Emissions tend to be highest during acceleration. In fact, the emissions of pollutants during acceleration on a mass per time basis are five to ten times higher, on average, than the emissions during idling.
Furthermore, the amount of emissions during acceleration is related to how you accelerate. Very hard or aggressive accelerations, or accelerations under heavy load (e.g., when driving up a steep hill) can produce higher emissions than do moderate accelerations.
So far, we have found that emissions for properly operating newer vehicles are very low during idling, and also are typically very low during steady cruising on level roads and with normal loads. Emissions during deceleration also are relatively low compared to acceleration and in fact are comparable to idling or cruising emissions in many cases. However, the emission rate during deceleration may differ somewhat, for example, for a car with an automatic transmission versus one with a manual transmissions. For a car with a manual transmission, deceleration emissions tend to be different depending on whether you keep the car in gear when braking or shift into neutral when braking.
The same car can produce very different emissions depending upon who is driving or how they are driving. The emissions from a trip with moderate accelerations and steady-speed cruising will typically be much lower than the emissions from a trip involving a lot of aggressive accelerations and speed changes. However, even modest changes in speed during high speed driving can produce short periods of high emissions.
Another important consideration is that emissions are typically extremely high when a cold car is first started. In particular, if the catalytic converter is not already warm from previous operation, the car is said to be in a "cold start" mode when you start the car. During a cold start, the catalytic converter is too cold for the chemical reactions that convert pollutants (e.g., carbon monoxide, hydrocarbons, and nitrogen oxides) to water vapor, nitrogen, and carbon doixde to take place. More technically, the rate of the chemical reactions is too slow at low temperatures to control the emissions. Thus, the emissions from the tailpipe are the same as the uncontrolled emissions from the engine during a cold start. Depending on the vehicle and whether you let the vehicle idle until warm or drive it to speed the warm-up, the cold start may typically last something like a little more than a minute to several minutes. The average emission rate during cold start is comparable to the average emission rate during acceleration.
A car that has been sitting in a parking lot for a few minutes and is still warm will typically not produce very high emissions during starting compared to the purely cold start situation.
A car that is out of tune may have poor timing of the sparks in the engine, resulting in situations that can either cause less efficient combustion (thereby increasing carbon monoxide and hydrocarbon emissions) and/or may cause increased emissions of nitrogen oxides.
Thus, some suggestions for low emissions driving include:
- moderate accelerationsOf course, the most important consideration in driving is your own personal safety and the safety of those around you. However, by keeping these suggestions in mind, you can make a contribution to improved air quality.
- maintaining steady speed at cruising or avoiding rapid increases in speed
- minimize the number of cold starts of your vehicle by combining several shorter trips into one longer trip.
- also, keep your vehicle well maintained, especially in terms of engine tune-ups!
Can one driver make a difference?
Millions of drivers contribute to thousands of tons per year of air pollutant emissions here in North Carolina and in states throughout the U.S. If only a fraction of them drive in a manner that lowers emissions, it is possible to achieve substantial reductions in emissions. This approach is known as pollution prevention. By modifying your driving to avoid the formation of air pollutants, you have prevented emissions to the atmosphere. This is usually a much cheaper solution to air pollution emission problems than implementing new air pollution control technologies. As many urban areas in the U.S. continue to face increasing pressure to reduce air pollutant emissions, it may be necessary to look to emission reduction solutions based upon improved public awareness and moderate voluntary changes in behavior.
On a periodic basis, we will post additional information here that you may find interesting or helpful and that we feel is technically accurate.
Last updated: January 18, 2002.
This web page was designed by H.C. Frey
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