Discussion in 'Fuel Economy' started by Appletank, Dec 3, 2019 at 8:05 PM.
Guy tunes his V8 vette with lean burn mode at under 2k rpm, 6th gear.
Don't know how eco it is to increase NOx emissions.
I've heard Holdens had a lean burn in the past, and that the subroutine was present in some North American GMs, just needed to be turned on.
IIRC NOx comes out at peak engine temperatures. Drop the fuel injected in low enough, there wouldn't be enough fuel to get it all that hot in the first place. One of the updates mentioned that they were monitoring exhaust temperatures.
Leanburn promotes sky high NOx emissions that are well beyond the ability of a Three Way Catalyst (TWC) to convert back to N2 at anywhere close to Regulated levels.
Many Vette's are currently rated in the high 20s range on the highway so pulling 40 mpg without resorting to any engine or vehicle modifications is as simple as driving it.
It was great to see the direction but not the emissions.
NOx emissions only spike if the cylinder temperatures spike. They mentioned monitoring the exhaust and engine temperatures, so if any anomalies show up they would see it. When the load increases, the ratio goes back to stoichometric so there won't be a lot of load in lean burn, which may raise temperatures. It only 'activates' in very low load scenarios where you don't need power.
Engineering Explained has a great video on lean burning and engine temperatures. His reasoning is that if you lower the amount of fuel injected enough, there's simply not enough fuel to generate high temperatures. Engine temperatures rising is more of a danger in two-strokes that require oil in the fuel for lubrication. Diesels are technically lean burning all the time with the lack of a throttle valve reducing the amount of air going in.
Exhaust gas recirculation is a known technique for lowering NOx, and its essentially doing a similar thing to lean burn, except by dumping excess inert gas into the cylinder that the fuel can't burn with to absorb heat.
To light off a lean burn mixture, you have to have high temps. It is the physics of the combustion. Diesels do in fact run lean burn all the time and they light off because of the higher compression yielding higher ignition temperatures. They also have extraordinary emissions hardware to reduce NOx along with soot formation that is also created at those high flame point temperatures.
Which is how many factory lean burning cars operated in the past, and all had higher NOx emissions. The CARB Civic VX had to have the lean mode deactivated to meet regulations. The last lean burn gas car sold in the US was the original Insight. It had a NOx trap to deal with emissions, and is the only gasoline car I know of to have such.
Burning fuel is only one source of heat in the cylinder. The other is the compression of the air. Compression generated heat is what ignites diesel fuel. Modern DI engines are running at higher compressions.
And diesels are NOx making machines. Excessive NOx was the result of VW's cheating. To meet current regulations, they need SCR with urea injection or a NOx trap. Many use both. The SkyActiv-D uses a lower compression to help with NOx production.
Exhaust gases are hot, and few cars have cooled EGR at this point. EGR reduces NOx by limiting available oxygen. Excess oxygen is needed with heat for NOx formation, which is the case with lean burn. EGR helps, but is not enough on its own. A commercial truck company nearly went bankrupt trying to meet regs with just EGR.
Controlling cylinder temps could lead to lower NOx. It is how a HCCI engine like the SkyActiv-X is suppose to to have lower emissions with less down stream emission controls. The engine just came on the market, so it will take time to see how effect it is outside a lab.
As for engine tunes, the manufacturer is balancing efficiency, power, emissions, NVH, and reliability. Change one of those with an aftermarket tune, and the others are likely to also change.
What does excess oxygen do if there's no fuel left to burn it? Oxygen only does anything if it reacts with something, it might as well be inert gas otherwise, which can also absorb heat. I have serious doubts if I put a droplet of vaporized gas on a spark inside a cylinder 1 meter in diameter, its going to make the air temperature very hot. I agree that going from 14.7 to 15 or 16:1 is going to make you have a really bad time. I'm talking about going even further to 17 or 18.
I checked a few places that made graphs for A/F ratios vs temperatures, and all of them show roughly the same thing:
15 to 16:1 is bad, but dive past it to around 17.5 and the temps and NOx are down again.
From my own research, Diesel's horrible emissions problems seem to stem from the molecule itself being terrible at burning properly, and the stratified injection not helping with its lack of good fuel/air mixing. Fuel on the outside of the droplets burn really well from being a mostly fuel-rich interface, producing even higher temp spikes caused by the compression (NOx), while fuel inside the droplets are mostly insulated and don't get burned properly, creating soot. Trying to rich-ify the mixture will only worsen the problems of insulated droplets. Might get less NOx though if you throughly soak the cylinder with diesel. That's coal-rolling territory tho.
