From the Experimental Aircraft Association
piperboy84. A bit of reading to wade through. If you have access to the actual 1996 magazines you will find photos and graphs of test results. In summary for those who don't
Substantial negative pressure waves can be generated in tuned aircraft exhaust systems and the timing of their suction can be arranged so as to improve engine power. Such improvement should produce more power, better efficiency and a cleaner combustion chamber.
The 4 into 1 collector exhaust systems appear to offer the best combination of low opening pressure, some pumping gain and good scavenging, though the crossover and Tri-Y systems can also obtain good scavenging during the overlap stroke.
The addition of a suitable megaphone to the collector of a 4 into 1 exhaust system usually produces an increase in the negative pressure achieved at the exhaust valve, but at a substantial penalty in noise.
The use of swiveling ball joints on the collector of a 4 into 1 exhaust system has a neglible effect on the EPG and provides an important vibration-isolation benefit to the system.
The optimization of pipe geometry for the crossover, Tri-Y and 4 into 1 exhaust systems can be found by study of the EPG.
Fast Fourier transforms, derived from the EPG, could facilitate the development of an electronic, active noise-cancelling muffler. Aircraft exhaust systems, by their limited RPM range, are particularly well-suited to such a muffler.
The Coanda nozzle did not produce a noticeable increase in power. Fabrication and durability problems make this nozzle of limited attractiveness.
Exhaust jet thrust was measured and calculated for several exit sizes, RPM's and fuel flows. It can produce significant thrust at high power settings, especially at cruising altitudes.
The stock camshaft used in an aircraft engine is typically optimized for reliability and tractability and is not optimized for the tuned exhaust systems tested here. To fully realize the potential benefits of tuned exhaust systems for the aircraft engine, the camshaft timing must be suitably altered by making exhaust valve closure occur later and the overlap period of longer duration and higher lift.9 Many of the scavenging systems here do not exhibit as much effect upon the intake manifold pressure during the overlap as might occur if the camshaft had greater valve overlap.
The effect of cylinder head temperature, mixture (EGT), and outside air temperature on the EPG were studied and found to produce minimal changes.
Further study should include the correlation of climb and cruise airspeeds with EPG's taken in flights controlled for power setting and aircraft weight. These should be per- formed using exhaust jet nozzles, megaphones, altered ignition timing and other variations.
Flight Reports - EPG and Aircraft Exhaust Systems 1
EAA Members Only
Flight Reports - EPG and Aircraft Exhaust Systems 3
Flight Reports - EPG and Aircraft Exhaust Systems 4