Reply
 
Thread Tools Display Modes
 
Old 02-06-2013, 11:46 PM   #1
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,715
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.
__________________
Advertisement

__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 02-07-2013, 12:04 AM   #2
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
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.
__________________

Marin is offline   Reply With Quote
Old 02-07-2013, 12:56 AM   #3
Guru
 
Insequent's Avatar
 
City: Brisbane
Country: Australia
Vessel Name: Insequent
Vessel Model: Ocean Alexander 50 Mk I
Join Date: Mar 2012
Posts: 1,426
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.
__________________
Brian
Insequent is offline   Reply With Quote
Old 02-07-2013, 08:03 AM   #4
Guru
 
djmarchand's Avatar
 
City: East Greenwich, RI
Country: USA
Vessel Name: Bella
Vessel Model: Mainship Pilot 34
Join Date: Aug 2012
Posts: 2,882
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
djmarchand is online now   Reply With Quote
Old 02-07-2013, 10:38 AM   #5
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,715
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.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 02-07-2013, 10:56 AM   #6
Master and Commander
 
markpierce's Avatar
 
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 10,265
Quote:
Originally Posted by djmarchand View Post
Eric, your supposition that "roughly half as much fuel is injected at half "throttle"" is not true.
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.
__________________
Kar-KEEN-ez Koot
markpierce is offline   Reply With Quote
Old 02-07-2013, 11:31 AM   #7
Guru
 
djmarchand's Avatar
 
City: East Greenwich, RI
Country: USA
Vessel Name: Bella
Vessel Model: Mainship Pilot 34
Join Date: Aug 2012
Posts: 2,882
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 is online now   Reply With Quote
Old 02-07-2013, 11:45 AM   #8
Master and Commander
 
markpierce's Avatar
 
City: Vallejo CA
Country: USA
Vessel Name: Carquinez Coot
Vessel Model: 2011 Seahorse Marine Coot hull #6
Join Date: Sep 2010
Posts: 10,265
Quote:
Originally Posted by djmarchand View Post
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.
Nevertheless, the boat runs at nearly half-speed at 1000 RPM.
__________________
Kar-KEEN-ez Koot
markpierce is offline   Reply With Quote
Old 02-07-2013, 11:57 AM   #9
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,715
"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.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 02-07-2013, 12:04 PM   #10
Scraping Paint
 
City: -
Country: -
Join Date: Oct 2007
Posts: 13,748
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.
Marin is offline   Reply With Quote
Old 02-07-2013, 06:28 PM   #11
Veteran Member
 
Third-Reef's Avatar
 
City: Santa Barbara, California
Vessel Name: Fleur De Lys
Vessel Model: 1988 Nova 36 Sundeck
Join Date: Dec 2011
Posts: 56
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 is offline   Reply With Quote
Old 02-07-2013, 06:57 PM   #12
Guru
 
City: coos bay
Country: usa
Join Date: Oct 2012
Posts: 1,203
Quote:
Originally Posted by Third-Reef View Post
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.
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
__________________
Britt
bfloyd4445 is offline   Reply With Quote
Old 02-07-2013, 07:09 PM   #13
Scraping Paint
 
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
Quote:
Originally Posted by bfloyd4445 View Post
If too lean engine damage can occur ... I think that is correct

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 is offline   Reply With Quote
Old 02-07-2013, 07:40 PM   #14
Guru
 
City: coos bay
Country: usa
Join Date: Oct 2012
Posts: 1,203
Quote:
Originally Posted by RickB View Post
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.
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
bfloyd4445 is offline   Reply With Quote
Old 02-07-2013, 08:32 PM   #15
Guru
 
djmarchand's Avatar
 
City: East Greenwich, RI
Country: USA
Vessel Name: Bella
Vessel Model: Mainship Pilot 34
Join Date: Aug 2012
Posts: 2,882
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 is online now   Reply With Quote
Old 02-07-2013, 09:35 PM   #16
Guru
 
City: coos bay
Country: usa
Join Date: Oct 2012
Posts: 1,203
Quote:
Originally Posted by djmarchand View Post
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
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



Stoichiometric air-fuel ratios of common fuels

FuelBy massBy volume [1]Percent fuel by massGasoline14.7 : 1—6.8%Natural gas17.2 : 19.7 : 15.8%Propane (LP)15.5 : 123.9 : 16.45%Ethanol9 : 1—11.1%Methanol6.4 : 1—15.6%Hydrogen34 : 12.39 : 12.9%Diesel14.5 : 1—6.8%Wood (0%mc)6.1:1source: wikipedia, http://en.wikipedia.org/wiki/Stoichiometric
__________________
Britt
bfloyd4445 is offline   Reply With Quote
Old 02-07-2013, 09:40 PM   #17
Guru
 
City: coos bay
Country: usa
Join Date: Oct 2012
Posts: 1,203
Quote:
Originally Posted by RickB View Post
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.

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
__________________
Britt
bfloyd4445 is offline   Reply With Quote
Old 02-07-2013, 09:56 PM   #18
Guru
 
djmarchand's Avatar
 
City: East Greenwich, RI
Country: USA
Vessel Name: Bella
Vessel Model: Mainship Pilot 34
Join Date: Aug 2012
Posts: 2,882
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 is online now   Reply With Quote
Old 02-08-2013, 05:42 AM   #19
Scraping Paint
 
City: Fort Lauderdale
Vessel Model: CHB 48 Zodiac YL 4.2
Join Date: Oct 2007
Posts: 3,804
Quote:
Originally Posted by djmarchand View Post
But it takes significant hp to run oil and cooling pumps, overcome friction in bearings and rings, etc. which is called parasitic losses.
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.
RickB is offline   Reply With Quote
Old 02-08-2013, 06:53 AM   #20
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 15,919
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?????
__________________

psneeld is online now   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 01:56 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012