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Old 11-10-2007
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Volvo Penta Running Rich

I have a VP MD 17 C which is running a bit rich from time to time. The engine takes air through the cockpit locker, which I have had fairly full in light of the cruising-with-kids "requirements". By chance I noticed today that when I opened up the engine compartment the engine seemed to run less rich. My thought is that I may have had to much stuff in the cockpit locker and thereby constricted airflow. Does this seem like a reasonable conclusion?

If not, any other thoughts?

Thanks,

Doffe
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Sounds like the logical answer

Having just taken a "Diesel in a Day" class, and gained just enough knowledge to be dangerous, that would make sense to me. Is the locker airtight? Or do you think the air intake was blocked by the gear?

Mary
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Old 11-10-2007
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Diesel motors consume a lot of air compared to a gas engines. Mary51 asked the appropriate questions. If the locker isn't vented, I'd experiment with operating the engine with the lid cracked open and if it seems to confirm that it helps, I'd add a vent for fresh air intake.
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The locker is not airtight. The engine compartment has air intakes from the locker and the locker has two equal sized intakes from the outside. My thought is that I may have constricted the airflow with the gear. The airintakes are set up so that you can fill the cockpit with water without water getting into the locker and ultimately the engine compartment. Doing so creates an less than strait shoot for the air. The intakes were no completely covered and it seems it would have to be more of a flow issue.
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Old 11-10-2007
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What makes you think the engine is running "rich"? A diesel is not like a spark ignition engine where the fuel and air are mixed and then compressed. Only air is compressed in a diesel and the fuel injected into the hot compressed air. Diesels always run lean - just more or less lean depending on load. If you are getting smoke, it could mean the injectors need servicing, or other engine problems.
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Old 11-10-2007
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If by rich you mean black exhaust smoke, I would look at the load side, the propeller covered in something unwelcome or too much friction on the shaft in some way. It could also be a restricted exhaust pipe.

But if you notice the difference by letting more air into the engine, then that problem should be cleared anyway, diesels need to breath free.
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Old 11-10-2007
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Quote:
Originally Posted by erps View Post
Diesel motors consume a lot of air compared to a gas engines. Mary51 asked the appropriate questions. If the locker isn't vented, I'd experiment with operating the engine with the lid cracked open and if it seems to confirm that it helps, I'd add a vent for fresh air intake.
I do not want to hijack the thread, but why would diesel engine consume more air than gas?
As a rule diesel engines for the same power are a bit bigger, but they run on lower RPM. The amount of air is proportional with the cylinder volume and frequency that volume is filled.
From cars:
1,6 l gas engine, 100 HP at 6000 rpm
2 l diesel engine, 100 HP at 4000 rpm
Both non turbo.
In this case the gas uses more air at rated rpms. In real life we would most likely run them on half the rated rpm most of the time.
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Old 11-11-2007
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tomaz, gas (spark ignition) engines control power by throttling the air intake and then mixing fuel in to maintain approximately ideal air/fuel ratio of about 14/1. At low loads, the air flow is significantly less than the flow rate determined by engine displacement and RPM. A diesel (compression ignition) engine does NOT restrict the air flow and thus the air flow IS determined by engine displacment and RPM, even at low loads. At rated power, both are probably moving close to the same air flow per HP, but at low loads the spark igntion engine is moving considerably less.
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If the motor were having to gasp for its air, it would run rich. The diesel injection system will inject according to revs, and makes no allowance for the motor having to work harder to get it in there.
I would leave the locker lid off.

A simple experiment would be to drill a wee hole in the engine compartment (or use an existing aperture), and put a wee length of clear tube through the hole, with the lowest part of the tube being the "U" of the U-tube. Seal the gap in the hole with putty or something. Now fill the tuble, (say) half way with water, and mark the tube.
Start the engine, rev up slowly,and you will see readily if you are pulling a partial vacuum in the engine bay.
Be careful you do not have too good a seal in the engine bay, even partial vacuum will be causing some serious forces on the planar surfaces in the engine bay, and indeed the hull. There is a lot of area in there.
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Tomaz-

Diesel engines are also running at much higher compression ratios than gasoline. A gasoline engine may be running at 8:1 or so... As a rough method of comparison, you can think of it as if you have an engine with a displacement of two liters, it requires 16 liters of air to operate. A diesel engine is running at much higher compression ratios, typically 15–20:1, so a 2.0l diesel engine at 16:1 compression would require 32 liters of air, or twice as much air. At any given RPM, given two equal displacement engines, the diesel will generally require about twice as much air.

BTW, the higher compression ratios of diesel engines is part of the reason for their heavier weight, when compared to a gasoline engine of the same displacement. The engine has to be built heavier to withstand the higher forces.

Doffe—

It sounds like the cruising stuff for the kids is causing a restriction in the airflow through the locker. Personally, I think that the engine should be vented directly, not through a locker. Many boats have the engine vents under a coaming in the cockpit, so that water entry through the vents is minimized. On a C&C 38 I was on a while back, the vents were located in the helmsan's seat.

Not allowing the engine to breathe properly will result in a lot of different problems, that can result in expensive repairs. Carbon build up in the engine is only one symptom. It can also cause soot to buildup in the exhaust system...which can lead to back pressure issues...and so on... you really need to solve the situation properly now... rather than later IMHO.
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Last edited by sailingdog; 11-11-2007 at 10:54 PM.
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