Diesel engine/prop/hull efficiency questions

That is true, however the turbo won't spin fast enough to produce enough boost to create additional horsepower until 1300 rpm's or so on most engines.

What exactly cools the Intake Air? The intake air is cooled off by an intercooler. The intake air coming out of the turbo is cooled by an aftercooler before it is ingested into the combustion chamber.

Do you think that this might somehow be related to the amount of fuel being burnt in the hours those run versus the amount burnt in a NA Engine?

I think fuel consumed has a good amount to do with how long an engine lasts but is not the entire factor. With a turbo, the more boost you cram into the engine, the more static compression, and the more stress you are putting on parts. Plus, if anything goes wrong on a turbo charged motor like an injector that doesn't open the less margin of error you have versus on a natural that's not producing as much compression and therefore heat in the cylinder. You cannot take most natural engines and put a large turbo on it and expect it to last a month. Many internal parts are made stronger to handle the turbo; connecting rods, pistons, valves, crankshafts, even the engine block. It is definately not linear, I have seen charter boats that always run low rpm's easy, and their engines definately don't last nearly as long as the boat with naturals next to them. They burn about the same amount of fuel per trip as the boats next to them because the one with naturals has to turn more rpm's to go the same speed. Also, in the older non-electronic controlled engines, the turbo engines have considerably larger injectors then the naturals and at low RPM's they don't meter the fuel as well and run as clean as a natural and in a pleasure yacht application a lot of time is spent running low rpm's. Also they don't stay or get up to proper operating temperature unless they're running higher rpm's creating more wear.

It came from airflow research's website which makes a good amount of turbo's that you do see in marine engine applications such as older detroit diesels. Yes it is explained in regards to gas engines. BUT, the concept is totally the same of how a turbo works and what it does for an engine. Yes the boost figures are different for gas engines and diesels and diesels generally run more boost (which doesn't necessarily equate to more HP, it is not linear). Boost is the amount of back pressure in the intake tract not airflow. If you increase the efficiency of the airflow in the intake tract of the engine, boost will actually go down (everything else equal) because more air can travel more efficiently into the motor. Also the more boost you create on an engine, the more heated the air intake charge becomes and you get to the point of where a bigger turbo does not create anymore HP then the smaller one.

None of the modern high output high speed diesels I am familiar with use Intercoolers. They all do use Aftercoolers and not all of these are directly sea water cooled.

A Turbo ( when the Turbine is turning fast enough to drive the Compressor at a speed at which it can compress air )provides combustion air at a volume that makes the Inlet Tract pressurized to somewhere above the atmospheric pressure. It is extra air that is being forced into the Cylinders not boost. Boost refers to the increase in pressure that is generated by the
turbocharger in the intake manifold that exceeds normal atmospheric pressure.

Static Compression is the ratio between the volumetric contents of the whole cylinder with the piston at BDC and what's left when the piston is at TDC. This is a mathematical calculation and will not vary depending upon the turbo's performance.

The Dynamic Compression Ratio is what varies with the varying charge air volume and fuel load.

If you lose an Injector ( doesn't open)on a Turbo charged or Naturally Aspirated engine you lose an injector ( other than splitting the end of a DD 2 Stroke injector)you will normally also experience a misfire and suffer a reduction in power. There will not be some catastrophic failure for a simple unit failure like that. If you have a tip go from water in the fuel or similar your real danger is filling the Oil system with Fuel and having a crankcase explosion. This can happen to both Turbo and NA EAngines.

I would tend to agree with this

Fuel metering with mechanical unit injectors and mechanically controlled delivery valves have always been variable at best.

Running the engine cold will always cause premature wear to components.

Would that be the same as Airesearch a division of Garrett?

Boost as I have explained above is, the increase in pressure that is generated by the turbocharger in the intake manifold that exceeds normal atmospheric pressure.

