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Old 08-20-2016, 12:21 PM   #41
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Eric is correct, torque is force x distance, and the main factor for a diesel is the force from a high compression combustion chamber. Stroke is important but the bore is where most of the power is made, the larger the bore the more room for fuel and air. Of course I know CID is stroke x bore as the main parts of the formula but the larger the bore the higher the torque is a good rule to follow as a norm.
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Old 08-20-2016, 01:03 PM   #42
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Lots of reasons for more torque from diesels: Higher comp ratio, longer stroke, turbocharging, no worries of detonation or pre-ignition, injection continuing well past tdc.

And yep, bore is a bigger deal. Displacement varies with the square of the bore, yet linearly with stroke. If you add a half inch to either, bigger bore gets bigger effect.

And someone mentioned BMEP/BSFC charts. That is the gold standard for nailing down engine performance and efficiency. The charts the eng mfrs give us are poor substitutes for the fuel maps they certainly have, yet do not release. Annoying.

Cummins and Cat give enough data that you can reasonably calc efficiency in boat service. But they could just publish the fuel maps.

For trawler service, a very rough way to check loading is to take your cruising hp and compare it to rated power. So using a Lehman 120 as example, if you cruise at 60hp that is 3-4gph. And rpm should be somewhere around where torque peak at WOT. Probably around 1500 to 1800rpm.

So if you are burning 3gph at 1700, the engine is in a sweet spot.
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Old 08-20-2016, 01:57 PM   #43
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"The larger the bore the more room for fuel and air .. yes but it's a issue of stroke too .. Displacement. And big time the compression but isn't the energy in diesel fuel less than gasoline? They aren't the same but I can't remember which is which. Also diesel don't burn at the same rate as gasoline. I think diesel is faster burning and that may have an effect on torque. Could be a plus for gas??
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Old 08-20-2016, 03:44 PM   #44
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No, I meant 80 percent of WOT. The boat is on plane at around 2700 and I often cruise her at 2800. WOT is 3550. This is SOP for these engines in this configuration.

With that said, I spend most of my time at 1300 rpm enjoying the view and the low Gph.

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Perhaps you mean 80% of rpm. Almost nobody runs at 80% of power output. That would be only 100-150rpm down from max. Maybe even less.
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Old 08-20-2016, 04:18 PM   #45
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The above is just opinion but if you care to look you'll find lots of short stroke engines with above average torque output.


Fletcher500'
Perhaps you mean 80% of rpm. Almost nobody runs at 80% of power output. That would be only 100-150rpm down from max. Maybe even less.
I think I'm pretty close. Although I'm fairly underpropped so my 2400rpm cruise is likely less than what the fuel and power curves show.
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Old 08-20-2016, 04:28 PM   #46
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"The larger the bore the more room for fuel and air .. yes but it's a issue of stroke too .. Displacement. And big time the compression but isn't the energy in diesel fuel less than gasoline? They aren't the same but I can't remember which is which. Also diesel don't burn at the same rate as gasoline. I think diesel is faster burning and that may have an effect on torque. Could be a plus for gas??
Backwards Willy. Diesel has more energy than gasoline. Burns slower too.
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Old 08-20-2016, 11:52 PM   #47
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Thanks bayview I couldn't remember.

I did think it burned slower because I thought it should but remembered it was backwards ... and it wasn't.

But I wonder if it burns sooner or later because it would seem that having the greatest force on the piston should optimize power if the downward force on the pistion occured a bit before halfway down from top dead center. And some engine cylinder bores are offset to maximize the advantage of having the downward force at the optimum time.
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Old 08-21-2016, 03:03 AM   #48
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Oftentimes the spark is actually set to just after TDC...this includes diesels with their compression ignition concept...

The theory I think is....it's much easier to fire just after TDC because it keeps the engine going forward/around...ie: an object in motion stays in motion..

Igniting the fuel air mixture too far down the stroke or before TDC has the engine fighting its natural motion and trying to tie itself up in knots

Also...someone else said it more scientifically but basically..there's no replacement for displacement. .'Merica has proven that time and time again in the racing world...

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Old 08-21-2016, 06:36 AM   #49
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Injection operating like spark timing is always Before TDC.

The injector sprays fine fuel dropplets into the cylinder where it needs both heat and time to vaporise and ignite before it can burn efficiently
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Old 08-21-2016, 06:37 AM   #50
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Gasoline engines are set to between 4 and 10 degrees before TDC to allow for flame to propagate and pressure to build. Sparks timing is variable and advances as RPM increases. My Lehman injection is timed for 20 degrees before TDC.
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Old 08-21-2016, 06:46 AM   #51
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No, I meant 80 percent of WOT. The boat is on plane at around 2700 and I often cruise her at 2800. WOT is 3550. This is SOP for these engines in this configuration.

With that said, I spend most of my time at 1300 rpm enjoying the view and the low Gph.
Many diesel vessels are run 200 rpm off the pins.

That is the rule of thumb I was brought up with.

Everything from fast planing boats to displacement tugs.

