newer engine air/fuel ratio ?

[adam728]

When ethanol content goes up, stoich goes down.

Which is why we often work in Lambda instead of A/F.

[grant6395]

As I understand it they maintain the a/f ratio for emissions and get fuel economy with egr.

EGR isn't used anymore, cam timing is played with instead. Alter exhaust gas residuals with exhaust timing and alter intake timing to change manifold vacuum (or lack thereof) to reduce pumping losses, and drive by wire because we're now at the point where engine power has little to do with throttle position, so throttle position as a primary input is no longer a valid means of control. It is now an effect, not a cause.

Air/fuel ratio is generally controlled "grossly" with the front O2 sensor, but the rear O2 sensor is used to trim that, because the whole point of feedback control is to keep the cat happy and the rear O2 watches what comes out of the cat. The rear O2 is no longer just a catalyst monitor, it's part of the fuel control and has been for at least ten years.

That is basically all there is to it: keep the cat happy. Fuel control is down to three things: Warm the cat up as quickly as possible, then keep it lit, and enrich as necessary under heavy load to keep it from melting. This is kind of the way it has been since 1975 but the controls and sensor tech has gotten a lot better in the past 40+ years.

This is how we are able to have, say, engines that run stoich up to near 20 pounds of boost (well, that and some serious CFD as regards to cooling flow in the engine), and engines that cruise and even idle at WOT in order to get maximum efficiency.

There are many new Toyota models with egr valves/ egr coolers and egr passages. Particularly hybrids. They also incorporate vvti. I’m a Toyota master tech and have issues with these systems all the time. Pretty common.

[Brian P]

Yes the stupid emissions have been killing fuel economy for quite some time now.

not really; with modern engine controls and engine designs you can have low emissions and decent fuel economy and good power in the same package. (not all engine manufacturers have it sorted out, though)

We're pretty happy with the Pentastar; it's reasonably economical considering the big box of a vehicle that it is in, and it makes the van go decently enough.

Consider this. The displacement and number of cylinders in the 3.6 Pentastar are quite close to those of the old 225 Slant Six. The Pentastar makes double the power the Slant Six did before they had to emission strangle it, and more like triple the power of a later emission strangled Slant Six. And it uses less fuel than a Slant Six would ever hope, and the emissions are lower than a Slant Six could ever possibly achieve.

We just did a 2500 km trip with the van, towing a 5 ft by 10 ft enclosed box trailer. It used 12.9 L/100 km (18.2 mpg US). An old B van with a Slant Six wouldn't do that even without the trailer in tow ... and it would probably struggle with the hills that the new van does with the cruise control set and never really breaking a sweat.

[John Wallace]

Which is why we often work in Lambda instead of A/F.

X2!

[vwchuck]

Brian P if you are forced to run at 14.7 AFR you are leaving 15% more fuel on the table. That is significant.

[MadBill]

Significant yes, but just one of the many expenses of acting in the interests of maintaining a breathable atmosphere.

[Brian P]

Brian P if you are forced to run at 14.7 AFR you are leaving 15% more fuel on the table. That is significant.

At full load, adding 15% more fuel is probably richer than optimum power for a combustion chamber and system that is optimized to run close to lambda=1. Adding 10% more fuel for an engine optimized in that manner probably only makes a few percent difference ... not significant for a daily driver.

At cruise, taking away 15% is probably close to lean misfire unless the system is optimized for it. Taking away 10% so that it operates in "lean cruise" is probably only worth a couple or three percent in improved fuel consumption (and nitrogen oxides would be sky high by comparison).



The main reason for production emission controlled engines to deviate from lambda=1 nowadays are (1) conditions where the engine won't start otherwise (cold start and the first few seconds of cold running), or (2) it will hurt something, at full load, be it catalyst or pistons or exhaust valves or turbo. The regulators won't allow otherwise ... but still, my emissions compliant Pentastar beats the old similar displacement Slant Six in every possible measure, and it will also beat the various V6 engines of similar size that came between. I doubt if there is much left on the table by fiddling with the tuning, a few percent at most.

[MadBill]

In the recent past (I think it's more stringent now) EU standards for NOx allowed extreme lean burn in DI gasoline engines, giving I believe typically ~ 6-8% better economy than the same engine in North American trim.

Of course the ultimate lean burn engine is the diesel...