency percent LOP). So even at LOP cruise, the IO-550 con- that’s only 70 percent of optimum. Even if sumes 573 hp worth of go-juice in order to you operate slightly LOP (let’s say at an THERMAL AND CHEMICAL LOSSES produce 195 hp (65 percent of 300), for an air-fuel ratio of 16-to-1), your efficiency is A quite different analysis (from efficiency of about 34 percent. Definitely just 89 percent of optimum, and that Fundamentals of Power Plants for Aircraft better, but certainly nothing to write brings total efficiency down to 49 percent by Joseph Liston) analyzes the various home about. times 89 percent, or 44 percent. thermal and chemical losses suffered by a Mechanical losses—Friction losses piston aircraft engine. WHY SO WASTEFUL? involving the reciprocating and rotational We’ve already seen that an internal- Here’s one breakdown of efficiency losses parts inside the engine consume a signifi- combustion engine is incapable of (from Performance of Light Aircraft by cant amount of power that could converting all the heat of combustion into John T. Lowry, Ph.D.): otherwise be delivered to the propeller. mechanical energy, limited primarily by its Otto cycle efficiency—the thermody- Mechanical efficiency varies with engine finite compression ratio. The rest of the namic efficiency of a four-stroke internal speed (lower losses at lower rpm), but is heat of combustion, as well as a small combustion engine—is limited by the com- typically around 88 percent, bringing total amount of additional heat generated by pression ratio (i.e., the ratio of cylinder efficiency down to 44 percent times 88 friction, is lost through the engine’s volumes as the piston moves from bottom- percent or 38 percent. exhaust and cooling systems. dead-center to top-dead-center). The Accessory losses—A certain amount of There are also some chemical losses. In higher the compression ratio, the greater engine power is consumed driving acces- theory, the combustion of pure hydrocar- the efficiency. For an IO-550 with a com- sories such as magnetos, fuel pumps, bon fuel at stoichiometric mixture should

pression ratio of 8.5-to-1, the Otto cycle alternators, vacuum pumps, hydraulic produce nothing but carbon dioxide (CO2)

efficiency works out to about 57.5 percent. pumps, air conditioning compressors, etc. and water (H2O). In reality, however, Volumetric efficiency—As mentioned Figure this robs 5 percent of the remain- there’s always some sulfur in the fuel, earlier, the ability of the engine to breathe ing power, bringing total efficiency down which is transformed by combustion to

in its full theoretical displacement of air to 36 percent. sulfur dioxide (SO2) and sulfuric acid

during each power cycle is restricted by a Other losses—This includes a grab (H2SO4). If the mixture is a bit on the rich variety of pressure losses at various points bag of other inefficiencies including side, the exhaust also contains carbon in the induction system: air filter, throttle blow-by past the piston rings, unburned monoxide (CO), which results from body, intake manifold, and intake valves. hydrocarbons in the fuel, humidity in the incomplete combustion, as well as some For most of our engines, volumetric effi- air, back pressure in the exhaust system, unburned carbon particles and some

ciency is around 85 percent, bringing total and so forth. Figure another 5 percent methane gas (CH4).

ILLUSTRATION COURTESY OF MIKE BUSCH www.eaa.org 31

30-32_savvyOCT.indd 31 9/10/14 11:16 AM Here’s how Liston breaks this all down: is fast becoming unobtainable because of to facilitate operation at or near Fuel energy, 100 percent the campaign to eliminate tetraethyl lead best economy mixture (i.e., consider- Exhaust, 51.6 percent (TEL) from avgas. Consequently, the trend ably LOP). Heat, 47.0 percent in recent years has been toward lower com- Mechanical losses—The biggest thing Chemical, 4.6 percent pression ratios that are compatible with that can be done to reduce mechanical CO, 3.1 percent low-lead or unleaded fuel. While this may losses is for pilots to cruise at low rpm

