"lean" & "rich" diesel mix temps.

Does a "lean" diesel mixture burn hotter than a "rich" one? I know there is no such thing as lean and rich in the realm of diesel but lets say does dumping more fuel into the combustion chamber increase the temp or decrease it.
I personally know (in MY mind at least) the answer but i saw this described incorrectly in a rather long thread and could not find any response correcting the original poster.
I may be wrong or missing something so i just wanted to make sure.
Mike P.

In simplest terms without taking anything else into consideration (turbo boost, etc), an engine running fuel rich, or simple over-fueling, will have cooler combustion temperatures.

Over-fueling: lower combustion temps, high smoke/soot, & low NOX

Under-fueling: higher combustion temps, low smoke/soot, & high NOX

Getting the right balance between low soot and low NOX is walking a tight rope for diesel engine designers.

I disagree, at least with a diesel engine in mind.

If I run with a light pedal (no smoke) I get cooler temps on the pyro.

Any smoke at all in the mirror from light to heavy (in each case is considered "overfueling" on a diesel) results in increased pyro temps which are taken in the manifold on my car.

I am not a mechanic however.

BKmetz said:

In simplest terms without taking anything else into consideration (turbo boost, etc), an engine running fuel rich, or simple over-fueling, will have cooler combustion temperatures.

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I put that statement first because I knew I would get a contrary opinion. There are a million variables that can change the operating parameters and get contary results. The basics are as I stated.

If I run with a light pedal (no smoke) I get cooler temps on the pyro.

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Your statement confirms my post. In this condition you are not running rich or lean, you are running your car correctly, the way it is designed to run. The ECU computer is doing its job and keeping your car in its "zone", keeping soot and NOX at a minimum.

I'm not sure the lean/rich temperature rules apply the same to a variable ration diesel as they do to a fixed ratio gasser. Because if that were the case, the diesel engine sitting idling running super, super lean would melt holes right through its pistons!

More fuel means higher temps in a diesel. If you can keep the boost high, it'll help lower the temps, contrary to a gasser.

I know Jeff at Rocketchips says the only way to get EGT back down is with a bigger turbo, which makes sense. EGT skyrockets sometimes to over 1600 degrees F on a RC3 car with big injectors on a hard launch!

Turbo or non turbo it does not matter. More fuel means hotter temps. The more fuel the hotter the cyclinder temps and the more power you have.

Greg

Your statement confirms my post. In this condition you are not running rich or lean, you are running your car correctly, the way it is designed to run. The ECU computer is doing its job and keeping your car in its "zone", keeping soot and NOX at a minimum.

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That sounds like a statement for a gas engine being at 14:1 ideal mix for a gas engine.

A diesel almost always runs lean since there is no throttle plate.

At idle it could be as high as 100:1 ratio (I'd say that's lean)

Even at full load the ratio doesn't get close to a gasser 14:1 unless you have some power mods as well as noticable black smoke.

This is one factor as to why a diesel is much more efficient than a gasser.

Pedal input on a diesel can be almost directly related to the fueling.

A light pedal would be a lean condition and produce cooler EGT.

I am aware that engine load would account for EGT, but generally speaking the more load on the engine, the more rich the fueling, and the more EGT. They are corelated I guess.

On a gasser a lean setting does increase EGT, richer generally cooler EGT.

But gas and diesel engines are completely different EGT wise because of no throttle plate on the diesel and a spark ignition on the gas.

A gas engine will always run in the "zone" as you say at 14:1 A/F unless something is fouled or power enhancements have been added. RPM and power are controlled by the throttle plate.

A diesel will vary the A/F ratio depending on operating conditions mainly based on the position of your right foot. RPM and power are directly related to the amount of fuel injected per stroke since the quantity of air going through the engine is not variable (no throttle plate, and to be simple not bring the turbo into play).

That is all true EXCEPT that there are some diesels that do in fact have a throttle plate. I am not sure how the older cars' throttle plays in (as found on an old 240D), but on the PD TDI it is mainly there to do two things: one force more air to be pulled in through the EGR , and two to tighten up the HUGE range of air/fuel ratio so that the planar O2 sensor can better work with the EDC16 system to keep enough leftover fuel (i.e. "rich" condition) to keep the catalyst hot and functioning.

The PD's air/fuel ratio is not nearly as wide as a "conventional" diesel. It is that fact that makes me think adds to the PD's hit in fuel efficiency and the smoke/smell that some people notice on the stock PD but did NOT notice on the stock VE. Although my PD seems pretty dog-gone clean to me.

Guys don't confuse the volume of heat with actual combustion temperatures.

At idle the engine is producing a low volume of heat HOWEVER the actual combustion temperatures are high(er) as a result of lean combustion (abundance of oxygen) and this results in the combination of nitrogen and resulting increase in NOx.

Under Full load (to demonstrate the range) combustion temperatures are lower (core of the combustion bowl) due to less oxygen available due to higher fuel volume being burned. This richer state of combustion lowers the actual temperature HOWEVER the volume of heat is higher. The higher volume results in more heat reaching the turbo thus the reason you will see higher TIT's(turbin Inlet Temperatures).

There is a very wide range of fuel to air ratios which are entirely dependant on the load the engine is under. Higher loads cause enrichment of that fuel air ratio but the ECU will limit the IQ, injection timing, Boost pressure and other factors to limit that fuel air ratio within the range of no more than 35:1 (in most cases) to avoid an over-fueling situation and excessive soot formation.

The ECU's smoke map determines how best to maintain the fueling control within the emissions limits.

