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.

 

[ivymike]

I believe it was to restrict fuel delivery until boost reached desired levels, to keep the A/F ratio appropriately lean (and avoid smoke).

 

[waross]

An aneroid valve is a device that measures the manifold pressure. It controls the fuel delivery. It prevents the fuel control system from delivering more fuel than there is air to burn completely. They were used on truck engines to prevent the plume of black smoke that occurs with full throttle application at low RPMs, before the turbo boost builds up.
GregLocock's post is probably correct in that the early aneroids often didn't work as planned and over-restricted the fuel at low RPM so much that in a tough spot, a truck engine often could not develop enough torque to get the truck moving. Construction sites, logging, gravel pits, starting on steep grades etc.
Many aneroids had to be removed and given a "flotation test" before the truck could do its job.
(Flotation test,- Throw it in the river and see if it floats!)
respectfully

 

[drwebb]

"

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

gives typical quantities of naphthalene related molecules in diesel"

Be aware that "napthenics" are saturated hydrocarbons (i.e. not aromatic) very different from naphthalene- a particular 2-ring aromatic compound that smells like mothballs. It is unfortunately confusing nomenclature, but while diesel fuel is made up of almost half naphthenes, there is little if any napthalene in it.

Likewise, the common solvent called "naptha" has no naphthalene in it.

 

[GregLocock]

waross, diesels do not work anywhere near stoich for most of their operating range.

Cheers

Greg Locock

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

 

[patprimmer]

I would have guessed that at full throttle, slightly richer than stoic would give maximum power irrespective of pollution, and about stoic or just leaner would give maximum power without excess smoke.

Like I say, just a guess, and obviously much leaner than stoic at part throttle.

I use the word throttle figuratively for the speed and power control device or the pedal on the right hand side of the floor.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[waross]

Hi greg

waross, diesels do not work anywhere near stoich for most of their operating range.

I knew that, actually.
I appologise if I wasn't clear in my previous post.
I was under the impression that the aneroid valve knew that also. However, in the pre-computer engines, the fuel charge at low boost would theoretically go rich of stochiometric when the pedal went to the metal. At this point I understood that the aneroid valve would act to hold the mixture at stociometric or less to avoid the smoke plume.

Didn't the aneroid valve hold the mixture to near stociometric at full throttle until the boost pressure built up?

To answer my own question, it was supposed to, but it didn't work very well and it often over compensated. In my area, in those days, most of the drivers and/or owners of turbo-charged work trucks gave the aneroid valve the flotation test. (Dump trucks, logging trucks, any trucks liable to face unusual conditions requiring full torque to get moving.)
Am I misusing any technical terms here. I thought that I understood what stochiometric meant.
respectfully.

 

[GregLocock]

Stoichometric is the perfect air fuel ratio, ie two oxygen atoms per carbon, one oxygen atom per two hydrogens.

Now, I think it is a bit of a furphy (red herring) in the case of a diesel, as they are effectively stratified charge engines, so the fuel does not come in contact with all the air. Nonetheless by the normal SI definition (mass air/mass fuel) a diesel is operating lean, even when it is belching black smoke. If it were truly operating at stoke then it would produce the same torque per unit swept volume as a gasoline engine, for the same VE (crudely), NA.

For instance, this 4 litre gasoline engine I happen to have under my desk, will max out at 383 Nm of torque, whereas an equal displacement non turbo four stroke diesel will struggle to reach 200 ft lb, or 260 Nm, according to this:

http://www.off-road.com/tlc/faq/engine.html

Cheers

Greg Locock

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

 

[tdirs1]

Suddenly Greg i just got it!!so the reason it is a red herring is due to the fact the diesel is injected into compressed and heated air into the cylinder and not into the airstream as a petrol.

 

[CCycle]

CI engines usually top out at 0.6 to 0.7 of stoichometric.
That is the number given by Heywood.
Any more fuel results in smoke.
The problem is to get the fuel in contact with the O2 and as mentioned above, the burn front will be in zones where the ratio is near stoichometric.
For this reason Direct Injected CI engines will never reach the specific power of a SI engine at a given input pressure.
The DI CI engine always runs lean which has serious consequences for catalysis.
Reducing cats do not work hence the problem in cleaning up NOs.

