Lean-burn engines; anyone make them?

[AP9]

Originally Posted By: rslifkin
Originally Posted By: 440Magnum
power output at a given throttle setting is also reduced so any time power is needed you have to enrich a little or flow a bigger volume of lean mixture


This is actually how lean burn saves fuel. By opening the throttle further to get a given amount of power, there's less pumping losses in the engine, increasing efficiency.


+1. The air:fuel ratio is not constant across all RPMs and BMEP/load/torque. Not even in our current stoichiometric engines.

 

[Cujet]

Originally Posted By: 440Magnum
its also very hard on the engine hardware. Spiky high combustion chamber temperatures and free oxygen running around to oxidize the overheated metal components.

Also, its something of a false economy. Yes, the mixture is lean, but power output at a given throttle setting is also reduced so any time power is needed you have to enrich a little or flow a bigger volume of lean mixture, either of which negates the gain. Staying nearer stoichiometric, designing for more complete combustion, and applying things like DFCO are much better paths to overall fuel efficiency than lean-burn.


Lean operation results in lower combustion temperatures, and longer combustion burn times. At modest RPM's lean burn results in lower cylinder head temperatures, as even though the combustion is slower, it completes well before bottom dead center.

Also, lean operation is, absolutely, without a doubt, a more efficient way to make power. There is no "false economy" with it. BSFC improves as mixture is leaned beyond stoic, and in a properly designed combustion chamber, will peak at between 17/1 and 20/1 air/fuel (depending on the engine of course) .

As noted, lean operation results in a power loss. This requires more throttle opening to regain the lost power. This has a positive result, in that pumping losses are reduced, further improving BSFC numbers.

Today, there is a way to achieve a lean burn engine, with low NOx. The engine cycles between stoic and as lean as 20/1. With short periods at stoic. The exhaust contains a Lean-NOx trap that converts the stored NOx during the short period of "rich" operation.

In a Ford 5.4L BSFC numbers are as follows:

1) Stoic 272 g/kwh
2) Lean burn 20/1 near unstable, 222 g/kwh
3) cyclic mode 255

 

[OVERKILL]

Originally Posted By: Cujet


Lean operation results in lower combustion temperatures, and longer combustion burn times. At modest RPM's lean burn results in lower cylinder head temperatures, as even though the combustion is slower, it completes well before bottom dead center.


The only part of your post I have an issue with it this. An engine running lean has HIGHER EGT's, not lower. A lean engine will cause the headers to glow (as many a hot rodder can attest to) unless we are looking at relatively lean vs really lean here

 

[rslifkin]

Originally Posted By: OVERKILL
Originally Posted By: Cujet


Lean operation results in lower combustion temperatures, and longer combustion burn times. At modest RPM's lean burn results in lower cylinder head temperatures, as even though the combustion is slower, it completes well before bottom dead center.


The only part of your post I have an issue with it this. An engine running lean has HIGHER EGT's, not lower. A lean engine will cause the headers to glow (as many a hot rodder can attest to) unless we are looking at relatively lean vs really lean here

As you go a little leaner than stoich, EGTs will go up a bit. Once you get over the hill, they drop off as you continue to go leaner.

 

[Cujet]

Originally Posted By: OVERKILL
Originally Posted By: Cujet


Lean operation results in lower combustion temperatures, and longer combustion burn times. At modest RPM's lean burn results in lower cylinder head temperatures, as even though the combustion is slower, it completes well before bottom dead center.


The only part of your post I have an issue with it this. An engine running lean has HIGHER EGT's, not lower. A lean engine will cause the headers to glow (as many a hot rodder can attest to) unless we are looking at relatively lean vs really lean here

This is incorrect. Peak EGT generally occurs at an AFR of around 14.7- 15.0 to 1 on gasoline. This is far too lean for maximum power and is dangerous under continuous WOT conditions. Many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry- about 15 to 1 for our purposes. If you go richer than 15 to 1, EGT will drop and if you go leaner than 15 to 1 EGT will ALSO drop. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will occur at an EGT somewhat colder than peak EGT.

