Diesel fuel/air ratio 7

[tdirs1]

Does anyone know what ideal fuel air ratio of a diesel is ?I know a petrol engine is 14.7 to 1.

 

[crysta1c1ear]

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Thanks Frankiee.

Let's take a look at your figures.

With a diesel fuel oil consisting of 86.5% C, 13.2% H, 0.3% S and 0%O

86.5/12=7.21 where 12 is the molecular weight of carbon.

So ignoring the 0.3% sulphur, we can pretend your hydrocarbon is C7.21H13.2. (I'm not saying the molecules are that short, I'm just thinking about the carbon hydrogen ratio.)

CxHy + a(O2+0.79/0.21N2) => xCO2 + y/2H2O + a*0.79/0.21N2
where a=x+y/4

in this case, a= 7.21 + 13.2/4 = 10.51

AFR = a(MWO2+0.79/0.21*MWN2)/MWfuel
AFR = 10.51 (32 + 28*0.79/0.21) / (86.5+13.2)
= 10.51 * 137.33 / 99.7
= 14.42

14.55 Kg air to Kg fuel
So a rough calculation for your fuel (ignoring the sulpher in the fuel, argon in the air, etc) gives 14.42 which is pretty close to the figure you gave - not that I ever doubted you, ..., more a question of trying to understand why things are the way they are.

So why the difference to the earlier estimate?

If hydrogens were about 2:1 with the carbons, we could expect an AFR a bit below 15 as indicated earlier.

7.21*2= 14.42.
The actual percentage hydrogen you give is 13.2, ie less that 2 hydrogens per carbon.

The picture above is a naphthalene molecule. Similar molecules in diesel allow the ratio of hydrogen to carbon to be lowered to less than 2 to 1. The article where I'm leaching the above picture from

http://www.zymaxforensics.com/forensicsprimer/mainpageorigin.htm

gives typical quantities of naphthalene related molecules in diesel. The quantites are significant. Bearing that in mind I think your figure of around 14.5 is better than my estimate of 15, for the reasons stated.

=

franzh (great posts on gas engines by the way franzh, thanks) mentioned that diesel engines (by which I really mean compression ignition) could run on just a smidgen of fuel. I thought about that.

Since the fuel is ignited by the temperature of the surrounding compressed air I assume that there is no limit to how lean the mixture can go - two molecules remote from each other will both ignite. It's just that going too lean would mean that friction would cause the engine to stop. On the other hand I assume that a spark ignited engine requires a sufficient concentration for a flame to propagate.

 

[tbuelna]

Direct-injected, compression-ignited engines always combust at near stoichiometric, if the engine is designed properly. The fuel injected will only combust when sufficient oxygen is present at the flame front. The fuel will continue to burn as long conditions are suitable within the combustion region (ie. oxygen, fuel and heat). If the available oxygen within the chamber is used up before the available fuel, soot is created. If the heat in the combustion zone is reduced sufficiently, due to expansion of the end gas and slow combustion, combustion halts and the remaining fuel and oxygen are dumped out the exhaust. If more oxygen is pumped into the woking chamber than is necessary for the amount of fuel injected, then engine efficiency is reduced.

The basic nature of a compression-ignited means that it can run "lean" or "rich" without doing damage to the engine. Ultimately, the amount of fuel delivered is dictated by the load on the engine, and not by the amount of air flowing past the throttle as in an SI engine.

 

[Dzltec]

When we dyno vehicles to fault find, we also check boost pressure and air fuel ratio with a wide band lamba senor at the exhaust pipe.

The ideal ratio is 18:1 for good power and very minimal smoke emmisions.

This has also been proven after we reconditioned the pump and injectors for this vehicle. Too low a/f and they get smokey, too high they can lack power. At a cruise situation they can go as high as 40:1.

You can go lower with a/f but the exhaust temps start to rise pretty quickly on a turbo application.

Hope this helps.

 

[SomptingGuy]

Does anyone know how low you have to go with a/f to kick off a DPF regeneration?

 

[turbomotor]

Hello tdirs1:

Your string generated a lot of discussion. We engineers are an opinionated lot, aren't we?

I'll add to the fray: Ivymike suggested some IC engine texts that may be helpful. My personal favorite is a book by Obert of U of Wisconsin - Madison. Obert's book includes a chapter on fuels for SI and CI engines that includes many neat hydrocarbons and the combustion data associated with those neat hydrocarbons. I find the tables to be very practical and useful when considering potential alternate fuels.

