Do higher NO_x numbers explain Eco's much superior fuel efficiency?

Are the NOx results in the table an average of the results from the EPA spread sheet?

EPA Tier 2 Bin 2 has a NOx threshold of 0.02g/mi for full useful life under the FTP test cycle. Non of the Prii tested exceeded that for any of the tests. The Eco gets 7.7% better fuel economy than the regular Prius.

The 1993 Civic VX is well known to hypermilers for its lean burn mode. It is rated 13.2% better than the regular Civic in fuel economy. Its age means that it fell under Tier 1 rules with a 0.6g/mi threshold on NOx. Even then the lean mode emitted too much NOx for CARB states, and the mode was disabled for those markets. The real Insight also had a lean burn mode. It needed a NOx trap, something otherwise only used on diesels, to keep its NOx emissions low.

Same question Bob, because your responding post is only comparing Toyota's new sensor to a heated oxygen sensor, which is pretty much the default one for all cars. The VX had a wideband oxygen sensor, which has 5 wires to a heated one's 3 or 4. The cost(>$100) to replace it can be daunting for such an old commuter car.

Which is why I suspect this air sensor isn't new, but just new to Toyota. It is a company that didn't offer a better than 4 speed automatic transmission on their popular Corolla until last year, and the 4 speed is still on the base trim.

 

I took the highest MPG row for each of the three model vehicles and made the table those three rows. I didn't average anything:

	Represented Test Veh Model	PRIUS Eco	PRIUS	PRIUS c
1	Test Vehicle ID	16-ZV2H	16-ZV1H	12-NP2H
2	Test Veh Displacement (L)	1.798	1.798	1.497
3	Rated Horsepower	96	96	82
4	Equivalent Test Weight (lbs.)	3250	3375	2750
5	Axle Ratio	2.83	2.83	3.19
6	THC (g/mi)	0.01172	0.01438	0.00464
7	CO (g/mi)	0.06442	0.08763	0.04183
8	CO2 (g/mi)	105.40951	113.86017	117.82708
9	NOx (g/mi)	0.00295	0.00187	0.00106
10	CH4 (g/mi)	0.00195	0.00224	0.00077
11	N2O (g/mi)	0.00135
12	RND_ADJ_FE	84.1	78.0	75.0
13	Target Coef A (lbf)	17.130	18.272	17.919
14	Target Coef B (lbf/mph)	0.28731	0.29545	0.18180
15	Target Coef C (lbf/mph**2)	0.01386	0.01379	0.01556

I learned about it by reading the 2016 Prius "New Car Features." I had not seen it in other Prius. As for non-Prius cars, I'll defer to your expertise.

The only non-Prius cars I have even a passing interest are those claiming to having higher MPG over a Prius. EVs are a special case because they are so simple, they instantly become boring. An efficient car with an engine tickles my curiosity.

Bob Wilson

 

plus the wideband O2 sensor and maybe the catalyst.

 

The difference in NOx output is too low to be actual lean burn. The fuel economy would have been even better if it were, and there would be a different emission control system on the Eco.

The Eco could be more aggressive with ignition timing, but the results might be falling within the standard deviation of the gen4's NOx emission levels.

Once the smog ratings are posted to Fueleconomy.gov, the engine codes of the cars can be checked.

 

I wonder if the Atkinson cycle timing is different Gen 4 vs Gen 3..?

Both engines have high, 13-to-1 static CR. And, late closing intake valve allows intake charge to bleed down to yield effective ratio around 11-to-1 on the Gen 3.

Perhaps the supplemented EGR on the Gen 4 allows higher effective CR (with less intake bleed down)?

And, the Eco 2 is some variant of this with still different Atkinson timing?

Just speculating......

 

The question of whether there are different engine controls should be easily answered by comparing the EPA engine codes between the Eco and another Prius. They aren't posted on Fueleconomy.gov yet, but they are on the a sticker inside the hood or front edge of the engine bay.

"The Eco could be more aggressive with ignition timing" was in error on my part. What I meant there was that the Eco might run a little to the lean side of stioch than the Prius, but it doesn't have a full lean mode like the Hondas and other earlier cars.

 

There are errors in 'Fueleconomy.gov' that I hope our friends at Toyota wake up and fix. For example, internal volume is off.

