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Old 12-27-2013, 10:57 PM   #1
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Rated total number of revolutions for a small block Chevy?

I was thinking as I was sitting down tonight I wonder if anyone has done a cost breakdown per individual revolution for a gas engine or diesel engine? I have read literally dozens of post about life expectancy of engines in number of hours. But have yet to read any post that has broke down the cost of each revolution. I would estimate that the cost would be very close, very close indeed. If one considers that diesel engines do last say 6000-12000 hours but only run at say 1/3 of the total number of hours as a comparable gas that runs at three to four times the total number of RPMs one might argue that a gas is just as reliable as a diesel. In fact, one might suggest running a gas motor at lower RPMs might yield the same total number of RMPs? No? I know many, many, many factors go into reliability, but I am curious if anyone has broke it down to the simple math. How much does it cost per individual stroke of the cylinder. This would put both gas and diesel on the same playing field. Don't get me wrong, I love diesel - the smell, the sound, the feel of a well running diesel. Whether car, boat, tractor - it doesn't matter.
Thoughts?
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Old 12-27-2013, 11:43 PM   #2
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I think there are too many variables, plus a fundamental difference in how the two types of engines are used; which is mostly expressed in the RPM range.

Odd fact that I learned years ago and almost never get a chance to mention is that the cylinder walls of a small block Chevy (Generation I, roughly 1955-1996) start to collapse at about 14,000rpm.

Which I mention in that it is illustrative of the entirely different paradigm of a gasoline engine.
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Old 12-27-2013, 11:51 PM   #3
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I've been told diesel contains more energy than gasoline. Regardless, doesn't it come between the relative cost of the different fuels?

On a displacement-type hull, hyper-revolutions are irrelevant/wasteful. Thus a slow RPM engine is more better. Aren't diesel engines better at slower RPMs than gasoline engines?
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Old 12-28-2013, 12:07 AM   #4
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The first thing you'd have to know is when the engine dies. Up until that information is available it's an ever changing factor.

I guess my biggest question would be--why would you want to do a calc like that? If you came up with a number of $.0000000001 per revolution, what good is it? It's not really a good indicator of anything. An engine that was run near WOT for much of its life is not likely to last as long as one that was run at hull speed, so IMHO that would be a much bigger factor in making any kind of 'gas vs diesel' comparison.

On my engines (Cat 3406C's) before I bought the boat I talked with the local Cat dealer to get his feedback about the engines. He said they were built to go a million miles in an over-the-road truck before they needed any real repairs. When I told him the number of hours on the engines at that time (around 800) he said that would be equate to about 8,000 miles in a truck, or not really even broken in yet.

I'll never put enough hours on my engines to factor in the cost of a rebuild. If I take good care of them by keeping the fuel and oil clean I shouldn't have any issues other than routine maintenance items.
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Old 12-28-2013, 06:28 AM   #5
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Most comparisons of engine life will use total fuel burned till overhaul , not piston miles.

Truck/boat comparisons usually use 1 hour is 50 miles .

Todays commercial big truck engines frequently get 1,000,000 miles before overhaul, but many are helped with bypass filtration (centrifugal) and synthetic oil.

It also helps , probably the most, that trucks run 125,000++ miles a year , so the slow death of rusting cylinders from non use over decades is not a problem.
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Old 12-29-2013, 12:45 PM   #6
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Most comparisons of engine life will use total fuel burned till overhaul , not piston miles.
Very true. The manual for the CAT 3116 in our Mainship suggests major overhaul at 30,000 gallons. Considering we burn about 300 gallons a year, we should be good for the next 100 years!
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Old 12-29-2013, 01:14 PM   #7
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I could bore you all with a bunch of numbers that shows that gasoline is cheaper overall except for repowering when necessary if you only run 100 or so hours each year.

But consider this:

Gasoline engines cost 1/3 to 1/2 of an equivalent hp diesel. They last 1/4 to 1/2 as long, probably on the lower end. Gasoline engines burn about 2/3 more fuel per hp output.

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Old 12-29-2013, 03:38 PM   #8
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I could bore you all with a bunch of numbers that shows that gasoline is cheaper overall except for repowering when necessary if you only run 100 or so hours each year.

But consider this:

Gasoline engines cost 1/3 to 1/2 of an equivalent hp diesel. They last 1/4 to 1/2 as long, probably on the lower end. Gasoline engines burn about 2/3 more fuel per hp output.

David
Really?? - - > "Gasoline engines burn about 2/3 more fuel per hp output."

Alternative Fuels Data Center Fuel Properties Comparison

http://www.afdc.energy.gov/fuels/fue...ison_chart.pdf
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Old 12-29-2013, 05:15 PM   #9
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Art:

I presume that you are referring to the statement in that link that a gallon of diesel has 113% of the energy in a gallon of gasoline. True.

But that says nothing about how efficiently that energy is turned into horsepower. Diesels are not throttled. A diesel sucks in a complete charge of air (well unless turbocharged) on each inlet stroke whether at full "throttle" or at light "throttle". The injector pump squirts just enough diesel to maintain the rpm set by the governor.

So it is very efficient as it is always moving the maximum amount of air in and out and just enough heat is added (as combusted diesel) to expand it to make the desired power.

