Diesel Engine

RCook said:

My comparisons were based on running much the same way. At planing speeds of 18-20 knots.

Very much the same 40% advantage in fuel economy - in fact somewhat more - when running at 6-7 knots.

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Seems you surely made correct choice for boat to engine match-up! Congrats on your improved efficiencies.

Mid 1960's Dad and I replaced 155 HP Nordberg Knight big straight six with a Perkins straight six. Results were very pleasing on a 37' 20K lb + SD hulled sport fisher boat. Hard to have accurately compared everything as the Nordberg basically was worn out. Suffice it to say that the single Perkins had our boat cruise 12 to 13 knots with not too much fuel use (I believe we were getting nearly 3 nmpg - memory on stats vague after 50+ yrs.).

Soooo... Yes for sure... for certain budgets, in certain boats, used for certain reasons, under certain conditions, with certain power of engine chosen, to travel certain speeds, turning certain prop-size/blade-count diesel engine is the smart way to go! And for same reasons... sometimes gasoline engine is the smart way to go. I like both type engines - my choice per boat is greatly dependent on conditions present.

chicagoq said:

I am just trying to understand what 'efficiently' mean.
Not trying to compare gas vs disel.
suppose i have two identical engines, both gas. Put them on two boats. Same gear etc, everything the same except one boat weights 10k and the other 5k lbs. Run both boats at 3000rpm for 500 hours. Would the two engines have the same wear and tear? Would they consume same amount of gas? If not, why?

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chicagoq,
It's hard for diesel orented guys not to talk about diesels.

The difference is that the boat weighing twice as much will burm twice as much fuel. For a FD hull it's almost totally as simple as that. For SP hulls it gets a little more complicated and for planing hulls even more complicated.

As for the wear on the engines the one pushing the boat weighing twice as much will have more wear. Maybe only a tiny bit more depending on operating temps and many other variables. But if the harder working engine was below it's continous power rating and the lower loaded engine was not suffering from the effects of light loading (and gas engines seldom have a problem w that) the engine wear should be nearly the same ........ I M O.

manyboats said:

bayview that's a statement. I've heard it many times. I don't believe it. Torque is force like voltage. There can be lots of torque w/o any movement of the shaft but no movement will happen w/o work being done. If your boat is moving work is being done and that's hp .. not torque. Torque is a static value. Torque increases the shaft speed or accellerates a car or starts a boat moving but hp is what poweres the boat.

An old wives tale said that long stroke engines had more torque. Don't belive that either. Buick went from very long stroke to short in 1953. All the short stroke Buick engines made more torque than the old long strokers that preceeded them. The same is probably true for Chevy's. I've always said it SEEMED the short strokers didn't have much torque because the torque was overshadowed by all the increased power .. hp.

But like you I do believe torque has something more to do w power than a static force. I think I used to know but have long since forgotten. I'm hoping someone can relight my extinguished flame.

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IMO -Torque is the movement initiation force available and applied. Horsepower is the working coefficient that keeps the movement ongoing once a prescribed level of movement has been attained... via applied torque.

Side note: Torque adds to and builds momentum to reach a set energy level desired. HP then works to uphold the inertia for continuing said energy level. In other words, regarding boats; torque is for acceleration to desired speed and an amount of engine's HP then comes into play for maintaining the speed reached. That said - Torque is created by HP, HP is not created by torque.

I'm sure someone has time to delve deeply into definitions of Torque and HP as well as their interactions of togetherness regarding making props spin through water. Unfortunately, right now. I do not.

djmarchand said:

Boattest.com recently did a review (well actually more of a features recital) of Mercury's new 3.0 liter 260 hp marine diesel engine which was derived from Volkswagen's TDI automotive diesel.


They "compared" it to a Mercury 6.2 liter 320 hp gasser which delivers roughly the same performance as the diesel. What was interesting was the pricing quoted:


The MSRP for the Mercury Diesel 3.0L V-6 260 is $33,533.
The MSRP for a Mercury 6.2L 320 ECT TowSports gas inboard is $20,550.


