Validated Thermal CFD in an Outboard Marine Engine Enclosure

Modern, high-performance, outboard marine engines operate in severe environments. They are typically mounted to a planing boat operating at high horsepower levels due to high hydrodynamic drag. The engine also experiences high vertical impact loads in rough-water conditions. In the ocean, corrosive salt water circulates through the engine to provide necessary engine cooling. Splashing water can be ingested into the combustion air inlets on the outside of the engine cowl (engine enclosure) and must be appropriately managed. In addition, the engine often operates in very warm climates with a sealed cowl wrapped tightly around it. The warm atmospheric air that flows through the cowl inlets and into the engine compartment must first circulate around the power head in order to cool thermally sensitive components such as engine controllers and ignition coils. In some applications, the same air stream mixes with fuel then participates in the combustion process inside the cylinder. At Mercury Marine, computational fluid dynamics, CFD, is used to aid the design of outboard engines that will operate robustly in these extreme conditions. One specific application for CFD is the management of the flow and thermal aspects of engine-compartment air flow. Studies can be done with CFD to assist product design decisions that aim to balance the need to protect thermally sensitive electronics and to efficiently provide the engine with the combustion air. The CFD simulation predicts the air flow behavior from the cowl duct inlets, around the power-head, and into the throttle body inlet of the engine. The simulation also predicts air temperatures, component temperatures, and heat flow to and from the air. The CFD model typically includes rotating components such as alternators and flywheels. A recent study was conducted to validate the CFD method. The CFD model and the dynamometer experiments were conducted with a mid-size outboard 4-stroke engine. The test engine was fully instrumented to measure air temperatures, air velocities, and component temperatures. The validation exercise included a detailed comparison of these values between the CFD predictions and the experimental results. A high level of agreement was achieved and a few lessons were captured for future implementation.