X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [67.8.182.29] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 5.0.9) with HTTP id 1135228 for lml@lancaironline.net; Wed, 31 May 2006 18:05:21 -0400 From: "Marvin Kaye" Subject: Re: Lean of Peak question To: lml X-Mailer: CommuniGate Pro WebUser v5.0.9 Date: Wed, 31 May 2006 18:05:21 -0400 Message-ID: In-Reply-To: References: MIME-Version: 1.0 Content-Type: text/plain; charset="ISO-8859-1"; format="flowed" Content-Transfer-Encoding: 8bit Posted for Gary Casey : The LOP discussion centers mostly on fuel injected engines, but John's observation with a carbureted engine is a good one. Left out of most discussions is the "rest" of the fuel distribution question. Yes, cylinder-to-cylinder variation is usually the big one, but there is also the issues of time-dependent (cycle-to-cycle) variations and in-cylinder variation. If you built a (carbureted) plexiglas intake manifold (I did) you would see something truly disgusting - droplets of fuel everywhere, little rivers running in all directions, puddles here and there and worse. I submit that the "roughness" that is usually considered the lean limit for carbureted engines is primarily from cycle-to-cycle variations in mixture. On the lean side of peak the power produced is directly proportional to the quantity of fuel. On the rich side it is mostly independent of the fuel and is dependent mostly on the quantity of air delivered. So when the engine feels rough it is mostly because at least one cylinder is lean of peak and the time-varying fuel delivery is causing a cycle-to-cycle power variation. And some engines are better than others and I suppose they will operate with all cylinders lean of peak as reported by Walter. There is an unlimited number of tricks that can be played with a carburetor installation to improve cylinder-to-cylinder and cycle-to-cycle distribution, but one you can try with any updraft Lycoming is to vary the throttle opening slightly. The airflow won't be affected, but the angled throttle blade will deflect the fuel delivery slightly, changing the engine behavior. Adding heat always helps. In-cylinder distribution is another variable, but a much more difficult one to manipulate. The biggest variable is the mixture at the spark plug during the ignition event and having two ignition sources helps reduce the variation. Incidentally, the best shot at improving this is to make sure the fuel is in vapor form during the intake stroke, increasing the chance it will be uniformly mixed with the air. Injected engines are at a disadvantage here and the primary method of evaporating the fuel is to spray it directly on a hot surface often by aiming it at the intake valve head and stem. During the dwell between intake strokes the fuel is evaporated. The worst time to inject fuel is during the intake stroke as that fuel has the least chance of evaporating. Our air bleed injectors don't do much as there is virtually no air pressure drop to provide the atomization power. However, that doesn't matter much as I have found that a solid stream is almost as good as a finely atomized one - it gets the fuel solidly deposited on the hot surface, speeding evaporation. The PRISM system continues to be interesting, but I wonder if the eventual market it will find is different than some think. Turbocharged engines have limited need as they operated at essentially a fixed altitude, so conventional fixed timing and crude mixture controls maybe aren't so bad. Think about the opposite application - one that might be the "standard" - build a high- compression large displacement naturally aspirated engine (where I came from "NA" stood for naturally, not normally aspirated) running the modest-octane lead-free fuel of the future. With most operation between 10,000 and 15,000 feet the PRISM system could make a truly remarkable difference in engine operation. And to add a little icing on the cake an engine operating under those conditions will emit very little CO and HC and even NOX emissions will be less than half those of a more conventional engine. The feds would be happy. FWIW Gary Casey