Continuous flow fuel injections systems

Continuous Flow Fuel Injection Systems By Randy Knuteson November 1998 ontinental Motors calls it their "Continuous Flow Fuel Injection System." Some technicians refer to it in less dignified terms, replete with colorful expletives. The...


Setting Pump Pressures
Don't be alarmed if it takes several attempts to set pump pressures. Both low- and high-end settings tend to chase each other. Fine-tuning these pressures requires a little patience. Best performance is achieved by setting the pump idle pressure (relief valve) to the higher side of the accepted parameters. During part throttle operations, with the fuel/air mixture properly adjusted, this setting generally provides for a slight fuel enrichment. However, setting idle pressures too high can also disturb the pump's equilibrium. While the engine may start fine, it may idle rough, or quit on roll-out due to an overly rich condition. Correctly setting unmetered pressures at idle is especially critical on turbocharged systems. Low unmetered pump pressures result in a leanness at mid-range cruise and full-rich climb configurations. Some try to compensate for this leanness by backing the aneroid out further in an attempt to achieve the desired top-end flow. Improper adjustment of the pump may manifest itself in high CHT and oil temps (confirmed by a high calibrated TIT temp). This can be remedied by adjusting the unmetered fuel flow at idle to the top specification.

When setting up the top end of naturally aspirated systems, be sure to fly the airplane, rather than rely on ground runs. Most naturally aspirated engines will not achieve redline rpm in static runs at any field elevation, but will do so on takeoff roll as air gets moving through the prop — it's just a fixed-pitched prop until the engine turns up to on-speed governor rpm. Make certain the governor is adjusted to redline rpm also. Use a tach checker, as most older mechanical tachs seem to deviate as much as 100 rpm. Remember, this system is rpm dependent and will require actual redline rpm and full rich mixture (regardless of altitude) to check redline/takeoff fuel flow. Turbo systems can be tuned by using static runs but pay close attention to those temps.

Turbo-charged engines that incorporate a pressure regulator require this unit to be deactivated when setting pump pressures. Remove the center hose to the regulator and cap the fitting. Also plug the detached hose. TCM recommends setting these systems so full power metered fuel pressure and fuel flow are "5 percent higher than the maximum specified limit." Upon successfully setting pump pressures, plumb the pressure regulator back into the system. Pressure regulators, (sometimes referred to as pressure controllers), serve to "regulate" or limit full power fuel pressures without compromising maximum fuel pressures at lower rpms.

Adjusting the Throttle/ Control Assembly
The pilot's sole control over the fuel system lies in the throttle and control (metering unit) assemblies. The metering-unit houses both the mixture and the main metering valves. Adjustments to these units are rather straightforward. Idle speed is increased by a clockwise rotation of the adjustment screw and conversely, a CCW rotation decreases idle speed. Mixture set up is accomplished by adjusting the length of a linkage rod joining the throttle plate to the metering valve. Once set, this arrangement then schedules fuel in relation to airflow. A 3/8 in. elastic stop nut serves to make this adjustment. A C/W rotation of this nut enriches the mixture and turning it, CCW leans the mixture. Mixture adjustments made to the GTSIO system on the Cessna 421 are the exact opposite and are an exception to this rule. Throttle bodies that include an appendage like Seneca's 360 system requires the usage of a straight-bladed screwdriver or allen wrench to adjust mixtures. On these control assemblies, a C/W rotation leans the mixture. When leaning to ICO, an rpm increase of 25 to 50 rpm (75rpm+ at 5,000 ft. field elevations) should be observed on the tach. A reading in excess of 50 rpm indicates an overly rich setting whereas, no increase in rpm reveals a lean condition. Between mixture adjustments advance the throttle to approximately 1,500 rpm for a short period of time to clear the engine in order to prevent it from loading up and giving false rich indications.

Manifold Valve Assembly
The next component in line is the manifold valve. It sits astride the backbone of the engine and, as mentioned earlier, serves several practical purposes. Although there are no field adjustments that can be made to this simple device, a few suggestions may help when faced with the grim task of troubleshooting.

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