At low loads you have maximum combustion bowl temperatures crossing over to lower combustion bowl temperatures but higher exhaust gas volume at maximum load. Injection timing controls to an extent the temperature of the combustion gasses at both ends of the load spectrum. In all cases fuel quantity controls the volume of those gasses which are registered at the Turbine Inlet.

For those who are running larger injectors, you have the fueling potential to melt the turbo, you can therefore fine tune the TIT's by using the injection timing to raise or lower your maximum temperatures. Advancing has the effect of lowering TIT's by causing the exhaust gasses to have more time to release energy in the form of work, however the more fuel you inject the less control you have due to higher fuel/gas volumes.

Confused?
DB

Drivbiwire said:

Confused?
DB

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Yes.
Correct me if i am wrong, but a 1000 degree flame only has the potential to heat up something else to 1000 degrees, no more.
If that is correct, then an enriched diesel mixture actually burns at whatever those elevated temperatures are (1600 degrees +), its not an "optical illusion" brought about by the shear volume of a lower temperature combustion. Volume does not explain those high temps. Volume will only affect the speed at which that mixture heats up the area around it to its maximum potential temperature.

Also, is not an engine, lets say running full load at 3000 rpms, pumping out the same volume regardless of whether it is running perfect (meaning utilizing all oxygen with no leftover fuel - lambda?) or rich? VE is the same, the only difference is the efficiency in the use of the available oxygen.

Keep it simple.

At idle the actual temperature of the flame due to extremely lean burn is very high if I recall in excess of 3000F in the core of the combustion bowl . The excess oxygen and nitrogen provide the ingredients for NOx formation. NOx skyrockets when combustion temperatures exceed 2900F, EGR is used mostly at idle since this is the highest NOx formation period, the EGR trims the amount of available oxygen thus lowering peak combustion core temperatures in the bowl so combustion temperatures fall below the 2900F NOx formation threshold.

As load increases fuel volume is increased thus "Enriching" the mixture and lowering the temperature slightly of the flame in the combustion bowl due to a reduction in available oxygen and nitrogen. This also results in lower NOx output. However the amount of wasted heat leaving the cylinder is increased which is why you will see higher temperatures when read at the turbos inlet.

Remember that the turbo increases the volume of air pumped through the motor and also increases the available air for combustion. This also provides the ability to have more losses exiting the cylinders which carries the wasted heat of combustion.

DB

DB

I'm sorry to say to you all that LanduytG is closest to having it right. Within the realm of Diesel engines, more fuel for a given amount of air results in increasing temperature. I will expound on this in a moment

I believe the confusion stems from the concept of stoichiometry and stratification.

A very basic observation in combustion is that, regardless of fuel, flame temperatures are at a maximum at a point very close to the stoichiometric air/fuel ratio. On fact, This peak occurs very slightly lean of stoichiometric, around Lambda = 1.1 for many hydrocarbon fuels. It its not entirely coincidence that NOx peaks also at the same Lambda value.

The second basic tenet one needs to understand is the difference between a homogeneneous charge and a stratified charge, because that is where gasoline and Diesel engines begin to diverge.

Gasoline engines (except specific stratified-charge designs) operate under a homogeneous charge, where fuel and air are pre-mixed at a uniform air-fuel ratio throughout the combustion chamber at the time of ignition.

In contrast, a Diesel engine, which injects the entire fuel charge within a short window in terms of crank angle, has a decidedly non-homogeneous, or stratified charge. This means that within the volume of the combustion chamber (i.e. the piston bowl), there are extremely lean and extremely rich regions, and everything in between. Combustion initiates at sites that are sufficiently hot and having air fuel ratios within the range of combustible limits, starting with the sites that are near stoichiometric.

Much research and development by engineers is focussed on making the mixing of fuel and air as rapidly and evenly as possible within this extremely short window of time, which has spurred the development of high-pressure fuel injection and injection rate shaping strategies, since the above-described stratification results in both unacceptable levels of NOx and soot emissions.

Limiting our discussion to Diesel engines, we first have to distinguish a global, or overall, air-fuel ratio, as well as a local one. A Diesel engine ALWAYS operates at a globally lean air-fuel ratio; the engine would reach a limit of unacceptable smoke emissions around 18:1. When an engine is idling or operating at low load, the global A/F ratio can be much higher, like 100:1.

Regardless, even through the engine is operating globally lean, there are sites as said before that are extremely lean and extremely rich. The interface of the flame with the rich regions are sites of soot production and the locale of combustion in the stoichiometric and lean-of stoichiometric regions generate much of the NOx.

The only time when combustion temperatures can go down in a relatively rich mixture in a Diesel engine is when the engine is grossly overfuelled, such that the extremely-rich sites I described above dominate in the entire combustion chamber, which is manifested by extreme smoking.

It is important here to note that, whether Diesel engine or gasoline, EGT cannot be taken as a direct, 1:1, relationship with flame temperature nor peak cycle temperature. A very lean-burning gasoline engine will have low peak cycle temps but soaring EGT. Similar analogies can be made for Diesel engines.

Take home message #1 is that you need to understand basic concepts of stoichiometry and how flame temperature is a function of this stoichiometry; how they are different between Diesel and gasoline engines; the difference between a homogeneous charge and a stratified-charge; and the difference between global- and local stoichiometry.

The take home message #2 is that within the normal range of air-fuel ratios seen in a Diesel engine, more fuel = more heat = higher combustion temperature.

Take home message #3 is that EGT does not tell the full story about peak combustion temperature.