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4316Re: [carfree_cities] More pollution from slower traffic

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  • carfreecrawford
    Feb 7 7:15 AM
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      Andras Toth continued:

      >So, putting aside all theoretical argumentation about traffic and
      >planning (in which I have always fully adhered to the opinions you
      >on this list):
      >Is it true or false that a car trapped in a traffic jam or slowed
      down by
      >sleeping policemen pollutes more on a given distance than a car
      doing the
      >same journey at an optimal speed/rpm whatever? If true, how
      significant is
      >the difference?

      Brake-specific fuel consumption of internal combustion engines
      generally reaches a minimum (optimum) at the engine RPM at which
      the greatest torque can be developed. Generally, this is about
      half the maximum RPM but varies quite widely. Further, the
      fuel efficiency of a car in any gear is generally best when the
      engine is running at the optimum RPM, but this is only for
      that gear. The most efficient gear is alwyas the highest gear
      except if steep hills are being climbed. That means that the
      top gear of a car is always the most efficient and that a
      speed of somewhere between 40 MPH and 65 MPH is almost always
      the most efficient speed in top gear (some sports cars may
      be most efficient at even higher speeds).

      Aerodynamic dag of a car increases as the square of the speed,
      so the energy required to overcome air drag over a given
      distances doubles as the speed doubles. The rolling resistance
      of pneumatic tires increases less slowly than the square law,
      but, unlike railroad wheels, increases as speed increases due
      to higher losses to hysteresis.

      One of the largest terms in fuel consumption is acceleration
      of the vehicle. This is because the energy used to accelerate
      the vehicle is always lost when the vehicle brakes (except
      that some hybrids recover a comparatively small proportion
      of that energy). In stop-and-go traffic, poor fuel efficiency
      is inescapable because the engine is idling some of the time
      (using fuel without doing useful work) and is accelerating
      the vehicle quite a lot of the time. When the vehicle must
      brake, that energy is lost.

      Thus, we see that a vehicle maintaining a steady 50 MPH is
      almost certainly more fuel-efficient than a vehicle caught
      in traffic. Fuel efficiency translates quite directly to
      pollution output, with these caveats:

      Cars equipped with catalytic converters emit far more
      pollutants when cold than once warmed up. There are two
      reasons: cold engines require much richer fuel mixtures
      to run, and the converter does not "light" until it reaches
      a certain, relatively high, temperature. Once this occurs,
      the vehicle runs fairly clean.

      When a car is accelerating, especially if the driver uses
      a lot of power for a fast start, more fuel, and a richer
      mixture, is required. This results in higher, and sometimes
      MUCH higher, emissions levels.

      So, there can be almost no arugment with the contention that
      modern cars pollute less for a given distance when moving
      in free-flowing traffic at highway speeds. (This equation
      would change somewhat if engine power were limited to, say,
      20 HP and top speed were limited to, say, 30 MPH; in that
      case, a car moving a steady 20 MPH would be operating most
      efficiently, and would, incidentally, be gettign FAR better
      fuel economy than modern, high-powered cars.)

      The argument against adding traffic capacity must thus be
      made on other grounds, of which there is no shortage.

      Hope this helps. For further info, see Mark's Handbook
      (published under varying titles for many, many years, and
      the standard reference for mechanical engineering).


      J.H. Crawford
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