Ah, have you looked into how Sky-X's HCCI, or rather SPCCI, works? They control temps via an intercooler and a supercharger feeding in excess air. I checked a few articles to refresh my memory: they prevent auto-ignition with a very lean mixture put into a high speed swirl, then add a bit of extra fuel near the spark to set up a pressure and heat wave hot enough to cause compression ignition. The entire point of SPCCI is to ignite ultra-lean mixtures, because normally ultra lean mixtures are a massive pain to spark-ignite properly.
NOx from combustion decreases as the air-fuel mixture becomes leaner, but catalyst efficiency decreases much more quickly. So while the engine is producing less NOx, the resulting tailpipe NOx emissions are much greater from a lean burning engine.
I type too slow
Excess oxygen will react with nitrogen to form NOx under temperatures that burning gasoline can easily reach.
You get low NOx with a rich mixture becuase the oxygen is used up, and also because the excess fuel cools the cylinder. With the high lean mixes, the low amount of fuel in the cylinder means the heat from combustion dissipates too quickly to support NOx production.
The problem with those lean mixes is that you need an engine designed to run with them as a minimum concentration of fuel is needed in order to for it to ignite with the spark, or you will have misfires. Honda reached 22:1 ratios and avoided the misfires by igniting some of the fuel at stoich in a prechamber, and by controlling airflow into the cylinder so that the charge was stratified with more fuel concentrated at the spark plug. The system also required a wide band oxygen sensor.
This Vette doesn't have such strategies available to it, so it is likely running in the bad range.
While NOx may not be high from an engine with such real lean ratios, but it is high enough to still warrant exhaust treatment, which runs into other issue. Three-way catalytic converters require a certain mix of pollutants to function efficiently. The isn't the case when the ratio is so lean. While you are making less NOx, more of it is going past the cat. To meet today's regulations, a lean burn gas engine will require diesel engine emission controls, which increase the cost and complexity. Besides, hydrocarbon emissions are also regulated, and those climb under lean burn.
It sounds too good to be true, so I'm waiting to see how it works out in the real world. I wonder how the increased cost of the engine compares to the reduced cost of emission controls.
Personally, I'm more interested in Mazda's PHEV with Wankel range extender.
Aye, I know jack shit about how catalytic converters work, so I bow to your expertise there. Though I am amused this conversation went from "Lean burning burns too hot and makes NOx" to "Lean burning burns too cold to get the converter to reduce NOx", though it seems more that the converter needs a very specific blend of exhaust gases to work, and any deviation mucks that up. On the other hand, if the guy claims that the Vette is also tuned for performance, there's a chance its converter isn't going to be happy either way unless it gets swapped out for a diesel-like one. Dunno if that's a cheap enough swap to be worth it.
Regarding SPCCI, Mazda claims that most if not all of the components needed to make the engine are off-the shelf components, so outside of retooling the factories it shouldn't increase the costs that much. And yeah, I'm very curious to see how Mazda will be able to SPCCI a Wankel since they're horrible at burning fuel completely. Though a PHEV version would do a lot to help keep it in its narrow efficient zone.
I had come across the fact that leaner mixtures eventually run cool, which makes sense from the less fuel, less heat perspective, while kicking around the idea of a fuel vapor system for a lawnmower. The concern there wasn't emissions, but excess heat damaging things. Their monitoring of engine and exhaust temps was more about the potential damage. Temperature doesn't tell you what is in the exhaust gas. What I learned of aftermarket tunes from hanging on a Sonic forum is that; no one talks about the emission changes, and they are stealth, it automatically or easily switches back to the factory tune for inspections and dealer work.
In addition to the right mix of emissions, cats also need to be hot, and they are heated by the exhaust.
Adding a charger will increase cost if you weren't adding one before. Eventually, the SPCCI will cost only a little more than a typical engine, but companies usually want to cover the R&D costs sooner rather than later.
The PHEV is just using a basic Wankel. It will run in a narrow range for emissions and efficiency, but likely still worse than a small 3 cylinder. The EV range will be over 100 miles on the EPA, so the Wankel will see little use for most people. Then being less efficient in hybrid mode becomes less of a concern as total fuel use drops. The Wankel being smaller, lighter, and smoother running becomes more important for such a PHEV design than absolute engine efficiency.
I think Mazda already uses a lot of superchargers, they'd used a lot of Miller cycle engines before. As for Wankel, I meant that either SPCCI or PHEV would be places the rotary would function much better in, in comparison to normal operation. You can basically cram a tiny triangle anywhere since its so dang small.
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