Fuel Mixture and Load

I've been meaning to start a conversation highly related to what is being discussed on the twins thread.


When a diesel is at WOT propped correctly it makes it's maximum hp and I assume it's fuel injection system is, at that point injecting the optimal amount of fuel. But that is only one point of operation and one that most of us only use or attain for brief periods so the fact (assumed) that the mixture is perfect there is not very meaningful to anyone here. The mixture at lower engine loads where we run our engines could be very important to everyone.

When we look at the load curve and the power curve a great difference will be apparent. Most power curves are convex. That is the power rises steeply at half engine speed and near the top end flattens out in a shape that could be considered a hump. The resistance of a FD hull is a concave curve. Rising slowly at first and then more and finally rising steeply to hull speed and a bit beyond.

So backing off from WOT very quickly the resistance drops dramatically but the power available remains close to maximum. Suddenly there's a great difference between required engine output and power available.

On an old fashioned mechanically injected engine the amount of fuel injected is controlled by the fuel injection control rod and I think that at 50% deflection of the arm connected to the control rod is half way between idle fuel amount and WOT fuel amount. So roughly half as much fuel is injected at half "throttle". It would seem to me that if I was at half throttle I would NEED to be at half rpm to have a proper fuel mixture. And the amount of fuel burned is proportional to the power output and way less than half power is at half throttle. Too many variables to have that just happen to come about and at many other engine speeds that aren't rising and falling exactly together there should be a lot of mismatches in load, speed and throttle position. Yet the mechanically injected engines only smoke from too much fuel on rare occasions. And that's just considering drastically too rich of a mixture. Perhaps a mixture of too lean is much more of a problem. Does the load, engine speed and injected amount of fuel mostly come out right across the span of loads and speeds? Dosn't seem likely to me.

Then there is the modern computer controlled engines and I suspect they have sensors to monitor the load, temps, and anything else pertinent and then adjust the injection to inject the perfect amount of fuel for any anticipated situation.

Well it looks to me like the computer controlled engine should be far better than their reputation and I'm surprised they are only 10 or 15% more efficient. And there should be lots of other benefits like far less fuel oil contamination of lube oil from rich mixtures and excess fuel leaking down the cylinder walls and into the lube oil.

But more important to me and probably most here is how far astray does the mixture get in actual mechanical engines propped correctly. I'm sure over propped engines run too rich. Will under propping result in better mixture control than correctly propped engines? I was under propped for a time in Alaska by 100rpm and the engine seemed to run better than otherwise but that's very subjective.

What can the most knowledgeable tell us here about our fuel mixture in our typical dynamic conditions.

I'd like to understand this better.

Eric--- What I know about this subject wouldn't cover the head of a pin. So all I can provide is an observed data point. And that is that when we bought our boat it was over-propped. Deliberately, as Grand Banks boats tended to be from the factory although our boat no longer had its original props. So WOT missed the max rated rpm by several hundred rpm.

If the over-pitching was causing a too-rich mixture this was never evidenced by any smoke or sooting on the transom or the transom-carried dingy.

We had the boat's propellers extensively reworked and this included a pitch-down to "correct" specs. So at WOT the rpm is the max rated rpm. There is been no change at all in terms of what comes out the exhaust since nothing noticeable was coming out the exhaust before. If there has been a reduction in fuel consumption it has been very subtle.

My understanding - and also note I'm no expert - is that gas and diesel engine speed controls fundamentally operate differently. For gas engines, opening the throttle supplies more fuel which gives more power or higher rpm, if not fully loaded.

For diesel engines, changing 'throttle' simply changes the selected rpm. The amount of fuel injected is always determined by the governor, and load. Basically enough fuel is injected to maintain your rpm/throttle setting. If you load the engine then rpm would fall except that the governor counteracts it.

If the governor malfunctions, then increased fuel flow will cause increased rpm, and diesel engine runaway can occur if you/it keeps adding more fuel.