In cruise conditions, such as a car on the highway, operation "lean of peak EGT" can result in as much as a 25% improvement in MPG, with some well configured engines. While resulting in LOWER exhaust valve temperatures.

Interestingly, engines configured with high compression, high swirl combustion chambers and staggered intake valve openings can very effectively run (at modest loads, such as highway driving) at leaner than 17 to 1 and often as lean as 20 to 1.

Remember, such a lean mixture burns slowly and won't detonate. Counter intuitive, I know. But true.

 

[OVERKILL]

Originally Posted By: Cujet
Originally Posted By: OVERKILL
Originally Posted By: Cujet


Lean operation results in lower combustion temperatures, and longer combustion burn times. At modest RPM's lean burn results in lower cylinder head temperatures, as even though the combustion is slower, it completes well before bottom dead center.


The only part of your post I have an issue with it this. An engine running lean has HIGHER EGT's, not lower. A lean engine will cause the headers to glow (as many a hot rodder can attest to) unless we are looking at relatively lean vs really lean here

This is incorrect. Peak EGT generally occurs at an AFR of around 14.7- 15.0 to 1 on gasoline. This is far too lean for maximum power and is dangerous under continuous WOT conditions. Many people think that the leaner you go, the higher the EGT gets. This is also incorrect. Peak EGT occurs at stoichiometry- about 15 to 1 for our purposes. If you go richer than 15 to 1, EGT will drop and if you go leaner than 15 to 1 EGT will ALSO drop. It is VERY important to know which side of peak EGT you are on before making adjustments. It is safe to say that peak power will occur at an EGT somewhat colder than peak EGT.

In cruise conditions, such as a car on the highway, operation "lean of peak EGT" can result in as much as a 25% improvement in MPG, with some well configured engines. While resulting in LOWER exhaust valve temperatures.

Interestingly, engines configured with high compression, high swirl combustion chambers and staggered intake valve openings can very effectively run (at modest loads, such as highway driving) at leaner than 17 to 1 and often as lean as 20 to 1.

Remember, such a lean mixture burns slowly and won't detonate. Counter intuitive, I know. But true.


OK, my position relative to what constitutes "lean" here is off. I was thinking WOT conditions, most of the cars I'm thinking of are running 12:1-13.5:1, not stoich, as stoich is "too lean" at WOT and results in glowing headers and can lead to potential engine damage, LOL! But stoich isn't technically lean, which is where the disconnect was. Fuel is added at WOT to both increase power output, lower combustion temps and control detonation. Too much fuel of course will lower power output, but peak power always occurs below stoich, I'm sure we can agree there. Go above that (heading towards stoich) and you start losing power, driving up EGT's to dangerous levels, can burn valves....etc.

I didn't know that EGT's dropped off though when you went extremely lean however, so I learned something there, thanks for that

 

[NateDN10]

To make sure I'm understanding this properly:
The reason gas engines in the US don't use "lean-burn" when cruising is not that high combustion chamber temperatures increase NOx production, it's that 3-way catalysts alternate between oxidation and reduction, and thus require AFR to oscillate from just rich of stoich to just lean of stoich?

 

[andyd]

Originally Posted By: NateDN10
To make sure I'm understanding this properly:
The reason gas engines in the US don't use "lean-burn" when cruising is not that high combustion chamber temperatures increase NOx production, it's that 3-way catalysts alternate between oxidation and reduction, and thus require AFR to oscillate from just rich of stoich to just lean of stoich?
I would say for all the reasons you list. Both the NOx and jittering of the AFR

 

[blackman777]

In Honda's ULEV engines the catalyst captures the NOx. Then after a few miles the engine switches to standard burn (14.7:1) to release the NOx as N2 and O2. (The SULEV/PZEV engines have lean disabled.)

