Chapter 10
Engine Maintenance & Operation
- Ignition system.
- Fuel-metering system.
- Induction system.
- Power section (valves, cylinders, etc.).
- Instrumentation.
If a logical approach to the problem is taken and the instrument readings properly utilized, the malfunctioning system can be pinpointed, and the specific problem in the defective system can be singled out.
The more information available about any particular problem, the better the opportunity for a rapid repair. Information that is of value in locating a malfunction includes:
- Was any roughness noted? Under what conditions of operation?
- What is the time on the engine and spark plugs? How long since last inspection?
- Was the ignition system operational check and power check normal?
- When did the trouble first appear?
- Was backfiring or afterfiring present?
- Was the full throttle performance normal?
From a different point of view, the powerplant is, in reality, a number of small engines turning a common crankshaft and being operated by two common phases: fuel metering and ignition. When backfiring, low power output or other powerplant difficulty is encountered, first find out which system, fuel metering or ignition, is involved and then determine whether the entire engine or only one cylinder is at fault. For example, backfiring normally is caused by:
- Valves holding open or sticking open in one or more of the cylinders.
- Lean mixture.
- Intake pipe leakage.
- An error in valve adjustment that causes individual cylinders to receive too small a charge or one too large, even though the mixture to the cylinders has the same air-fuel ratio.
Ignition system reasons for backfiring might be a cracked distributor block or a high-tension leak between two ignition leads. Either of these conditions could cause the charge in the cylinder to be ignited during the intake stroke. Ignition system troubles involving backfiring normally are not centered in the basic magneto, since a failure of the basic magneto would result in the engine not running, or it would run well at low speeds but cut out at high speeds. On the other hand, replacement of the magneto would correct a difficulty caused by a cracked distributor where the distributor is a part of the magneto.
If the fuel system, ignition system, and induction system are functioning properly, the engine should produce the correct bhp unless some fault exists in the basic power section.
Valve Blow-By
Valve blow-by is indicated by a hissing or whistle when pulling the propeller through prior to starting the engine, when turning the engine with the starter, or when running and blow-by past the intake valve is audible through the carburetor.
Correct valve blow-by immediately to prevent valve failure and possible engine failure by taking the following steps:
- Perform a cylinder compression test to locate the faulty cylinder.
- Check the valve clearance on the affected cylinder. If the valve clearance is incorrect, the valve may be sticking in the valve guide. To release the sticking valve, place a fiber drift on the rocker arm immediately over the valve stem and strike the drift several times with a mallet. Sufficient hand pressure should be exerted on the fiber drift to remove any space between the rocker arm and the valve stem prior to hitting the drift.
- If the valve is not sticking and the valve clearance is incorrect, adjust it as necessary.
- Determine whether blow-by has been eliminated by again pulling the engine through by hand or turning it with the starter. If blow-by is still present, it may be necessary to replace the cylinder.
Cylinder Compression Tests
The cylinder compression test determines if the valves, piston rings, and pistons are adequately sealing the combustion chamber. If pressure leakage is excessive, the cylinder cannot develop its full power. The purpose of testing cylinder compression is to determine whether cylinder replacement is necessary. The detection and replacement of defective cylinders prevents a complete engine change because of cylinder failure. It is essential that cylinder compression tests be made periodically. Low compression, for the most part, can be traced to leaky valves.
Conditions that affect engine compression are:
- Incorrect valve clearances.
- Worn, scuffed, or damaged piston.
- Excessive wear of piston rings and cylinder walls.
- Burned or warped valves.
- Carbon particles between the face and the seat of the valve or valves.
- Early or late valve timing.
Perform a compression test as soon as possible after the engine is shut down so that piston rings, cylinder walls, and other parts are still freshly lubricated. However, it is not necessary to operate the engine prior to accomplishing compression tests during engine buildup or on individually replaced cylinders. In such cases, before making the test, spray a small quantity of lubricating oil into the cylinder(s), and turn the engine over several times to seal the piston and rings in the cylinder barrel.
Be sure that the ignition switch is in the OFF position so that there is no accidental firing of the engine. Remove necessary cowling and the most accessible spark plug from each cylinder. When removing the spark plugs, identify them to coincide with the cylinder. Close examination of the plugs aid in diagnosing problems within the cylinder. Review the maintenance records of the engine being tested. Records of previous compression checks help in determining progressive wear conditions and in establishing the necessary maintenance actions.
Differential Pressure Tester
The differential pressure tester checks the compression of aircraft engines by measuring the leakage through the cylinders. The design of this compression tester is such that minute valve leakages can be detected, making possible the replacement of cylinders where valve burning is starting. The operation of the compression tester is based on the principle that, for any given airflow through a fixed orifice, a constant pressure drop across the orifice results.