The real benefit of the diesel engine control setup is that it inherently leads to fuel efficiency. Once you set the rpm via the 'throttle' then the governor ensures that just enough fuel is injected to maintain that rpm at that load. Variations in diesel engine fuel efficiency then relate to how good the governor design is, along with their airflow dynamics. Modern engines can have real-time feedback to optimise this process rather than mechanical governors.

Eric, your supposition that "roughly half as much fuel is injected at half "throttle"" is not true. As others have noted diesels are different from gassers.

A naturally aspirated diesel is always sucking in maximum air volumes at a given rpm, unlike a gasser which is throttled. Also the fuel injector and governor inject just enough fuel to regulate the rpm at the throttle setting unlike a gasser where the throttle "throttles" the air intake. So for a diesel at idle, lots of air is being injected with a tiny amount of fuel to maintain idle rpm.

So in neutral, open up the throttle to half, say 1,500 rpm. The governor opens up the injection pump a little more but not much more as there is no load. Then slam it in gear (not a good idea, but just for illustration), the governor senses a drop in rpm and opens up the injection pump and starts dumping in fuel, much more fuel and might smoke for a second or so. When the rpm gets back to its set point, the governor backs off fuel nijected but not near as much as before slamming in gear.

Another point often missed is that a diesel is always running lean except when accelerating or at wot. Running lean doesn't matter unlike a gasser which would stumble and die if leaned as much as a diesel because the heat of compression ignites the diesel unlike a gasser which needs a narrow fuel/air ratio for the spark plug to ignite it.

So in summary, fuel/air ratio is mostly an irrelevant concept for a diesel. It is always running lean except at wot or accelerating.

David

OKOKOKOK GOOD. The light is ON.

So what I've been missing is the governor function. I thought the governor only came into play at engine speeds above rated speed like 2500 for a FL. Now I see from Insequent and djmarchand's posts the governor is always balancing load, rpm and fuel such that the resulting amount of fuel is perfect for those conditions. Seems like a lot of magic for a simple mechanical thing attached to the hand of the helmsman. But the air/fuel mixture is controlled on a gas engine mechanically but the aerodynamics in the Venturi's are, perhaps, not that simple.

The other thing I think I've been missing is unlike a gas engine that requires a fixed mixture (correct mixture) the diesel's correct mixture can be over a wide range unless full power is required. So if two trucks are going up a hill whereas one is in 3rd gear and the driver's foot is at upper-mid throttle whereas this combination produces a good balance of fuel and engine speed and the other truck is in 5th gear and the driver's foot is hard on the throttle pedal pinned to the floor (WOT) and both trucks are climbing the hill at the same speed. These are mechanical engines. We all know the driver in the taller gears isn't doing his truck any good but in this situation is the governor choosing the correct amount of fuel for both trucks?

If so or even if the answer for the above is yes only for the truck in 3rd gear would there be a different amount of fuel injected if the engine was electronically controlled? I assume there would be less fuel injected into the engine in the truck in 5th gear w electronic controls.

djmarchand said:

Eric, your supposition that "roughly half as much fuel is injected at half "throttle"" is not true.

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Yes, not true. For the Coot's JD:

RPM HP gal/hr
1000 40 0.4
1600 63 1.3
1800 68 1.7
2400 80 4.0

What surprises me is that at near-idle RPM, the engine is generating 50% of its potential horsepower. Less surprising is consuming fuel at ten times the rate when generating only twice the horsepower.

Mark:

Your table shows wot hp at 1,000 rpm but your fuel consumption is for prop horsepower which is a fraction of wot hp. But at 2,400 rpm, prop hp and wot hp are the same.

Eric:

Common rail electronicaly controlled injection doesn't change the basic principles, it simply does a more precise job. The high pressure injection creates a more precise spray pattern and some engines actually pulse the fuel injection several times which resultis in smoother combustion with less knock. Listen to a newish Dodge diesel truck vs an old mechanically injected one.

