Reciprocating Engine Cylinder Cooling Fin and Baffle and Deflector System Inspection | Aircraft Systems

Reciprocating Engine Cylinder Cooling Fin and Baffle and Deflector System Inspection

Engine Cylinder Cooling Fin Inspection

The cooling fins are of the utmost importance to the cooling system, since they provide a means of transferring the cylinder heat to the air. Their condition can mean the difference between adequate or inadequate cylinder cooling. The fins are inspected at each regular inspection. Fin area is the total area (both sides of the fin) exposed to the air. During the inspection, the fins should be examined for cracks and breaks. [Figure 1] Small cracks are not a reason for cylinder removal. These cracks can be filled or even sometimes stop-drilled to prevent any further cracking. Rough or sharp corners on fins can be smoothed out by filing, and this action eliminates a possible source of new cracks. However, before reprofiling cylinder cooling fins, consult the manufacturer’s service or overhaul manual for the allowable limits.

Engine Cylinder Cooling Fin Inspection
Figure 1. Regulating the cooling airflow

The definition of fin area becomes important in the examination of fins for broken areas. It is a determining factor for cylinder acceptance or removal. For example, on a certain engine, if more than 12 inches in length of any one fin, as measured at its base, is completely broken off, or if the total fins broken on any one cylinder head exceed 83 square inches of area, the cylinder is removed and replaced. The reason for removal in this case is that an area of that size would cause a hot spot on the cylinder; since very little heat transfer could occur.

Where adjacent fins are broken in the same area, the total length of breakage permissible is six inches on any two adjacent fins, four inches on any three adjacent fins, two inches on any four adjacent fins, and one inch on any five adjacent fins. If the breakage length in adjacent fins exceeds this prescribed amount, the cylinder should be removed and replaced. These breakage specifications are applicable only to the engine used in this discussion as a typical example. In each specific case, applicable manufacturer’s instructions should be consulted.

Cylinder Baffle and Deflector System Inspection

Reciprocating engines use some type of intercylinder and cylinder head baffles to force the cooling air into close contact with all parts of the cylinders. Figure 2 shows a baffle and deflector system around a cylinder. The air baffle blocks the flow of air and forces it to circulate between the cylinder and the deflectors. Figure 3 illustrates a baffle and deflector arrangement designed to cool the cylinder head. The air baffle prevents the air from passing away from the cylinder head and forces it to go between the head and deflector. Although the resistance offered by baffles to the passage of the cooling air demands that an appreciable pressure differential be maintained across the engine to obtain the necessary airflow, the volume of cooling air required is greatly reduced by employing properly designed and located cylinder deflectors.

Cylinder Baffle and Deflector System Inspection
Figure 2. Cylinder baffle and deflector system

Cylinder Baffle and Deflector System Inspection
Figure 3. Cylinder head baffle and deflector system

As shown in Figure 4, the airflow approaches the nacelle and piles up at the top of the engine, creating a high pressure in the top of the cylinders. This piling up of the air reduces the air velocity. The outlet at the bottom rear of the cowling produces a low-pressure area. As the air nears the cowl exit, it is speeded up again and merges smoothly with the airstream.

Cylinder Baffle and Deflector System Inspection
Figure 4. Differential air cooling

The pressure differential between the top and the bottom of the engine forces the air past the cylinders through the passages formed by the deflectors. The baffles and deflectors normally are inspected during the regular engine inspection, but they should be checked whenever the cowling is removed for any purpose. Checks should be made for cracks, dents, or loose hold down studs. Cracks or dents, if severe enough, would necessitate repair or removal and replacement of these units. However, a crack that has just started can be stop-drilled, and dents can be straightened, permitting further service from these baffles and deflectors.

Reciprocating and Turbine Engine Cooling Systems
Reciprocating Engine Cooling System Maintenance
Maintenance of Engine Cowling
Cylinder & Exhaust Gas Temperature Indicating Systems