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

I've noticed the latest Passat ad shows auto-stop too. Recently the NHTSA has announced efforts to add, active safety features to their ranking. So I'm suggesting we're going to see more across all manufactures and models.

BTW, I tried to get these safety features in 2009 decoupled from the extra $6,000 of 'car bling.' I came very close to walking away from a good deal. Safety is important and the sooner manufactures realize their customers will shop for safety, the better.

Bob Wilson

 

Back to the OP:
Nitrogen Oxide (NOx) emissions troubles arise with lean mixtures & rapid combustion. NOx isn't necessarily a heat issue- as diesel EGT's are significantly cooler than gasoline EGT's. NOx is more dependent on combustion processes running on the lean side of stoichiometric air fuel ratio (14.7:1).

Slowing down the combustion process for a more thorough burn & to allow more time for multiple injection cycles are some of the reasons most light-duty diesel engines have steadily decreased their compression ratios (CR) from the +22:1 range (pre-Euro regulations), to 18:1 in the 90's, then 16.5:1 in the mid-'00's- to Mazda's class-leading SkyActiv Diesel 14:1 CR (c. 2011 JDM debut). Even with this drastic reduction in CR over the decades, scrubbing NOx from exhaust gas requires diesels to utilize exhaust urea injection, SCT, or other exhaust after-burning treatments to meet reduced NOx emission requirements.

I can't find any exhaust after-treatments being utilized in the Gen 4.

Why the big fuss over Nitrogen Oxides? NOx lead to respiratory & cardiovascular illnesses, they are a big component of smog & acid rain formation, & they have been shown to cause premature deaths. This is why NOx emissions are heavily scutinized in the West.

If there is a real difference in NOx emissions, it may help explain why the Eco model is rated to return better mpgs. A leaner-burning engine (as evidenced by increased NOx) would be a more credible source of improved fuel efficiency than the deletion of a spare &/or rear wiper, marginally lower RR tires, etc...

Bad news is this MPG gain appears to come at the expense of higher NOx emissions. Even though the NOx increment is small- the vector is in the wrong direction.

Perhaps this offers yet another incentive for buying a non-Eco model?

YMMV. Drive safe!

 

I completely agree, Bob.
Some lawyer, cleverer than I am, might be able to discover that a client (innocent third party) has been injured because the price of safety equipment on a given car was artificially inflated by the requirement of buying the other stuff . . .
Ken

 

I went back and added the Gen-3 data from the EPA Test Vehicle Database:

	Represented Test Veh Model	PRIUS Eco	PRIUS	PRIUS c	PRIUS (Gen-3)
1	Test Vehicle ID	16-ZV2H	16-ZV1H	12-NP2H	10-ZW1H
2	Test Veh Displacement (L)	1.798	1.798	1.497	1.798
3	Rated Horsepower	96	96	82	98
4	Equivalent Test Weight (lbs.)	3250	3375	2750	3375
5	Axle Ratio	2.83	2.83	3.19	3.27
6	THC (g/mi)	0.01172	0.01438	0.00464	0.003985
7	CO (g/mi)	0.06442	0.08763	0.04183	0.358224
8	CO2 (g/mi)	105.40951	113.86017	117.82708	130.8100637
9	NOx (g/mi)	0.00295	0.00187	0.00106	0.0031105
10	CH4 (g/mi)	0.00195	0.00224	0.00077	0.0007799
11	N2O (g/mi)	0.00135			0.01
12	RND_ADJ_FE	84.1	78.0	75.0	66.6 (72.4)
13	Target Coef A (lbf)	17.130	18.272	17.919	20.234
14	Target Coef B (lbf/mph)	0.28731	0.29545	0.18180	0.01993
15	Target Coef C (lbf/mph**2)	0.01386	0.01379	0.01556	0.01874

• I used the California metrics but also added the (Federal MPG) who were not measuring CH4 or N2
• Smaller axle ratio means the Gen-4 engine turns faster at any given speed, higher Coef B?
• Other two roll-down coefficients are significantly smaller.

