Diesel Engine Overload: Restricted Air Intake?




I just read your tech tips about overloading diesel engines. It sounds like you are describing my dilemma.


I do not feel I run my engines hard. But every day back at port after the fishing day, my deckhand has to scrub like mad to get that stubborn black soot off the transom. I even have a fairly new Awlgrip paint job, bright white so it really gripes me everyday for the extra work to keep the boat looking beautiful. My engine is apart now and the mechanic found a valve seat that was not working properly, that is why we could hear the ping noise out the exhaust. When we get things back together after the repair we plan to do new sea trials under the more realistic conditions that you also elude to in your article. I am also expecting not to reach full RPM at WOT. The props will then be repitched accordingly.

Here is another detail that my mechanic and I are discussing, and, something I don't think you mentioned in your article. The supply of air into the engine compartment:

The manufacturer specs say these engines require 950 cubic ft/min of air per engine. Also that the square inches of opening for air to the engine space to be 250. In my boat I have about 250 sq in total. I guess this would mean that I am actually supplying my space with just half the recommended air intake ????? What do you think about this?


So lately I have been trying to learn about blowers. I have found a company on the web that makes blowers for this application but there is still a lot of information I need to get from them. In addition to some small hull vents I have 2 main vents that go from engine room up through the cabin roof. These vents are 8 inch diameter galvanized pipe that are slid into a wooden, fiberglassed box basically. The reason for the metal piping I'm sure is to satisfy Coast Guard requirements.

The blowers I'm looking at are ELL 215 or ELL 250. The 250 does 1059 cubic ft/min there website is A blower (or blowers) would save me from cutting holes in my boat which I really don't want to do.

Do you think I am on the right track with the air issue or just stick with the prop tooling to get my problem under control?




Yup, restricted intake, whether it be engine room air shortage or dirty air filters, absolutely can cause smoke. Fortunately, the engine room problem is an easy one to test for. Do a sea trial with the engine room doors wide open (on a nice day) and see if you get full RPMs. If you do, you need to chase the problem no further and even if it isn't THE issue currently causing you problems, you do need a good clear air supply and free flowing exhaust for your engines. Use the test that I describe above to figure out if you need some prop work or just the intake modifications will cover it.

Since you are local, Krueger and Sons near Fisherman's Wharf does good prop work at a good price.

Either way, your mechanic has pointed out a problem that deserves deeper investigation. My personal tendency would be to open up more engine room venting, but you should ask on Tony Athens has done hundreds of re-engines, some of which had iffy intake and exhaust specs. He can give you a view on what works and what doesn't. Generally, I'm a big fan of just giving the engine manufacturer the environmentals that they ask for. But I know this isn't always possible/affordable.

-- James


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