quote:

Originally posted by Rusty:
The volume of air is a function of displacement, not compression ratio. Turbochargers don't have much boost at idle, so don't add to the volume.

Very true, BUT, diesels do not have throttles so it is taking in a full charge of air every cycle whereas a gas engine will have a high vacuum at idle, reducing the intake charge.

This I found on the net:

Air/Fuel Ratios

* Diesel engines produce very little carbon monoxide, since combustion takes place in an oxygen-rich environment
* Diesels do not run at the same stoichiometric ratio as gasoline engines (14.7:1)
* Diesels run about 50% leaner than gas engine stoichiometric mixtures.
* Diesel engines need about 18 times more air by mass than gasoline to maintain a smoke-free tailpipe.
* Smoke-free operation is usually around 22:1 air-to-fuel ratio (AFR)
* As additional fuel is added, more air is needed to stay smoke-free
* Black smoke indicates we are producing particulate matter by not burning all the fuel
* Even with black smoke, you can still add more fuel to produce more power, but combustion efficiency goes down
* White smoke is extreme over-fueling
* Some white smoke is normal during cold start-up
* Cylinder temperatures are too low to burn all the fuel, resulting in some blue and/or white smoke

Diesel Cooling

* Diesel engines run cooler at idle than gas engines
* They take a lot longer to warm up
* In fact, most of the time you need some engine load to fully warm up the engine
* Cooling systems are also very large on diesels, which adds to warm-up times

One big reason for cool-running (as opposed to gas engines) is that diesel engines have no throttle plates, so during intake stroke, more air enters the cylinders and this helps to cool the engine internally.

So in regard to this statement;

quote:

The reason idling diesels is a no no is that they are hard to keep warm at idle.

So what?

I can understand a cool engine being an issue on a gas job due to plug fouling, but since compression is the igniter, so what if it runs cool? That's a good thing isn't it?

OK, a little more research answered my question and provided some good reasoning from DD:

“How long should a diesel engine be warmed up before driving off?”

Your question ranks right up there with the most commonly asked questions we receive. It is also, at least in my opinion, the most commonly misunderstood aspect of the operating parameters of a diesel engine. The short answer is we have always recommended that if the engine is going to be idling, low idle, longer than five minutes, that it be shut off. The reasoning is a little more complicated but without getting into engineering 101 basically the following is the layman’s reasoning.

Diesel engines were designed to operate at peak efficiency running wide open. As there are no spark plugs, cylinder temp., obtained by compressing air in the compression cycle, is necessary to ignite the fuel. At low idle the cylinder temp. drops leading to incomplete combustion. This incomplete combustion leads to carbon build up on the valves, in the oil (crankcase), on the fire deck of the head and the dome of the piston along with fire rings on the piston. Engine “slobbering”, what appears to be oil, out of the air box drains and the exhaust are other side effects. As worn oil control rings will also give you both of the last two items this is often cited as the cause of an engine “slobbering”. The irony is that the carbon build up in the oil will lead to worn oil control rings.

Now the reasons folks give for letting the engine idle - “I need to keep up cab heat / AC in the winter / summer.” If you are going to be away for less than 10 minutes the engine will keep more heat being shut off than idling for that amount of time. The AC unit will not maintain temp at low idle. “Truckers all leave their rigs running while at a truck stop.” They aren’t paying for the fuel or the repairs. “When you’ve been running all day you need to let the engine “cool” down for at least ten minutes before shutting it down.” Simply not true!!

quote:

Originally posted by MWrench:

quote:

Originally posted by Rusty:
The volume of air is a function of displacement, not compression ratio. Turbochargers don't have much boost at idle, so don't add to the volume.

Very true, BUT, diesels do not have throttles so it is taking in a full charge of air every cycle whereas a gas engine will have a high vacuum at idle, reducing the intake charge.

Agreed - but airflow in DI or ported injection engines will be higher than carburetted engines. The real issue isn't airflow but the much higher volatility of gaso.

quote:

Diesel engines were designed to operate at peak efficiency running wide open.

Not true. Diesels are designed to run at optimum design power, which is not "wide open". Most ARE designed however, to be able to run constantly at full throttle, whereas few gaso engines are.

quote:

At low idle the cylinder temp. drops leading to incomplete combustion. This incomplete combustion leads to carbon build up on the valves, in the oil (crankcase), on the fire deck of the head and the dome of the piston along with fire rings on the piston. Engine “slobbering”, what appears to be oil, out of the air box drains and the exhaust are other side effects. As worn oil control rings will also give you both of the last two items this is often cited as the cause of an engine “slobbering”. The irony is that the carbon build up in the oil will lead to worn oil control rings.

Partially true, but misstated. Most carbon build up in ringbelts and valves comes from issues with the motor oil, not the fuel, and from high temps, not low (although varnish is a concern at low temps - and that does have the potential to stick oil rings). And carbon doesn't build up in the crankcase due to incomplete combustion; that leads to dilution of the crankcase oil. Incomplete combustion means the carbon and hydrogen in hydrocarbon fuels are unchanged. Either the entire molecule oxidizes (combusts) or it doesn't. And I spent years as a fuels and lubricants engineer, and never encountered "carbon buildup" in the engine oil.

quote:

“When you’ve been running all day you need to let the engine “cool” down for at least ten minutes before shutting it down.” Simply not true!!

That's true, but disingenious. Turbocharged engines should idle long enough to bring EGT down to at least 350°F before shutdown.

quote:

“When you’ve been running all day you need to let the engine “cool” down for at least ten minutes before shutting it down.” Simply not true!!

quote:

Originally posted by Rusty:
That's true, but disingenious. Turbocharged engines should idle long enough to bring EGT down to at least 350°F before shutdown.

