Can 2 cars with same MPG have diff. emissions output?

<div class='quotetop'>QUOTE(efusco @ Oct 10 2007, 11:31 AM) [snapback]523653[/snapback]</div>

Assuming that we aren't considering evaporation out of the tank. . . .

While the ratios of various compounds might be different and some compounds might be considered more severe in terms of "pollution", I'd think that the total amount of carbon, oxygen, and hydrogen would be the same. If the fuel contained hydrocarbons only, then the "perfect" output of the "cleanest" possible emissions ICE car would be entirely carbon-dioxide and water. Other compounds are formed from various non-hydrocarbon compounds in the fuel, or incomplete combustion of the hydrocarbons, leaving you with exhaust that contains things like carbon-monoxide, or hydrocarbons in the exhaust.

A car that gets really good MPG might not be getting all the possible energy out of the fuel because of incomplete combustion, but might make better use of the energy that it does get. This would give you an increased MPG (due to the efficient use of the energy obtained) with increased "pollution" in the form of compounds other than carbon-dioxide and water.

So while a car with identical MPG might be considered "dirtier" because of increased "bad" compounds, I'm not yet convinced that such a car would "output more greenhouse gases". I don't know enough about the contributions of carbon-monoxide, various hydrocarbons, and other possible pollutants to the "greenhouse effect".

 

Easily. Even really good engine designs are still far from ideal, and much of the energy produced by the engine is lost as heat through friction, plus most engines spend much of their life running at non-ideal speeds. There is no direct correlation between mpg and low emissions. Common diesels are an example of an efficient engine design that produces a lot of particulates, which of course brings up the question of what is really considered "emissions".

Here is a limiting example: Let's say we have a car that contains an engine that consists of a sealed one gallon container which we have filled with gasoline. With the gasoline sealed into the container, the car can't move, so it gets 0 mpg, but it also has zero emissions.

Tom

 

<div class='quotetop'>QUOTE(claytonsuch @ Oct 10 2007, 11:23 AM) [snapback]523650[/snapback]</div>

Yes if you look at the NO(x) distribution and possibly if you look at the HC and CO(x) distributions. A higher combustion temperature engine will tend to be more efficient in converting gasoline to propulsion energy. However, higher temperatures can result in more NO(x) compounds and these lead to 'acid rain'. This by the way is one the major challenges for diesel engines, dealing with NO(x).

As for HC and CO(x), it is harder but not impossible to get different ratios. It is a function of quench such as having a lot of surface area relative to the volume that can lead to more 'bad stuff' coming out the exhaust . . . if it is not caught by the catalytic converter. Diesels have gotten better about complete combustion but they used to be pretty bad about generating soot, solid C.

For now, we'll assume the catalytic converters are equally effective.

Bob Wilson

 

We should be careful of what type of emissions we are talking about.

If we are talking about poisionous emmissions like Nitrogen Oxides and Carbon Monoxide, then the quality of the emissions systems in the car can make a huge difference between two similarly MPG-rated cars.

Here is a bit of information on what a catalytic converter does (lifted from Wikipedia's page on catalytic converters).
1. Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx --> xO2 + N2
2. Oxidation of (toxic) carbon monoxide to (less toxic) carbon dioxide: 2CO + O2 --> 2CO2
3. Oxidation of unburnt carcinogenic hydrocarbons (HC) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 --> 2xCO2 + yH2O


However, if you are looking strictly at the "geenhouse impact" of a car's emissions, i.e. the amount of Carbon Dioxide that is produced, it's pretty much even for two similar MPG cars.

It's a pretty simple (and alarming!) chemistry problem. For every 1 gallon of gasoline consumed, nearly 20 lbs. of Carbon Dioxide is produced! This is true of all cars, regardless of the quality of their emmissions systems. (As a point of fact, the better the reduction of Carbon Monoxide by a car's catalytic converter the more Carbon Dioxide that is produced.)

Therefore if we are talking strictly about "greenhouse gas emissions," it turns out that MPG directly effects how much is produced, and similarly rated MPG cars will produce similar amounts.

 

Here is the EPA website that gives pollution info for cars
http://www.epa.gov/emissweb/

 

Hi Claytonsuch,

A little OT here. But the Yaris does not have similar EPA rating. The difference is about 10 mpg (the Prius gets 28 percent better mileage than the Yaris) at highway speeds. So, the Yaris will emit more CO2 as well.

 

Hi Clayton...,

No problem. We have trolls come through here everyonce and a while. I thought the appearance of "Yaris" in the same paragraph with your original question might have been such a comment, in a more deceptive manner. A third pary reader might infer that the two cars are equivalent in mileage.

My pet pieve at the moment is the apples to oranges comparison of the newer smaller cars to the Prius on mileage. The Prius is lager, heavier, and quicker, besides the better gas mileage.