What I was referring to with engines running cold is, in the US a lot of yachts will rarely see cruise RPM's..... You could let a DD 671TI-16v92TI as well as others CAT 3208t's, cummins 6bta's etc etc. , idle at the dock all day and it will never see past 140 degrees, you could let it warm up to 140 degrees, then take the vessel out all day and never see past 1,000 rpm's and 140 degrees water temp in the Intracoastal or trolling in the ocean, and the only way those engines will see proper operating temp is if run at cruise. Whereas the same natural engines would be run a little higher rpm-wise, as well as smaller injectors that are running a higher duty cycle (say 20%, versus 5% on the turbo motors (for example!) and they tend to meter fuel better the higher the duty cycle, and the naturals tend to run closer to operating temperature......180 degrees in most cases..... so in general yacht usage this is more of a reason the naturals will last much longer.......

"...as well as smaller injectors that are running a higher duty cycle (say 20%, versus 5% on the turbo motors (for example!) and they tend to meter fuel better the higher the duty cycle,..."

Where on Earth are you getting this stuff?

Various Factory Certified CAT, MAN, and Detroit Diesel mechanics over 25 years of experience as well as over 300 engine startups on new vessels and 1000's seatrials with used vessels with factory trained engine surveyors. Where are you getting your stuff?

Let's use a hypothetical situation so you can get a real idea of what I am talking about. NOW, this is just an example. Let's say we have two different 1980 45' Hatteras SF's. Vessel #1 has 8v71 naturals with the factory N70 injectors (70 liters of fuel per hour), Vessel #2 has 8v71 TI's with the factory M110 injectors (110 liters of fuel per hour). Both vessels are going trolling for the day and want to do 7 knots.

Vessel #1- is at 1000-1200 RPM's due to running less pitch propellors and for example running a load factor of 25% to make 7 knots , considering a natural has a fairly linear RPM to power output ratio across the graph, the injector is probably running a 25% duty cycle. It is also running at 180 degrees because it is at a higher rpm and load factor then vessel #2. Cruise speed at 1950 rpm's would be 15 knots

Vessel #2 is running at 700 rpm's to maintain 7knots because it is running more propellor pitch because then vessel #1 because it makes 40% more torque and horsepower on the upper half of the RPM range. Vessel #2's injectors are probably running no more then 10% duty cycle because they are much larger, load factor is much less at 700rpm's, and the turbo is not spinning fast enough to greatly increase airflow and horsepower. Vessel #2's RPMvsPower Output ratio is not linear and greatly increases after 1400 rpms as the turbo boost starts increasing, so the injectors wouldn't start really running over 20% duty cycles until the rack sees a decent amount of boost/load usually around 1400rpm's on DD's. Engine temp would never exceed 150 degrees ALL day long, until rpm's have come up over 1,000 rpms. Cruise speed at 1950 rpm's would be 23 knots.

Can you now understand how this works? Both situations are pretty close to what you'd see in real life on a 45' Hatteras equipped with those engine combinations.

Hi,

If you have the correct temperature thermostat there should be no way you can run your engine at 140 Deg F even if it is idling.

I second the previous posters question as to where some of the statements you are making concerning this facet of Marine Engineering really come from.


And by the way in the second line of your second paragraph of the post above--- There is NO such thing as a naturally aspirated 8V 71. They all have a Roots Blower

It has been more than 25 yrs since I had an 8V 71 apart but seem to recall that the Injector number was a capacity in millilitres not litres.

I certainly hope you know more about the duties and responsibilities of being a Captain than you do the engines that push you around for 15,000 N.M. a year.

What happens when you get to 15,001 miles?

I know they ALL have a roots blower, but the blower is the same displacement as the engine it is on, so it does not produce any additional airflow. Any of the detroits that are not equipped with a Turbo, are referred to as "Naturals" by Detroit Diesel Corporation.

Post another topic "What water temp do your 2 stroke detroits run at, below 1000 rpms" and ask other captains whether any 2 stroke non-electronic detroit diesel reaches 180 degrees under 1000 rpms. Actually I will do it.

I have run tons of detroits (as well as other mechanical motors in yachts and electronic diesels) NONE of them do, this is with the proper 180 degree thermostats. They never reach operating temperature in a yacht under 1,000 rpms unless they have an ill-maintained cooling system (worn raw water impellor, clogged heat exchanger, etc.) where it would overheat at higher rpm's. Over the road trucks put a piece of vinyl over the grill in the front of the truck during the winter to block some of the airflow over the radiator to maintain operating temperatures.