Yesterday I set the cruise on my Civic Hybrid at 46 miles/hour for a nice leisurely drive.

The interstate was mostly empty, and the car returned 74 MPG..... that's what lean-burn can do.

 

[artificialist]

Would it be possible to use urea/DEF with a lean burn gasoline engine? It seems to me like it would avoid some of the limitations.

I don't actually want such a car, I am just exploring ideas.

 

[blackman777]

I thought urea was for burning-off the Soot which gets trapped in a diesel particulate filter.

As such it would have no relevance to a gasoline engine, which has no soot (according to the EPA anyway).

 

[itguy08]

Originally Posted By: JHZR2
In our infinite wisdom, we trade TONS of CO2 and other garbage for a tiny bit of NOx.


Yes but NOx can't be taxed nor can it further the global warming, err, climate change scam.

 

[blackman777]

Insightful! More CO2 == more credits sold.
Originally Posted By: OVERKILL
OK, my position relative to what constitutes "lean" here is off. I was thinking WOT conditions, most of the cars I'm thinking of are running 12:1-13.5:1, not stoich, as stoich is "too lean" at WOT and results in glowing headers

Honda's lean burn disables itself at WOT, or pretty much any throttle setting more than 1/4th. Probably to prevent the very condition you describe. Their lean run only works during light power demands (like how VCM works).

 

[NateDN10]

Originally Posted By: blackman777
I thought urea was for burning-off the Soot which gets trapped in a diesel particulate filter.

As such it would have no relevance to a gasoline engine, which has no soot (according to the EPA anyway).

Nope. It reduces NOx to N2 and H2O.
http://en.wikipedia.org/wiki/Diesel_exhaust_fluid

 

[AP9]

Originally Posted By: NateDN10
Originally Posted By: blackman777
I thought urea was for burning-off the Soot which gets trapped in a diesel particulate filter.

As such it would have no relevance to a gasoline engine, which has no soot (according to the EPA anyway).

Nope. It reduces NOx to N2 and H2O.
http://en.wikipedia.org/wiki/Diesel_exhaust_fluid


+1. The DPF is totally separate from SCR aftertreatment.

 

[blackman777]

Huh..... both my insight & Civi hybrids trap & then burn-off NOx without ever using any urea. They just switch switch to rich-burn mode. I wonder why that tech wouldn't work in diesel.

How does the DPF burn-off the trapped soot?

 

[jrustles]

Originally Posted By: blackman777
Huh..... both my insight & Civi hybrids trap & then burn-off NOx without ever using any urea. They just switch switch to rich-burn mode. I wonder why that tech wouldn't work in diesel.

How does the DPF burn-off the trapped soot?


Whats on the Honda is a NOx trap- it's the other solution to DEF.

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

Manufacturers commonly opt for the SCR DEF route, because NOx traps are very expensive parts and DEF becomes the operator's cost. It's basically a special catalytic converter.

DPFs are like big cans full of a substrate with a high surface area, designs vary, but essentially the soot particles attach to the surface area of the substrate. They progressively get more and more loaded with soot until a 'burn off' happens, either by being lit off by raw fuel injection (injecting fuel on the exhaust stroke in a diesel) or by high loads with high exhaust temps in which the soot burns like coal.

They are notoriously problematic because they don't always light off and regenerate as intended in some real world operation.

 

[blackman777]

Ahhhh..... now I understand why Mazda designed their diesel engine with low compression (14:1), so it doesn't need all that junk. It likely saves them a lot of money.

 

[jrustles]

Originally Posted By: blackman777
Ahhhh..... now I understand why Mazda designed their diesel engine with low compression (14:1), so it doesn't need all that junk. It likely saves them a lot of money.



Well, it is junk indeed if the problem can be solved

For saving money, it really saves the motorist money by eliminating the NOx problem altogether using clever engineering without cost to the consumer. I doubt the engineering man-hours were free for the company, though

 

[blackman777]

The company saves money by not installing Soot filters or Urea catalysts