Warming A Diesel - Study Hall

[Ex-Calif]

Alan,

Excellent post. I'd like to explore the advice regarding warming up a diesel.

We start our diesel and get undeway with no dealy. We operate at around 1,000 RPM for about 3-5 minutes. Here's my concerns.

Oil Pressure - If you are running anything higher than 30W the oil can be very viscous until it is warm. Oil splashing is generally how the cylinder walls are lubricated and cooled. Operating at a high load with highly viscous oil could cause piston skirt scrubbing leading to engine deterioration, higher oil consumption and blow by. Secondly, bearing clearances are also very tight at start up and open slightly with temps. Tight initial clearances and highly viscous oil could cause bearing scrubbing and in a worst case bearing spinning.

Piston and Cylinder Thermal Stability - At start up the clearances between the piston and the cyclinder are at their greatest. Some cylinders are cut with a taper because the upper end is hotter and will expand more at the top than the lower end. A "cocking" of the piston is possible at low temps making the above cylinder scrubbing problem worse.

Thermal Expansion - Metals expand in proportion to the heat applied over time. If I am applying a lot of heat to the cylinder head right after start up there is a risk of cracking as opposed to a gentler warm up.

I am interested to hear your opinions on these statements in relation to diesel engines. I agree that you dont' want to start and run in idle for 10 minutes but on every internal combustion engine I have owned and operated I have always allowed a period of low power operation until oil and metal temperatures are stabilized, or on the way to being so.

[Alan Wheeler]

Hi Dan. Good questions. The answer is , it depends!
OK, so first the oil. It is important to run a Diesel on the manufacturers specified rating. These engines are very different to a Gasoline engine, which can benifit from thicker oils. Diesels do not do well on a heavier weight oil. In fact it can create damage. Most of the lower reving British and US engines tend to run on a mono 30W oil.(once again there are exceptions) A mono30 is a good oil weight and mono oils are very stable. Far more stable than multi grade oils. Be very wary of introducing oils of heavier weights than specified and even more wary of introducing oils with a wide rating range, like 15W50 for say,(unless the maker has specified that), or damage as you have stated can occur. Many people believe it is the oil company that designs the oil specs. But actually it is the engine manufacturer that specifies to the oil manufacturer what they want for their engine. The Oil man may make something that exceeds that spec, but the weight will be as asked for the design and tolerance. So most of the issues you have stated are actually thought of and taken care of by the oil weight.
Mostly Japanese engines are the ones that take the multi grade oils. But once again, the engine maker specifies what weight for his engine.
In regards to splash, very few larger engines use a splash design. (I do presume you are talking about the Crank/bigends splashing into the sump oil.) It is normally found on very small engines and very old engines. The larger engines get the oil splashed onto the cylinder wall by the oil dripping down from the Wrist or Gudgeon pin area of the Piston.
Heat. A very good concern here. Gasoline engines are very prone to heat stress, especially around the valve seat area, where cracking can take place. And Alloy heads in earlier years were a real problem. But a Diesel does not produce the heat the same and the vast heatsink of cast Iron heads and Blocks allow the major stress area's to have the heat soaked away quite fast. However, this is why I also stated no more than 1000RPM. This is about the right speed to allow a little load and not too much stress to be placed on the engine and not too much heat produced all at once. It is a balance between creating a load to produce heat and not produce too much that it causes damage. 1000RPM is about right for this. The main concern we are try to avoid is glazing. To prevent glazing, we have to produce a certain amount of wear so as those Rings remain bedded in. Allow the engine to idle cold for a long period and she will start smoking like a train with a 2 pack a day habit.

[seafox]

Hey Alan, where is the first post??

[avazquez]

As usual Alan has the concise answer..Thanks.

What I do is that before leaving I warm up the Perkins 4108 at 1000 RPM in forward gear until operating temperature is reached (180-195F). If I still have to wait for someone and it takes more than say 15 minutes I turn off the engine and when we leave the engine is warm already. I never leave it at idle for more than 5 minutes. But how do you charge the batteries with the alternator when anchored?

[Alan Wheeler]

Hey Alan, where is the first post??

If you mean the "paper" look in the "study hall" thread sticky above.

But how do you charge the batteries with the alternator when anchored?

Just because you run the engine once or twice NOT doing a loaded warm up, will not destroy the engine. So the odd time you have to run the engien to charge while not being able to place in gear is fine.
In regards to this warm up business, even a smoking engine can sometimes be brought back from that, by being hard on it. Like if you have an engine that produces Blue smoke badly when you start it, (depending on what is causing the smoke) you can be very tuff on the engine and actually improve it. Start the engine, get it straight into gear and take the revs to 50%, then as the temp rises, get it to max throttle, allow it to come to full temp and then bring the throttle back to idle, cool and shut down. Doing this a few times will dramaticly reduce smoke. Providing it is a glazing issue and depending on the engine. Some engines that have high chromium cylinder sleeves will not be affected. Engines with cast steel cyclinder sleeves respond well to this treatment. There is not really anyway of knowing what was applied. Some engines like Fords, can have either. Does this treament cause wear?? Absolutely! that's the whole point. You are trying to wear those rings and make them bed back in. Will this treatment damage the engine?? not likely!.
The only other alternative is to dismantle the engine and give it a cylinder hone. Which would you prefer to try first :-)

[avazquez]

I followed those instructions from a previous discussion and smoking has diminished substantially....

[colemj]

If you are going to state a specific rpm, like 1,000, I think you need to state the operating or max rpm of the engine. For mine (max 3,600), 1,000 rpm is barely above idle.

Mark

[Ex-Calif]

Originally Posted by colemj

If you are going to state a specific rpm, like 1,000, I think you need to state the operating or max rpm of the engine. For mine (max 3,600), 1,000 rpm is barely above idle.

Mark

Good point - And as Alan says it depends. In this case on the horsepower / torque curve for the motor. I'd say a good rule of thumb would be 1/3 rated power until temps start to rise.

Some engines want the oil and water temps up quite a bit. Unless you are operating in the winter around freezing temps I would say that if the oil and water temps are coming off the pegs well then it's time to go to whatever power you need for the conditions.

In regards to charging batteries on anchor - what the heck - it's a good excuse to go around the bay - LOL.

Seriously I don't charge on the hook but if I did I would consider something above reverse idle (1/4 - 1/3 power) while at anchor or on the mooring. Using reverse would avoid having the boat swing around.

[Alan Wheeler]

For mine (max 3,600), 1,000 rpm is barely above idle.

It does not matter what the max RPM is. Even an engine with a max of 4600 will still have an idle no higher than say 750RPM at the most. If you go back and read the post again, you will see that the 1K is for reasons of Alternator and allowing some load till the engine warms enough. Placing the engine at 1K will allow the prop to bite and allow some work to be done by the engine. Even a high reving engine will notice the prop working at this RPM. You will notice all other rev ranges are in percentages. This is because they are more important to the actual rev range of the engine.

And as Dan suggested reverse, no it really doesn't matter if any of this is done in forward or reverse.