Marmot and K1W1, have either of you operated ANY Detroit Diesel or other mechanical diesel in a yacht? Do you have any real world experience with these engines in operation in a yacht or sportfish?

So you think the Blower displaces 8 x 71= 568 cu in?

I would say the answer to this is yes.

"... have either of you operated ANY Detroit Diesel or other mechanical diesel in a yacht? Do you have any real world experience with these engines in operation in a yacht or sportfish?"

Rather a substantial amount in both yacht and commercial applications from 2-71s up through the largest EMDs over a long period of time.

If you are finished trying to smokescreen the issue with a bunch of rubbish about duty cycles and airflow, would you like to make another attempt at explaining how your magic engine gets close to 50 percent efficiency at idle? This is especially interesting now given the volumes you have written about how badly these things run at low rpms.

It has not much to do with which engines, most all diesels sip fuel under 1,000 rpms. Most diesels at no load, idle will burn less then 1/2 gph. It has to do with at which speed the particular hull is most efficient. Each hull is different but each one is most efficient closest to it's displacement speed and takes the least amount of hp for the speed. Once you pass that speed you're applying more power with very little gains in speed until you overcome the water and the hull climbs onto plane (given it's a planing hull).

You can also adjust displacement speed for each hull depending on where the weight is located in the vessel (ie. fuel). Generally in a total displacement hull, you will remove fuel from the stern tank(s) first and it works well in planing hulls until you get to the point where the bow starts digging in and plowing. Look at Broward's range figures in and of itself. It gets a heck of a lot better range at 10 knots which is only 2 knots slower then 12 knots.

Look at Searay's for example, they have a low displacement speed. It doesn't take much to move them to around 7.5 knots but once you get to that point, they won't go hardly any faster regardless of how much rpm's you give it until they get on plane.

Another example..... I worked on a long range 97' steel motoryacht. It drew 9' of water. It cruised at 8-10 knots depending on current. It had twin 250 HP Gardener diesels for power and used 1.5 GPm at cruise speed 1050rpm's (it maxed out at 1200 rpm's if I remember correctly.) and much less at slower speeds. Needless to say it only took about 400hp to move that massive and heavy boat 8-10knots........It came to the US from England on it's own bottom

Needless to say, in relatively calm seas, I believe the range figures published by Broward and have done a lot of deliveries at those speeds.......

"Most diesels at no load, idle will burn less then 1/2 gph. It has to do with at which speed the particular hull is most efficient."

That is truly a bizarre statement. One last time before I give up a lost cause. Go find the propeller curve for that 16V2000 and get back to us would you.

Most diesels at no load, idle burn less than 1 gph. That is just running with NOTHING attached to it. No propellor, 0 load. You're joining 2 totally different statements together.

Each hull has a speed that it is most efficient and requires the least amount of power to push. On this particular Broward you lose 20% efficiency just by speeding up 2 knots to 12 knots. It's probably even more efficient at 8 knots but nobody would be interested in seeing that in their specs

"Most diesels at no load, idle burn less than 1 gph. That is just running with NOTHING attached to it. No propellor, 0 load. You're joining 2 totally different statements together."

How can you possibly make such an absurd statement? What are you doing, averaging every diesel under about 500 hp, including the single cylinder units?

Your original suggestion that I still claim is BS, was that your 2000 hp engine burned 3 gph with the clutch engaged. I challenge you to look up the propeller curve for that engine and tell me what the horsepower load is at 600rpm. That isn't joining different statements, it is getting back to what we were talking about before you started blowing smoke and writing science fiction stories. And while the concept might be difficult for you, there is a difference between efficiency and fuel consumption.

By the way, the blower on a 71 series engine delivers just over 1.3 times the swept volume. In other words, it delivers more than 30 percent more air than the engine displaces.

The propellor curve that is published starts at 1200rpm's, not 600. I can tell you what the engine computers were saying was 3 and 3-4gph. Go find a vessel with 16v2000's and take it away from the dock and tell me what it tells you. I don't know what more to tell you. I know what the boat burns, I know what the engine displays read, why do I need to go find a propellor curve and this and that? It's irregardless to me. I also ran a 45' Cabo with c-18's and it burned 3-4gph total at idle with c18 cats 1015hp each, and did 8 knots at that speed.

