NAMBA April 2020 Web.pub

Modern pipes have two or more diffuser cone sections and a larger diameter center section to expand the exhaust. Fig. 3 shows some standard, commercial pipe designs with a parallel header section and two-stage diffusers in the larger sizes. The Nitro Engine: Part 4 – Exhaust By Lohring Miller NAMBA Safety Chairman Flow through the engine gets a lot of attention. With one exception, I believe this is of less importance than the factors in the previous articles. That exception is the exhaust system. The development of tuned pipes has more than doubled the power of our engines. With open face exhaust a 1960, 10 cc (60) engine could develop around 1 hp at 10,000 rpm. Todays tuned pipe 11 cc engines develop over 4 hp at more than double that rpm. How does this happen? The tuned pipe acts like a no moving parts turbocharger, converting wasted exhaust energy into a pumping action. This does three things while the exhaust port is open. First, the opening exhaust port sends a high pressure wave down the pipe. Second, the low pressure (“suction”) pulse this creates draws mixture up the transfers. Third, the returning high pressure pulse supercharges the cylinder just before the exhaust port closes. See Fig. 1 for what a Quickdraw Hot Pipe does in a simulation of a custom 32 cc Quickdraw Pioneer engine. The exhaust pulse starts at close to 3 times atmospheric pressure just after the exhaust port opening (EPO). The low-pressure pulse reduces the pressure to between 0.2 and 0.4 times atmospheric pressure around bottom dead center (BDC). Finally, the return pulse supercharges the cylinder to between 2 and 2.4 times atmospheric pressure after the transfers close (TPC) and just before the exhaust port closes (EPC). PROPWASH 4 April 2020 Fig. 1 - Exhaust Port Pressure vs RPM The late Bill Wisniewski is responsible for developing tuned pipes for model engines. His design, Fig 2, is described in the March 1967 issue of Model Airplane News (http:// modelenginenews.org/people/wiz/wiz_pipe_man.html) , and is still used today. It consists of a parallel header and two cones, a diverging cone to expand the exhaust gas into a low pressure wave followed by a converging cone to compress the gas into a reflected high pressure wave. Note that comparatively little flow is involved. All these waves are pressure waves traveling at the speed of sound. The exhaust flow is regulated by the stinger and is much slower. Fig. 2 - Bill Wisienewski’s Pipe Design Fig. 3 - MACS Marine Pipes Some pipes have a parallel center section. This is primarily to place the converging cone at the right distance from the exhaust port so the pressure wave returns at the right time. The lower the rpm, the longer this section needs to be. Fig. 4 shows a pipe for a lower rpm engine with a broad center band and a single stage diffuser.