Lean Burn engine
 

Hi there, got a stupid question...

anyone ever tried to tune the engine below 2.5k to 3k rpm like a lean burn engine:
this means afr above 20:1 to max 28:1
it's only possible under low rpm, cause such lean mixture would burn quiet slowly --> at high rpm the mixture would still burn when the exhaust valve opens...
therefore high ignition advance would also be a must...
furthermore knock sensors...
and a wideband lambda which is possible to detect levels above 20:1
anyone tried so far?

what I am aiming for is:
better fuel economy :facepalm: for low revs

thx for your thoughts

Are we talking about a stock n/a engine on 91oct? 20:1 is ridiculously lean I highly doubt it would work, but I'm definitely keeping an eye on this thread for possible explanations proving otherwise:)

Anything above 16:1 and you will lose power big time, to the point of misfiring.

This... well 16 ish... i run 16.5 and when it is hot, I miss alot. I also get great milage, but when it is hot, I am hosed, so I had to richen up a bit. Good luck with 20 breah.

i have a stock engine, but turbo charged (95 ROZ)
of course i know that igniting the leaner mixture is not a simple task, i already have a better ignition system
the loss of power is cause the ignition has to be extremly advanced, otherwise the peak pressure is reached beyond the optimal point (10-20 degree after tdc)
probably a cylinderpressure sensor would be useful...

but i want to know if any1 tried so far?

thx for your thoughts ;-)

One of the problems with Miatas is 2500 rpm is like 40mph or something. Don't seem likely to be cruising at that speed.

I was thinking about this, using table switching for this, having a cruise map and when you punch it a "go" map. But I want to get an EGT sensor before I start messing.

There was a P-40 pilot in WW2 that figured out how to set his shit to lean burn, then he went and taught all the other P-40 pilots about it. They noticably extended the range of the P-40. The concept is proven, but it's prone to making mistakes. The ignition is advanced significantly to start the burn early, and the burn completes shortly after TDC, effectively providing more power for the down stroke than the resistance of compression on the up-stroke. The problem can be the transition from lean-burn to stoich burn (where you have to bridge the detonation range of the AFR) - WW2 planes that were just cruising pretty much set the RPM and stuck to it.

IIRC, I thought the prius was set as a lean-burn engine. The EPA doesn't like them though.

I had a "semi lean burn" fuel table on a MegaSquirted Civic a while back - that car liked to cruise at 18:1 to 20:1.

Matt what was your spark timing in this cruise area?

damn that's impressive. I can get just around 17:1 in my miata then I get misfiring. My old subaru would misfire at like 15.5:1 haha. 20:1 would be sweet.

This topic was featured in hot rod i think?
Yeah it was:
http://www.hotrod.com/techarticles/e...ine/index.html
Also:

http://www.legendarycollectorcars.co...clusive-video/
There is more to it than better mpg; Smokey Yunick actually made power with his setup.

damn. Ok. I'll have to bring this up with the old timers at work.

Yes I've been heavily interested in the adiabetic engines smokey has built, but that in itself in a whole another way to go about it. Hes heating up the fuel to 400f+ tempatures before it goes into the engine.

True, but heating up the fuel enough vaporizes it, allowing the air and fuel to mix properly, avoiding detonation allowing higher AFR. It is one way to skin a cat, but I think I read something about honda using lean burn technology.

One time I drove on the highway following Saint_foo here up to a dyno day. in the rain going +80mph my fuel map needed work and I was hitting over 17:1 before it would need more fuel. Got pretty good mpg IIRC.

I typically highway cruise at 16.5:1.

I read about the honda "lean burn" at one point. It wasn't as extreme as Smokey's IIRC, just sorta using vtec in an economy mode and leaning it out bigtime like we try to do with our standalones, but with honda's better tech. cylinder head and cam timing. Worked great on those old civics but then the EPA didn't like the emissions and killed that idea. Back when I was ignorant of emissions and had my link ecu I went thru the NJ inspection with a very lean 17ish AFR, thinking it'd pass no problem as I wasn't using much fuel. WRONG. I forget what goes sky high when you go lean but something was off the chart.

I read an article a while back where Honda had developed a test engine that could run at an equivelant 65:1 afr. What they did was machine a cup into the top of the piston then using direct injection they squirted into the cup. This gave them around 20:1 afr in the cup but the average for the cylinder was 65:1. When you wanted more power the engine would run in normal Stoich mode on the full volume of the cylinder.

