Is driving slower also driving greener?

[1993CivicVX]

Originally Posted by McPatrick

I was going 55 mph on a 2.5 hour trip yesterday and while doing that I was wondering if someone that did the trip doing 55 mph (and thus saving gas) compared to someone doing that same trip doing 65 mph would drive 'greener' or less green when it comes to total emissions for the whole trip.

Going slower sounds greener, but maybe the engine burns more efficiently at higher speeds and thus the total of emissions for the whole trip would be less?In other words, do saving gas and saving the environemnt go hand in hand or not?

Since no one answered your question, I believe the answer to be yes in most cases. With a possible exception that I will go into, driving slower and thus using less fuel is greener. There *is* a corollary between burning less gas and producing less emissions. The only case where this might not be true is with engines that enter "lean burn" mode where more NOx is produced when the engine is working less hard as compared to when the engine is not in lean burn and thus at full fuel consumption with the pedal to the floor: in this scenario it's getting worse fuel economy but perhaps the overall emissions would be less because fewer NOx gases are being produced. But I think given the increase of all the other emissions, it's probably still greener to drive slower and thus get better fuel economy than driving faster in an attempt to stay out of lean burn by burning more gas and producing more emissions of the non-NOx variety (such as CO2).

To clarify: there are ways to manufacture engines to get better fuel economy with the drawback of producing more harmful emissions. Cars with lean burn engines get better fuel economy but when they are in that gas sipping lean burn mode they are producing more noxious nitrous oxide gas (maybe they've refined this with Hybrids, but that is the case for the 1992-1995 Civic VX). So as long you are mashing the gas pedal, and thus driving faster, you won't be producing as much NOx (or any?) But does the increase of all the other emissions from mashing the gas pedal outweigh the reduction in NOx output?

So the question is, does a Honda Civic VX or HX produce more emissions at 65 or 75mph than they do at 45 or 55mph? Any thoughts? Any way to know?

[R.I.D.E.]

The lowest emissions would occur at the highest mileage. Revolutions per mile are a constant factor of distance travelled (assuming no pulse and glide). The vortex produced by the reduced lift of one of the two intake valves in the VX, when in lean burn, allow better fuel distribution in the air-fuel mixture. NOX emissions are a function of higher combustion chamber temperatures, and less than perfect fuel distribution in the fuel-air charge.

One of the developmental pathways in internal combustion engines is Homogenous Charge Compression Ignition. This is being researched and developed currently. The concept is if you can perfectly mix the air and fuel before combustion you can have a gasoline engine that runs on compression ignition like a diesel. When HCCI is achieved (it has been done under certain circumstances)the exhaust emissions are so clean no aftertreatment is necessary(needs no converter).

The difficulty is in achieving HCCI over all the parameters of engine operation, and the first successes may be in applications where the operational parameter of the engine is in the very narrow range of speed and load that is illustrated in the graphs of fuel consumption per unit of power produced. The range is usually about 75% of full throttle in the range of 1200 to 2400 RPM. This is the heart of the mileage improvements when using pulse and glide, where each combustion pulse utilizes a full charge of fuel and air which produces the maximum power for the amount of fuel and air available for combustion.

Two factors affect total emissions for a given distance. Total engine revolutions, and cylinder volume per combustion sequence. Lower speeds with correspondingly lower drag require less sustained horsepower which means less effective compression per power pulse and lower thermal efficiency.

Imagine you have a car with an infinitely variable transmission that would allow you to keep the engine speed constant regardless of the vehicle speed. Now you can have a greater amount of air in the cylinder for each combustion pulse. My wifes Murano (CVT) will accelerate from 0-65 MPH while the engine speed stays exactly 1700 RPM. With the engine speed as low as possible for the speed of the vehicle you have the ability to have each combustion pulse occur with the cylinder completely full of air-fuel mixture. This is where the efficiency is greatest.

When you are only trying to maintain a constant speed the horsepower required at normal freeway speeds is not suffecient to allow you to operate the engine at 75% WOT. Only at engine speeds barely above idle speed would that be possible. With an infinitely variable transmission you could achieve this more efficient state. The trend is towards more gears in transmissions, but ultimately the transmission will be capable of infinite ratios.

The EPA estimates vehicle mileage can increase by 80% through powertrain improvements alone, with no further engine development. They estimate 120% with predicted engine developments. HCCI capable engines are 25% more efficient than regular gasoline engines, on par with diesels.

regards
gary