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Estimation or verification of emissions from vehicles or vehicle fleets in actual operation has been a topic for discussions for a long time. The main argument is whether to verify the emissions from vehicles in-use by actual measurement, or to rely on estimations based on emission levels specified by regulations. The use of emission factors given by recognized international data bases such as Mobile, Copert, "German Handbook" etc. is a further alternative. Establishment of "local emission factors" is becoming more and more interesting.
For testing of light duty vehicles, emission laboratories have been in use in Europe, Japan and the U.S. since the 1960´s, while for heavy duty vehicles the test resources have been very limited due to high costs for a laboratory and in addition lack of stringent emission regulations for heavy vehicles. During the past few years there has been an increased interest in testing heavy duty vehicles partly to make it possible to compare different engine/vehicle technologies and partly to verify emission reductions when tighter emissions standards for these vehicles are introduced. Further, as car emissions have been reduced, the overall emissions from heavy duty vehicles have become more important. Finally, as alternative measures related to traffic (public transport) are introduced, there is a need to be able to accurately evaluate the potential benefits.
Testing of heavy duty vehicles have also been introduced not only to measure exhaust emissions but also for determining total energy consumption from certain advanced technologies such as hybrid buses (where a conventional combustion engine is used together with batteries). Only then would it be possible to compare energy consumption between conventional buses and hybrids.
Lately, focus has also been directed to unregulated pollutants and particle emissions from vehicles using alternative fuels or vehicles with/without particulate filters. In addition, measurement of the number of particulates and size distribution has been discussed.
All present emission regulations for heavy duty vehicles are applicable to the engine to be installed in the vehicle. As an example, the European directive (88/77/EEC and with later amendments) specifies emission requirements as "…relating to the measures to be taken against the emission of gaseous and particulate pollutants from diesel engines for use in vehicles" (later amended also to cover gaseous pollutants from gas engines). The same approach is also applicable in the U.S. The manufacturer of the engine is responsible for the emission performance of the engine and also holds the Certificate of Conformity, the approval of the engine.
The different regulations are well specified and the method for analyzing the various pollutants and the test procedure/sequence to be followed are described in detail. Maximum permissible emissions are also specified in the regulations as limit values. In reality, an individual engine is mounted in an engine test cell and tested for verification of the exhaust emissions according to given specifications by the regulation.
During the test procedure the engine is tested under various conditions, specified by a driving or test cycle. The test cycle could either be a "Steady-state" test or a "Transient" test.
A Steady-state test is the measurement of emission performance at given fixed load points (power and torque) of the engine. The total emissions are then calculated as the average emissions from each fixed test point where each test point also has a specified weighing factor. In Europe the present driving cycle is called ESC (European Stationary Cycle) and consists of 13 fixed load points. Before the ESC test cycle was introduced a similar driving cycle called "13-mode" test was used.
The Transient test is a test procedure closer to real world driving. The driving pattern consists of a simulation of actual driving and the engine is governed to follow a "driving trace" of sudden acceleration, deceleration, idle and constant speeds in a well specified order. In Europe a new transient driving cycle is being introduced for Euro IV diesel engines (mandated for 2005 for new engines) and is called ETC (European Transient Cycle). This driving cycle has however been used for certification purposes for engines fuelled by CNG (Compressed Natural Gas) since the year 2000 (Euro III requirement).
When a Certificate of Conformity is issued for a specified group of engines (engine family), a manufacturer of a vehicle can use a suitable approved engine together with a drive line including a transmission. Theoretically, the same engine can be used for different types of heavy duty vehicles and in combination with any type of transmission. It is obvious that the driving conditions for a city bus, a long hauler truck and a truck used for construction work in a city are quite different and therefore the emissions during real world driving are also quite different.
The result from tests according to regulations is given in grams/kilowatt-hour (g/kWh) or in grams/brake horsepower-hour (g/bhp-hr).
It is a well known problem to estimate exhaust emissions generated under one set of conditions when actual emissions were measured under completely different conditions. As mentioned above, exhaust emissions (expressed as g/kWh or g/bhp-hr) are normally generated during testing of an engine and where the "load points" (torque at given engine speed) of the driving cycle are related to the engine performance of the tested engine.
Testing a heavy duty vehicle, during transient conditions, on a chassis dynamometer, gives the possibility to test at any conditions simulating different road resistance (tires, number of axles), wind resistance (frontal area) and load of the vehicle. In addition cold start could also be simulated as well as different gear shifting points and types of transmissions. The chassis dynamometer test is a test of a complete vehicle or a vehicle combination and is in principal carried out in the same way as specified for light duty vehicles (passenger cars) by the European, the U.S. and Japanese regulations.
The results from chassis dynamometer testing is given in grams/kilometre (g/km) or grams/mile (g/mile). From there the result could also be easily recalculated to e.g. grams/ton-kilometre.
The basic difference described above is one of the major reasons for the difficulties to recalculate exhaust emissions from g/kWh to g/km.