At least for these pairs of diesel/gassers the price is not 4-5 times higher for the diesel. OTOH, I don't think that the Mercury diesel will last the 5-15,000 hours quoted above. I think that if used the same way, both the Mercury diesel and the equivalent gasser will probably last roughly the same length of time. I did note that the EPIC 23V that boattest used to compare the two engines was propped very lightly for the diesel, which probably helps with longevity.


I also take issue with the "a bit more fuel for the gasser" statement above. It is a lot more fuel. The Mercury diesel produces 20 hp per gph of fuel at wot. The equivalent gasser produces about 12 hp per gph. At about 90 hp where boattest reported the "best cruise" for that engine, the diesel burns 4.3 gph, the gasser would burn 7.5 gph, 74% more.


David

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Interesting report. Wonder if Boattest worked as an unbiased outside lab, in other words - any bias applied regarding gas or diesel? And, ouch! The fuel use stats sure do not resemble previous years' reports I've read... but... there are some really great new-age design improvements on diesels being accomplished. On gas engines advancements are also in process.

You can have torque without rotation...

Your 2:1 transmission will double the engine's torque minus a bit for gear loss.

If you look up your engine spec sheet, the max performance will be about where peak torque output is. You can add more air and fuel and get more HP and a bit more speed, but lose in efficiency. This is assuming that it is properly propped, BTW.

Short stroke vs long stroke.
It depends on the RPM operating range of your engine. If the bore is wider than the stroke, that's a short stroke, high RPM engine.

longer stroke than bore, and that's a lower RPM engine.

The energy to reverse the piston in the cylinder is lost, so doing it less often is usually more efficient. If you lighten the piston crown / apron, and have everything as light as possible you can compete on a short stroke engine. But with everything cut to the bone, you lose the sledgehammer durability. That's why the older engines maxed out at 1800 RPM. Later it went to 2300 and then to 2800 as Turbos came about. Even though the HP output went up, they had to work hard to move the torque curve past about 1600 RPM.

Then the short stroke diesels came around with 3400-4800 RPM bands, usually with turbos and sometimes after-coolers. All the Yanmar engines are short stroke high output engines.

RCook said:

My comparisons were based on running much the same way. At planing speeds of 18-20 knots.

Very much the same 40% advantage in fuel economy - in fact somewhat more - when running at 6-7 knots.

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But I still suggest that the engine efficiency may not be fair.

My example. My 1986 37 Silverton convertible came with 3208s custom from the factory. I never heard the stats on any other (but there might have been) but gassers with carbed 454s.

My boat got 1NMPG at 20 knots while the gassers got 1/2NMPG. The diesel had a lower top end of about 3 knots...but who cared.

Yet when these same boats were repowered with the newer fuel injected Mag 5.7s, their mileage got within a few tenths of a NMPG that the old diesel was. Had I repowered with newer diesels...the spread may have increased again.

One of the few boat models that I have direct knowledge of with similar sized powerplants of gas/diesel.

Of course the diesel Silverton was something like 3000 pounds heavier and sat 4 inches or more deeper in the water...but boat for boat...that's my experience with boats/motors as close as you can get.

Good question - were the engines and boats comparable vintage/technology/design?

My Volvo is a 24-valve electronically controlled 260hp 1997 KAD44P. Boat is a 25'7" moderately deep-V (19 degrees at the transom), 11,000 lb (loaded).

My Bayliner 2859 buddy's 454 gasser was a fuel-injected 2001. He repowered it with a relatively simple (no electronics) 300-315hp 2002 or 2003 Yanmar 6LP. His boat 28' 9", a few degrees less deep-V, 13,000 lb loaded.

Engines very comparable vintage.
Both boats moderate-speed planing hulls.
We cruised the Inside Passage together, at the same speeds.

As for the comparison of gas vs diesel Bounty 257's - those numbers came from the naval architect who owned Bounty and built my boat.

