Why does a lean engine run hot?

[DFINITLYDISTRUBD]

The bit that causes melted pistons is that lean mixtures extend the combustion duration due to the slow flame speed,

I don't buy that... it is my experience that

lean mixes should burn faster and hotter...run too lean you detonate which hammers the hell out of your engines internals....run to rich your changing plugs but the engine's fine....it's really no different from cranking the oxygen too high on an acetylene torch eventually you reach the point where you get machinegun like reports or one big bang that's the leanest of lean super hot super quick.....course at the other end is the sooty pure acetylene flame which burns fairly cool but very sluggishly...even the sound of this flame lighting is very sluggish and sounds a lot like a bubble in mud.

[marcringuette]

I understand your reaction, but the textbooks seem to be saying that lean burning is slower, not faster.

 

From the book "Lean Combustion" by Derek Dunn-Rankin, p.104 : "A disadvantage of lean operation is that the burning rate is reduced compared to combustion under stoichiometric conditions. ... The reduction in burning rate results in an increase in the overall combustion duration, which in turn leads to increased heat transfer losses to the cylinder walls and a decrease in the overall engine thermal efficiency."

 

Of course, once the cylinder walls are heated up by this effect, preignition can start happening and the usual process of "flame front from spark outward" is messed up. Before that point, though, lean combustion will start with the spark, as intended, but will be slower and less consistent, with some remaining regions of fuel-air mixture not burning until later in the piston stroke.

 

Anyhow, because I was only seeing the "no evaporation of excess fuel" explanation for why lean engines run hot, I wanted to introduce this seemingly important alternate explanation. Late burning of fuel leaves heat around in the cylinder for longer and causes a thermodynamically less efficient cycle. Cylinder walls heat up, and badness begins.

 

Cheers

Marc

 

Ref: search inside "Lean Combustion" book for term "combustion duration"

at URL amazon.com/gp/reader/012370619X

[dingotj]

I was wondering if someone could shed some light on the effects of lean fuel mixtures in a diesel engine. I've had people tell me that it works the same as gas enginges and it burns hotter, and I've had people tell me that they work differently and it burns cooler. Thanks for the help

[marcringuette]

Regarding lean mixtures and diesel engines, I found

a couple of points to consider using search inside

amazon.com/gp/reader/012370619X for "diesel", on page 99,

and also at en.wikipedia.org/wiki/Diesel_engine .

Please bear in mind that I'm all theory, all the time. :-)

 

Diesel engines never throttle the intake air, so at part load

they're always running very lean, and even at full load they're

significantly lean of stoichiometric. They don't seem to have

the problems with slow or incomplete burning of lean mixtures

that Otto cycle engines do -- in fact, somewhat the opposite.

Local pockets of lower temperature where the fuel is not fully

atomized can cause incomplete burning and soot; this happens

more often with the richer mixtures, hence the "black smoke limit"

when you stomp too hard on the accelerator despite the fact that

this richer mixture is still lean of stoichiometric.

 

So, theory suggests to me that for diesel, lean mixtures won't

run hotter.

 

 

Marc

[alexander]

Lets start with the fact that its not an alternative explanation, but it's another piece of the puzzle :naughty:

 

yes lean mix can burn slower then a rich one, the problem is that in order to describe why burn rates go up as the mix becomes leaner and leaner (until it does not burn at all), requires an explanation that touches on everything from chemistry to thermodynamics, and it's something that is actually quite hard for me to understand, but yes, some leaner mixes will burn longer then the richer mixes....

 

( if you feel like reading into the science of it, here's a google book to start with: Combustion: Physical and Chemical ... - Google Book Search )

 

it's way aboe anything i will care to read to answer a post...

[alexander]

Diesel engines never throttle the intake air

you want me to take a picture of my dad's intake manifold in his tdi, or will you just trust my word... they throttle air like any other car does....