And to your question about two 18 wheelers loaded the same, climbing the same hill at the same speed, one in third and another in fifth. Both require exactly the same horsepower at the rear wheels. The one in third may consume slightly more diesel because the engine is spinning faster and the internal losses will be higher. That is the argument for overpropping a slow speed trawler. But the effect is rather small and has nothing to do with the type of fuel injection.

David

djmarchand said:

Mark:

Your table shows wot hp at 1,000 rpm but your fuel consumption is for prop horsepower which is a fraction of wot hp. But at 2,400 rpm, prop hp and wot hp are the same.

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Nevertheless, the boat runs at nearly half-speed at 1000 RPM.

"Nevertheless, the boat runs at nearly half-speed at 1000 RPM."

Mark I think that has mostly to do w the fact that hull resistance is so low at that speed.

March,
Thank you ever so much as I now have a much better understanding of all of this admittedly still grouping around a bit t quite a bit but w lots more scope.
And yes those older Dodge's are AWFULL.

David and Brian-- Thank you for your explanations. For whatever reasons gas engines have been easy for me to understand from the outset but other than knowing that the heat of compression is what ignites the mixture and that there's a governor in the injection pump diesels have always been something of a mystery to me. Your posts have been very helpful.

The reason a diesel can run so "lean" is the it has direct injection. So at the point of injection it is not lean at all. As the spray of the injector propigates across the combustion chamber it combusts as it meets more O2 untill it is all combusted. Black smoke is created when there is too much fuel and it does not all burn. The heat of compression for ignition eleminates the need to have a uniform mixture that a spark plug can ignite. The temperature is so high and since there is O2 present, where ever fuel is injected it combusts.

Third-Reef said:

The reason a diesel can run so "lean" is the it has direct injection. So at the point of injection it is not lean at all. As the spray of the injector propigates across the combustion chamber it combusts as it meets more O2 untill it is all combusted. Black smoke is created when there is too much fuel and it does not all burn. The heat of compression for ignition eleminates the need to have a uniform mixture that a spark plug can ignite. The temperature is so high and since there is O2 present, where ever fuel is injected it combusts.

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I think that the compression of the air causes it to heat up then the fuel is injected and combustion takes place. Your engine runs very slightly lean then as the throttle is advance the fuel amount is increased resulting in more engine rpm and power. Without the proper cylinder presure and heat your engine will not run. Each stroke unless turboed only contains enough air fuel to fill the cylinder. If too lean engine damage can occur if to rich smoke is the result. I think that is correct

bfloyd4445 said:

If too lean engine damage can occur ... I think that is correct

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I know that is not correct. As long as you guys keep talking about mixture in a thread about diesels you will continue to keep swapping nonsense.

RickB said:

I know that is not correct. As long as you guys keep talking about mixture in a thread about diesels you will continue to keep swapping nonsense.

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ok, so diesel engines just burn fuel alone or is it air? No, can't be just air cause that dosent cost $5 a gallon. If the pump needs both then we are talking about a mixture right? But your right, I may be getting my engines confused but for a fact diesel engines still need a 14.5:1 ratio of fuel for best economy. Correct? ....maybe

Britt:

The fact that diesels can run with 30:1 or whatever is why they get good economy, particularly at lower loads. A gasoline engine has to run at 14.5:1 for clean combustion. So it has to throttle its fuel/air mixture so that the thermo cycle is compromised by a partially filled cylinder.

Diesels don't because the temperature of the air on the compression stroke lets diesel combust at almost any ratio less than stoichiomietric. Stoichiometric is the theoretical ratio (about 14.5:1 for gasoline) where there is exactly enough molecules of O2 to burn/combust/oxidise each molecule of diesel.

A diesel will produce black smoke when the fuel/air ratio gets below stoichiometric, ie too much fuel for the available air. We can talk about how and why that happens on another thread.

