X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [72.255.27.234] (account marv@lancaironline.net) by logan.com (CommuniGate Pro WebUser 5.0.6) with HTTP id 916118 for lml@lancaironline.net; Fri, 06 Jan 2006 20:53:18 -0500 From: "Marvin Kaye" Subject: Re: Idle power descent? To: lml X-Mailer: CommuniGate Pro WebUser v5.0.6 Date: Fri, 06 Jan 2006 20:53:18 -0500 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 : > From Regarding the subject of low power descents adversely affecting the piston/cylinder clearance I have always found a flaw in that argument. The idea proposed is that removing the heat source (going to low power) will cause the cylinder to contract around the piston. However, the flow path for the heat is from combustion INTO the piston and then out through the skirt to the cylinder and finally to the surrounding air. Seems to me if I reduce the heat input the first thing to cool will be the piston and then the cylinder, not the other way around. How could the piston possibly stay hot and the cylinder decide to cool off? The only problem I've seen from prolonged low-power operation is from too much oil getting on the plugs, causing a fouling problem. This is caused by a low pressure in the cylinder, encouraging oil to flow above the rings combined with a lack of combustion in the cylinder that will burn the oil. This effect is a problem during ground idle as well, so if you don't like idle descents then you must really not like idling on the ground. In theory, the best way to do a low power descent is to leave the throttle open (naturally aspirated engine) and pull the mixture. The manifold pressure will remain high, reducing the volume of oil pulled into the cylinder and the engine will be perfectly happy just pumping air. I don't have the nerve to do this as I don't like shutting the engine off in flight (what happens if the mixture cable falls off, I guess), but it would make the engine happy. Incidentally, all modern car engine controls shut off the fuel during a deccel and there has been no known issues. There was also a question about "backfiring", more technically correctly called "after-firing": It is commonly caused by an intermittent very slow combustion caused by excessively rich or lean mixtures combined with a low cylinder pressure. One cylinder might misfire, depositing a load of unburned fuel in the exhaust system. The metal is probably not hot enough to light the fire, but another cylinder might have a "slow burn", creating an open flame in the exhaust, lighting the unburned mixture. It's no big deal, but some people don't like the noise (but we, as kids, liked it a lot). Gary Casey