CH4, 1.5 percent be wonderful for the environment, it sure and high manifold pressure, rather than Other thermal, 12.2 percent doesn’t help the thermodynamic effi ciency vice versa. Small additional gains are pos- Conduction to air, 7.2 percent of our engines. sible through the use of high-lubricity Conduction to oil, 1.6 percent One bright light on the horizon is the synthetic oil to reduce friction losses, but Radiation and misc., 3.4 percent prospect of moving from fixed-timed mag- the leading all-synthetic oil (Mobil AV 1) Mechanical, 36.2 percent netos to sophisticated, computerized was pulled off the market in the 1990s Friction losses, 4.3 percent electronic ignition systems capable of pro- due to its inability to control lead depos- Brake horsepower output, tecting engines against detonation by its, and even semi-synthetics like 31.9 percent varying ignition timing. The incorporation AeroShell 15W-50 have lead-deposit Again, this fi gure agrees pretty well with of variable ignition timing and detonation problems, particularly in small-sump our earlier 34 percent fi gure for the sensors should permit the use of higher engines like the ones used in the Cessna IO-550-B at 65 percent LOP cruise. compression ratios even with unleaded TTX and Cirrus SR22. When the lead is fuel. It may take a few more years before ultimately removed from avgas, all-syn- CAN WE DO BETTER? any such systems make it through FAA thetic oils may come back in favor for What, if anything, can we do to improve certification, but the prospects for piston aircraft engines. this dismal efficiency? Well, don’t expect improved efficiency are significant. Accessory losses—The conversion to any miraculous improvements of large Even more exciting is the recent advent electronic ignition systems may also pro- magnitude. But every little bit helps, and of certificated diesel engines for piston vide some small benefits by eliminating there are certainly a few areas where the aircraft, which run on Jet A and have the mechanical losses involved in driving potential exists for improvement. 18-to-1 compression ratios that offer much dual magnetos, although this may be par- Otto cycle effi ciency—As we’ve seen, the greater thermal efficiency than any spark tially offset by the requirement for basic thermodynamic effi ciency of an inter- ignition gasoline engine. electronic-ignition engines to have dual nal combustion engine is a function of Volumetric efficiency—Small improve- alternators. The trend toward all-electric compression ratio. Unfortunately, high- ments in this area are possible through the airplanes with no pneumatics or hydrau- compression engines have traditionally use of tuned induction systems, large lics may also help slightly. required high-octane gasoline in order to intake valves, venturi-style valve seats, For now, the best thing you can do to avoid detonation, and high-octane gasoline ram recovery air scoops, and turbocharg- improve efficiency is to lean aggressively ing. Auto engines have (considerably LOP if feasible), and to even gone to multiple cruise at low rpm and high MP rather than intake valves per cyl- vice versa. In the foreseeable future, fur- inder, but the weight ther improvements may be possible and complexity might through the use of variable-timing elec- make this impractical tronic ignition systems and installation of for aircraft engines. higher-compression pistons. Efficiencies Mixture losses— in the area of 40 percent are possible, Major strides have but don’t expect much more than that already been made in from spark-ignition engines, at least this area, partially any time soon. through pilot educa- tion to encourage the Mike Busch, EAA 740170, was the 2008 National Aviation use of lean mixture Maintenance Technician of the Year, and has been a pilot settings, and partially for 44 years, logging more than 7,000 hours. He’s a CFI through improve- and A&P/IA. E-mail him at mike.busch@savvyaviator. Figure 2: Thermal and chemical breakdown of effi ciency losses by Joseph Liston. 1) Exhaust ments to engine com. Mike also hosts free online presentations as part [Heat], 2) Exhaust [Chemical], 3) Conduction to Air, 4) Conduction to Oil, 5) Radiation and instrumentation and of EAA’s webinar series on the fi rst Wednesday of each Misc., 6) Friction, 7) Net Power Output mixture distribution month. For a schedule visit www.EAA.org/webinars.

32 Sport Aviation October 2014 ILLUSTRATION COURTESY OF MIKE BUSCH

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