[alexander]

They don't seem to have

the problems with slow or incomplete burning of lean mixtures

yes they do... and its not incomplete burning of lean mixtures... lean mixtures burn longer and fuller, rich mixtures burn incomplete, its not pertaining to fuel, it's to the chemistry and the dynamics of the process :naughty:

[marcringuette]

dingotj, here are two articles I just ran across regarding fuel/air ratio

and diesels, focusing on the flip side of the lean mixture problem,

over-fueling and how it leads to overly hot exhaust temperatures.

Sorry that there's a bit of marketing-speak mixed in with the good stuff,

but the articles are a lot of fun.

 

bankspower.com/techarticles/show/25-why-egt-is-important

bankspower.com/techarticles/show/24-nitrous-oxide-the-diesel

 

 

Marc

[alexander]

by the way, you can run gasoline egines lean with addition of gases too, there are some papers on running a lean engine with injection of propane into the mix, to increase the rate of burn and keep temperatures steady... i am not paying 30 some odd dollars to read it, but it was the essence of what they meant from the preview posted...

[jab2]

you want me to take a picture of my dad's intake manifold in his tdi, or will you just trust my word... they throttle air like any other car does....

Not being able to see all the plumbing on my Tdi due to all the cosmetic plastic covers ,I cannot comment on this.

 

What I'm however sure of is the Mercedes Benz OM352 engine in my Unimog does not use a throttle valve to adjust engine speed. The air goes from the filter unrestricted into the inlet manifold. That is also true for all other truck (lowish tech) diesel engines I have seen. The speed of the engine is regulated by the injector stroke length, and therefore fuel volume of the mechanical diesel pump, a quite nifty and expensive little block of engineering I might add.

 

It might be different on the new electronic injector engines which I know has a lot of electronic control and sensors.

 

But then even though there may be a butterfly valve in the inlet track it does not mean that it is there to control engine speed. There are patents on the web where throttling of the intake air charge is proposed to reduce vibration, knocking and noise at idle speeds, yet power control is still done by way of injector timing (electronic injectors).

 

Sorry for being a little OT, but my 2c worth after having had to fiddle with a diesel that ingested water contaminated diesel ±200km from the nearest proper town.

 

BTW, I would love to get detail on the engine controls of modern diesel engines, if someone has a link.

[alexander]

We just had to do some work to the header on my dad's tdi, well he snapped a glow plug when attempting to replace it, so we had to take the head off, so it could be machined out, and trust me, that butterfly throttle valve is connected directly to the throttle assembly (via a wire) and is most certainly there to control the throttle, like in every other engine.

 

Here's what makes NO sense, why would they leave the intake wide open all the time? It makes no sense mechanically, and i have worked on 1 or 2 motorized vehicles before (yeah, those numbers are closer to hundred if not thousand), its one of very few ways to control the mix, how are you going to control air, if you are wide opening the throttle? Now you may see the valve not being directly attached to throttle, it is an electronic valve, so unless you were running the engine, you wont see it move when someone presses on the accelerator. Unless its a jet engine of some sort, i dont see that you can control engine throttle without a throttle body that will limit air intake... any engine in a car, be it boxter, rotary, diesel, natural gas, 2 stroke, 4 stroke, 5 stroke, inline, v, w or otherwise (just not electric or fuel cell) must have a throttle body, that or my understanding of engine mechanics and chemistry and physics of propellants needs to be drastically changed, and memory of many hours spent at the shop working on them erased...

[alexander]

OOoh, i finally figured what system you guys are talking about, you guys are talking about EGR systems that are or rather can be used for throttle management.... yes you can have no air restriction valve.. the thing is you still limit your air intake by controling the amount of exhaust gases you recirculate into the intake, you are not doing this by physical means, but you are restricting air flow still

 

sheesh i thought i was going crazy here...

 

it is infact not a very very common system, that's why i couldnt get what in the world you guys are talking about, and yes it does happen on some bigger diesels and some gasoline engines, if implemented properly it may increase the efficiency in a couple of areas, and diesels do benefit from it quite a lot, yes, though it limits their power. The new Jetta TDI's have this system if i'm not mistaken, my dad's has an earlier EGR system, while it recirculates the gases, it is not the sole means of controlling air intake, and still has a butterfly valve, but with proper ecu software, you can eliminate the need for the valve, yes... once again, you do loose power by decreasing the specific heat of a power stroke with exhaust gases though, so while some cars have this, not all cars do...

