Old Iron

28 08 2008

Today we’ll cover some of the realities of older engines that will help you decide the best route to take as your engine comes up for rebuild or replacement.

One of the most challenging aspects of using older diesel engines in a marine operation is managing part-load operation to avoid “wet stacking” and “fuel dilution” that can occur.

First though, let’s cover the terminology: “Wet stacking” is incompletely burned fuel, that has left the the cylinder through the exhaust valves. This fuel “paints” the inside of your exhaust manifold with a thick, black ooze. This fuel will soon will find any leaks in your exhaust system, and on the outside of the engine this fuel looks a lot more like lubricating oil, than diesel fuel. This appearance has even been the trigger for un-needed engine repair jobs. “Fuel-dilution” on the other hand is unburned fuel that goes down past the cylinder’s piston rings, diluting the lubricating oil with fuel,and reducing the viscosity of the oil.

Diesel engine ratings determine not only what an engine is capable of, but also the way it must be used. For example, an engine rated for generator use must carry a load that is no less than eighty percent of its maximum rating, eighty percent of the time, for best results.
Following this eighty percent rule, for example, a 100 kW marine generator set would not be allowed to produce less than 80 kW.
Conditions come up when there just isn’t enough load to put on an engine. One example of this would be a main propulsion engine that is rated at 400 horsepower (hp) at 2100 rpm, and the engine is periodically used to power hydraulics at only 700 rpm. A few minutes of this isn’t so bad, but  hours of producing only 20 hp, or five percent of its rating, will harm the engine.
Another example is a generator set engine that is rated at 60kW to power a vessel’s heavier electrical loads, but only produces 12 kW at night when the crew is using the electric range and doing laundry.
Plainly, some engines can tolerate part load operation better than others. There is a qualifier for this statement though; many, if not most diesel engine powered boat engines are ten to fifteen years behind the “State of the Art” in fuel efficiency. This list focuses on the older engines that are in the 20-500 horsepower range.
A list of general rules follows:
-Newer (technology) engines are better at part-load, than older ones. One good reason for this is the ongoing advances in engine design and materials. Another reason is that the ever more stringent emission standards are helping to insure that all engines do a better job of burning their fuel in the cylinder, and not in the exhaust manifold, even during light loading conditions.

-4-stroke engines handle part load better than 2-stroke engines, and one good reason for this is that there is more time for the fuel to burn before the exhaust segment of the combustion process.

-4-stroke engines with 3-ring pistons will do better than 2-ring pistons.

-Direct fuel injected engines, do better than pre-combustion chamber engines because the injection pressure is much greater and the fuel is more finely atomized.

-Air-cooled engines, sometimes do better than water-cooled engines at part load operation because the cylinder temperature of an air cooled engine tends to be 10-15 percent hotter.

-Electronically controlled engines, perform part load operation better than mechanically governed engines because electronically controlled engines usually inject fuel at higher pressures.  The electronic engine’s control system also, much more quickly, cuts back the amount of fuel injected as the load tapers off.

-Square-cut piston compression rings often work better for lightly loaded engines than the tapered Keystone-style rings. The reason for this is that square cut rings are not as dependent on cylinder pressure to force the rings against the cylinder wall during light loads.

-Naturally aspirated engines sometimes work better for light loads than turbocharged engines because their cylinder compression ratios are usually higher.

-Small bore engines, that is, the smallest bore engine that is sufficient for your application, work better than large bore engines because it is easier to control leakage in a small cylinder.Can we say then that the ideal marine diesel engine would be a technologically advanced, air-cooled, electronically controlled, 4-stroke, direct injected, naturally aspirated, with three-ring pistons and square cut compression rings?

Unfortunately there is no such marine engine in production. However, we can say that as engines are reconditioned or replaced we can keep the above attributes in mind, as part of a strategy to deal with part loading problems.
Finding the minimum “loaded” RPM for your model and series of your engine is important. Begin by getting the engine manufacturer’s guidelines. Every engine company that is involved with marine diesel engines also publishes an Installation or Application Manual. Next, pound the dock a little and check with those that use similar equipment under conditions like yours.
For main engines, eliminate all idling under 1200 RPM. For those hydraulic systems that run off of the main, changing the sizes of pulleys can let your main idle a little faster while your pump turns a little slower.
There are two ways to size generators for a fishing vessel. The first is to buy a gen set that will handle more load than you have. This works great for tired crews, because it doesn’t matter what order the loads are applied to the generator. It also works well if there is a smaller “hotel” gen set for cooking, lights and laundry. However, having just one large gen set can easily lead to light loads.
The second way is to do a very careful power survey and then install a smaller capacity gen set that will require sequential loading. The way this works is that the largest loads are put on the generator first and then the next largest and so on. This enables the use of a smaller set that can help to more easily manage the loading.




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