More diesel efficiency discussion

This is perfectly on topic.

You told the guy that he would have to have his DD injection pump adjusted after R&R'ing his injectors or he would risk having high EGTs because there would be too much air because the turbo is the same size and the fuel quantity is less. That is the most amazing nonsense I have read in a long time.

Would you like to tell us more about the "IP" on a DD 2-stroke? And then tell me how you put more air in after burning all the fuel? If you are trying in some way to say that there is no need for more air than is required to combust the fuel then you are way wrong again. One of the reasons a diesel is so efficient is there is far more weight of air than is required to merely burn the fuel. Again you seem to be thinking about the way a gas engine works, where a very specific fuel/air ratio is required to produce power.

If "SC" means supercharger, a turbocharger is a supercharger when it raises the pressure of the charge air above atmospheric pressure. The reason you can get more power out of a diesel with a supercharger is because you can burn more weight of fuel in a greater weight of air. The blower on a DD doesn't deliver much boost, it is a scavenging pump designed to provide air at a pressure high enough to overcome exhaust backpressure to scavenge the cylinders and that is about it.

The topic is reducing the size of the injectors in order to operate more efficiently at a lower power setting by providing a "cleaner" spray pattern, and in effect, to derate the engine. There is some merit to the idea but if it is really cost effective is debatable. Having problems with high EGT resulting from injecting less fuel is certainly not a factor. Getting "crisper" performance" from the same turbo is also very debatable because with less mass of exhaust and consequently less energy in the exhaust the larger turbo simply won't be as efficient as it will be operating outside its design range. Marooned on a different island if you know what I mean.

The purpose of scavenging is to remove all the exhaust gas from the cylinder. Once that job is done, overlap is used to cool the exhaust valves. It doesn't matter if it is a 2 or 4 stroke engine. If the designer wants supercharging and not just scavenging, he closes the exhaust before the inlet to increase the weight of the air charge.

If by SC you are trying to say mechanical supercharger, then you are correct, there is little point in wasting power to drive a mechanical supercharger on a 4 stroke marine diesel. They are a waste of power on a modern marine engine. Two stroke engines need some sort of scavening air supply at loads below that at which there is sufficient exhasut gas to drive a turbocharger, that is why the large EMD 2 strokes use an over-runing clutch to mechanically drive the centrifugal compressor until enough exhaust mass and heat is available to drive it with the turbine. Mechanical superchargers work great on cars and other applications that require frequent large load changes. That pattern doesn't often apply to marine diesels.

There is some sort of wierd circular logic there ... Boost pressure is designed into the system. A certain gas flow through the engine at rated power will deliver a certain charge air pressure at a certain temperature to deliver the weight of air required to burn the weight of fuel needed to produce the design rated power.

With fuel flow less than at rated power, the temperature and mass of the exhaust will not be sufficient for the turbocharger to deliver the designed flow of charge air. In this condition EGT will be lower than at rated power. The more fuel that flows, the more exhaust gas is produced at a higher temperature and the more charge air is delivered, those parameters are linked and one follows the other. You could easily say that higher EGTs create higher boost pressure and not the other way around. They are inextricably linked until the design parameters are exceeded one way or the other.

There is no magic way to increase charge air flow without adding fuel so of course EGT goes up, like I said before, it is a measure of engine loading. If in your imaginary engine there was some external means to increase "boost" without adding fuel then of course the charge air temperature would go up if the aftercooler was inadequate, but as it went up its density would decrease accordingly, and since we have less fuel in the cylinder along with less weight of air the EGT still won't increase. Maybe you know of one but I don't know of any production engines that have some magic device to independently increase boost pressure at low power settings ... I mean, what is the point?

Some of this info is false.

A supercharger is not the same thing as a turbo. A supercharger is gear driven and then either turned by the motor with gears or with a belt. A turbo is only turned with exhaust airflow. They are not the same and you are right a turbo charger is more efficient then a supercharger in most all cases.

EGT can also be lowered with more fuel or excess fuel. If the motor is tuned towards the rich side, meaning more fuel then what is needed for the combustion of all of the air in the cylinder, the excess fuel will also lower the EGT as does lowering the load factor at a given rpm. They run excess fuel in racing turbo charged diesel vehicles all of the time to lower EGT.

Gimme a break ...

A supercharger is a device to increase the weight of charge air in a cylinder above that supplied by "normal aspiration." Hence the term, "super" which means above or better. A supercharger can be mechanically driven or driven by a turbine as on a turbosupercharger. Understand the concepts before you start nitpicking terms.

I never said tuirbos were more efficient in most all cases. Each has its place.

Oh geez, here we go. Discounting the theatrical performance of pulling tractors, the concept of a rich mixture cooling the combustion is a gasoline thing. In a normally operationg diesel engine adding fuel will increase power output and exhaust temperature up to the "smoke point" beyond which poor combustion reduces power output, wastes fuel, may dangerously increase EGT, and pours out large volumes of black smoke.