Fair enough ...but goes to show you that not all boats and engines pan out to simple concepts.


If 40% gains could always be achieved by diesel ..I would think it would be a much harder sell by the boating industry....even 3rd party "experts"....

Manyboats:
torque is not just static it is continuously available at different levels over all RPMs. Torque is what does the work HP measures that work. Without torque at every RPM the engine would stop.


Torque and HP are interrelated but HP does not exist without torque while torque may exist without HP.

Let me thow some data into the fire:

A 50 year old NA DD 6-71 makes about 18 hp per gph of fuel. A similar era carbureted gasser makes about 10 hp per gph.

A modern common rail, Tier 3 diesel makes about 20 hp per gph. A modern high compression, EFI gasser makes about 13 hp per gph.

So both have made significant improvements over the years in fuel economy, but the relative advantage of the diesel has stayed about the same.

Automotive engines should continue to improve with things like variable valve timing, cylinder cut out at low loads, highly turbocharged low displacement engines, etc. But these mostly affect automotive use.

Diesels may also improve, but I think that the improvements will be minor and in the case of automotive diesels they may get worse due to NOx emissions controls, a la VW.

David

There is nothing about diesel engines in the OP's question.

bay,
Hp = the energy that moves things. HP is the force that will lift 3300lbs so high in a given amount of time. Look it up. Hp dosn't measure .. it is power. One hp is a specific amount of work done.

Your last sentence may be true. But torque does not do any work.

manyboats said:

The difference is that the boat weighing twice as much will burm twice as much fuel.

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That a real factoid, all by itself?

Or does that need a boatload of qualifying assumptions to be true?

-Chris

ranger42c said:

That a real factoid, all by itself?

Or does that need a boatload of qualifying assumptions to be true?

-Chris

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At least a few assumptions ....such as speed...that's why we HAVE a trawler forum.......for us slowpokes.

My trawler is 2X as heavy as my sportfish but burns 1/10 as much to go 1/3 the speed..... so that rule is snapped right there.

But all things being equal except 2 times the weight (speed, naval architectural specifics, etc)....I'd have to let a physics wizard come up with the correct answer. Finding 2 vessels that meet that criterion is...well I would think impractical except in computer modeling.

Not much difference in a FD boat.

If it takes 40hp to push a given FD hull of 5,000lbs at a certian speed a hull of 10,000lbs will require about exactly 80hp to push it at the same speed. Quite similar for SD too but a few more variables.

So Chris asking "That a real factoid, all by itself?" Yes as a rule of thumb. But by doubling the weight things like PC, wetted surface and many other things may not increase/decrease at exactly the same rate. so it won't be exactly 1-1 but quite close.
And if you compare different hulls and engines it can be very different.

One of the most common expressions of power required to drive a slow boat is X amount of power per ton of displacement. In that expression it's basically 1-1.

david: the basic reason for the difference between gassers and diesel is the energy content of the fuels. despite engine improvements diesel fuel will always have greater energy content.

Hp is not force. HP is work done per unit time done by the force called torque

bayview said:

david: the bBothasic reason for the difference between gassers and diesel is the energy content of the fuels. despite engine improvements diesel fuel will always have greater energy content.

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BTU content is part of it, but is not the biggest difference. Diesel is about 7 lbs per gallon and gasoline is about 6. Both have about the same BTU per lb, so that gives diesel about 7/6 more BTU per gallon or about 17% more.

The bigger difference is that diesels are never throttled. They always suck in the maximum amount of air that the valves and intake ports can deliver. When that air is heated by injecting fuel it expands and produces more power from that fuel than a gasoline engine would because the gasoline engine is almost always throttled so the air in the cylinder is much less, so less power is produced for the fuel burned.

As a corollary, a gasoline engine has to operate near the stioichiometric ratio which is the ideal air to fuel ratio where all air is used in the combustion and little is left over. A diesel always operates at excess, and at light loads a lot of excess air, sometimes ten times more than is needed to combust the fuel injected. The high compression ratio makes it possible to ignite the diesel as it is injected whereas a gasoline engine needs to operate near stoichiometric for it to be ignited with a spark. It won't ignite if it has too much air.