Bob Wilson

 

The volume measurement for EPA classes could be a different method than what is used for marketing. What GM posted for gen1 Volt rear cargo on the Chevy site was 8 cubic feet less than what was on Fueleconomy.gov. It was likely the difference between measuring the height up to the roof and measuring it to the top of the rear seats.

The engine codes and smog rating simply isn't up yet, and it be due to a longer review process than for fuel economy numbers.

 

I bought a month of access, $75 or $3/day, and have already done a first pass of the 'New Car Features.'

What stood out was the fuel-air-ratio sensor that is not in the Gen-3. Unlike the oxygen sensors that are 'binary', this sensor has a linear response which means finer tuning can be done on the mixture. In effect, keeping the car on the lean-side, mixture while still meeting emissions limits.

Bob Wilson

 

Lean burn is a proven method for increasing fuel efficiency, and nearly every manufacturer made use of it in the past. It is actually one of diesel's 'secrets'. The majority of modern gasoline cars no longer make use of because of the large amounts of NOx it produces.

The NOx numbers are only a little higher than the other trims, and it isn't high enough to push the car into a dirtier bin, like the gen1 Insight was compared to the gen1 Prius. There are three possible explanations for the higher results.

1. There is no difference between the car's ECU software, and the results are actually statistically the same.
2. The Eco runs a little more on the lean side of stoichiometric for the air fuel ratio. This alone would not explain the Eco's fuel efficiency gains over the others given that the car still falls into T2B2.
3. It has a true lean burn mode, aka the Insight and VX. If so, it also has additional emission control equipment, like what a diesel has, in order to get the NOx down as low as it is. This will be easy to check from the EPA engine code or parts catalogue.

I think we are starting to hit the wall in terms of high fuel efficiency with low emissions, and Toyota has had to use a wide range oxygen sensor as one of the ways to do so. It will allow finer tuning of the air:fuel ratio, and may be required to properly adjust the valves between economy and power settings. It is needed for a lean burn mode, but isn't evidence of that on its own.

Note: Stoichiometric air: fuel ratio is 14.7:1. Honda's with lean burn could go up to 22:1.

 

...or better cat converter? But I am going with statistical equivalence for now, must be variations in fuel quality if they changes batch of fuel between cars, not to mention warm up state and ambient temps might add in.

 

The Insight was able to go really lean. It had an LNT like a diesel.

 

Thanks for the info on the presence of a LNT.

In the early days I was also informed that there was a time limit on how long the ECU could run at an AF of 22 to protect the catalyst from meltdown.

The ECUs that use switching NTK O2 sensors (in another car make that will remain un-named) can run AF ratios out to around 16 as the base AF table is provided with an -ve offset table which is then added to the base table. Usually the tables are in units of Lambda.
NO2 can be reduced by EGR and it does deliver this benefit along with lower pumping losses resulting even higher MPG numbers but you may start to run into detonation issues which may require forged PTFE clad, ceramic top pistons instead of metal-matrix, cooling the exhaust gasses being recycled is an option up to a point. Piston cooling with oil jets were provided for in the Gen3
Trailer-hitching could become the next limit.

As I had the programming files for an MDE2 ECU ( for a 4.6L V8) I tested the above ( no piston change) and raised its MPG from 24.2mpg to 41.6mpg. Detonation became an issue running this tune for about 5 minutes upper limit on RPM was 1750.

 

Lean burn engines running hot seems to be a myth. It is well known that they produce more NOx. While NOx formation requires a high temperature to start, which is present in the engine cylinders no matter the fuel ratio, the reaction itself is endothermic; it will cool down the exhaust as it occurs.

I'm guessing the damage people saw from improperly running lean burn was actually from engine knock.

 

I owned a 1994 Civic VX purchased new and gave it to my dad when it had over 210,000 miles and stayed in the family until it had well over 300,000 miles. The engine was a 1.5 VTEC-E engine and it really obtained 50 plus mpg with the 5 speed manual tranny. I understand the lean burn was when the engine was running under 2000 RPM's or so if I recall...

 

So, are you suggesting that those of us who want more of the safety features than are available on the II-ECO might want to wait a bit to see if Toyota puts new tech into the other Prius models, and get even better MPG?