According to Broward's website the vessel in this post holds 9,000 gallons and at 10 knots it has a range of 7,800 NM. Obviously the engines are not at 600 rpm's to attain that speed. So they're showing a fuel burn of less then 12 gph at that speed 9,000/780 hours= 11.54 gph. and at 12 knots 6,000NM= 18 GPH. Why would it be so hard to imagine that at dead idle the motors wouldn't burn less then 12GPH? Do you think that Broward Marine would risk a lawsuit from a buyer because they published range figures that are totally offbase? Why don't you e-mail Broward Marine and ask them what their burn rates are and what RPM they are seeing that consumption if it's so unbelievable. Also you're looking at the M93 propellor curve chart for fuel consumption and that is the 2400HP version. Detroit Diesel also makes an 1800 and 2000hp version of the 16v2000. I would think consumption would be considerably less on the 1800hp version at 1200 rpm's

Hi,

I am glad there is such a mix of types posting here, it makes life interesting in a warped sort of way sometimes.

There has been some good reasoned technical data provided in response to some absolute drivel and falsehood being posted as fact.

I will not waste any more keystrokes or any more of YF's bandwidth by continuing to reply to ridiculous statements still pouring forth after several attempts to explain errors or gain further info as to how and where this info is sourced are simply ignored and more junk is posted.

"Also you're looking at the M93 propellor curve chart for fuel consumption and that is the 2400HP version."

You seem to be backpedalling so fast you've lost track. The original poster asked about a boat with those engines. I responded to that post. You chimed in with some nonsense about the 2000 hp version burning 3 gph. If you go back and read you will see that I replied to your post with: "Using the figures supplied by DD for the 16V200-91 (2000 hp @ 2350 rpm)..." I don't think there is much room for confusion on that issue.

So far your 2 -3 gph has increased to 3 -4 gph, the 71 series engines don't blow exactly the same volume as they displace, no engine made by DD can obtain 49 percent efficiency and lowest BSFC at idle, and load factor is a ratio. You don't seem to understand how propellers are loaded and think that if a boat goes fast enough it will use less power ... Instead of supplying data you keep responding with anecdotes about how fast a particular model of fishing boat goes.

I give up. K1W1 pretty much nailed it.

I understand how propellors are loaded as well as vessel speed is not directly related to RPM either. An Engine/vessel is propped for a 100% load factor at rated RPM in this case 2350 rpms. Some vessels from the factory may turn 2380rpm's, some 2330 rpm's and that is still within DD specs. Anyways, a propellors load is not linear throughout the RPM range. There is slippage and some propellors are more efficient at certain speeds then others. For example a 4 blade prop will almost always make more speed on the top end then a 5 blade prop. Less drag. 5 blade props are usually more efficient at slower speeds. If you look at 10 different vessels with 16v2000's, each one will show a different load factor at idle in gear. Heavier or less efficient vessels will show more load, lighter more efficient hulls will show less load. The Broward hull is obviously lighter and has more of a flat bottom (as they always have) to achieve the top end speeds they are. Load factor is directly related to fuel burn. a 16v2000 at 1800rpm's is not going to burn nearly the same fuel in neutral with no load attached to it, as it is going to burn turning 1800rpm's and pushing a 60ton vessel. The higher the load at a given rpm, the more fuel needed to maintain that rpm. ALSO each propellor has a certain amount of slippage which is usually around 20%, but that is also not linear through the rpm range and some props will slip more at different rpm's and be more efficient at others.

The FACT of the matter is that none of you so-called engineers have provided ANY published technical data that has refuted Browards range quotes. All I have seen is 1 propellor load/fuel consumption curve that starts at 1200 rpm's and only shows 2 other RPM points not even in 100 rpm increments. Why don't you guys come up with a real RPMvs fuel burn curve that starts at idle and goes through the scale in 100 or 200rpm increments or ANY published technical data besides a fuel chart from a dyno and not even installed in a yacht application. You're the ones smoke screening, I haven't seen one factual published article to back up your claims. Just guestimations based on 3 reference points on a very limited propellor curve.

Second post of this thread dated 07-16-2008 at 01:16PM Refute it if you are capable.

And at the risk of being repetitious, load factor is a ratio.