David

djmarchand said:

Britt:

The fact that diesels can run with 30:1 or whatever is why they get good economy, particularly at lower loads. A gasoline engine has to run at 14.5:1 for clean combustion. So it has to throttle its fuel/air mixture so that the thermo cycle is compromised by a partially filled cylinder.

Diesels don't because the temperature of the air on the compression stroke lets diesel combust at almost any ratio less than stoichiomietric. Stoichiometric is the theoretical ratio (about 14.5:1 for gasoline) where there is exactly enough molecules of O2 to burn/combust/oxidise each molecule of diesel.

A diesel will produce black smoke when the fuel/air ratio gets below stoichiometric, ie too much fuel for the available air. We can talk about how and why that happens on another thread.

David

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isnt the ratio 14.7:1 for gas, 14.5:1 is for diesel.

30:1 whatever? ..........you must be refering to the compression ratio not the reactant ratio for optimum combustion i was referring to

RickB said:

I know that is not correct. As long as you guys keep talking about mixture in a thread about diesels you will continue to keep swapping nonsense.

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iiiieeee what an idiot i am. I see the issue. In my mind i was thinking of a lean mixture meant that there would be less fuel for lubrication of valves injectors and this isnt so. Less fuel means lower engine rpm resulting in less demand for lubrication. My mistake. A DI engine dose not have problems in this regard because the fuel input ids what controls the RPM....

Ok Rick, you finally beat it into my head

No, I was speaking of 30:1 air to fuel ratio. A diesel can operate quite efficiently at that ratio.

The peak BSFC (brake specific fuel consumption) of a diesel engine at prop loading conditions (ie low loading at low rpms) is in the 70-80% of wot rpm range or about 40-50% of rated hp. I suspect that this is about 20-30:1 air to fuel ratio.

If it weren't for internal losses, a diesel would probably be most efficient at very low loads, ie a little bit of diesel burning in a lot of air. But it takes significant hp to run oil and cooling pumps, overcome friction in bearings and rings, etc. which is called parasitic losses. Those parasitic losses are related to rpm and are mostly independent of actual hp produced at the output shaft.

At light loads the parasitic losses are a big part of the total so BSFC is high. Only when significant shaft hp are being produced do the parasitic losses get small enough as part of the total so that BSFC peaks at about 40-50% of rated hp.

David

djmarchand said:

But it takes significant hp to run oil and cooling pumps, overcome friction in bearings and rings, etc. which is called parasitic losses.

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Parasitic losses are those associated with external "stuff" driven by the engine and not required for its operation. Things like belt driven auxiliaries, alternators, raw water pumps, air compressors, and so on.

The losses you are talking about are internal losses and are the result of friction and pumping losses. They are measured by "motoring" the engine- driving it with an electric motor - and measuring the power required to turn it over at rated speed.

Wouldn't a reason they don't operate that efficiently at really low rpm is that it takes a lot of energy just to get the compression required?????

"That is the argument for overpropping a slow speed trawler. But the effect is rather small and has nothing to do with the type of fuel injection."

Not really,

The argument for "overproping" moving the engines actual cruise RMP down considerably so the HP required and the HP produced at that lower RPM are more closely matched.

As noted diesels work best at heavy load , not just at peak RPM but also a lower RPM loaded highly..

In other words if your boat needs 50 HP at the prop, it could be created with a the MFG preferred top speed prop at a higher RPM than a prop selected on experience for your boat.

Creating 50 HP at say 1900RPM (where the engine might male 110HP is noisy ,not fuel efficient and does not load the engine properly.

Select a cruising pitch and diameter prop and at 1500 or 1600 the engine may only be able to create 60-70 HP, a far better load factor with 50 HP required.

The GB folks stock over pitch must have figured their boats would be cruised at modest displacement speeds and set cruise RPM accordingly.And the owners could enjoy low noise , low fuel burn and longer engine life.

I guess they thought the 1nm and 1/2 NM per gallon cruisers would select a different boat to go fast.

