More diesel efficiency discussion

The amount of fuel a cylinder needs in a compression ignition engine is directly proportional to the amount of air that is in the cylinder. You could "de-tune" the engine to give less power, but that will affect the rest of the propulsion/drive system. The short answer is no, not easily, or cheaply.

Actually yes you can, and on 2 stroke diesels like DD's it's relatively easy. I ran a 58' Hatt YF with 8v71TI's that DD put 90lph injectors in and took out the 110lph injectors (LPH= liters per hour). They just adjusted the fuel rack differently and we had to take out 2-3" of pitch to achieve 2350rpms at top. Fuel consumption at 1950 rpm's went to 30 gph (both) from 45-50gph. Speed dropped from 15.1knots to 13.5 knots (this was a heavy yachtfish). Fuel consumption at low speeds was amazing and the engines never sooted up or ran dirty at any speed, no matter how many hours at 600 or 800rpms. They also were very crisp when it came to giving them throttle since the turbo's were still larger. HP went from 535hp to 472hp (rated by DD). This conversion was done by DD of Daytona when they did major overhauls on the engines and I ran the boat for 3-4 years afterwards......

The short answer is NO. It will greatly increase your low end hull speed fuel burn efficiency because the smaller mechanical injectors can meter the fuel better and you'll get a more efficient fuel burn underneath the turbo's. You will also get cleaner air boxes when doing a lot of low speed running. On the particular boat I was on that is the reason DD went to the smaller injectors is because the boat did mostly hull speed running. The 2" of pitch you won't even notice at hull speeds. Meaning at 800 rpms, the boat will do within a few tenths of a knot of what it did before. On a DD this will have no harm on ANY of your driveline components. Speak to a good DD diesel about the subject and see what they have to say.

I have done the great loop in entirety in a 75' Hatteras MY and most of your cruising will be done at hull speed from Ny to Mobile if you're over 45' or so. And a lot of times it doesn't pay to run at cruise because you're waiting at the lock for the slower boat behind you anyways. Let me put it to you this way, we averaged 8.5 knots the entire time and even the boats that were running at cruise as much as possible (a 40' Tiara, a 42' Searay) ended up at the same marina's as us each night. We averaged 1.65 GPNM the entire trip and that included running at a higher cruise 2050rpm's for 30mins every 6hrs (recommended by 2 different DD dealers to do), 2 days of running at cruise at 100 gph (beaufort to Coinjock, and Lake Michigan to Chicago) we had the large injectors and the 12v71TI's we had runs a little fuel rich IMO at lower speeds, and this included anchoring 70% of the time at night running a 20 or 27kw generator 24 hours a day because we couldn't fit into a lot of marina's being 75' long and a draft of 6'. Our fuel consumption could've been knocked down much more if we ran at 800rpm's instead of at 1,000 but there comes a point where time is a factor. At 1,000 rpm's we did 9.5 knots, at 800 rpm's 8.5 knots

There are many different varieties DD used, DD may have used 12 different injectors even for the same engine 6v71 for example. Call a dealer and they should be able to tell you in a minute or two, I would also ask them what they think on going down an injector size and they should have a good recommendation as well as the price. I want to say they're around $125US +/- each. On a trip of the magnitude you're talking about, they most likely will pay for themselves many times over.

You have to remove the props, send them to a prop shop and re-install- if they are correct on the first time in adjusting the props kudos to them, if not do it again. Proper prop install with a propsmith means you have to haul the boat. You also have to buy a new set of injectors- or a rebuilt set, and R&R those injectors. An injection shop will adjust the internal settings of the IP. Take a seatrial with a mechanic to see that the engine has the proper load at all RPM's. Running the same turbo gives "crisp" performance?- decreasing the amount of fuel will require less air (PSI) and either under fueling or to much air means adjustments may be needed- watch out for high EGT's. Top speed will drop- as will speed across the board. Look at all those things and see if the cost of those is greater than the fuel you'd save- then decide. If those things seem "easy and cheap" in the long run then do it.

The ecconomics of this seem marginal when all is done. I've also been chased by enough seas to be loathe to give up any kts. My big question though is what is the effect on the engine warranty when this is done.

When it was done on the 1 vessel I ran, they did it at major overhauls and it carried the same warranty as if it wasn't done. I would guess his engines are long out of warranty. It's basically a commercial tune (de-rated) that they have used in gensets and so forth where they're looking for better fuel burn, more torque, and HP isn't as much as an issue is what they told me. The fuel burn is considerable, we went from 15 knots at 50gph 3.33 gallons per NM, to 13.5 knots at 30gph or 2.22 Gallons per NM. Engine life is supposed to be extended as well because you're not putting out as much HP out of the same internals.......

It sounds like someone is confusing a diesel with a gas engine. A turbocharged diesel has valve overlap, both exhaust and intake valves are open at the same time to improve scavenging. Clean air is blown through the cylinder to clear out the exhaust and cool the exhaust valves.

There really can't be "too much" air in the cylinder of a diesel. "Underfueling" only reduces power output and lowers EGT. There is nothing to adjust because reducing fuel input is how you reduce power and/or engine speed, reduce it enough and the engine quits, that is how you stop a diesel.

The only thing that will increase EGT is increased fuel and load. "Overfueling" means you have more fuel than there is air enough for complete combustion. This will lead to high EGT in a turbocharged engine because fuel is still burning when it passes the exhaust valves.

Only a gas engine suffers high EGT from a lean mixture at high power, not a diesel.

I might be confused, but would you please tell me where the intake valves are on a 6-71? Higher boost pressure in a diesel certainly does raise EGT's and IAT's. My original answer to the thread's question was that it was not "cheap and easy". That's a statement that is qualified by what the original poster wants in the long run, what kind of hours/miles he runs per year, and what he wants to give up in return for burning less fuel.

I think it will greatly benefit him. He mentioned he wants to do the "great loop". For those that don't know and/or never done it, the Great Loop is around a 5,000 nautical mile trip.

I have also used "sea slide" it is a product that is applied over the bottom paint. When it is wet, it is super slippery and your hand glides right down the bottom. I have used this and did record over a 1.5 knot increase at cruise speed with it on there. The boat went from a 17.5 knot cruise to a 19.3 knot cruise at the same rpm's. I ran the boat 950NM's from ft. laud to St. Croix and the speed never dropped off. My customer who requested it, says it is still working a year later.

There aren't any of course. The inlet valving is performed by the ports and the same scavenging sequence occurs as in a 4 stroke with inlet valves. Both inlet and exhaust are open at the same time and fresh air is swept through the cylinder so it is full of air. There cannot be "too much" air for the amount of fuel injected.

High boost pressure from a turbocharged engine can only occur at high power settings because that is the only time there is sufficient fuel, heat, and mass of gas available to spin up the turbine to produce boost. You can't have high boost at low power so of course EGT goes up with power. EGT is used as a measure of power output on a diesel, and it can only go up as fuel input goes up.

"IAT" that's the air temperature measure on a gas car engine, right? Maybe that's related to the idea that a diesel exhaust gets hotter with a "lean mixture." On a diesel, inlet temperature is measured at the air intake, before the compressor and if that temperature goes up, power goes down and EGT along with it until an extremely high temperature is reached. If you mean charge air temperature, the air temperature after the turbo and charge air cooler (if one is fitted) then yes, engine and EGT will increase but this is a separate phenomenon than high EGT resulting from lean burning of the fuel.

Lean burn in a diesel means cooler temperatures because there is less fuel to heat the air in the cylinder.

Marmot- Of course there are not intake valves, but you implied there was. Most 4 stroke diesels tend to be low rpm and there is little valve overlap (in comparison) - otherwise you would lose the compressed intake air. DD and other 2 cycle diesel engines need the scavaging effect (supplied best by the supercharger). Of course you can have too much air in a diesel- and compressing air increases the intake air temp (IAT) (combined gas laws) this is why we have charge air coolers- also called intercoolers or aftercoolers (depending on a variety of factors). You only need a bit more than enough air to burn the fuel- any more than this will needlessly heat the air and compressing air take energy. High IAT's and high boost pressures can also certainly occur with lower "power settings"-(as you say) the amount of boost is not set by the IP or injectors- but by the turbo/SC and it's systems. One of the major issues of DD engines is burned exhaust valves.
The original poster wanted to know if this was possible and I replied it was not cheap and easy. He will decide in the long run if this suits his needs, and that is depended on the price of fuel, the work/cost/time to "convert" and if needed "reconvert". If there is next buyer of the boat most likely they will like to be able to restore the engine's previous horsepower and the boat's capabilities. The survey of the boat during the sale on both the engines and the whole boat will show that the engines have been detuned, the boat does not reach the manufacturer's stated speed, and be a cause for questions to arise if the boat can in fact be restored to what it once was. All these things need to be taken into consideration. Despite my earlier quote, I'm not confused about DD engines, nor diesel engines in general. I lived in Micronesia for a decade, and I learned there to give a person a way out first. I'll stop there to conform to the decorum of the forum.

What on Earth are you talking about Bamboo? Turbocharged engines use valve overlap to provide better scavenging, timing of the exhaust valve closure combines with charge air pressure determines the weight of air captured in the cylinder. This applies to 2 strokes as well as 4 strokes.

Wow ... where did you get that information? And in comparison to what?

Your use of automotive terms indicates to me that you are still confusing gas engines with diesels. What the heck is IP? Tell me how you can get high boost pressure at low power settings on a turbocharged engine?

So did I but you keep getting further into a load of nonsense about how a diesel works. So for you claim a diesel can have too much air for the amount of fuel and that will cause high EGT; a diesel can suffer from high boost and high temperatures at low power output; reducing the capacity of the injectors will lead to high EGT because there is too much air. On top of that you say that most 4 strokes "tend to be low speed," which is nonsense, and that valve overlap is reduced on a turbocharged engine while quite the opposite is true. That is just plain wierd stuff. And if you inferred that I think DD 2 strokes have intake valves you are really off base my friend. Having operated DD 2-strokes from 2-71s to 16-645s and nearly everything in between, I think I know the difference.

I don't work on car gas engines. IAT means what I posted- regardless of the engine type. The comparison is to two strokes. Low rpm means just that- as opposed to high rpm. 5000 RPM diesel is a high rpm diesel. Sorry I omitted "larger" before the 4 stroke in my post. (IP)= Injection Pump. Power is determined by fuel and the air it burns- once all the fuel is burned adding more air does nothing for power. Boost pressures are not set by the injectors or the injection pump. Saying that higher boost pressures do not raise egt's is false. DD's need more scavaging than 4 strokes- and only a few non 2 strokes have a SC. You introduced the intake valves in a discussion limited to DD's. I said nothing about turbocharged diesels needing less overlap- I said 4 strokes need less. None of this helps Mr. Bevins in his questions stated at the start of the thread- so let's keep it on topic.

I'm not getting into the middle of diesel speak. But an 8v71 TI at a given RPM is going to ingest exactly the same amount of airflow regardless of which injector is in it. It will also going to spin the turbo just as much.

But, if he converts the boat he will have all of the old injectors which can be put right back in, in about a days time for both 6v-71's and adjust the rack. It would not be a major expense to convert the boat back. As for the loss of speed, most people won't even notice the knot difference or so on a boat with some age. As for the props, if you have a spare set you can just leave those with the normal pitch and throw them back on......All in all to swap back is not a big deal. The boat I was on, sold twice in that configuration and was welcomed and not an issue with either buyer. In fact I still talk to the last owner on a regular basis and he loves it and wants me to run it from Charleston, SC. to St. Martin this spring. Another thing is there was never any fuel smoke or any smoke on cold startup no matter what the temperature was outside. Those honestly were the best running set of detroits I ever ran when it came to throttle response, cleanliness and everything else and I've run at least 500 different sets of detroits. Let me take that back, the DDEC's I have ran, ran exactly like them and those were non-ddecs