Which would be better for the environment? A dirty engine that gets good mileage, or a clean engine that gets bad mileage? Here's my reasoning. Exhaust emissions are measured in parts per million, right? So it's possible that a "dirty" engine that gets really good mileage would actually put out less total emissions than a "clean" engine that gets less mileage, simply because there's a higher total volume of gasses moving out the tailpipe of the "clean" engine! (think "air pump" on older Ford trucks)

So, I guess the question is, is it really correct to measure the concentration of pollutants in the exhaust, or would it be better to measure the total amount of pollutants emitted, leaving concentration out of it?

I'm surprised people haven't rediscovered the air pump. Is your vehicle failing smog checks? Rig up an air pump, dilute it down, and go pass. Would it not be that simple?

 

From my take, a cleaner engine is much better.

If you look at the emissions charts from the 1970's vs. today, the difference is enormous - HUGE drop in total emission concentrations. Almost to the point where today the output is scarcely noticable as opposed to early 1970's and earlier where output was easily measurable and actually dangerous from a lone tailpipe.

 

That's not really what my intent was to compare. Of course those old 1970's motors had bad emissions. Look at the technology of the era. Carburetors, no computers to speak of, leaded gasoline fouling everything up, etc.

 

I read that an older engine with carbon build up has a higher comp ratio. Is this true? If so this exsplains why older engines made to use reg gas often run better with a higher octane.

 

Yes, to a certain extent. It also makes a perfect place for a hotspot to develop and cause preignition. Never mind the fact that most of these older motors were retuned by some shadetree mechanic long ago to run rich, with a non-optimal timing curve, causing the carbon to build up that much faster. Or the fact that they've probably got oil leaking into the combustion chamber from the rings or valve guides.

I make one reference to an air pump (aka smog pump) and everybody thinks I'm comparing to older engines. I'm not!

 

Maybe you could rephrase it.

But to give it another shot, presuming you mean head to head comparison. Air pollution is rated in how much fuel you burn - period. Mass in equals mass out. The total pollutants will be depndent on the emissions of each engine assuming you drive both engines the same distance.

 

Yes, I guess I should re-phrase it.

Say you had two engines. One is a modern, emissions controlled engine. We'll call this the "clean" engine. The other has all the same computer controls, modern technology, etc, but the engineers were given free reign to calibrate the thing for mileage, not emissions. We'll call this one the "dirty" engine.

So the "dirty" engine would get better mileage, and use less fuel than the "clean" one. It is in the using less fuel that the would actually turn the "dirty" engine into the cleaner one of the two if one looked at the total amount of pollutants exiting the tailpipe, even though the concentration would be higher.

 

Well again, if you could demonstrate that the "dirty" engine burned less fuel in proportion to the greater emissions it produced then, yes, it would be a less polluting engine overall.

Not to killjoy you though, I think you'll find that a very efficient engine will produce on average less emissions than a less efficient engine regardless. The pollutants are a result of the trace gasses in the atmosphere, the nitrogen in the atmosphere reacting at ultra-high combustion chamber temps, and particulates resulting from incomplete burns. Of the pollutants, probably 90+% of them can be removed using the catalytic converter - a device which won't impact fuel economy to any extent worthy of note. The rest can be removed via careful fuel metering - which will improve fuel economy as well.

The two are inexorably intertwined.

 

Everything can be easily taken care of except NOx -- and that's one of the main block to increase engine efficiency.

However, there are some backdoors -- for instance, VW are planning a diesel with precisely metered NH3 injection into the exhaust gases to combat NOx.

In any case, to answer your question, my guess is a 20% - 30% more efficient engine, and perhaps as much as 10 times as much NOx. NOx mostly matters in congested urban areas with dry climate such as Los Angeles.

 

ppm measurements are used for smog checks, yes, but grams per mile is the standard that new cars have to meet when it comes to original certification testing. In fact, the EPA mpg estimates are not measured from how much fuel goes in the tank, but the quantity of exhaust gasses coming out of the tailpipe during those certification tests.

The original question does bring to mind the trade-offs that are engineering reality, instead of political fantasy as far as emissions are concerned. The first systems (1966 CA, 1968 Fed) did measure HC and CO in ppm, and the air pump helped and "cheated" at the same time. Mixtures were also leaned a little and timing retarded, resulting in higher NOx, which wasnt addressed until 1971 CA, 1972 Fed with EGR or elimination of vacuum spark advance. Cats, O2 sensors, high-energy ignitions and EFI have all further reduced HC, CO and NOx.

Todays tradeoff NOx vs CO2. The less hydrocarbon fuel you burn, the less CO2. Diesels are inherently more efficient, so lower CO2. The problem is higher NOx.

California passed a "feel good" law about CO2, yet does not allow diesel passenger cars because of NOx.

Jim