Cooling the combustion process with additional fuel is not a design feature of a diesel engine. Extracting the maximum energy from the minimum fuel is.

Capt J I applaud your efforts to assist MBevins. Marmot your argumentative tone is unbecoming and a detriment to the thread and forum IMHO. (In my humble opinion). You have contributed almost nothing to MBevins quest for answers. I'm sure now we will all hear another diatribe from you.

Perhaps. But is is far better than to tell MBevins that he will need to pay someone to adjust the IP on his DD to avoid high EGT if he uses smaller injectors. Unbecoming, but at least it is not misleading and inaccurate.

As CaptJ told him, just put in smaller injectors and leave it at that. You sir, were the one who came back with a lot of nonsensical prattle about how that would negatively impact his operation.

You have my most sincere apologies if I offended your gentle sensibilities but I suffer fools poorly, especially when they offer bad advice.

I notice that MBevins seems to have left this discussion about the same time the snippyness began so all the debate might not have helped him so much.

Capt J also said to adjust the props- which I pointed out to do it correct means a haul, and at least one seatrial. One day of shop time, one haul & install props, plus divers pulling the props to get them to the shop (or two hauls), prop shop costs, injector costs, plus anything else- I told MBevins to consider these. The last survey I drove for the vessel was two knots under stated speed and the deal was off until it made that speed- which meant two more hauls plus prop shop work. This made the buyer suspect and lowered the final sale price- certainly more than the savings in fuel MBevins will realize. I also said IP instead of rack. You came back with intake valves. You offered no advice to him- just pissing match comments on other's posts on subjects that were of little substance to the main issue. If you thought EGT's were not going to be an issue- could you have just said "don't worry about high egt's with smaller injectors"? yes- but you chose to be a@@ instead. Fools such as yourself often think highly of themselves, and yet you added nothing of substance to the main issue here.

I agree.

Cutting the props aren't that big of deal after the change, do the seatrial record the top end numbers and bring the numbers are propellor size to a prop shop and they'll usually get it right there. It's about 1" pitch for every 50 rpm's you want to gain usually. Divers remove props here pretty reasonably in south florida for a boat his size.

Yep divers do. I have had a set that took the divers 6 hours to pull and they broke three pullers (cheap ones I assume), however. I bought a 4" propsmith for the boat so that I would not have that problem in the future. Sometimes even on land with a prosmith you have to use heat and they still don't wanna budge. I worked for the Viking Service Center for three years and have pulled/installed over a 1000 sets. All most likely a non issue on the boat in question. MBevins probably has 2" to 2 1/2" shafts, and they should provide no issues. I hope we hear from him in regards to if he changes injectors, how it performs, and after some time he reports as to if in his case it was worth it in his view.

Two questions, one simple answer and one very subjective answer.

Reducing the capacity of the injector reduces the maximum fuel delivery in a given time and therefore the maximum power available from the engine. So, in a one word, yes, you will reduce fuel consumption at full throttle.

What are you giving up? In a word, horsepower. Horsepower and all that goes along with it. You can achieve identical results by just throttling back. That reduces the amount of fuel injected and therefore reduces total fuel consumption, speed, and acceleration.

If you reduce the size of the injectors you will, in effect, limit the maximum power output, what the boat driver sees as full throttle setting. Doing so of course reduces the maximum speed of the boat and the rate of acceleration at full throttle.

There are reasons why just reducing the throttle with your current injectors has a downside. Most mechanical devices are designed to perform best at a certain rating or setting. The injectors supplied with your propulsion system are designed to flow the amount of fuel required to produce rated power at full throttle. The nozzles are designed to atomize fuel most efficiently at the pressure and flow developed at that power setting. That is the optimum configuration. Any variation from that point will produce less than optimum performance which might be indicated by engine roughness, noise, vibration, or sooting, all due to less than ideal fuel delivery which results in less than ideal combustion. Those conditions lead to higher fuel consumption relative to the amount of power produced i.e., brake specific fuel consumption.

You can avoid many of the problems associated with poor fuel injection characteristics at low throttle settings by optimizing the injectors for that power setting. Since your injectors are “unit injectors” and combine the fuel pump, metering, and atomizing in a single unit, they are optimized for the flow required at a certain power output and rpm.

Now, this is where it gets into the very subjective aspect and pocketbooks and opinions color the discussion. You can very simply change to a smaller injector and change nothing else. That will give you better low power combustion characteristics at the lowest cost for the amount of fuel saved. If you are happy just cruising along at part throttle, then that may be a very attractive alternative. Just realize that you are gaining a small savings in fuel by giving up maximum speed and acceleration but may gain a cleaner exhaust and smoother running.