All of which is to say that the excess air nature of a diesel engine is responsible for about 50% better specific fuel efficiency over gasoline and the BTU content another 17%.

FWIW in the laboratory you can run a diesel on gasoline. You need a fuel injection system that won't lock up due to lack of lubrication and high pressure injection system to keep the gasoline from vapor locking.

And I guess I am now just spouting off and should shut up ;-).

David

bayview said:

Hp is not force. HP is work done per unit time done by the force called torque

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Yes you're right. Except for the torque part. You can't do work w force. Movement is necessary to do work.

I think what you're saying is that torque causes the piston to go down, the force felt on the con rod and then to the crankshaft that causes rotation. And the torque is applied to the output shaft and produces rotational movement that does work. So hp is created from torque. Hmmmm But the movement only happens if the resistance allows it. But torque didn't cause the piston to go down. Torque is rotational is it not? ... a twisting force. Not a pushing force. The crankshaft turns because the force from the explosion forces the piston down and that force is turned into rotational effort by the mechanical components of the engine. So torque is created by the expanding force and then turned into rotational force (torque) by the engine. In a motor electrical force (voltage and current) is turned into mechanical rotational force that can do work. But the electrical force creates torque w or w/o shaft movement.

1. So hp is not created by torque.
2. And torque is not created by hp.
3. Torque is created by forces in the engine turned into rotational forces that is applied to a shaft.

I think I'm chasing my tail.

Pressure is turned into mechanical movement that converts the pressure into rotational movement that can do mechanical work. The mechanical twisting force in the shaft is torque. And the hp is the work done by movement caused by torque.

What do you think is true?

bayview you say "HP is work done per unit time done by the force called torque". By that you're saying torque can do work. But you can have lots of torque w/o doing any work.
Perhaps you're saying you can apply torque to a machine (gear and a propeller or gear and a pump) and do work. But the work done is not hp. Is hp only a measurement of work? That must be it. But it can't. Because we decide how fast a boat can go by how much hp we apply to appropriate machinery .. like a propeller. We can't propel our boat w torque.

"I think I'm chasing my tail.

What do you think is true?"

That part.

Well I got someth'in right.

So here are the actual definitions of torque and horsepower. Seems fairly obvious to me that an engine produces both of them and both of them are necessary for an engine to do anything worthwhile.

Torque
Noun
Full Definition
1 : a force that produces or tends to produce rotation or torsion <an automobile engine delivers torque to the drive shaft>; also : a measure of the effectiveness of such a force that consists of the product of the force and the perpendicular distance from the line of action of the force to the axis of rotation
2 : a turning or twisting force
Origin: Latin torquēre to twist.
First use: circa 1884

horsepower
noun
: a unit used to measure the power of engines
Full Definition
1 : the power that a horse exerts in pulling
2 : a unit of power equal in the United States to 746 watts and nearly equivalent to the English gravitational unit of the same name that equals 550 foot-pounds of work per second
3 : effective power <intellectual horsepower> <computing horsepower>

Well I got someth'in else right .. the expression/word twist.

And I got someth'in else right. You need the rods, crankshaft and piston to convert the force/pressure into rotational effort that excerts torque to the shaft.

So can you power a boat w torque?

David:
don't in cylinder fuel injected gassers run un throttled?? Far less pumping losses then.


Way back in the last century the military had diesel multi fuel engines that would burn diesel, kerosene or gasoline.

I remember a dragline in a mine in western Alaska that started on gasoline and switched to diesel when it warmed up. Haven't thought about that for a very long time.

Some old Diesel engines used a 'pony engine' which was often a gasoline engine that you started first, and then use it to start the diesel engine. Once the diesel is running, you shut down the pony engine. I've only seen one and it was an OLD engine, and I was 10 or so, so it's ancient by todays standards.