Can't seem to find the full article any more though.

I can't remember exactly where it was, but it was nothing special. I suspect the cylinder head design was more important - the Civic CX could go even leaner.

If Smokey's hot air engine wasn't a hoax - which I'm not ruling out either - there were a lot of details about that motor that Smokey kept hidden from the media. My own guess is that the engine was a homogenous charge compression ignition engine. That would fit a lot of the published details - that the engine ran extremely lean, and that the air was heated to almost the point of autoignition - as well as some educated guesses - such as that the engine appears to have been a total bitch to tune, given how several companies he'd licensed the technology to couldn't get a production version working.

Nitrogen oxide emissio(NO, NO2) from the higher EGT.

They do to a point and then start falling again.

http://www.s14.net/photopost/data/50...r-co-chart.gif

I had thoughts of putting a big injector right infront of the turbo to heat up the fuel charge using that injector only to feed the engine while trying to lean it out, be for cruise only of course, ideas ideas

I set mine up for lean cruise 15.5:1 but I had issues at low gear low rpms with stumbling.

Lean burn engine and Miata don't mix. Although I have been able to get mine to cruise around at 16.5:1, the throttle response is terrible even with big asynch pulses. My guess is that the engine is just really dry, and when you throttle in hard it takes ALOT of fuel to wake it up.

May require another table for enrichment. Or an asynch specific to the ranges your running super lean. Even still best theoretical fuel mileage is at 15.2:1-15.4:1. Then I don't think the next fuel economy hump comes until after 20:1. Best to shoot for 15.2:1 and save yourself some time! I made a chart for tuning practices around here somere... Aha! Attached!

Where did you get the "best theoretical fuel mileage is at 15.2-15.4" value?

It's high peak temperatures in the combustion cycle, not high EGT's that causes high NOx.

VTEC-E disables one intake valve below a certain RPM (around 2300 RPM IIRC), inducing a strong swirl and turbulence in the cylinder to help homogenize the mixture. That's how they were getting away with such high AFRs.

Another lean-burn idea shot down by the EPA is/was VW's SFI engines, where a "pocket" of fuel was stratified in the cylinder and ignited. Also has high NOx so it's not used in the US.

Thermal dynamics book on otto-cycle engines. The actual burn quality goes way down as you go above 15.5:1. So while your not using as much gas per metered air, the energy your extracting per gas volume is disproportionately lower. This goes back up for some reason at 20:1. I would guess that is because all the engines they've been able to run at 20:1 have special modifications for lean burn such as direct injection.

In the cylinder itself there is a stratification of gasoline charging. At the bottom of the cylinder there maybe 9:1 air, while at the top 22:1. When you change the overall AFR your reshuffling this whole column, and the burn characteristics are different all the way through. These burn characteristics may be less and less ideal to the point where the burn doesn't produce the proper conditions to fully combust the fuel.

Engine designers have tried for decades to increase the ability of the engine to burn leaner and leaner charges. EGR was the first of such inventions. This system forced already exhausted air that was mostly inert into the chamber to reduce intake pumping losses and to increase the pressure in the chamber to combust more the fuel. Likewise, cylinder head designs began incorporating quench regions into the cylinder head to break down the stratification of charge in the chamber so they could run leaner and burn the mixture faster. Now we have the next generation technologies such as special piston designs, direct injection, and so on to push the envelope farther!

With our current technology level we are forced into using the AFR's the engines were designed for which is around 15.3 for economy and 12.7 for power. There isn't any easy way to make that range wider unless we fit some of the newer technologies. Some people even remove some of the technologies on their engine like EGR! I've also seen people rounding the quench regions on the cylinder head to "reduce sharp edges" which also reduces turbelence/charge mixing. I don't know which has the harder effect, but I wouldn't go mucking about in the chamber without a really good reason! I'd also keep the EGR on if possible. Good for mileage, emissions, and longevity. Even if it is against the racing pop culture ;)

/end rant

I'm at 15.4 in cruise and getting 28mpg with a 400lb tire trailer at 85-90mph on road trips, lol. Jeff did not appreciate that in his car with the 5-speed/4.30, and I'm certain he appreciated my cat-less exhaust.