Obviously adjustments were made for different situations, but that was considered "typical" loading rpm unless specifically rated for 24/7 on the pins, or came with specific loading instructions.
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Old 08-21-2016, 08:36 AM   #52
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Diesel is different from gas in that the fuel is injected over a pretty long duration. Injection process might begin at 20deg btdc, but actual injection will start later, and combustion start even later than that. It is all timed so pressure starts rising from combustion after tdc. But the big difference is that injection continues at a measured rate so as piston descends and volume increases, the pressure is kept somewhat constant until injection stops. That way peak cyl pressure can be kept below structural limits. This also helps efficiency somewhat by limiting heat transfer by having heat added later in the process.

In a gasoline engine combustion starts with the spark and the rate of pressure rise is not actively controlled, only limited by the rate of the flame front advancing. Too much pressure or heat and flame initiates spontaneously with bad results. Thus raising compression ratio, which directly increases efficiency, has its limits. Also closing down the throttle to reduce output effectively reduces compression ratio, with significant drop in efficiency.

Fun stuff..
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Old 08-21-2016, 08:55 AM   #53
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IMO 200 off the pins is bad advice in general. For 30 HP per liter engines it was fine but get to 60 HP per liter and you are pushing things hard. Makers do not warranty that level of running on recreational engines for any high duty cycle.
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Old 08-21-2016, 09:14 AM   #54
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"200 off the top" as a cruise point is a very crude tool, but it does apply pretty well to lots of engines.

A 2300 rpm Detroit or Cat, 2100 is considered ok, provided it can make it to 2300.

A 3800 rpm Yanmar, I would not like to run it at 3600 continuously.

If an engine can't make its rated rpm, then you need to give it even more margin.
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Old 08-21-2016, 09:33 AM   #55
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The concept of ignition timing tends to confuse people. It is easier to understand when you think of combustion as a chemical reaction that happens over time. In other words it takes time for the process to happen. As an engine increases rpm the time component of each stroke shortens. To visualize this consider an engine at 1 rpm. It would take 30 seconds for the compression stroke at 1 rpm. Now let's double the engine speed to a whopping 2 rpm's. The compression stroke now happens in 15 seconds. Get it?
Ignition timing advances so that combustion has enough time to happen before the exhaust valve opens thus maximizing the power generation of that cycle...
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Old 08-21-2016, 09:38 AM   #56
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Here is a video by one of my favorite YouTube channels. The subject matter is always automotive in nature but this guy is very capable and he does a good job of explaining the subject matter. I can watch this stuff for hours and there is lots of material. This video is on Horsepower vs Torque...
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Old 08-21-2016, 10:26 AM   #57
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IMO 200 off the pins is bad advice in general. For 30 HP per liter engines it was fine but get to 60 HP per liter and you are pushing things hard. Makers do not warranty that level of running on recreational engines for any high duty cycle.
That is in the actual Cummins manual for max continuous power. Does it mean you should run it there??? No it does not. But they do feel the need to define it. And since it is defined, I would assume it is warrantied. My 4LHA Yanmar was rated at 240hp@3300rpm. Again the manual said 3100(190hp) continuous(Gives you an idea how non linear the power curve is with that last 200rpm representing 50hp). I ran my Yanmar at 2800rpm and that yielded 15kts in that Pilot. I run my Cummins around 2300-2400 yielding 16-17 knots. Cummins rated at 315@2800rpm. Both boats were/are fairly underpropped so they seem quite happy.
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Old 08-21-2016, 10:57 AM   #58
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That is in the actual Cummins manual for max continuous power. Does it mean you should run it there??? No it does not. But they do feel the need to define it. And since it is defined, I would assume it is warrantied. My 4LHA Yanmar was rated at 240hp@3300rpm. Again the manual said 3100(190hp) continuous(Gives you an idea how non linear the power curve is with that last 200rpm representing 50hp). I ran my Yanmar at 2800rpm and that yielded 15kts in that Pilot. I run my Cummins around 2300-2400 yielding 16-17 knots. Cummins rated at 315@2800rpm. Both boats were/are fairly underpropped so they seem quite happy.
That may be so for low power models but the cummins specs i remember said not to exceed 1 hour out of eight.

it is also worth mentioning that the max rated load is not a requirement just allowed. Some folks have the idea that their engines should always be run at very high load.
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Old 08-21-2016, 11:10 AM   #59
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I think the one hour out of 8 is for WOT or anything above the max continuous range....hence the term "max continuous". And I fully agree with the rest of your sentiment. Just because it says you can does not mean you should. Basic common sense tells you the harder you run it, the more wear it is causing.
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Old 08-21-2016, 11:10 AM   #60
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The common cummins 5.9 liter engine run at 200 hp ( 34 hp per liter) will last a very long time while the same run at 400 HP (68 HP per) not so much.

Interestingly overhaul expected time sometimes is discussed in terms of fuel used which directly relates to work done. On that basis both rated engines would be the same. One just uses fuel 2X faster and races to overhaul time faster.
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