Effects of Running Diesel at Low RPM

[rsmith]

Those of us that fish spend up to 12 hours a day at less than 1000 rpm trolling.at the end of the day when you push the throttles up all that smoke is from unburned crap that's coated the exhaust valves and port area. These areas were never designed for combustion. If you crank it up and burn it off before the buildup gets too bad you may be ok. I wasn't so lucky. After years of offshore canyon fishing in the North Atlantic with 4 hour runs to the fishing grounds I decided to leave the boat in Fla. With manatee areas between my dock and the inlet and only 11nm till the fishing grounds it hardly makes any sense pushing the throttles up. It's gentlemen fishing at its best. After a few of these trips we got caught in a typical Florida afternoon white out squall. Cranked it up to go home and one engine started pouring out white smoke. When we pulled the head one of the valves was torched actually melted down to the stem. The rest were coked up with the residue from not running the engine hard enough.
Thermostat may keep up the coolant temp to prevent acids and condensation in the oil but the only answer for low combustion temp it to run em hard

[krush]

Thermostat may keep up the coolant temp to prevent acids and condensation in the oil but the only answer for low combustion temp it to run em hard

Or put bigger props on to load the engines more at lower RPM.

My pyrometers are mounted after the turbo and the reading doesn't budge at idle in neutral. But when I put in gear, the needle climbs up a bit...something I did not expect. And when I'm running at 1800RPM cruise, the needle isn't much higher than 1000rpm in gear. Now, I'm assuming that the values are correct....and I need to verify the instrumentation calibration.

[Pete]

On DD engines, the volume of blower scavenger air is directly proportional to the RPM of the engine, as it is gear driven. The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm. Thus, requiring more power at a lower engine RPM will have little influence on the turbo output to the blower and thus the scavenger air volume. Granted the purpose of the turbo is to raise the input air pressure on the blower, but at low RPM that is being discussed this is minor. It would be very interesting to see a RPM versus turbo output pressure versus scavenger air pressure graph to better understand the precise relationship.

But what we do know empirically is that low rpm operation, especially in an over propped setup, does carbon up the combustion chamber and exhaust, as everyone sees the increased black smoke when the air flow and temperature are increased by higher RPM operation. That black smoke is the carbon deposits of unburnt fuel, some in the combustion chamber, other on the pistons, rings, exhaust valves, exhaust manifolds, where they do no good.

Circulating back to those million mile over the road engines, they do not lug along at 1000 to 1200 RPM for hours, then blow them out for 30 minutes. Simple logic will tell us, if you want similar results from a marine engine, best idea is probably to emulate the operating environment that produces those results.

Pete

[Avenger]

Wow!!!

[Top Hatt Craig]

"The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm" are you sure? Isn't the expanding combustion gas the source of power to drive the hot side turbine against the load of the cold side compressor/turbine? The scavenger air is not much over atmospheric, there's very little energy there. Throttling up against a load produces (as you know) more expanding gases that accelerates turbines, providing boost. I'll betcha' if we could rig a test where a very large electric motor were to spin the crankshaft of a DD at 2100 rpm with no fuel injected, we wouldn't see near as much, if any manifold boost compared to our boat at 12 knots or a truck climbing The Grape Vine. Regards, Craig

[Avenger]

You don't need an electric motor. Spool one up to the governor in neutral and you will find zero manifold pressure from the turbo. No load = no boost.

[kelpy]

You are correct, Running the engine up to high idle, say 2350 RPM, will show no boost. The burning fuel provides the energy to spool up the turbos. Later engines had bypass blowers to eliminate them at high boost.

[Seapig100]

Over the road trucks burn about 8 gallons an hour. So to emulate this I need to be right about 1500 rpm as a guess. This is by far the worst place to be on my boat. So I have a choice burn 4 gallons an hour per engine or run them up to 1900 and burn 17 gallons an hour per engine. I would bet that I get many more miles from the engines if I burn 4 gallons an hour.

[Pete]

Run the engine up in neutral to no load limit and take a look at the exhaust outlet. Where is that air volume coming from that shoots the exhaust water 50 feet or more across the water. Also, what is manifold pressure at WOT under load?

Seapig, the solution to your dilemma is to take pitch out of your props such that 1500 RPM produces the cruise speed you desire. You will give up WOT speed in doing so, but that is the trade off.

Pete

[krush]

The scavenger air volume is the primary contributor to the air volume exhausted through the turbo to increase the spool rpm. Thus, requiring more power at a lower engine RPM will have little influence on the turbo output to the blower and thus the scavenger air volume. Granted the purpose of the turbo is to raise the input air pressure on the blower, but at low RPM that is being discussed this is minor. It would be very interesting to see a RPM versus turbo output pressure versus scavenger air pressure graph to better understand the precise relationship.

Volume and mass flow are different things. Volume per rotation through the positive displacement blower on a detroit is practically the same at all loads and RPM. However, mass per rotation varies greatly.

You don't need a graph to figure this stuff out. A boost gauge will tell the whole story.

I'm not here to argue on the interwebs LOL do what makes you feel good.