Also, it may be hard to find, but Pratt & Whitney used to publish an aeronautical handbook that had gas properties for products of combustion (100%, 200%, and 400% theoretical air) of a CnH2n hydrocarbon with air. I have found this data to be very useful, especially when evaluating the available energy in the exhaust products of a turbo diesel motor.

Dick

 

[tdirs1]

Wow some good info fellas thanks,especially about oxygen levels in exhaust.Ive been toying with the idea of installing a lambda sensor to keep track of oxygen levels but ive not had a base reading to go for.Ill tell you all the base of what im doing,im building a Golf a2 with a 16v 2.0 tdi engine in from an a5 Golf,it should run Bosch edc 16 management but ive decided to run the earlier Bosch edc 15 which doesnt require a motorised throttle body for egr.I ve ported and flowed the cylinder head had some injector nozzles flowed,installed a 4 branch stainless exhaust manifold and a Holset HX35 hybrid turbo.I will be using it primarilt on the road so good emission will be essential and occasionally be heading for the drag strip.The power i am aiming for is between 310 to 330 hp.

 

[tdirs1]

I will be using a lambda sensor and reader to keep track of emissions.There are some pictures of my turbo install on tdi club website on the news from the batcave thread.

 

[Rob45]

SomptingGuy:
My guess is you can't go low enough on A/F to kickoff regeneration of the diesel particulate filter since going low drops the exhaust temperature.
That's why Cat is using the fuel-added afterburner.

 

[ivymike]

Somtinguy, you should start another thread. I'll do it for you, in fact. thread71-131992

 

[Rob45]

Franz:
You made a comment 'way up-thread that isn't what you'd call entirely true:
" Now, since almost all of the major OEM diesel engine manufacturers use EGR..."

In fact, I only know of Cummins employing EGR; Detroit may, also, but the two of them together don't produce nearly as many diesel engines as does Caterpillar, who do NOT use EGR.

 

[franzh]

No challenge from me on raw numbers even if I would have to think about it for a bit, but most of the worlds advanced diesel engine builders have acknowledged that the use of EGR, Urea injection, aftertreatment, particulate traps, will be necessary for 2007+. Cats use of their advanced engine management system has been successful, but my inside sources say it was a real challenge to get and keep it there. Cat makes a great product and their market share is well deserved, but this is no place for semantics.

The diesel industry has its work cut out for itself for the next two iterations of emission certification. I read Diesel Progress monthly, and my related trade journals, plus sit in on our engine development meetings, and the topic of diesel emission attainment is a top and hot topic. Almost everyone agrees that EGR is the most successful means of meeting '07, even with its problems. I feel we havent seen the full breadth of the research needed for the next 10 years.

Let the hounds loose, the advanced diesel technology will persevere.

This is certainly a new topic area, Rob45, why don’t you kick it off?

Franz

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[UWH]

Rob45

As far as I know pretty much all the major diesel manufactures except Cat use EGR to meet Tier III EPA emissions.

Cat took a bold step in developing their ACERT technology instead of going the EGR route. And they paid something like $7K per on-highway truck engine in penalties a couple of years ago because they weren't ready with the technology.

I believe the major hurdle with EGR going forward is the size of the air-to-air aftercooler needed to cool it and overheating in souther climes.

 

[SomptingGuy]

I was under the impression (from the industry rags) that EGR and SCR (with urea) were competing technologies and that there was a broad split between Europe/USA as to which to use. The problems with each approach being that EGR compromises efficiency; SCR requires some method of policing the urea addition.

 

[franzh]

EGR is used pre-combustion, urea is used post combustion. There is no reason they cannot be used together. Your second comment about policing the urea is certainly valid.
Franz

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[turbocohen]

Urea is a huge nono upstream, too corrosive.

 

[tdirs1]

Seems to me Audi got the ratio pretty much spot on judging by the lack of smoke and noise on the R10 at Sebring.

 

[waross]

I'm puzzeled by the comments that the stoichiometric mixture is not or cannot be controlled on a diesel engine. I was under the impression that that was the purpose of the aneroid valves used with early turbo chargers. Didn't the aneroid valve hold the mixture to near stociometric at full trottle until the boost pressure built up?

 

[GregLocock]

No

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[ivymike]

the a/f ratio IS controlled, just not to the stoichiometric a/f ratio!

 

[tdirs1]

Whats an aneroid valve?