Power assembly

The term power assembly refers to an Electro-Motive Diesel (EMD) engine sub-assembly designed to be "easily" removed and replaced in order to restore engine performance lost to wear or engine failure. Typical of heavy-duty internal combustion engines used in industrial applications, EMD engines are designed to allow the cylinder liners, pistons, piston rings and connecting rods to be replaced at overhaul without removing the entire engine assembly from its application location. This increases engine value, reduces downtime and allows the engine to be returned to true new engine performance. Other terms such as cylinder pack, liner pack, cylinder assembly and cylinder kit are used in the engine industry to describe similar assemblies. In the large-engine industry, the term "power assembly" has also become generic and is often used to refer to the assemblies used in non-EMD engines where "power pack" may be the preferred term, although both terms are functionally the same.

Because of the size and weight of the engine assembly and the difficulties of removing and transporting them for repair, they are typically serviced on-site in stationary applications and in the ship or locomotive in transportation applications. Designing the engine for "easy" service is done out of necessity rather than the desire to increase engine serviceability. Power assemblies are large and heavy and overhead lifting equipment sufficient to lift the fixture and assembly are required.

An EMD power assembly consists of the following components:

In an EMD diesel engine, since two power assemblies share a common connecting-rod journal, and since the power assemblies are directly opposite each other rather than staggered as in a typical V-type engine, two different power assemblies are required in a single engine. The difference between the two assemblies is in the connecting rods. One connecting rod "big end" has to fit inside that of its companion rod and the two types are referred to as "blade rods" and "fork rods". The "fork rod" is logically the "master" as only it has a "rod cap", in this specific case referred to as a "basket", whereas the "blade rod" is logically the "slave" as its "big toe" is designed to fit completely within, and is guided by, and is retained by the "fork", and both are retained by the single "basket".

Several situations can require power assembly replacement. Most are due to failure within the power assembly itself such as a dropped valve, broken piston or internal coolant leak. Less common are replacements to repair catastrophic failures such as broken connecting rods or a "hydro-locked" power assembly that has been broken or knocked out of the cylinder block when the cylinder filled with coolant during engine operation and the inability of the piston to compress the liquid caused catastrophic failure. Complete power assembly replacements, where all of the assemblies in an engine are replaced, are least common and are normally done as part of a comprehensive engine overhaul.

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