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

[claytonsuch]

Hello everyone,
This may be a silly question: Can a non-hybrid gasoline vehicle with similar EPA ratings, "pollute" more than a Prius?

Say the Prius and the Yaris average the same MPG for a 50 mile highway trip – I know this is unlikely, but hypothetically speaking. Would the Yaris still output more greenhouse gases during the trip? And if so is it because the Prius doesn’t always idle when stopped or maybe Otto vs. Atkinson?

I have been thinking about this question for the last 24 hours or so, and I am interested in what you guys think/know about this.

Thanks
-Clayton

[efusco]

Absolutely, there's actually a lot of difference...and yea, the Otto/Atkinson is one thing as the amount of 'reburn' of is higher in the Atkinson, the Catalytic converter type matters as well. Plus secondary systems like the fuel tank bladder in the Prius reduce evaporative emissions from the gas.

When you look at a comparison of something like the Honda Insight vs the Prius the Insight's emissions rating is worse than the Prius even though it gets better FE and doesn't idle at a stop.

[Danny Hamilton]

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

Absolutely, there's actually a lot of difference...and yea, the Otto/Atkinson is one thing as the amount of 'reburn' of is higher in the Atkinson, the Catalytic converter type matters as well. Plus secondary systems like the fuel tank bladder in the Prius reduce evaporative emissions from the gas.

When you look at a comparison of something like the Honda Insight vs the Prius the Insight's emissions rating is worse than the Prius even though it gets better FE and doesn't idle at a stop.
[/b]

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".

[qbee42]

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

[bwilson4web]

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

This may be a silly question: Can a non-hybrid gasoline vehicle with similar EPA ratings, "pollute" more than a Prius?

Say the Prius and the Yaris average the same MPG for a 50 mile highway trip – I know this is unlikely, but hypothetically speaking. Would the Yaris still output more greenhouse gases during the trip? And if so is it because the Prius doesn’t always idle when stopped or maybe Otto vs. Atkinson?
[/b]

Yes if you look at the NO(x) distribution and possibly if you look at the HC(n) 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(n) 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

[cheule]

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.

[sgb_pa]

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

[claytonsuch]

Thanks for all the replies! Also thank you for the links, I will be reading for a while!
-Clayton

[donee]

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.

[claytonsuch]

<div class='quotetop'>QUOTE(donee @ Oct 10 2007, 07:32 PM) [snapback]523920[/snapback]</div>

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.
[/b]

What I was trying to get at is -- If a gas ICE (no diesels) only car had the same mpg as the Prius for a short trip, could one have worse emissions than the other? i.e. Is there a direct correlation between mpg and emmisions? and it seems that the answer to my question is: yes and no. I am still reading about this topic -- my knowledge of chemistry is pathetic!

Perhaps mentioning the Yaris in my 'scenario' detracted from the underlying question was attempting to ask.

Thanks for your interest
-Clayton

P.S. Priuschat is awesome!