During the early stages in the development of these systems, problems arose with momentary overboost of as much as 6 to 10 inches of Hg during throttle advancement. To compensate, pilots were instructed to slowly advance the throttle to 32-in., wait for a few seconds, and then continue to the rated 35-in. of MAP. This surge in overboost was dramatically reduced to 2 to 3 inches of Hg with the development of the GTSIO-520 engines. By modifying the bore size of the wastegate actuator, designers found they could minimize the time it took for oil displacement and wastegate actuation.
Safeguards are required to prevent the overboosting of an aircraft engine. Overboosting occurs whenever maximum rated manifold pressures are exceeded. Properly adjusted controllers and proper throttle management serve to preclude the onset of overboost. However, despite these measures, inadvertent overboost still happens. An incidence of overboost can take place when the engine oil has not yet reached operating temps and the throttle is rapidly advanced for takeoff or when a pilot on short final quickly advances the throttle for a go-around and the prop governor lags or the wastegate sticks. Overboost can also occur at full throttle and below critical altitude when exhaust gases expelled are more than capable of driving an uncontrolled compressor to engine damaging speeds.
What differentiates between Overshoot and Overboost? Answer: Duration, or the length of time manifold pressures exceed their maximum rate. "Overshoot" occurs when manifold pressure increases above maximum rate by one or two inches, and then returns immediately to the maximum rated. Lycoming states that if overshoot does not exceed two inches and three seconds duration, it may be disregarded. TCM says, "If the amount of overboost is from three to six inches, the system should be checked immediately for necessary adjustments or replacement of the malfunctioning component." As defined by Textron Lycoming, "momentary overspeed" is "an increase of no more than ten percent of rated engine RPM for a period not exceeding three seconds."
A table for computing overspeed by engine model is included in Lycoming SB#369I. Stringent inspection criteria are specified for engines subjected to inadvertent overspeed. These procedures include a thorough examination of the cylinders, valves, guides, seats, counterweights, etc.
How do you spell relief?ÉP.R.V.
The Pilots' responsibility is to ensure that manifold pressures stay within their prescribed limits. But, humans are known to be fallible. With this in mind, thoughtful engineers sought to incorporate a secondary safety feature known as an absolute Pressure Relief Valve ("pop-off valve"). This valve is strategically positioned between the compressor outlet and the fuel injector/carburetor to prevent engine-damaging surges in manifold pressure. The valve head is normally held in the closed position by a spring/bellows combination. It is set to off-seat at approximately two inches above normally rated MAP. Continental Motors compares the operation of this spring-loaded valve to a safety valve in a steam boiler. As excessive pressure builds, the valve opens and stays open until the manifold pressure again falls within acceptable limits. The aneroid-style bellows allows the valve to "crack" open at a specified pressure regardless of the operational altitude of the engine.
In the Mooney 231 (TSIO-360-GB/LB), and the Piper Turbo Arrow and Senecas (TSIO-360-E/EB/F/FB/KB), the Absolute Pressure Relief Valve functions as a turbocharger control/boost limiter. These engines incorporate a "fixed" wastegate arrangement and do not rely on other servo control systems. In this design, the PRV is set to vent any compressor discharge pressures exceeding one inch Hg above rated manifold pressure. This "crack" pressure increases as a function of altitude, thereby maintaining a constant manifold pressure as relief flow past the valve head varies. This "modulating" valve successfully compensates for variations in compressor discharge air-flow.
Continuous Flow Fuel Injection Setup
continued...page 2 of 3
Fuel system adjustments
It's important to remember that adjustment may require multiple tries to stabilize the system at the desired pressures. One adjustment typically affects other parameters in the following manner:
- Adjustments to the high end have a significant impact on the low end pressures.
- Adjustments to the low end have a significant affect to the operating range.
The four adjustments that can be made to the fuel injection system are:
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