Continuous Flow Fuel Injection Systems
By Randy Knuteson
ontinental Motors calls it their "Continuous Flow Fuel Injection System." Some technicians refer to it in less dignified terms, replete with colorful expletives. The proclivity of human nature has always been to focus on the negative. But regardless of an occasional negative encounter, the positive attributes of these systems are easy to identify. You need not look far to appreciate the commendable features of the TCM Fuel Injection System. For many years, the factory has pointed to "simplicity" as being the "single-most significant feature" of their systems. Some would concur. Others would strongly disagree. The goal of this article is to provide some information to clear up any misunderstandings that may have landed you in the "disagree" camp.
Continuous Flow Fuel Injection Systems
By Randy Knuteson
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Setting up the System
Is "Fine-Tuning" Required?
Some shops make the erroneous assumption that a freshly overhauled fuel system need only to be bolted to the engine and flown away. They say "after all, wasn't the system overhauled and calibrated to factory specs?" Yes — however, variations in induction systems, supply systems, and operating environments mandate a final "tweaking" of the installed components. Still, some tenaciously hold to the mind set that "it should be set up for my airplane out of the box."
For this reason, many Repair Stations have difficulty persuading technicians to use calibrated hand-held gauges while setting up their Continental systems on the airframe. While TCM strongly recommends the usage of externally plumbed gauges, some technicians still consider this procedure nothing more than a mere suggestion. Failure to follow this "suggestion" often results in lackluster performance, frustration, and dissatisfied customers. Always use these gauges if you expect to appreciate the system's full-performance capabilities. Teledyne's Service Information Directive 97-3 provides clear instructions for final adjustments once fuel components are installed on the airplane.
Verify the accuracy of the following gauges before making adjustments to the fuel system: Tach, MAP, Fuel Flow, Oil Temp, Oil Press, CHT, EGT. Don't rely solely on the ship's gauges when setting pressures and flows. Rectify any discrepancy if fou nd. Errant gauge readings, if significant, can result in annealed rings, compression loss, and cylinder detonation. Tee in the calibrated gauges. Pressurize the system with the boost pump and inspect for leaks. Chock the wheels, set the brakes. Finally, before attempting any adjustments, allow CHTs and oil temps to reach their normal operating values as spelled out in the POH.
Be sure to limit the duration of full rated power run-ups, and cowl the engine to direct prop-blast across the cooling fins of the cylinders. Carefully monitor CHT readings during all ground runs. Allow the engine to stabilize for 10 to 15 seconds, and take your readings, but always hold high rpm runs to a minimum especially with newly installed cylinders. Never exceed 420ûF CHT or 210ûF oil temps. Failure to take these precautions could dramatically shorten the life of your cylinders. After full powered runs, it is also imperative that the engine be allowed to run at 800 to 1000 rpm for a few minutes. This practice allows the engine temperatures to stabilize prior to engine shut down.
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.
If an engine continues to lope on at ICO, it can usually be attributed to either scored mixture valves in the metering unit or a manifold plunger that refuses to seat. Typically, if the engine runs on indefinitely, the problem can be attributed to scored mixture valves. However, if it continues to run for a short period, then the problem lies somewhere between the manifold valve and the cylinders. To further isolate the cause, remove the hose feeding the manifold valve at the metering unit outlet fitting. With the mixture valve against the ICO stop, engage the boost pump and look for any indication of fuel flow from this fitting. Be certain to capture any leaking fuel in a suitable container and use caution if the engine or exhaust is heat-soaked. Leakage in excess of 20 drops per minute at 10 psi of pressure is unacceptable. The metering unit should be sent in for repair. If you're still uncertain as to the cause, interrupt the hose feeding the manifold inlet and plumb a ball-valve between it and the metering unit. Then, with the engine running at idle speed, use the ball valve to manually cut off fuel flow to the divider. If the engine continues to run, the problem most likely rests at the manifold valve. Other contributing causes of poor idle cut off may stem from a leaking primer system — plugged or partially plugged injection nozzles, allowing the engine to run on residual fuel pooled in the induction, or in the case of a turbo-charged engine, a leaking aneroid seal that dumps fuel back into the upper-deck.
A fuel system that has been fine-tuned for a specific engine/airframe may require a periodic re-trimming. As the engine breaks in, and operating parameters change, and cockpit gauges lose their accuracy, it becomes necessary to perform a "check-up."
Continental encourages an "operational verification" any time an engine is installed, a 100 hr. or annual is performed, or whenever a fuel component is replaced or adjusted. This may seem "excessive," but it assures of a smooth running engine to TBO.