Importance of Good Engine Room Ventilation
Why A Diesel Engine Needs So Much Fresh Air
Diesel engines require a lot of clean, cool air – typically 170 cubic metres (6000 feet3) per hour for a 40 hp engine. Any shortage will make a diesel engine work harder, limit rpm, waste fuel, increase carbon build-up and cause extra engine wear.
Active and Passive Ventilation
Cool air is denser, and therefore has more oxygen, so it’s essential to supply as much clean, cool, dry air as possible into the engine room. Air at 7°C (44°F) has 14% more oxygen than at 50°C (122°F). Lower temperatures also aid engine cooling (through radiation) and the alternator(s) to run cooler and more efficiently.
A good ventilation system should maintain the engine room temperature within 16°C (30°F) of the ambient outside temperature (e. g. a maximum engine room temperature of 41°C (105°F) in 25°C (77°F) weather.
Passive ventilation relies on air finding its own way into the engine compartment, typically via a cowl vent or a scallop-shaped vent. Air flow can be restricted by insect screens, convoluted piping, fenders etc. piled high in a lazarette. Often, no allowance is made for hot air to vent from the hot engine room (or off-gassing batteries).
Active ventilation uses blowers to increase inflow and, in a well-designed system, outflow. Only fans rated for continuous duty should be installed for engine room ventilation; these are typically “squirrel cage” fans (axial fans).
Inline bilge blowers, though often used for engine room ventilation, are designed to run for short periods to vent fumes and vapours (e. g. gasoline, propane, hydrogen). They are not intended for continuous duty and typically have a service life of only 300 – 350 hours.
Drawings and text excerpted from page 80, Chapter 6 Maintenance – Breathing