One of the troll tactics have been to say the Hybrid system has no advantage on the Highway. Which is true on a level highway, and of cars that only have Hybrid systems to improve fuel economy. Where the Prius has the Atkinson engine, aerodynmic shape, and CVT that allows the engine to come down to really low RPM's at highways speeds besides.

So, please forgive me, but you can see where my Troll senses were allerted?

 

Here is a good example of the decoupled nature of mileage and emissions;

All units are g/mi

Vehicle-----------CO---NMOG-----NOx----PM----Total ---HCNM+NOx--CO2---2008 EPA Mileage
'06Prius----------0.1---0.009------0.01---N/A---0.119-------0.03-------170---------48/46/45
HyCam-----------0.1---0.006-------0------N/A---0.106-------0.01-------N/A----------33/34/34
Lex GS450h-----0.1---0.008------0.01---N/A---0.118-------0.02-------305----------22/23/25
Frd Esc Hy-------0.1---0.006------0.02---N/A---0.126-------0.01-------N/A----------31/30/29
Lex RX400h-------0----0.007------0.01---N/A---0.017-------0.02-------315----------27/26/25

and the diesels just for fun:

Vehicle-------------CO---NMOG-----NOx----PM----Total ---HCNM+NOx--CO2---2008 EPA Mileage
'07 E320 Bluetec--3.27--0.066-----0.12---0.003--3.46-------0.169-------318--------23/26/32
'07 GL320 CDI----0.04--0.0032----0.48---0.003--0.555------0.99--------428--------18/20/23
'07 Jp Gr Ch CRD----0----0.003------0.5------0-----0.503------0.94--------442--------18/20/23
'06 Jp Lib CRD------0.7---0.062-----0.5-----0.04---1.302------1.24--------N/A--------19/21/23
'06 VW Jetta Tdi--0.04--0.0194----0.31---0.046--0.415------0.748-------N/A--------30/33/37

I was quite surprised that the Hybrid Camry, and Lexus hybrids are so good on emissions. Better than the Prius even, despite the lower mileage. Of course this ignores CO2. I was also surprised how bad the diesels were. It will be very interesting to see how the '08 VWs look. If the Bluetec Mercedes is any indication it may be disappointing.

EPA numbers from: http://www.epa.gov/otaq/crttst.htm
CO2 numbers from: http://www.vcacarfueldata.org.uk/

Rob

BTW wasn't trying to bash Prius emissions. I assume this is just due to HyCam being a newer design, and the price of the lexus's allowing more sophisticated emissions controls.

 

Hi Rob,


I can believe the Jeep CRD numbers. The few times I have been behind one I could see the exhaust. Allot worse than a recent TDI. Although the numbers do not look that much different. It must be a slow speed thing. Which is when I have been behind the Jeeps.

 

A good case in point is the 2002 Corvette vs the 2007 C6 Corvette. The 2007 C6 gets slightly worse gas milage yet has a higher greenhouse gas score than the 2002 C5 Vette.

 

<div class='quotetop'>QUOTE(claytonsuch @ Oct 10 2007, 11:23 AM) [snapback]523650[/snapback]</div>

To answer your question Clayton, yes, two cars can have the same fuel mileage and very different emissions levels. Take VW's 2.5L gasoline engine for example. These are both installed in the same car and get the same fuel mileage.

This is the 2006 EPA certification data from http://www.epa.gov/otaq/crttst.htm

Engine ---------------------------CO---NMOG-----NOx----PM----Total (g/m)-----Standard----2008 EPA Mileage
2.5L Gas (GVWXV02.5257)--0.41--0.0033----0.000----N/A------0.4133--------T2 B2---------19/22/28
2.5L Gas (GVWXV02.5253)--1.26--0.0290----0.029----N/A------1.3034--------T2 B5---------19/22/28

The first 2.5L gasoline engine is the CARB engine sold only in states with CARB emission regulations. The second 2.5L gasoline engine is the Non-CARB sold in the remaining states.

The difference is in the emission control systems. The CARB engine has a more advanced emissions system with 3 O2 sensors for finer control of combustion. The Non-CARB engine has the normal emission system with only 1 O2 sensor.

This is not a hybrid / non-hybrid thing. Just because a vehicle is a hybrid doesn't necessarily mean it has low emissions. For example the Insight was pulled from the US market because it did not meet the new CARB standard.

However, you can't really get a good idea of how clean a vehicle is by looking at the EPA score at www.fueleconomy.com because the EPA (with their Air Pollution Scores) set their ranking by the maximum emissions allowed by the vehicles classification instead of the actual measured emissions from EPA testing. That is the problem with the EPA classification and the reason it misleads a lot of people. It is better to go to the EPA's website and look at the actual test data.

 

<div class='quotetop'>QUOTE(claytonsuch @ Oct 16 2007, 01:27 PM) [snapback]526514[/snapback]</div>

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