One of the problems with direct injecting gasoline is if you ever got air in the lines, you can cause it to ignite in the lines and blow it apart. The direct injection pressures were much lower on gasoline due to it's lower flash point. Also it's viscosity is thinner and more easily atomized by the injector. If I recall the direct injection vettes in late 50's had 200psi injection pressure. Compared to 1400-1500 on Cat, JD, perkins, etc. Deutz had a pop pressure of about 3200 psi which atomized the fuel better, resulting in more power for the same fuel. At those pressures, when testing an injector, you had to be careful since it could inject the diesel fuel through your skin.

fast forward to today, the Common Rail Diesels use a rail pressure of 30,000-40,000 psi and piezo injectors. At those pressures, the materials have to be tool steel hardened so they don't erode.

All the old D-8 Cats in the mines I was in had pony motors (or pony engines) but I'm quite sure the dragline (Wakasha sp?) was a dual-fuel engine. May have been just a pony motor starter. They said it started on gas and then switched to diesel but those words could have been applied to either.

The starting engines on the D-8's were 2cyl flat head 4 stroke engines that used the coolant of the mother engine to pre-heat same and the exhaust ran through the main engine too for pre-heat. The pony's had a multi-speed gear box and clutch to turn the flywheel in starting and the engine (main) had a half compression setting. In cold weather it could take 10 minutes to start up. On a 60 degree day only a minute or two was required. When it was 20 or 30 below wood fires were built under the oil pan.


Marin your post #51 was helpful.

I have seen an International diesel engine on a generator that started on
gasoline and was switched to diesel when warmed up.

Have used a pup (pony?) engine to start dad's D-2 Cat.
Also seen one on a 80 hp Cat engine in a troller, no fast start with that one if
you stalled in a tight spot.
I think most of the older Cats were started this way.

Ted

bayview said:

David:
don't in cylinder fuel injected gassers run un throttled?? Far less pumping losses then.

Way back in the last century the military had diesel multi fuel engines that would burn diesel, kerosene or gasoline.

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Nope, all gasoline spark ignition engines are throttled to maintain the proper air to fuel ratio.

Gasoline has to be mixed in a fairly tight air to fuel ratio for it to spark ignite, which is an entirely different mechanism from compression ignition. Modern gasoline engines operate with just a few percent of excess air (from stoichiometric) at all power loads. Diesels operate at as much as ten times excess air at light loads.

Stratified charge or lean burn engines can have a higher air to fuel ratio. The gasoline is injected into a pocket of the piston crown. I think that the pocket is near stoichiometric to get it to ignite, but since it is surrounded by mostly air, the overall ratio is high. This reduces combustion temperature and reduces NOx.

David

Exactly right

RCook said:

I'd suggest the advantages of diesel are not limited to larger boats.

Our boat has been built with 315hp 454 gas, and with 260hp Volvo diesel. It gets about 40% more miles per gallon with diesel. A buddy with a 2859 Bayliner repowered from 454 gas to Yanmar 6LP diesel, and saw about the same advantage in fuel economy and thus range.

Even though we are only 26 feet, with limited fuel capacity (ours is 110 gal) the efficiency of diesel vs gas can be a huge advantage. Cruising the distances of the Inside Passage, diesel gives us far greater range on a tank, and thus much more flexibility in the routes we choose to wander. For us, the $7,500 higher initial cost for the diesel has been well worth it.

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One of the problems with planing hulls is weight. Added fuel adds weight and the decrease in fuel efficiency due the added weight of fuel. So in your case even if you doubled your fuel capacity you would probably only increase your range by 50%. This was the problem faced by me with my 28, carrying an additional 42 gallons in jerry cans never gave me the full measure in increased range. Another added value of the diesel is torque in big waves, especially with a planning hull. If I had kept the 28 it would have ended up with a Yanmar in it for the very same reasons.

This is kinda outside the ops question, but nobody mentioned the safety factor. Gas explodes, diesel fuel does not.