Appendix D: Vehicle
D.1 Background
The impact of vehicle traffic on local air quality in New Zealand has come under much attention in the last five years. To a large extent, the general perceptions of the public at large on how vehicle emissions affect the environment, and what should be done about it, are drawn from what is seen to be happening overseas. Internationally efforts to regulate vehicle emissions started over 30 years ago, but urban air quality is still a problem in many countries.
In 1996the MOT embarked upon the Vehicle Fleet Emissions Control Strategy (VFECS), a programme of work to define how road transport affects local air quality, thereby giving direction to the appropriate means for its control. The relevance of this to the fuel quality debate is that the work has indicated that fuel quality, engine and emissions control technology are only some of the factors contributing to the vehicle-related air quality problem. Vehicle numbers, traffic density and driving conditions are all significant factors, and increasing congestion may even outweigh the benefits from improvements in vehicle and fuel technology.
This work involved the development of two main outputs:
- The Vehicle Fleet Emissions Model (VFEM)
- Environmental Capacity Analysis (ECA)
D.2 Vehicle Fleet Emissions Model
The Vehicle Fleet Emissions Model (VFEM) was developed to predict the average fleet performance, or "fleet weighted average emissions rate" of the key exhaust contaminants (CO, NOx and PM) and their projection over time in response to fleet turnover.
The fleet weighted "average" vehicle, at a particular date, reflects the full range of vehicle types, fuels, age and engine/emissions control technology in the vehicle fleet at that time. For example the model predicts that in 1999 for instance, the "average" vehicle under freely flowing central urban driving conditions emitted 11 g/km of CO. For 2005 say, the corresponding emissions rate for the average vehicle under the same driving conditions will be lower (around 8 g/km) because of the introduction of cleaner vehicles due to fleet turnover.
The primary structure of the VFEM is not the age or source of the vehicle, or other such factors conventionally used to characterise emissions performance, but the interaction of the engine technology with road design and traffic driving conditions. In this, the characterisation process for emissions measures relates directly to road types and driving conditions. This is combined with fleet profiles which consider vehicle type, age and emissions control technology (both what is installed on any particular vehicle and how effectively it actually functions), fuel type and fuel economy.
The projections from this model indicate that the emission rate of a particular pollutant per kilometre increases significantly and exponentially with the degree of traffic congestion. For the above example of central urban driving conditions, the emission rate of CO per kilometre from the "average" vehicle in 1999 under free-flow conditions increases by a factor of three in congested flows, when the traffic volume approaches the capacity of the roadway. This is illustrated in Figure D.1 below. There is a significant increment again for the running period immediately after a cold start, which can represent a significant proportion of a typical local urban trip.
The projections are based upon the New Zealand fleet evolving generally as per international practice in emissions standards through the future, for new entrants to the fleet. The effect of improving vehicle technology on overall emissions rates, is governed by the rate of turnover of the fleet, so it takes time for the net effect to make a contribution to what is essentially a local area air quality problem. The direct benefits of the new, cleaner vehicles are not captive to the area of need.
Figure D.1: Predicted CO Emission Rate (VFECS)
→ Full size version of Figure D.1 [28 kB JPG]
The conclusion drawn might be that, in contrast to improvements in vehicle technology, improvements in fuel quality would be immediate and be effective nationwide but as discussed elsewhere, the direct impacts of fuel quality on emissions are fairly limited. The more important effect is that they "enable" new emissions control technology. However, the fleet turnover is influenced by a number of factors, which have often have much more to do with affordability and availability of vehicles (economic and trading considerations) than transport policy or fuel quality.
In summary, what the VFEM points to is that the emissions output of vehicle traffic as a collective source in the local airshed is dependent on more than tailpipe performance indicators. It is the product of three factors;
- Vehicle technology: the average "Fleet Performance".
- Road network density: the amount of potential traffic activity within a given area air-shed.
- Traffic density: on each road corridor in the network, the congestion influence on actual per kilometre vehicle emission rates.
It can be seen from Figure D.1, for the CO example, that the benefits of fleet improvement over 10-15 years can be countered by the local increase in traffic volumes, moving the corridor flow from the interrupted to the congested condition, and this is a contemporary trend with the increasing demand for travel in our cities.
New Zealand's air quality is discussed in more detail in Section 4.2. Appendix E presents an overview of the primary air contaminants and their sources.
D.3 Environmental Capacity Analysis
The main outcome of the VFECS programme has been the concept of "Environmental Capacity Analysis" (ECA). Traffic engineers refer to the "capacity" of the road network, that is, it can carry a finite volume of traffic before reaching the congested state. By analogy, reference may also be made to the capacity of the urban air-shed, which can absorb only so much vehicle emissions loading before concentrations reach pollution levels of concern. This capacity is at its lowest under calm, or stable air conditions.
This concept of capacity can be used to represent a limiting benchmark for managing the emissions activity in a given urban airshed, in a manner that recognises the different circumstances of different localities, both currently and as they change through the future.
The ECA framework is built around the city traffic network modelling process, a facility routinely maintained by every urban management authority. The emissions factors produced by the VFEM are designed to be integrated with any traffic model, in a way that can calculate the vehicle emissions loading for each link in the network, for the current and future projected traffic flows. The road network provides the skeletal structure in this inventory process, allowing direct comparison of the local emissions outputs from the various sources in the vicinity, for each pollutant.
D.4 VFECS Policy Outcomes
The primary objective of VFECS was to ensure that the right solutions are employed, to suit the nature of the air quality problems attributed to vehicles. The greater part of New Zealand does not experience air quality problems, so the policy recommendations were designed to target the improvement where it was needed. A number of measures were advocated at the national level, aimed at ensuring that fleet-wide performance improved over time in line with global auto technology developments. These mainly concerned the formalisation of new vehicle emissions standards, and review of fuel specifications to ensure they would be compatible with current and prospective engine technologies.
D.5 Acknowledgements
This appendix was based on an article prepared for New Zealand Science Review by:
Ian Moncrieff
Fuels & Energy Limited
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