Gimme Some Air!

 intake openings

Engines, like people, need to breathe.

The human body is an amazing machine, running continuously for decades, requiring nothing more than water, air and fuel —the latter in the form of food. Gasoline and diesel engines require the same three ingredients to function. In both cases, air is a critical element in the mix, because a reduction in the amount of air below the ideal level can cripple a motorperformance.

Providing the necessary amount of air to marine engines, installed within as small a space as is practicable, isneasy. The engine room needs as much air as possible, meaning big openings, but we want to keep out as much salt water as we can, meaning small openings. These requirements demand a careful balance of the two extremes.

Engines need air for combustion (combustion air), and engine rooms need it for ventilation (ventilation air). Both are enhanced by an air supply that is fresh, clean, cool, continuous and abundant. To perform at their peak, power plants require enough air to achieve the ideal fuel-to-air ratio. Anything less means wasted fuel, increased maintenance and reduced power. While most of the enginecooling comes from the water that circulates through the block and heat exchanger, a significant amount is also rejection to atmosphere," which means simply that the air in the engine room is heated by the engine. Insufficient ventilation makes the engine room and engine hotter, decreasing performance.

 intake openings

Most yachts bring air aboard through openings in the hull vents or superstructure. The air finds its way to the engine room through baffles, louvers and filters designed to limit saltwater intrusion, and from die engine room into the motors themselves through filters, separators or both. This system, as opposed to pulling combustion air directly into the engines through dedicated ductwork, greatly decreases die likelihood of salt water finding its way into the power plants. It also uses the combustion air for ventilation, meaning less total air flow is required.

One downside of this system is that the combustion air is warmer than it would be if it were directly ducted from the outside, and this decreases each enginehorsepower output. Thatwhy yousee external scoops and direct ducting to the engines on many raceboats and some high-performance yachts. The price paid for those few extra horses, however, is the risk of a slug of salt water entering the engine, something thatnot worth it to most yacht owners.

The best solution, then, is to take enough air into the engine room for both combustion and ventilation needs, through the smallest intake openings possible. Then, exhaust the heated ventilation air through other openings, also as small as possible and located away from the intake openings to avoid recirculation. This system also involves fans or blowers on both the intake and exhaust sides, in order to get more air through smaller openings and to change the air passing through the engine room more often, which enables a lower ambient temperature here and, in turn, provides for better engine performance.

Ideally, the intake blowers should include ductwork that delivers outside air to the bilge at one end of the engine room. Exhaust blowers would then suck air from the top of die space at the other end, encouraging a flow path over and around the motors for maximum heat extraction. The best systems include dampers and electrical switches interfaced with the engine roomfixed fire-extinguishing equipment to shut off the flow of air in case of fire. The only time you donwant lots of air being pumped into the space is when itfeeding a conflagration.