Energy and Efficiency: Why are our piston aircraft engines so @#$%*! inefficient?

sories such as magnetos, fuel pumps, alternators, vacuum pumps, hydraulic pumps, air conditioning compressors, etc. Figure this robs 5 percent of the remain- ing power, bringing total efficiency down to 36 percent. Other losses—This includes a grab bag of other inefficiencies including blow-by past the piston rings, unburned hydrocarbons in the fuel, humidity in the air, back pressure in the exhaust system, and so forth. Figure another 5 percent loss, bringing total efficiency down to 34 percent (which agrees with our earlier figure for an IO-550-B at 65 percent LOP). THERMAL AND CHEMICAL LOSSES A quite different analysis (from Fundamentals of Power Plants for Aircraft by Joseph Liston) analyzes the various thermal and chemical losses suffered by a piston aircraft engine. We’ve already seen that an internal- combustion engine is incapable of converting all the heat of combustion into mechanical energy, limited primarily by its finite compression ratio. The rest of the heat of combustion, as well as a small amount of additional heat generated by friction, is lost through the engine’s exhaust and cooling systems. There are also some chemical losses. In theory, the combustion of pure hydrocar- bon fuel at stoichiometric mixture should produce nothing but carbon dioxide (CO 2 ) and water (H 2 O). In reality, however, there’s always some sulfur in the fuel, which is transformed by combustion to sulfur dioxide (SO 2 ) and sulfuric acid (H 2 SO 4 ). If the mixture is a bit on the rich side, the exhaust also contains carbon monoxide (CO), which results from incomplete combustion, as well as some unburned carbon particles and some methane gas (CH 4 ). www.eaa.org 31 ILLUSTRATION COURTESY OF MIKE BUSCH Figure 1: Functional breakdown of efficiency losses by John T. Lowry, Ph.D. 1) Otto Cycle, 2) Volumetric, 3) Mixture, 4) Mechanical, 5) Accessory, 6) Other, 7) Net Power Output Here’s how Liston breaks this all down: Fuel energy, 100 percent Exhaust, 51.6 percent Heat, 47.0 percent Chemical, 4.6 percent CO, 3.1 percent CH 4 , 1.5 percent Other thermal, 12.2 percent Conduction to air, 7.2 percent Conduction to oil, 1.6 percent Radiation and misc., 3.4 percent Mechanical, 36.2 percent Friction losses, 4.3 percent Brake horsepower output, 31.9 percent Again, this figure agrees pretty well with our earlier 34 percent figure for the IO-550-B at 65 percent LOP cruise. CAN WE DO BETTER? What, if anything, can we do to improve this dismal efficiency? Well, don’t expect any miraculous improvements of large magnitude. But every little bit helps, and there are certainly a few areas where the potential exists for improvement. Otto cycle efficiency—As we’ve seen, the basic thermodynamic efficiency of an inter- nal combustion engine is a function of compression ratio. Unfortunately, high- compression engines have traditionally required high-octane gasoline in order to avoid detonation, and high-octane gasoline is fast becoming unobtainable because of the campaign to eliminate tetraethyl lead (TEL) from avgas. Consequently, the trend in recent years has been toward lower com- pression ratios that are compatible with low-lead or unleaded fuel. While this may be wonderful for the environment, it sure doesn’t help the thermodynamic efficiency of our engines. One bright light on the horizon is the prospect of moving from fixed-timed mag- netos to sophisticated, computerized electronic ignition systems capable of pro- tecting engines against detonation by varying ignition timing. The incorporation of variable ignition timing and detonation sensors should permit the use of higher compression ratios even with unleaded fuel. It may take a few more years before any such systems make it through FAA certification, but the prospects for improved efficiency are significant. Even more exciting is the recent advent of certificated diesel engines for piston aircraft, which run on Jet A and have 18-to-1 compression ratios that offer much greater thermal efficiency than any spark ignition gasoline engine. Volumetric efficiency—Small improve- ments in this area are possible through the use of tuned induction systems, large intake valves, venturi-style valve seats, ram recovery air scoops, and turbocharg- ing. Auto engines have even gone to multiple intake valves per cyl- inder, but the weight and complexity might make this impractical for aircraft engines. Mixture losses— Major strides have already been made in this area, partially through pilot educa- tion to encourage the use of lean mixture settings, and partially through improve- ments to engine instrumentation and mixture distribution to facilitate operation at or near best economy mixture (i.e., consider- ably LOP). Mechanical losses—The biggest thing that can be done to reduce mechanical losses is for pilots to cruise at low rpm and high manifold pressure, rather than vice versa. Small additional gains are pos- sible through the use of high-lubricity synthetic oil to reduce friction losses, but the leading all-synthetic oil (Mobil AV 1) was pulled off the market in the 1990s due to its inability to control lead depos- its, and even semi-synthetics like AeroShell 15W-50 have lead-deposit problems, particularly in small-sump engines like the ones used in the Cessna TTX and Cirrus SR22. When the lead is ultimately removed from avgas, all-syn- thetic oils may come back in favor for piston aircraft engines. Accessory losses—The conversion to electronic ignition systems may also pro- vide some small benefits by eliminating the mechanical losses involved in driving dual magnetos, although this may be par- tially offset by the requirement for electronic-ignition engines to have dual alternators. The trend toward all-electric airplanes with no pneumatics or hydrau- lics may also help slightly. For now, the best thing you can do to improve efficiency is to lean aggressively (considerably LOP if feasible), and to cruise at low rpm and high MP rather than vice versa. In the foreseeable future, fur- ther improvements may be possible through the use of variable-timing elec- tronic ignition systems and installation of higher-compression pistons. Efficiencies in the area of 40 percent are possible, but don’t expect much more than that from spark-ignition engines, at least any time soon. Mike Busch, EAA 740170, was the 2008 National Aviation Maintenance Technician of the Year, and has been a pilot for 44 years, logging more than 7,000 hours. He’s a CFI and A&P/IA. E-mail him at mike.busch@savvyaviator. com. Mike also hosts free online presentations as part of EAA’s webinar series on the first Wednesday of each month. For a schedule visit www.EAA.org/webinars. 32 Sport Aviation October 2014 Figure 2: Thermal and chemical breakdown of efficiency losses by Joseph Liston. 1) Exhaust [Heat], 2) Exhaust [Chemical], 3) Conduction to Air, 4) Conduction to Oil, 5) Radiation and Misc., 6) Friction, 7) Net Power Output ILLUSTRATION COURTESY OF MIKE BUSCH