Experimenter is a magazine created by EAA for people who build airplanes. We will report on amateur-built aircraft as well as ultralights and other light aircraft.
Issue link: http://experimenter.epubxp.com/i/434207
EAA Experimenter 25 The second category includes aerodynamic improvements: reducing drag and improving fl ying qualities. Then there is the eternal desire to reduce weight at every feasible opportunity. Let's take a look at some of Klaus' modifi cations. PROPULSION SYSTEM IMPROVEMENTS Klaus' plans to improve his Long-EZ were thwarted a bit in the beginning when his engine wouldn't run right. "It acted like a fuel-injection issue, like excess fuel would get injected and cause a rich miss," he said. "It was hard to find and hard to fix. I struggled with it for almost two years." The methodical Klaus admitted that he had been looking in the wrong place—fuel injection. Instead, it turned out to be poor intake design. He had finally accomplished step one of his four-step process: He had understood the situation. Now that Klaus found the general source of the problem, he had to return to step one: Find out what exactly was happening inside the intake. Klaus outfi tted the Long-EZ's intake with a pressure sensor for a portable oscilloscope and went fl ying. But Klaus had to get good measurements. He advises that magnetos make it very difficult to understand what's going on with other parts of the engine. Klaus said, "Their weak and short-duration spark delivered at greatly fluctuating de- grees causes so much scatter in exhaust and intake pressure waves as well as Lambda (mixture) values that they mask other problems." A precisely timed, powerful electronic ignition removes a lot of variables in the pressure data and shows other issues such as fuel atomization more clearly. Klaus is a recognized expert and advocate of electronic ignition for light airplanes. He is appalled that after three decades of automotive use, aircraft engines don't use elec- tronic ignition as standard equipment. In addition to helping with clear measurements of what's going on in the engine, "electronic ignition provides an immediate 10 percent reduc- tion in fuel consumption with 5 to 10 percent increase in power," he said. He uses Plasma III systems that are trig- gered directly at the crankshaft, reporting that "these sys- tems have around 0.5 degree of timing accuracy and can be varied by 1/10 of a degree. Their spark energy is about three times that of any mag." Klaus applied another automobile technology to improve his Long-EZ engine—a tuned intake. How do they work? Ambient air gathers speed as it rushes into the intake pipe during the intake stroke. At the end of the intake cycle when the inlet valve is closed, the high-velocity air hits the valve and compresses. This high-pressure air can't go into the en- gine, so it bounces back through the intake pipe. Then it hits the plenum on the other side and bounces back toward the engine. This pressure wave travels back and forth until the valve opens again. Figure 2 shows one of Klaus' oscilloscope readings for this pressure wave. Note that the big dip is the piston sucking. If the high-pressure wave happens to hit the valve at the exact mo- ment that the valve is opening, then it acts like a supercharger! In order to accomplish this feat, we need to tune the frequency of the pressure wave so that it hits the valve as it is opening. In cars, this frequency is af ected by engine speed and manifold length. You pick an engine speed where you want the ef ect to peak, and then change the manifold length appropriately. A longer intake manifold gives the best tuning performance at low speeds; a shorter intake manifold gives the best ef ect at higher rpm. Tuned intakes have been on cars and motorcycles for a long time, but they are relatively recent additions to light aircraft engines. Aircraft engine intakes are somewhat easier to tune than cars or motorcycles because their rpm range is smaller. On the other hand, the tuned intake must fi t inside an aircraft cowling and works best over a limited altitude. The main variables that are considered in tuning an aircraft engine intake are altitude plus the length and the diameter of the intake tube. Because car and motorcycle intakes do not have to be ef cient at widely varying altitudes, the traditional way of designing a tuned system is to use a dynamometer—a workbench instrument that measures an engine's torque. Klaus explained, "When done on a dyno, you have to make dif erent intake manifolds, which is a huge pain to do. But [air] density changes the resonance. So you can use the airplane as a dyno and fl y to dif erent densities and watch the resonances change." This is the best method to design a tuned intake because doing it on a dyno would take years building dif erent tubes for dif erent altitudes; and altitude is almost impos- sible to simulate on a dyno. Klaus reported that understanding, fi xing, and testing his tuned intake took two years. Once the intake was tuned, Klaus turned next to the exhaust on the Determinator. Tuned exhausts have been used on a variety of engines: automobile, motorcycle, aircraft, and even model aircraft. An untuned exhaust sometimes has the problem that the exhaust gas from one cylinder can travel out and then up the exhaust mani- fold to a second cylinder's exhaust. When that second cylinder's exhaust valves open, its exhaust gas is met with high pressure Figure 2: An example of an oscilliscope reading showing the effect of Klaus' tuned intake. Photography courtesy of Light Speed Engineering