Cruising RPM research
- Thread starter smokey73
- Start date
The generator should always run at constant RPM. Load is still a relevant concern. If the electrical load is low, the mechanical load will be low and the diesel will inject very little fuel in order to hold RPM which is not good for the diesel. As the electrical load increases the mechanical load on the generator input shaft increases and the amount of fuel injected is increased.
The power of a diesel is controlled by the amount of fuel added. There is no throttle plate to restrict airflow.
The power of a diesel is controlled by the amount of fuel added. There is no throttle plate to restrict airflow.
I think real diesel geeks monitor exhaust gas temperature to get an idea of "load". RPM isn't that good a measure. If your prop falls off you can easily get to max RPM but the load will be minimal.
Black smoke on the exhaust is a symptom of being overloaded (over fueled)
Black smoke on the exhaust is a symptom of being overloaded (over fueled)
@JohnShannon Correct. However, if you have kept up with the rest of this thread and can reasonalby determine that you have a properly sized engine with the proper reduction gear, a properly sized prop and a clean hull an prop. The Propellor Power Curves for your particular engine gives a "reasonable" approximation of load on the diesel. If your prop is fouled or fallen off, of course, your approximation falls apart. That is the purpose of this thread.
Very few of us will go to the trouble of installing the instrumentation to determine true load but knowing a good approximation is good enough. For instance, I learned that for a Yanmar 4JH2E you have to get way up in RPM on the Propeller Power Curve (much higher than I imagined) to get to =>60% power, which many believe (not all though) is better for diesels that don't like to be lightly loaded. For example, at my previous normal cruising RPM of 2750 (75% max RPM) I previously assumed I was at about 75% load. As it turns out, the load is more like 45% @2750 RPM. My fuel burn should be about 1.25 gal/hr (which is the case from experience). If I desired to cruise at 60% load, I'd have to up it to almost 3100 RPM which would then increase my cruise fuel burn to 1.7 gal/hr. Probably not enough bang for the $$. I won't get enough of a speed increase in doing this so I"ll probably stay at about 2750 RPM for the most part. Will that change how I cruise. Maybe, maybe not, but I will continue to be diligent to run at near max RPM for a few minutes every day when I am cruising.
In the next few days I intend to go out and do rigorous RPM vs Speed runs and see what the differences are and from that can not only tell the best cruise RPM, but can determine best RPM for range when that is a trade off that I might need to make in the future.
That is the purpose of this thread. I learned a lot. Maybe you did, maybe you didn't but I think others may have.
Very few of us will go to the trouble of installing the instrumentation to determine true load but knowing a good approximation is good enough. For instance, I learned that for a Yanmar 4JH2E you have to get way up in RPM on the Propeller Power Curve (much higher than I imagined) to get to =>60% power, which many believe (not all though) is better for diesels that don't like to be lightly loaded. For example, at my previous normal cruising RPM of 2750 (75% max RPM) I previously assumed I was at about 75% load. As it turns out, the load is more like 45% @2750 RPM. My fuel burn should be about 1.25 gal/hr (which is the case from experience). If I desired to cruise at 60% load, I'd have to up it to almost 3100 RPM which would then increase my cruise fuel burn to 1.7 gal/hr. Probably not enough bang for the $$. I won't get enough of a speed increase in doing this so I"ll probably stay at about 2750 RPM for the most part. Will that change how I cruise. Maybe, maybe not, but I will continue to be diligent to run at near max RPM for a few minutes every day when I am cruising.
In the next few days I intend to go out and do rigorous RPM vs Speed runs and see what the differences are and from that can not only tell the best cruise RPM, but can determine best RPM for range when that is a trade off that I might need to make in the future.
That is the purpose of this thread. I learned a lot. Maybe you did, maybe you didn't but I think others may have.
Fail to see the point of generator comparison. Is that about recharging batteries with the main engine? Our small auxiliary engines last for a long time and thousands of hours of use. They run most efficiently when hot and at around 85% to 90% of WOT RPM. I don't charge batteries with the diesel unless motoring somewhere.
@Benny17441 Agreed. If you read the whole thread, the generator comparison was just a data point and acknowledged that it might not be a good comparison but it was the START of the "research." The thread is not about recharging batteries with the main engine. It is about the "best" cruising speed and what is BEST for the engine longevity coupled with fuel efficiency and just "feeling right" As you state, running at higher load is generally considered better for any diesel (either as a generator or as a main engine) but the question was how do you determine the load. It was previously postulated that RPM is a good indicator but after more research and finding a better resouce for the info (the Propeller Power Curves) for the engine it bacame obvious that 75% WOT as measured by 75% of Max RPM is not the same as 75% load. It is only about 42% load as shown by the Propeller Power Curve for my paricular auxiliary (would be different for other auxiliary engines) That is the reason for the thread. It seems you already knew that from your last sentence so it may have not been useful for you but others, like myself wanted to understand a better relationship between RPM and load. Later in the thread if you read on, you will see that if you have performance curves for your engine, you will probably find that it takes a lot closer to Max RPM to get to 75% load than you imagined (3100RPM on a 3600RPM engine to get to even get to about 60% load) That is the leason learned from the research. The Generator comparison has caused some confusion and I apologize for that if it has implied that it was a direct comparison. It was just a start and one data point.
yah some of us are bored retired engineers. i just follow mack boring's suggestion: runthat diesel hot and hard every time on the water that the boat is out of its slip.