Bob Wilson

 

I wouldn't call the reported NOx difference small, but tiny. It isn't enough to bump the car into the next smog bin, and it is still less than the reported gen3 results. We are in the range of diminishing returns, and we likely can't get a gasoline ICE much cleaner without adopting emission controls from diesels. Until we get evidence of changes in the engine and emission systems between the models, I'm going with the results for the Eco and others within the range of normal for the car.

 

If the eco archives higher MPG simply from ECU tuning towards lean burn then why wouldn't all the models get this? Also, don't lean burn engines run hotter and suffer higher wear? Toyota had lean burn engines back in the 80s. No doubt this is newer tech, but what are the down sides?

 

if this is true, and i'm not sure it is, will it have reduced power?

 

It is just one guy's guess, not a fact.

Toyota outlines the steps they took that result in the Eco, not everyone want to believe that those facts are would improve MPG that much. They invent conspiracies from that lack of belief.

 

What did Toyota say for the eco being 4 MPG better?
1. Special LRR Tires (but eco seems to be showing up with the same tires as the other models)
2. No rear wiper
3. No spare tire
4. Li batt reduces weight a few lbs
5. Anything else?

 

I think they are using more expensive glass to reduce heating/cooling costs.

"The Prius Two Eco also has heat-insulating windshield glass to keep the car cooler in the summer, and a positive temperature coefficient ceramic heater to warm the cabin quickly in the winter." - 2016 Toyota Prius - NY Daily News

 

Lowered?

 

If, and it is a big one from my view, the Eco runs lean enough for better fuel economy, then it likely needs additional emission controls, and thus a higher cost.
Lean burn can actually cool the cylinder; NOx formation is endothermic. You need high temperatures to get the reaction going, but these aren't higher than what is seen in a cylinder with stoichiometric air:fuel mix. A 2 cycle engine that relies on oil mixed in the fuel for lubrication could see higher wear when running lean.
Many manufacturers had lean burn engines. They stopped using the technology when emission regulations got stricter. The most recent car to use it was the original Insight. Honda equipped it with a LNT. The only other cars to use them are diesels.

It would reduce reduce power in a simple engine. In a car engine, the lean mode was only engaged are certain time; mostly steady state cruising. Call for power from the engine, and the air:fuel mix goes back to stoichiometric.

Higher tire pressures.
When the pressures drop on the Sonic, it can reduce the fuel economy on my commute by 2mpg.

 

I got:
What did Toyota say for the eco being 4 MPG better?
1. Special LRR Tires (but eco seems to be showing up with the same tires as the other models)
2. No rear wiper
3. No spare tire
4. Li batt reduces weight a few lbs
5. Higher tire pressure specs
6. Low E glass for summer heat
7. Ceramic heater for quicker heat-up in winter
8. ?

Anyone trying to set per tank distance records in a non-eco better take that rear wiper off!

 

I will believe that--but wouldn't you figure that would be at least optional on the higher models?

 

I did'nt meditated a lot on the decision of Hyundai for Ioniq/Niro hybrids to create 2 separate cooling circuits, one for the engine block and one for the head (and Active Grill shutter). Beeing a direct injection engine, so with NOx problems, do you think that dual circuit could sort in the same time two problems, NOX and lean burn, allowing for an optimal reduced cooling for the engine block, and so getting more easely that 40% efficiency ?

 

I didn't know direct injection engines had higher NOx emissions. They still have a throttle plate and valves to control the amount of air that gets into the cylinder, and thus limit the amount of unburned oxygen available to react with nitrogen. Diesels simply don't have a throttle plate. Gulping in as much air as possible, which leads to lean conditions in the cylinder for NOx formation.

Direct injection are higher compression. That allows more work to be made from the burning fuel. The separate cooling loops in the Ioniq and others is to keep the head are cooler to reduce knock and pre-ingition for even higher compression ratios.

 

Conjecture on your part, right? You are saying that you feel fairly certain that there is a significant design difference, therefore you feel, when and if Consumer Reports ever tests both versions, they will see a big difference.

I agree it would be very nice if Consumer Reports tests both versions and give us eco MPG difference on their test. However, it is perhaps wishful thinking that CR has the time to test both versions heads up like that for us.

 

Whew, at least they are not as delusional as some here who think Toyota has a lean burn technology they did not put all Prius for some reason.