Engine wear at different RPM's

[Ziggy]

Originally Posted by Captain Bill

I'm at home right now and my engine manual is on the boat, so this may not be a perfect quote, but my manual says that the ideal cruising RPM should be 80% of Maximum Continious RPM. It also states that if operating for long periods below this speed that the engines should be run at this speed for 5 minutes every hour to prevent carbon buildup and cylinder glazing.

Exactly right. Here's an excerpt from the Yanmar 3YM30 manual:

When operating the engine at low speed for long periods of time, race the engine once every two hours. Race the engine with the clutch in NEUTRAL, accelerate from the low speed position to the high speed position and repeat this process about five times. This cleans out carbon from the cylinders and the fuel injection valves.
NOTICE: Neglecting to race the engine will result in poor exhaust color and reduce engine performance.
Periodically operate the engine near maximum speed while underway. This will generate higher exhaust temperatures, which will help clean out hard carbon deposits, maintain engine performance and prolong the life of the engine.

[sailorboy1]

Doesn't say it is better to always run the engine hard, just that every once in a while to run it up to blow out the fuel related deposits.

Lets not confuse the ineffectivity of burning fuel with the mechanical proterties of wear and tear. Engines are more than just cyclinder walls and fuel injectors.

[foggysail]

Many years ago when I was younger, a friend purchased a car from an older couple. The car had about 50K miles on it, all driven at low speeds. He thought he had a winner!

It turned out that he did not. His driving habits were similar to most of ours, operate your car at higher speeds on highways. Well, his engine broke down requiring major repairs. Each cylinder piston's upper most ring was broken. The thought was (and I believe it was correct) the wear over the early years caused a ridge to form in the cylinder walls at the end of the piston stroke. When the engine was operated at higher RPM, the pistons were driven higher in the cylinders as a result of connecting rod bearing wear and higher upwards momentum. That slight additional travel allowed the upper rings to get crushed against the ridge.

Best advice is if an engine is going to be operated at low RPM, it should occasionally be ran nearer to the high RPM limits.

Foggy

[Ziggy]

Originally Posted by Don Lucas

Doesn't say it is better to always run the engine hard, just that every once in a while to run it up to blow out the fuel related deposits.

Lets not confuse the ineffectivity of burning fuel with the mechanical proterties of wear and tear. Engines are more than just cyclinder walls and fuel injectors.

Right, and that's also the point of Capt Bill's post. Low RPM does not mean more wear, only carbon buildup and glazing. You can avoid these problems by racing the engine from time to time. Incidentally, carbon and glazing could lead to other problems later on.

[Bash]

I've had three different Yanmar mechanics over the years, and they've all been insistent that the engines should be run at high revolutions periodically. For example, I've been told to spend ten minutes at WOT at least once a day.

I've never been particularly attentive to this advice, but I've also done everything possible to avoid running the engine at idle to charge the batteries.

[djmarchand]

The question about harming a diesel engine by running at low speed gets debated on this and other boating forums periodically.

First lets separate wear from glazing and carbon build up. Wear results from several factors: rpm, bmep (brake mean effective pressure, a measure of load), temperature, lube oil age and oil pressure. So running at high rpm at wot in gear with a fouled cooling system, a bad lube oil pump and with old oil will lead to a lot of wear.

The diesel gurus on boatdiesel (marine diesel shop owners and engine developers) generally agree that running a diesel at low load, as long as it reaches operating termperatures, will do no long term harm. It generally takes about 30-40% of wot rpm to put enough load in gear on a marine diesel to keep it up to operating temperatures. For the typical small Yanmar in sailboats that is about 1200-1500 rpm.

Since sailboats operate at displacement speeds and their engines are often sized to push them up to hull speed plus a small margin for adverse waves, we usually operate at about 80% of hull speed. That is an efficient and comfortable speed and typically is at about half of the engines rated horsepower and 75% of maximum rated rpm. But running at 1/3 of rated rpm won't hurt them although it will be pretty slow.

I have had the injectors on my small Yanmar foul up. Running the engine hard at 3000+ rpm cleans them up. I have cruised for hundreds of hours at 2,400 rpm. If I run it up to 3000+ every hundred hours or so it blows out any accumulated soot and cleans up the injectors.

Finally I don't buy the argument that diesel engine rings NEED high combustion chamber pressures to seal them to the cylinder walls otherwise they will allow combustion gasses to blow by. There is a simple way to check for blow by. Put your finger next to the outlet of the small hose that comes from the valve cover on your Yanmar and most diesels. Feel for blowby pulses. Rev the engine in neutral (ie light load) up and feel for any pulse. Good engines with good compression will not show any blowby. Sure worn rings will produce blowby but it doesn't decrease when you put the engine in gear and put a load on it.

David