I think you guys are way overthinking this.
Running a diesel at very low RPM's (or unloaded) will cause it to run cool. This will cause carbon buildup. That's bad.
But once the thermostat fully opens at 170F-180F which is at about 60% of WOT for most engines - the engine's not going to get any hotter if you run it faster. So carbon buildup isn't going to be any worse.
So as long as the temp gauge is above 170F, is there are reason you should run at 85%? Sure the engine's rated for it and can do it, but on a lot of boats today 85% is inefficient because you are bumping up against hull speed. You're using a lot of fuel just to drag a big wake and see almost no speed increase.
On my boat, the sweet spot on a 4JH4-HTE is 2100 RPM which is 67% of the 3100 rating. The temp is over 170F. Fuel mileage is good. If I speed up, I gain a measly 0.3 kt while burning 30%+ more fuel. I've got 2300 hours on the engine and 95% of that is at 2100 rpm. No smoke. Starts instantly. No work on the injectors. Never cleaned or replaced the exhaust elbow. No work on the turbo. I do run up to WOT about once a week for 10 minutes. I never see smoke when I do. So I'm not sure it matters.
Am I missing something? If it's not carbon damage - what exactly is being hurt by running at 67%?
Running a diesel at very low RPM's (or unloaded) will cause it to run cool. This will cause carbon buildup. That's bad.
But once the thermostat fully opens at 170F-180F which is at about 60% of WOT for most engines - the engine's not going to get any hotter if you run it faster. So carbon buildup isn't going to be any worse.
So as long as the temp gauge is above 170F, is there are reason you should run at 85%? Sure the engine's rated for it and can do it, but on a lot of boats today 85% is inefficient because you are bumping up against hull speed. You're using a lot of fuel just to drag a big wake and see almost no speed increase.
On my boat, the sweet spot on a 4JH4-HTE is 2100 RPM which is 67% of the 3100 rating. The temp is over 170F. Fuel mileage is good. If I speed up, I gain a measly 0.3 kt while burning 30%+ more fuel. I've got 2300 hours on the engine and 95% of that is at 2100 rpm. No smoke. Starts instantly. No work on the injectors. Never cleaned or replaced the exhaust elbow. No work on the turbo. I do run up to WOT about once a week for 10 minutes. I never see smoke when I do. So I'm not sure it matters.
Am I missing something? If it's not carbon damage - what exactly is being hurt by running at 67%?
I'm an engineer, but not bored or retired. I think I learned the most about my motor from James Taylor. He says ... "Damn, this traffic jam! How I hates to be late. It hurts my motor to go so slow, I say damn, this traffic jam!"
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Funny thing is I'm perfectly happy to be sailing at 3-4 knots if that is the best I can do and I'm out for pleasure. But as soon as I turn on that engine, I'm not running any less than 3100, 3200 rpm. It's time to make tracks! But I do tend to set a speed goal rather than an rpm goal. If I'm against the current or there is little current, I run the rpm up so I can make at least 6 to 6.5 knots. If I'm going with the current, I slow down the engine just to reduce noise and still do my speed goal. Same thing with sail assist ... I'll slow down the rpm, mostly to assure that the boat rides flatter and quieter. This way, I think I'm getting about the right balance and the engine is happy! Don't want to hurt my motor, you know!
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Critical observation, and well put.
An axiom to that is my reflection on 23 years with my boat and its M25 Universal diesel. I have posted many times on posts about fuel gauges, that in addition to, or in my mind instead of one, that each of us should learn how much fuel each of our engines use: fuel consumption is critical. That said, regardless of whether you "baby" or "haul" your engine, based on your own personal use, you will soon learn that you can reasonably predict your own fuel consumption. I simply keep a spreadsheet with fuel refills and learned, early on
yikes: - back in 1998!!!) that my engine uses 0.5 gph. This is a real world figure based on keeping records. Of course it includes slow speeds and fast speeds, because that's how you measure fuel consumption. I use it to determine how much I need at my next refill, so I don't spill any fuel out the vent, and can check the health of the engine. Any significant changes to that figure gets me to check on just why it's different. 23 years, same number. Here's my spreadsheet: