Would changing to smaller injectors save fuel?

[Eventide]

See if this makes any sense: my DD 8v71 Naturals have N70 injectors. I believe that N55 injectors are also available for these engines. Now, '55' is 21% less than '70' arithmetically. Would it therefore follow that I might get 21% more (better) mileage if I changed out to the lower rated injectors? Apples for apples, I cruise at 1400 RPM, so would I likely get 21% better mileage if I stayed at the same RPM using the N55 injectors? Does this make any sense? Thoughts? Thanks.

 

[Boss Lady]

Fuel demand is directly related to load demand. You cannot cheat physics. Your fuel burn will be identical. You will just have to push the throttles further forward to get to 1400 rpm and then you will run out of throttle should you need full rpm. Diesels do not have throttling loses like a gas engine does. You can get better mileage on gas engines playing around with carbs and fuel injection throttle bodies, but this does not work on diesel engines. Gas engines control the amount of air that enters the engine, diesels control the amount of fuel. Distinctly different mechinisms.

 

[Pete]

Boss Lady, agree with everthing you noted, but have one question. Given the variable speed governors on these DD would you not advance the throttle controls to the same physical position for 1400 RPM, and the governor would then open the injectors more to get the required fuel?

Pete

 

[luckydave215]

The rack would need to advance more to "open" the smaller injectors farther. It still results in the same amount of fuel, and the amount of fuel is what counts.
Like Chris said, doing this would limit your ability to use full rpm if you needed to, it would act in the same way as installing a mechanical limiter on your "throttle" handles.

 

[Boss Lady]

You would run out of available fuel adjustment before you get to 2300 rpm, with all things being the same, e.g. props, hull weight, etc. since the engines can no longer make full rated power (I think these make 330 HP with that injector) with the smaller injector size, he may only get to 1800 or 1900 rpm under load. Either way the rack runs out of room.

 

[Pete]

Let me try again. I am just trying to verify my understanding of how our governors work. If you change to a smaller size injector then the injector will need to "open" more to deliver the fuel required to achieve any given RPM. Since the throttle control at the steering station is just an "RPM request" to the governor, you would put in the same position as you always did for a given RPM. The governor would then open the injectors to the point required to achieve the requested RPM. Thus, the only difference seen at the steering station would be at some point pushing the throttle further forward would not do anything since the governor would already have the injectors fully open.

Pete

 

[Paul45c]

If you were fine with lower speeds, though, and detuned your props to make full rated rpm you would definitely save fuel.

 

[Brian Degulis]

All previous posts are acurate in a real world terms you proubably would not notice any differance in fuel consumption. There may in fact be a very slight improvement caused by the smaller orfice in the 55 injectors they do atomize a little better. If you were to go from 90 injectors to 55 you would see a cleaner runninnig engine but even then the fuel savings would proubably be so slight it would only be noticable in a labratory.

If your looking for maximum fuel economy and you don't care about reducing the maxiumum HP the engine will produce. Then you could de rate the engine by re setting the governer. If your currently set for 2300 max RPM you could re sett to 1800 RPM. Then spec out a prop for the new lower RPM and lower HP the de rated engine will call for a prop of larger diameter or greater pitch. You will gain more by increasing diameter but you have to have the hull clearance to do that if not adding pitch will also work just not quite as well. The result will be a lower RPM for the same cruise speed and an engine closer to rated load at that speed. With this you will see a noticable savings in fuel consumption.

Brian

 

[Eventide]

Thanks guys. What I am trying to accomplish is the best fuel economy possible running my 50' Yachtfish at about 9 nmph. I don't care about going real fast, and if I have to outrun weather, or whatever, then jumping up to maybe 13 nmph is just as good to me as going 18 nmph (current top end, blowing $100 dollar bills out the exhaust with fuel consumption).

I am generally OK with trawler speeds and am looking at the next five to fifteen years when it looks like the forces of darkness are going to try to force the price of diesel up to European levels. I know that I have unnatural levels of masochism in me as it is but if diesel goes toward $5 a gallon then I won't be able to mentally tolerate it and my beautiful Hatty will be gone/sold and I am then looking for a single engine trawler. That entire situation would make me miserable so to stave it off as long as possible I want to be able to extract as much economy and mileage as I can from my DD 8v71Ns, hence the question about changing the injectors.

I do appreciate all your input and I am wondering if the post about de-rating the engine, maybe using smaller injectors and re-setting the governor / rack and changing the props might lead me in the direction of accomplishing my goal - which in the long run might enable me to keep my Hatty.

Again guys, great responses. I love your ideas. Please keep them coming.

 

[REBrueckner]

Brian says: "...The result will be a lower RPM for the same cruise speed and an engine closer to rated load at that speed. With this you will see a noticable savings in fuel consumption..."

If this means there would be some (minor, I think) improvement in fuel efficiency due to the somewhat greater efficiency of a larger diameter prop, that sounds good. But to go 9 knots the engine would still need to develop the same horsepower as before except for the "slight" improvement in prop efficiency.

I swing 28" x 32" props with a 8V71TI...even if I could get another 2" diameter, and I don't have enough clearance for that, that 1/14 diameter increase gave a 1/14 improvement in efficiency, it would not be much...unless somebody was running lots of annual hours....I guess it might be 5% more efficient???....anybody know more about prop efficiency???

And another question: If I take off my turbos, cut my injectior size...ADJUST GOVERNOR...ETC...I'd have to reduce pitch OR DIAMETER...does that make me less efficient?? How can that be?? ... I find that hard to believe given that a prior long thread suggested turbos was 10% more efficient...

 

[Genesis]

I wouldn't touch the engines.

If you want to run at a somewhat-higher RPM, de-pitch the props. That will accomplish unloading the engines while leaving the reserve there, and its cheaper to put the props back than it is to put the injectors back.

 

[Brian Degulis]

Actually if you adjust the governer to a lower RPM you would actually require a larger prop not because the HP is reduced but because the prop RPM is reduced. Remember if you double the prop RPM you must quadrouple the HP to drive it. Of course if you don't change the RPM and reduce the HP then the prop must be smaller.

On de rating to a lower RPM you not only get the efiecency of a larger prop you also run the engine at a lower RPM for the same cruise so you get some savings there remember an engine uses fuel even when it's just running itself like idle neutral.

The other gain is that de rating or lowering max RPM puts the engine closer to rated load thru the entire RPM range. Running at or near rated load gets better efiecency than being grossely under loaded as is the case with most of our boats at displacement speed.

This whole idea is very misunderstood but if you look at the prop and engine power curves you can see how it works. My last boat was a 55' trawler with 671 naturals rated 240 HP @ 2300 RPM I derated to 200HP @ 1800 RPM. The original prop size was 27X24 if I remember corectly? And the derated engine called for a 30x22. When I did that my savings in fuel @ a 9 knot cruise was just under 2 GPH. My cruise RPM went from 1800 to 1450 so it was quiter and hapier at the lower cruise RPM. Of course the downside was I lost the use of 40HP per engine. That didn't matter as it was HP I couldn't or wouldn't use anyway.

It's basically customizing and engine for your intended use.

Brian

 

[Boatsb]

If fuel = energy than the lower amount of fuel would = less energy. Just running at the lower RPMs will save fuel and even though there may be a very small savings in fuel use at a slow speed I don't believe it will offset the cost of the injectors in the life of the engine. Add to that the etra costs of re tuning, Maintaining a non stock engine and I see a loss. If it can be de tuned to another version that was avaialable itmay be worth it but based on pure physics I see not savings coming that way.

 

[Boss Lady]

If you intend to only run the boat within certain parameters, then really look at the most efficient set of props for your intended rpm range and power production within that rpm range. To do this right, you will need to install the smaller injectors and retune, then install larger diameter or more aggressive pitch props to fully load the engine at the new WOT rpm and cruise rpm. This will be the same as installing smaller engines if you derate the HP of the engines. You will get better mileage, but it is mainly from the more efficient props. To be honest you could skip the smaller injectors and just reset the govenors for the lower rpm and install wheels that will fully load the engines at the HP/rpm. Start by looking at the HP/prop load curves for the 8v71Ns that you have. To the best of my memory the curves look something like 120 HP available at 1400 rpm and prop loading is only 80 hp, so you could pick up a couple knots of speed but would never reach the old WOT. This would be the cheapest route to go, and you would only be out the cost of the wheels, and if you don't like the result then just sell them and put the old ones back on.

 

[Pete]

So far in this discussion, the engine cooling system has not been mentioned. The amount of cooling sea water going through the engine is directly proportional to the engine RPM. More horsepower at a lower RPM equals more heat with no additional cooling water. Detroits do not have excess cooling capacity built in so this is another factor to consider. Engine loading on the propeller curve is what keeps everything cool at cruising speed. Loading at maximum available horsepower at any given RPM will give the same marginal cooling results seen when running at WOT on the propeller curve.

Pete

 

[Boss Lady]

Good point Pete. However on these Naturals, the power produced at 1400 rpm is 1/3 the rated power, so even if fully loaded you cannot overheat them. I guess the amount of raw water being pumped should be verified just to make sure, but I do not see a problem IMHO.

 

[REBrueckner]

I'd really like to understand the correct answer to the original question better.

Here is what I think I understand:

(a) Original question: Fuel consumption at the same cruise/hull speed will not be reduced by 21% just because the injectors are that much smaller.

(b)To maintain prior cruise speed with existing props, cruise RPM would have to be increased if smaller injectors are installed.

(c) Larger props are more efficient than smaller.


Brian's post about his trawler (#12) is very interesting because he ended up swinging larger props at lower RPM. Was cruise/hull speed maintained. I can't explain this;it sounds like magic! What does "detuning" mean here?

I'd like to hear more since in general just using smaller injectors should require higher RPM to achieve the old cruising/hull speed with even the same size props, let alone larger ones.

 

[Genesis]

Using smaller injectors with no other changes will do very little to change fuel consumption. You will still need the same RPM and you will still produce the same horsepower - the injector helix will just be further advanced.

If you make other major adjustments (e.g. less pitch in the wheels, etc) you may be able to get the BSFC curve into a more advantageous range, which WILL improve fuel consumption for distance travelled. To know, you'd have to have a BSFC map .vs. output and RPM - I don't have 'em for that engine.

 

[Eventide]

Great feedback and great ideas, guys. Thank you very much. I think I love what Brian did with his former trawler, de-rating his 6-71s and changing out his props. Doing that he improved his fuel consumption by about 2 gallons per hour, which is significant over a long distance. Thank you Brian!

If there was some way to do that with my 8-71Ns, or maybe even reduce fuel consumption a little more since they are larger engines, it would be great. I would also be happy with lower RPMs, like Brian, and with maybe going a little slower, perhaps 1.1 times hull speed over long distances, or about 8 nmph.

In other words, I'm very flexible about how I get there if there is some way to materially reduce fuel consumption. I have so much invested in this boat in terms of systems, effort and love that I want to do whatever it takes to keep her until I start drooling, at which time I want to be pushed over the stern into the Gulfstream to be part of the food chain so I won't have to go into a nursing home and into that long, tortured decline toward the black abyss (but that's a whole 'nother topic, I visit the nursing home every day and have for the last 15 months and it is truly a living nightmare).

Again, thanks for all your thoughts.

Of course, per Karl's post, this might all be for naught if we did a really hard cost/benefit analysis, but that is for another day. Right now I am getting a terrific education and we're all just brainstorming.

 

[Brian Degulis]

I think I would re set the governers for a lower max RPM I went from 2300 to 1800 because 1800 is the work boat continious duty RPM. But you could go even lower just remember the lower you go the larger the prop has to be (you need the hull clearance) and the more you reduce the usable HP of the engine.

I wouldn't go to the 55 injectors the improvement from that would proubably not be measurable.

Brian