Look at the fuel consumption curve of any popular diesel. The venerable Lehman makes about 16 hp per gph of diesel burned. The modern Cummins QSB common rail turbocharged engine makes about 20.

You can't find similar fuel consumption curves for marine gasoline engines-wonder why? But from tests done by guys such as boattest.com you can look at their wot hp ratings and the actual measured fuel consumption at wot. It works out to about 12 hp per gph for a modern, fuel injected engine. Older carbureted engines do about 10 hp per gph.

So comparing modern gassers to modern diesels you get 20/12 = 67% more hp per gallon. Comparing older gassers to older diesels it is 16/10 = 60% more.

And to answer another question, diesels last longer because they (usually) have heavier bearings, operate slower, they run cooler at partial load due to all that air being pumped through and diesel itself lubricates the piston rings; all of which reduces wear compared to a gasser.

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Old 12-29-2013, 05:33 PM   #10
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Can't argue with you David. I'm satisfied having a diesel.
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Old 12-29-2013, 05:40 PM   #11
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...and diesel itself lubricates the piston rings; all of which reduces wear compared to a gasser.
Ah, you were doing so well until that statement.

If you have fuel lubricating the cylinder wall and rings, you have some major injection issues and probably more than a few piston and ring problems as well.

A poor spray pattern (one bad enough to reach the liner wall) can quickly lead to piston seizures, melted crowns, and overheated cracked liners.
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Old 12-29-2013, 07:11 PM   #12
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We really need more diesel owners exposing the downside of gas engines, plenty the other way.

Nothing wrong with liking a gas engine, but thank I'll keep the 4 diesels I own, have plenty of experience with both.

Now step away from the Kool-Aid and stop trying to convince yourself otherwise.

BTW diesel has been around longer.
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Old 12-29-2013, 08:49 PM   #13
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BTW diesel has been around longer.

Well if THAT'S the criteria...


STEAM POWER RULES!
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Old 12-29-2013, 09:03 PM   #14
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Yep ....still looking for that diesel power for my 12 foot dingy....

There are applications that each are suited for...one is not superior to the other in all aspects...
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Old 12-29-2013, 10:04 PM   #15
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Originally Posted by SCOTTEDAVIS View Post
We really need more diesel owners exposing the downside of gas engines, plenty the other way.

Nothing wrong with liking a gas engine, but thank I'll keep the 4 diesels I own, have plenty of experience with both.

Now step away from the Kool-Aid and stop trying to convince yourself otherwise.

BTW diesel has been around longer.
So has your boat! And, I like it, a lot!!

That said; there are newer style engine types and boat styles that I like too. Certain items work well into certain circumstances!

Happy Boating Daze - Art
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Old 12-29-2013, 10:25 PM   #16
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I've owned and used to depletion lots of gas and diesel engines. Back in the day we bought light duty diesel trucks, 3/4 and 1 tons, because it made economical since. They cost more, required expensive maintenance, but returned better fuel milage and longevity. Today the tables are absolutely turned. A chevy 6 liter gas engine will go 300,000 miles with less than good maintenance, and still keep doing the job. It only takes a 5 dollar filter and 6 quarts of oil, maybe every 10,000 miles if you are so inclined. It wont get 20 mpg in a 1 ton truck but the diesels wont either anymore. And, I have yet to own a PowerSmoke that made it to 250,000 without major mechanical work. Ditto for the 6.7 Cummins or the Duramax. EPA has killed them. The new gas engines are incredible, especially the Chevy LS. I am not an advocate of gas engined boats, but I sure could be. Especially when looking at the big picture, overall cost of ownership. Gas is cheaper, oil and filters are cheaper, and, if you wear one out replacement is WAY cheaper, plus, any car mechanic can fix it. I'm just sayin.... and I own lots of diesel powered items.
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Old 12-29-2013, 11:05 PM   #17
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Diesel?

Quote:
Originally Posted by SCOTTEDAVIS View Post

BTW diesel has been around longer.

In which capacity? I am curious as to what engine you are discussing? 1860's were first gas motors, diesels were 1880...steam though was one of the first engines ever produced.
I love diesel for all of it's qualities. I use to sell diesels, I've had diesel powered cars, trucks, tractors, stoves, furnaces all powered by diesels. Unfortunately the budget didn't allow for a well kept diesel.
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Old 12-30-2013, 05:41 AM   #18
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>Unfortunately the budget didn't allow for a well kept diesel.<

Just as the higher maint requirements and expenses may have affected the engine in most used boats.
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Old 12-30-2013, 08:34 PM   #19
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My opinion is that if you buy a well built boat with modern gas engines you will not use it enough or live long enough to wear it out. If you are circumnavigating diesel is the only way to go. Short of that, gas kinda makes since.
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Old 12-30-2013, 11:41 PM   #20
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Having a history of employment with then Standard Oil of Calif, related to fuel docks. Working the fire extinguishes on a number of incidents (4) ,of exploding boats seeing burnt mariners, thinking of open fire heating, and such has me asking this: Have gas engine applications in boats arrived at such a safety point that the discussion of gas over diesel is mute in terms of product safety?
Asked from one who profiles gas powered boats that moor near me in all instants. Who has sworn off gas other than small outboards,
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