I saw 31mpg once with the trailer and it's an aero brick. My daily can't break past 26mpg on road trips, without the trailer.

Well there ya go, you learn something every day. I can go home now.

I am of the racing pop culture that dislikes EGR

I wonder if there's some useful way to use the VTCS and VVT to create a leaner/lower gas consuming mixture.

1. decrease port cross section and increase tumble by closing the VTCS butterflies
2. midly advance the intake cam to increase overlap a little and reduce pumping losses
3. require more throttle position to further reduce pumping losses

Sounds a little like an Atkinson cycle, y8s.

does the turbo make it a miller?

I have a 65% probable plan to make a mpg efficiency experiment using a B6 or BP (whatever's cheaper at the time), removing 2 cyinders, throwing my ol' spare 1544 and seeing what happens. Smaller engine would require more throttle so less pumping loss there, plus less internal friction, etc. If I do a BP and can find someone who f'ed up a VVT head...that may be cool. But I also figure I'll try only using one intake valve on the 2 cylinders to increase swirl (find a spare cam and grind off the appropriate lobe).

Just read through the articles about that smokey guy,
increasing the airtemp with hot cooling water then exhaust gases is indeed the correct way to go for a better efficiency (all thermodynamics guys probably know that)
for not knocking just lean it out... (at my work we test natural gas engines, and sometimes we have to do a knocking test to evaluate new knock software... this is achieved by going from lambda 1.9 down to 1.7 YES we put in MORE FUEL to produce knocking!!)
when I put some mods to my engine, i'll let you know ;-)

EDIT: and the natural gas can be added before the turbos or after them --> the injection isn't the problem

Holy cow!! Talk about pop culture engineering. What a load.

Exhaust Gas Recirculation is the same thing as eating shit. From a thermodynamic standpoint (not "Thermal Dynamics"), it is just about the worst thing you can do. It was invented in the 70's to lower NOx emmissions to try to comply with the 1970 Clean Air Act. That was about the same time those wonderful big block V-8s went from 10:1 to 8:1 compression -- again, not good from a thermodynamic standpoint. EGR was definitely NOT about trying to accomplish lean burn or increasing engine efficiency.

The other stuff is mostly good information. Note that cylinder head quench regions -- which is a technology that allows you to run higher compression without detonation -- is also pretty old. Widely used in the mid-60s but fell out of favor because, again, high NOx. The change to big-block MOPAR cylinder heads is probably the most obvious example.

Wonderful head engineering these days to introduce swirl while still preserving flow. Charge stratification is the enemy (unless you do it on purpose to preserve a combustible mixture in a specific area near the spark source -- which is what a lot of the discussion above is about).

EGR . . . 100% bad. Rule of thumb . . . don't eat shit.

where'd you read that EGR is bad for cruise? Heywood?

EGR is fine in cruise . . . at least the way it is currently implemented with ECUs (it was god-awful in the 70s). But I certainly take issue with the assertion that EGR improves thermodynamic efficiency. That is total BS.

I think the asertion is that EGR makes the engine produce less power per stroke thus requiring you to open the throttle more. increasing throttle angle reduces pumping losses and improves volumetric efficiency.

you effectively limit your fresh air intake while reducing the restriction of the intake path.

at WOT, the above is meaningless because you can't open the throttle more to reduce pumping losses.

Agree. There are a couple of other things that are done with modern EGR systems:
1. Because a mixture with non-combustible exhaust gas burns more slowly, spark advance can be increased.
2. Because a mixture with non-combustible exhaust gas burns more cooly, there is less conduction head transfer into the engine block or waste heat out the exhaust pipe.

I think I'm just overgeeking and focusing on the thermodynamics rather than on the real world need to reduce NOx while utilizing ECU algorithms to keep efficiency as high as possible. Certainly, if you are running a stock ECU you would be foolish to disable EGR.

The real question in our minds is, does EGR improve overall efficiency (and thus MPG) at cruise.

NOBODY is suggesting it's good at WOT.