When I wrote that injectors are “… optimized for the flow required at a certain power output and rpm” that statement is particularly applicable to DD unit injectors as they are mechanical devices which create a pressure rise relative to the speed at which they are operated. The rate of pressure rise and the pressure above the valve which “pops” to allow fuel to reach the nozzle has an effect on the spray pattern and degree of atomization which is one of the factors that determines the quality of combustion.

Gains are made in combustion quality by sizing the injector to suit the power desired and incrementally smaller gains are made by altering the propulsion system to allow that smaller injector to operate at the original rated speed.

Whether it is worthwhile to you to spend a great deal of time and money to change propellers to allow the engine to turn up to the original rated rpm at a lower boat speed is completely subjective. In my personal opinion, it is not worthwhile. It is poor value returned for considerable expense and the risk of repeating the exercise to “get it right” when “right” really doesn’t give you much more than pulling the throttle back or resizing injectors will do.

Engine and propeller shops will be very happy to help you optimize your propulsion system. They will do the right thing for a price and make it all match. Does your operational profile indicate the return on that investment is rational from an economic viewpoint? Only you can answer that.

As an engineer, and boat owner, I would install the smaller injectors and keep the old ones preserved for the next owner who may not share your operational desires. The high cost and experimental nature of re-propping is not, in my opinion worth the tiny amount of additional fuel saved in an obsolete engine that is not known for its fuel efficiency in any event.

The "B" means "bypass blower." The discharge from the turbosupercharger is valved so as to bypass the blower rotors above a certain power level and feed directly to the airbox. This reduces the load on the mechanically driven blower as well as providing more charge air to the cylinders for more power.

After reading this post/inquiry I'd say Marmot knows his DD 's

Hi,

Read a few more threads, this YF Member has a wide ranging knowledge on a lot of things and isn't afraid to point out others mistakes.

I can't believe I missed this thread, must have been asleep for a few days.

Thanks K1W1, your check is in the mail.

Reading back over the thread I did find two overlooked statements that deserve comment, one by CaptJ and the other by me.

That isn't quite correct because while the blower may be a near constant displacement pump, a turbo is not. As the load on the engine increases the speed of the turbine will increase and the weight and volume of air it delivers also increases. When charge air pressure (density) increases, exhaust gas volume also increases and it is far above what the engine would "pump" if it were operated at the same rpm with little or no load.

The second statement was mine about not knowing of any engine that used some external device to produce higher charge air pressure at low loads and there is one, the mention of which usually just annoys people. Large slow speed 2-stoke engines normally incorporate an electrically powered centrifugal blower to provide scavenging air for starting and very slow running. Both conditions where the turbochargers don't produce much if any airflow.

The second statement was mine about not knowing of any engine that used some external

Marmot,

I am not sure if you know this but Volvo Penta uses belt driven roots compressors as superchargers for certain models of their engines. This has been going on for many years as the TAMD42 4-liter may have been the earliest (parasail boats love this). But now certain D4 and D6 (4 and 6 liter engines) have this option depending on the application. Certain stern drive models and many IPS models have this. The acceleration is very much like that of a gasoline engine with little to no lag on throttle advance.

Hi,

The VOLVO setup is in conjunction with a normal Turbo Charger and does not have any effect on the engine at low load ( this is the area of operation MARMOT was writing about) as it is controlled by an electric clutch and from memory comes into play at around 1500 RPM and above.

I first encountered the VOLVO setup with a pair of KAD 44;s now we are building a new trnder with a pair of D6's so will be able to see what they are like in a while.

Will be interested in hearing how they perform. I am not a Volvo guy (aside from driving one) but my limited knowledge the 40 series is that the blower provides scavenging and a bit of boost up to a fairly high power then cuts out as the turbo provides sufficient flow to maintain a high power output. Given the type of hulls these things live in I figured they were made that way to provide rapid throttle response then get out of the way when the turbo shows up, much the same as the DD bypass system only with less parasitic loss since the blower is clutched in and out by the ECS. Larger DDs (EMDs) use an over-running clutch on a centrifugal blower/turbo to do the same thing but the changeover is purely load based rather than rpm which is a calculated point that depending on propping may or may not leave some efficiency on the table.

Volvo use of compressors

K1W1 and Marmot,

Sorry for my misunderstanding and not following the post better. I also have not had a lot of exposure to the 1,000HP and up class of engines as you have.
The original 4 liter 42 series with a compressor was engaged based on RPM, but the KAMD44 had a computer with a boost sensor to aid control. Sort of an electronic smoke limiter perhaps? The compressor usually provides about 10psi of manifold pressure as you start to accelerate, and then cuts off as you both were aware when the turbo is up to speed.
I hope the D6's impress you K1W1 with common rail and (I am guessing stern drive?) compressors. There are some D4 and D6 models without compressors (cost and application) but not many. Pretty quiet engines and no smoke at all.
I suppose you might know the Lazzara 92' with the four D11 IPS 2 units have compressors as well? Besides the quick acceleration, they help move the boat quickly in joystick mode as well. No bow or stern thrusters are used.