X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Received: from [65.173.216.69] (account rob HELO [192.168.95.252]) by logan.com (CommuniGate Pro SMTP 5.1.5) with ESMTPSA id 1804184; Thu, 01 Feb 2007 10:27:00 -0500 Message-ID: <45C2065A.6040504@Logan.com> Date: Thu, 01 Feb 2007 10:25:14 -0500 From: Rob Logan User-Agent: Thunderbird 1.5.0.5 (X11/20060925) MIME-Version: 1.0 To: lml@lancair.net Subject: Re: IO-550 fuel injection system References: In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit posted for Gary Casey -Rob I've been fairly intimate with the Lycoming (Bendix/Precision Airmotive) fuel system, but I'm not an expert regarding the Continental system. However, maybe a description of the basic control methodology would clear the air - or at least remove a vapor lock or two.. The Continental system is what some describe as a "speed-theta" system as opposed to a "speed-density" (Honda and Chrysler) or "air-flow" system (used in most cars and by Lycoming). The concept relies on a positive-displacement pump turned by the crank to pump (actually "meter") a constant volume of fuel per revolution. Then a valve connected to the throttle restricts (or allows a bypass in the case of the Hilborn racing system) the flow in proportion to throttle angle (the "theta" of "speed-theta"). A very simple system in concept, but not so straightforward in execution. Remember that the fuel, under atmospheric pressure in the fuel tank, must snake its way through the cabin into the fuel selector valve and from there through the electric pump, pushing open the check valve that allows it to bypass the pump. Then to the gascolator and through its screen and finally it gets to the engine driven pump. The pump is a gear-type pump sized to deliver somewhat more fuel than the engine could ever use - which means it is fairly small as these pumps go - much smaller than an oil pump. A gear pump works by "sucking" fuel into the gaps between the gear teeth as the separate, carrying it around the pump and then squishing out from between the gear teeth as they mesh on the other side. Not too elegant. Since there must be clearance around the teeth there is always leakage present with fuel flowing backwards around the teeth (remember, the pump discharge pressure is higher than the inlet pressure). Above the pump is an orifice that bypasses some fuel directly to the tank through the fuel selector valve. I believe this is a fixed orifice, but I'm not absolutely sure. From the pump the fuel goes through the fuel flow transducer and then to the metering valve. This is a carefully-designed valve that restricts the flow more at low throttle openings and less at high openings, with the shape of the transition carefully matched to the flow capacity of the throttle at each angle. From there it goes to the "distributor" which has a relief valve that requires a certain fuel pressure before fuel is allowed to go to the injectors. This valve actually has a separate flow area (in the form of a slot) that goes to each injector. These slots gradually open as the flow is increased until at some flow substantially more than idle they are wide open, connecting all injectors to a common source. Finally the fuel makes it to the injector, which is another fixed orifice that has the downstream side vented to atmosphere (or compressor discharge pressure in the case of a turbocharged engine). So what does all this mean? First, the pump delivers a fixed VOLUME of fuel, so if there are any vapor bubbles in the inlet the MASS flow of fuel is instantly reduced. Deliver too much vapor to the pump and the leakage around the gear teeth will increase (more volume of vapor will flow through the leakage than liquid) to the point that the pump won't deliver any fuel at all - a "vapor lock." The pressure at the pump inlet is below atmosphere because of the restrictions of all the stuff in the line from the tank. How to turn fuel into vapor? Lower the pressure and increase the temperature. The pressure can be low because of a nose-high attitude during climb, restrictions in the filter, or a low fuel level in the tank. The temperature goes up as the fuel is heated in the engine compartment - the fewer components exposed to that heat the better. How to fix it? Turn on the boost pump as that will pressurize (supercharge) the engine pump inlet, preventing vapor formation. But the engine fuel "pump" is actually a metering pump, delivering a presumably precise volume of fuel and the resulting flow into the engine is a result of fuel flow through a combination of restrictions and bypass orifices - in essence a delicate balance of forces. When the metering pump is supercharged, the former pressure rise across the pump becomes a pressure drop - the pump is actually "holding back" against the pressure of the electric pump. The flow due to internal leakage now reversed in direction and the pump delivers more fuel than before. How much more? Depends on the magnitude of the leakage and all the other operating conditions, like altitude, engine speed, boost pressure and throttle position). If the engine is already almost too rich it might flame out. If it is lean it will just get richer. If the flow is predictable, then why is there so much effort required to "set up" the system for each engine? I'm not sure, but the internal leakage of the metering pump surely varies considerably from unit to unit. The large throttle valve changes air flow dramatically with very little opening when at idle. Also, since the fuel delivery is volume-based there can be significant flow changes based on fuel temperature. Fuel density easily changes by 10% from cold to hot. I hope this description helps - I hope it wasn't so simplistic to be patronizing or too obscure to be useful and if there is anything incorrect I would appreciate hearing about it so I can be corrected. The principle of operation of this fuel system is the main reason I installed a Lycoming engine in my ES. Gary Casey ES157, N224SG