Here's my take:

1. EGR is for emissions control. It is intended to reduce NOx by reducing peak combustion temperatures.
2. EGR has been in use for almost 40 years. Its current implementations are highly refined. Modern ECUs take advantage of EGR to hit highly efficient cruise design points where you can use more open throttle angles (higher VE) and increased spark advance.
3. If you were working a clean sheet engineering design to maximize Otto cycle efficiency where emmissions was not a concern, EGR would not be part of your design.
4. If you are running a stock ECU, keeping EGR (and everything else) intact will maximize your efficiency.
5. If you are running an aftermarket ECU, you're going to need a heck of a lot of test time to optimize use of EGR. Without an engine test cell or a lot of proprietary data, I don't know how you would replicate the efficiency achieved by the OEMs. You're probably better off disabling EGR and just focusing on the basics.

YMMV (pun intended).

Blower, ideally, for a Miller.

http://www.eng-tips.com/viewthread.c...=147390&page=1

"Vehicle efficiency gain limit due to EGR affecting piston suction losses is
8.24738372 percent"

"So if you can gently suck in air and recycled exhaust gas instead of struggling to suck clean air against a partial vaccuum there is a real fuel saving to be made. I'm not suggesting you could make an 8% fuel saving. I'm suggesting there is a maximum of about 8% available (for given assumptions) and that if EGR is even 1/4 efficient at helping to make that fuel saving, then you gain a couple of percent in fuel economy when cruising along, and it is a gain worth having - though its worth saying again: you cannot make the same saving twice."

Redact your statement sir, and head to the end of the line.

nuff said.

Upon reconsideration, I think the proper analogy is "if you're going to eat shit because the government says you have to, use lots of flavorful sauce so that it tastes like crawfish."

Now, off to the end of the line.

I looked at Millerizing my turbo setup with VVT. The idea being:
raise compression ratio than I would otherwise run (i.e. choosing 9.4:1 pistons instead of 8.6), then reducing VE / reducing dynamic compression with "non optimal" cam timing, and raising the boost pressure.

The theory is that doing "some of the work of compression" in the turbo (i.e. higher boost), is better because the charge gets intercooled, then doing the rest of the compression in the cylinders on the compression stroke, would result in more powah due to having a high expansion ratio with a reduced effective compression ratio (resulting in lower peak compression temperature thus reduced tendency to knock).

The problem I saw is that, if you look at a family of torque curves at different intake cam settings, low and high RPM torque went up and down (reduced VE), but the midrange RPM torque sort of mostly overlapped. That is, the change in VE in the midrange RPMs where you would need the knock resistance the most, was the RPM range least affected by the cam timing changes. So I said I'll just go with 8.6:1 pistons, if this idea doesn't work I'll be stuck with a knocky engine.

Having said all that, my cam timing in cruise is full retard even though the cam timing which produces max torque at those RPMs is some advance. That means VE is reduced requiring a bit more throttle opening. Because my max advance anywhere in my map is nowhere near what the VVT is capable of, I considered moving my intake cam 1 tooth so I could have even more retard than stock, in order to retard even further.

You can reduce VE by either over-advancing or over-retarding the intake cam. The advantage of over retard is you don't have possible issues with the intake charge fuel blowing out the exhaust. Thoughts?

I like your ideas including jumping a tooth on the intake cam sprocket. I also think another point of the miller is to retard the intake valve to the point it's closing somewhat far into the compression stroke (against the typical supercharger), resulting in a longer power stroke compared to the compression stroke. Kinda sorta.

EDIT to be a little clearerer: For arguments sake say you close the intake valve halfway up the compression stroke. You are now compressing half the engine capacity; It is like you have a .9l engine fueling but with the power stroke applied twice as long. The miller's I know of (millenia S) I believe use a supercharger that's always under positive pressure so the intake charge doesn't...I dunno...go backwards too far to where it's not supposed to go? Plus the intake duration is much longer...it's not just a standard duration lobe that's retarded. It opens when it typically should, just closes later. I'm sure you could get some of the miller effect however just retarding your standard cam.

I'm pretty sure I'm super close to starting one of these builds. 55mpg+ or bust.

TD-03 turbo on a 1990 1.6L

...Just thought i'd let you guys know.

:facepalm:

As I opened this thread, I thought "which jackass bumped this thread?? It better not have been RTC..."

Then I see Yank in here.

One of the few on here who would actually do it. Report back mang.

:laugh:

I think thats a lofty goal.
Check out this guy:
https://forum.miata.net/vb/showthread.php?t=441727
Put a 1.3L festiva engine in his Miata and did a bunch of cool stuff and ended up with ~45MPG.