Usually about 1 HP to 50 LBS of boat is required for this speedy style cruising.

May I suggest readers acquire a copy of:

Internal Combustion Engine in Theory and Practice | The MIT Press

Then revisit the discussion after digesting its contents.

Fred:

We have had this debate before on the merits of overproping. Let's do it again but with facts rather than opinions. Take a look at the Cummins spec sheet for the 6BT 210 hp engine at: http://www.sbmar.com/Engines/PDF/6BT/6BT%20210%20Power%20Curve-%20Nov%2000.pdf

This is an engine used in some of our trawlers and is a real workhorse. The spec sheet is one of the few that has fuel consumption curves for both wot loads and prop loads which makes it easy to compare the two.

So look at the prop curve and you will see that 100 hp is produced at a bit more than 2,000 rpm and it takes about 6 gph or 16.7 hp per gph. That is the place many trawler owners would operate at fast semi displacement speed, say a GB 36 at 8+ kts.

Now overprop the same boat and engine severely so that the 100 hp is produced at about 1,200 rpm which is on the wot output curve. The fuel consumption is about 7 gph.

Now these are extremes and no one should think about overpropping that much, as you will quickly ruin the engine. And I have no doubt that overpropping a little will actually result in fuel consumption below 6 gph. But not by much- maybe to 18 hp per gph or about 5.5 gph.

So I think that a 10% fuel consumption improvement is all you are ever going to see by overpropping. In my mind that just isn't worth the risk of overloading your engine if you ever push the throttles forward.

David

RickB said:

May I suggest readers acquire a copy of:

Internal Combustion Engine in Theory and Practice | The MIT Press

Then revisit the discussion after digesting its contents.

Click to expand...

Why confuse an evergrowing thread with the facts

djmarchand said:

Fred:

We have had this debate before on the merits of overproping. Let's do it again but with facts rather than opinions. The spec sheet is one of the few that has fuel consumption curves for both wot loads and prop loads which makes it easy to compare the two. So I think that a 10% fuel consumption improvement is all you are ever going to see by overpropping. In my mind that just isn't worth the risk of overloading your engine if you ever push the throttles forward.

David

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David

Further, a surveyor worth his salt will not approve an engine/prop setup that cannot achive rated RPM.

What if the rated max rpm is a range? My Volvo Penta manual WOT is 2700-3000. Mine can achieve 2975 and I still think it is a bit over propped as my idle rpm hull speed is too high.

Northern Spy said:

I still think it is a bit over propped as my idle rpm hull speed is too high.

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What is your idle rpm and speed?

sunchaser said:

David

Further, a surveyor worth his salt will not approve an engine/prop setup that cannot achive rated RPM.

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Surveyors don't "approve" anything and it wouldn't show up as a "safety" issue so insurance companies wouldn't care either.

Marin's statements about older GBs coming from the factory kinda dispute your statement as I doubt in the day surveyors were giving thumbs down to brand new GBs.

Spy,
I'm going to assume you actually have an appropriate idle rpm. I think that propeller speed v/s boat speed affects the idle boat speed. Lets say you have a direct drive gear and another identical boat has 10-1 reduction. I think the 10-1 reduction boat's idle speed will be considerably slower than the direct drive. This is admittedly a very extreme example but sometimes extremes will make it obvious what small differences fail to show.

Could it be that you have a 1.5-1 gear? My W30 has a 2.57-1 ratio and has an acceptably slow idle boat speed. And I idle at 900. I have the idle set a tad high so I can get underway soon w/o any danger of my engine quitting while shifting gears. The engine idles nicely at 700 but I could back out of my slip and possibly into another boat if I set it at 700.

The reason surveyors won't or don't like to survey a boat that isn't propped properly is that a proper evaluation cannot be made of the boat unless rated rpm can be obtained.

Manufacturers never recommend over propping.

Overpropping is not adult behavior.