The reason for this is the cookoff of the oil in the turbo. by shutting down the diesel, without lowering turbo temps, you will cause the oil to cook inside the turbo. This cooking will eventually wear out a turbo sooner. 3 minutes cooldown is generally considered long enough to accomplish this.

quote:

The reason for this is the cookoff of the oil in the turbo. by shutting down the diesel, without lowering turbo temps, you will cause the oil to cook inside the turbo. This cooking will eventually wear out a turbo sooner. 3 minutes cooldown is generally considered long enough to accomplish this.

Don't some engines have a turbo oil reservoir designed to gravity feed oil to it to prevent the cook-off?

quote:

Most ARE designed however, to be able to run constantly at full throttle, whereas few gaso engines are.

Aha! This explains why diesels are so popular in large boats. I always wondered what with the fuel and water thing being so finicky.

Cylinder wear is directly proportional to engine temp. There are a bunch of studies on this, the conclusions are all the same.
80-90% of all cylinder wear takes place with in the first minute of startup….

Okay, so for all of us "less mechanically inclined" members reading this, please answer a couple of questions in laymens terms:
(1) Do you try to run your diesel motor when you know you won't be using the coach itself, i.e. a snowy winter season?
(2) If so, do you need to take the coach on a 1/2 hour ride to do so? I know that running the coach down the road is like my collector cars in as much as it gets ALL the fluids and parts working. But from what I read here, it is a bad thing to just start it up and run it for 15-20 minutes, though mine will come up to temperature in that time period. Can you just sit in the coach and run the motor at a higher RPM?
(3) If you do not have the opportunity to "run the coach down the road" is it better to just let the coach sit until you can? Even if it is a three to four month period?
(4) This is a little off the base of this thread, but don't forget those generators in your coach! They need to be exercised and you don't have to go down the road to do so....

quote:

Originally posted by chrisW:
Cylinder wear is directly proportional to engine temp. There are a bunch of studies on this, the conclusions are all the same.
80-90% of all cylinder wear takes place with in the first minute of startup….

To add to that.....I had a valued instructor who was a retired engineer from Wright Aeronautical Engines. His A&E license was a single digit number, and signed by Orville Wright. He went on a discourse one day, and said "90% (or maybe it was 95%) of engine wear occurs during cold starts." He rattled off study after study and remedy after remedy. The subject was oil dilution devices, pre-oilers, warmup heaters, etc. Aircraft engine makers, airlines and the military did a lot of research on this.

Our portable and stationary diesel engines at work were rigged so they would not run until they had cranked enough to build up oil pressure. I think I remember the SOP on the V12 (gasoline) tank engines in the Army being that way, too.

quote:

Originally posted by Bruce & Kathleen:
Okay, so for all of us "less mechanically inclined" members reading this, please answer a couple of questions in laymens terms:
(1) Do you try to run your diesel motor when you know you won't be using the coach itself, i.e. a snowy winter season?

Not unless you can put 10-20 miles on it.

quote:

(2) If so, do you need to take the coach on a 1/2 hour ride to do so? I know that running the coach down the road is like my collector cars in as much as it gets ALL the fluids and parts working. But from what I read here, it is a bad thing to just start it up and run it for 15-20 minutes, though mine will come up to temperature in that time period. Can you just sit in the coach and run the motor at a higher RPM?

If it's not going to be run 15-20 minutes under load. leave it alone.

quote:

(3) If you do not have the opportunity to "run the coach down the road" is it better to just let the coach sit until you can? Even if it is a three to four month period?

Yes

quote:

(4) This is a little off the base of this thread, but don't forget those generators in your coach! They need to be exercised and you don't have to go down the road to do so....

They should be run under load also

If you store your diesel fill the tank up to limit condensation and add a fuel treatment against "algae" growth.

quote:

Originally posted by Marvin+Doris:
If you store your diesel fill the tank up to limit condensation and add a fuel treatment against "algae" growth.

Amen.

So what is the "Truth About Low Mileage Engines"? Or, is a low mile diesel much better than a high miler.

From this post I've gathered that there are believers in both camps of high vs low mile diesels and they are split pretty evenly. In fact, both may right as it seems accepted that most wear happens at cold start up. Therefore, the question isn't so much about high or low miles as it is about number of start ups. Perhaps then diesels should have a Start-ometer???

We've learned that some components (soft parts) will wear with age, so low miles can be a false security that these items should last. We've also learned not to idle a diesel more than five minutes and that it is a good idea to let turbos cool before shut down.

The final three things that resonated throughout are maintenance, maintenance, maintenance.

quote:

Originally posted by noble97monarch:

From this post I've gathered that there are believers in both camps of high vs low mile diesels and they are split pretty evenly. In fact, both may right as it seems accepted that most wear happens at cold start up. Therefore, the question isn't so much about high or low miles as it is about number of start ups. Perhaps then diesels should have a Start-ometer???

I am in neither camp, as the length of time sitting and length of trips can weight things in either direction.

However, the Start-0-Meter could be useful. Airlines have always rated their aircraft by both hours as well as cycles. A cycle being one takeoff, one landing, one pressurization and one depressurization. The cycle count is used in evaluating engine wear (particularly piston engines), skin fatigue, landing gear stress, etc.

And, you guessed it, short flights are harder on airplanes than the same number of hours on long flights.

Annnnnd.........airplanes don't like to sit. In the seventies, we parked a lot of planes in Roswell NM. We kept a crew of mechs there to fire them up, taxi them around, and cycle all the systems. The program went so far as to cycle every switch or pot a dozen times.

I only mention the airline thing because we kept such good records and even had reliability engineers that constantly analyzed the data to try to make the planes last longer. Information was shared with other airlines and manufacturers, who also had reliability engineers. They learned what worked.

Oh, yeah. We did oil analysis, too. And filter media inspection.