 

[GregLocock]

260/383= 0.68

funny that!

Cheers

Greg Locock

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

 

[globi5]

Just to clarify:
Reducing cats do work in cleaning up NOx but require urea.
(They sell it as AdBlue:

http://en.wikipedia.org/wiki/AdBlue)

 

[SD22]

Franz (et al): I know this is an OLD post but I found it via a Google search.

Air throttled diesels ARE around. I drive one ... a Nissan SD22 naturally aspirated four cylinder. It has a Bosch inline injection pump with a PNEUMATIC governor (and a centrifugal governor). The throttle pedal is cabled to an air throttle body. This provides a vacuum signal to the pneumatic diaphragm on the injection pump ... which in turn positions the fuel rack depending on the vacuum vs. spring signal.

-Philip

 

[Rob45]

UWH wrote:
"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."

Another big hurdle may well turn out to be the corrosive products of combustion that condense out in your shiny new aluminum intercooler...

 

[JackFu]

A Hyndai diesel tubocharged engine is also equiped with a throttle. if there is no throttle, when the intake air is boosted, the pressure at the EGR outlet may be higher than the pressure at EGR inlet, so EGR function is invalid. I think that a throttle is used before the EGR outlet can control the pressure, as to ensure the EGR function working.

-Fu

 

[kenre]

SD22,

Thats the engine i seen it on!! but this engine was fitted into a Holden Panelvan..... No idea why!
Any ideas why they used that system? must have something to do with reducing black smoke from overrich mixtures.

Ken

 

[blondesgetit]

Pump systems use a throttle. Fuel injectors are self throttling. I'm probably going to over-simplify but in Henry Ford's world that was the way to progress.
In electronic fuel injection, fuel is injected thru a narrow port that determines PSI. This is where the mathematics begins and it ends at the injector tip. Everything between is a created environment, subject to assigned parameters and machining variation. Black smoke is indicative of poor surface finish. It allows fuel to pocket. This alters the oxygen mix when it releases and is not likely to coincide with combustion. Black smoke is the result of unspent fuel releasing due to vibration. Regarding the use of test fluids - Do any of you drive a vehicle? Go park it anywhere for 3 months and look at the parts. Water of any kind is unacceptable; alcohol can be corrosive. Viscor is the correct fluid BUT it needs to be circulated.
Within the injector, conductuity and tempurature is held relavent to incoming fuel and air. The goal is to keep the fuel mist airborne and at a sustained pressure while any over delivery is rapidly burned to a carbon which is systematically directed to a dump port that recycles any unspent waste. Composition weight assists the process.

 

[SD22]

Ken: The Nissan SD22,23,25 naturally aspirated often have the air throttle/pneumatic governor Bosch inline pump. But depending on delivery location, these same engines may also be fitted with the more familiar Bosch VE rotary pump which does not require an air throttle because the VE has a mechanical throttle governor.

The RBD-MZ inline pump uses an air throttle for NOX emissions. Remember, these little diesels have 23:1 compression, indirect injection, and base injection timing of 20 degrees. Combined, these three factors contribute significantly to NOx emissions. So ... reduce compression at low engine loads by throttling intake air (reduces compression). Were you to retard base injection timing significantly, exhaust soot increases (not desireable).

In the USA for emissions reasons, the SD22 was fitted with the air throttle/pneumatic governor pump and EGR.

-Philip

 

[oxilume]

TDIRS1,

Your answer is simple. It is the LEL and UEL of a diesel and air mixture;

LEL = .6%
UEL= 7.5%

see:

http://en.wikipedia.org/wiki/Explosive_limit

If you are going to conduct any R&D with flammables you better know the LEL and UEL with respect to the oxidizer. 70 or 90% Hydrogen Peroxide is much different then air. Likewise, if using O2 don't forget that air is only 21% O2.

When running hydrocarbons through a high temperature plasma system thoroughly having control of the oxidizer and fuel is critical if you don't want to launch the plasma torch and yourself into sub-orbit.
see:

http://www.plasmawhirl.com

Todd

http://www.oxilume.com