[jab2]

Alexander, trust me the MB OM352 does not have any throttle butterfly nor EGR technology. The air goes through a cyclone and then a filter straight into the tappet cover (top black stopper behing MB sign in this photo). It has an internal intake manifold. If you look at this photo of it, you will notice a pivot plate left of the red oil level dipstick, connected to a spring and a shaft going into the picture to the left of the engine (rear side). This shaft is connected on the left side of the engine by solid linkages to the floor throttle pedal and by cable to the hand throttle lever. Following the linkage from the plate to the right, it passes behind the oil filter and connects on the rear of the injector pump (Unit bolted to side of engine in photo). Behind the injector pump is also the set screw for the idle speed. The right side of the pump, were the 6 injector lines exit contain the 6 variable stroke pistons that meters the fuel. The left part of the pump (with the vertical split) is the stroke drive unit into which the throttle linkage go. As you press the throttle it translate the movement into a larger stroke for the fuel pistons. On the left bottom (out of view) is a little cap which can be removed. Inside is a screw which you can adjust to increase the fuel quantity by adjusting the slope of the "delivery map" mechanically. This is the way this engine is tuned to various power outputs for different uses. On my Unimog I can still go higher in power output, but does not have a EGT gauge, which is essential to prevent melt down on long full power usage.

 

BTW, one of the classical, read old, reasons for diesel being lighter on fuel in town driving was that because they have no throttle butterfly there was no pumping loss like on a part throttle gas engine.

[goku]

you want me to take a picture of my dad's intake manifold in his tdi, or will you just trust my word... they throttle air like any other car does....

 

what is "tdi" ?

none of my diesel tractors throttle air intake, or restrict air flow

[DFINITLYDISTRUBD]

Turbo Direct Injected

 

Is a Volkswagen/Audi group diesel motor.

[alexander]

first, jab, props for having a unimog, you certainly dont see a lot of those in the states, and too bad, they are some cool machines, and very, very reliable, i might add

 

ok, ok, i think i finally figured it out, after doing much research and scratching my head i got around the hurdle of controlling air intake in diesels...

 

i think it really depends on the maker, and i think most, in their later development and testing have refrained from using air throttling in diesels... there is very little need for it, because the burning process is controlled solely by the amount of fuel that is injected and its timing, vs the mixture ignition... its mainly due to the timing of when the fuel is injected, i was stuck in the gasoline engine world, but i think i'm over it... my dad's tdi still has a valve, i think the newer jetta's no longer have that design, i will check (i know someone who has a new one :) ) but my understanding of engines is yet again progressed :)

 

I loves learning

[jab2]

Alexander, how's your access to USSR (Russian) military vehicles. The ones I've seen also do not use throttle bodies.

 

All the diesels I have ever seen in such detail, did not utilise a throttle body as power control is as you mentioned by way of fuel quantity. The newer diesels, Tdi/CDI etc, are so covered in decorative plastic covers that it is virtually impossible for one to get a clear view of the engine auxiliaries without disassembling all those stuff, so I do not have a clue how my Tdi operate. I have also read that one of the reasons for lower fuel consumption for a diesel is that it has no pumping loss due to absence of a throttle body.

 

Yes, the Unimog is a nice vehicle. A pity I'm losing mine to rust. My 1979 model (416.162) was imported into SA as a running chassis under agricultural equipment to circumvent the arms embargo in place during the apartheid era. It was give a mild steel body with square tubing, angle iron and sheet metal by the military supply organisation. Due to cost and presumably "low life expectancy" it was not even galvanized. At the moment it is not running and I do not have a covered space for it, so it weathers the full force of sea wind. That is really eating away the metal. I also do not have money to work on it now, so have to see it disintegrating by the day. Hopefully the chassis/drivetrain will be more immune to the rust ants.