The most realistic standardized method to measure exhaust emissions from actual vehicles is by the use of an emission laboratory, following specified test procedures. During the test sequence external parameters such as ambient temperature, humidity and air pressure (simulating altitude) can be fully controlled. In order to verify emission performance for newer technology vehicles, they must be compared during exactly the same driving conditions with older technology vehicles. This can be accomplished by chassis dynamometer testing. To reduce uncertainty during the measurement it is important that all instruments, equipment and emission analyzers meet high quality standards and that the laboratory itself can assure high quality testing.
Chassis dynamometer testing can be briefly described as follows.
A vehicle is placed on a chassis dynamometer and the dynamometer is adjusted to simulate driving on the road (uphill, downhill, flat, weight of the vehicle etc.). In front of the vehicle a "TV-screen" is placed, and on the screen a selected driving pattern will be presented for the driver of the vehicle as soon as the test starts. The driver is sitting in the drivers seat and during the test he/she has to follow the driving pattern presented on the screen by the use of accelerator pedal, brake pedal, clutch and gear box (in case of a manual transmission). The driving pattern will show the speed related to time continuously and also the gear to be used at any given point. In front of the vehicle a fan is placed simulating cooling of the engine during driving on the road. The vehicle is then connected at the tailpipe by a flexible tube to an exhaust gas sampling system. From the sampling system the exhaust is diluted by ambient air in a "dilution tunnel" and then diverted to the system for analyzing the various pollutants.
At the start of the test sequence the exhaust gas is sampled simultaneously with the start of the driving pattern. During the test the driving distance is recorded. After the test, emissions are analyzed and calculated according to specifications.
Results from chassis dynamometer testing of heavy vehicles can be used for verifying emission performance from vehicles in real world traffic or for research and development.
The main objective with chassis dynamometer testing is to verify exhaust emissions as close as possible to real world conditions. There is no specified "standard driving cycle" used for approvals or certification tests, but there are several standardized transient driving cycles in use world-wide. Since the driving pattern and load of the engine is different for each driving cycles, the result from emission measurement will not be the same when different driving cycles are used.
Among the most common transient driving cycles the following could be mentioned:
In Europe:
Braunschweig city driving cycle
Simulated European Transient Cycle (ETC) also called FIGE
Dutch Urban Bus Driving Cycle (NL-82)
Millbrook driving cycle (for London buses)
In the U.S.:
EPA Urban Dynamometer Driving Schedule (UDDS)
New York Bus (NYBus)
New York Composite
Orange County Bus Cycle (OC Bus)
In Japan:
New Mode (JE05)
In Chile:
Santiago driving cycle
It has become more frequent that cities are designing own driving cycles to either develop local emission factors and to verify the pollutants from the traffic sector or to use the driving cycle when measuring the emissions as indicators for verification of the pollutant reduction when various measures are introduced.
Regardless the high level of detailed specification for tests carried out in emission laboratories, it will never full replicate in-use operating conditions. A further alternative is to use a system for measurements of the emissions from vehicles when they are used under normal operating conditions on the road, so called on-board measurement. However, one method can not replace the other, rather they are good complements for a full verification of the actual emissions emitted. By the use of OBM it is possible to monitor the emitted emissions from a vehicle when it is used in actual operation on the road. OBM has recently been established in the U.S. as "an official tool" to verify whether an engine is meeting requirements set by the regulations. Under a program called Not to Exceed (NTE) starting with model year 2007 vehicles will be required to meet emissions limits during any mode of in use driving.
With OBM it is possible to verify emissions from e.g. buses operating on a selected route and to evaluate the reduction of emissions if buses using alternative fuels are introduced on the same route or if dedicated bus lanes will be introduced as an alternative. It is also further easy to verify differences in emission performance from identical buses but operating on different altitudes if OBM is used.
Disadvantages with OBM are that the accuracy might be lower compared to when testing on a chassis dynamometer, and the system could be complicated and/or time consuming to install and calibrate on the vehicle. When measurement according to OBM is carried out, the test vehicle will follow the normal flow of the traffic instead of a well specified driving cycle as for chassis dynamometer tests, thus the repeatability for OBM might be less.
In the future OBM will play an even more important role for measurement of the emissions from especially heavy duty vehicles. Programs are under development both in Europe and in the U.S. to validate different systems for on-board measurement. In Europe, the system is called Portable emission measuring system (PEMS). The ultimate target is to use the OBM as a scanning tool for verifying whether heavy duty vehicles are meeting prescribed emission standards.
Remote sensing can not be considered as a field test of vehicles in-use, but is a good tool for scanning emissions. The major use of remote sensing is briefly described below.
There are two general uses for remote sensing, enforcement and monitoring. The two major applications for enforcement are "Clean Scanning" and identification of gross polluters. The monitoring applications are generally referred to as evaluation of I/M program. The USEPA has recognized remote sensing applications for evaluation of I/M program as the potentially most powerful instrument. Another application for monitoring is verification of emission performance of vehicle fleets made by the fleet owner with the main objective to identify engines in need of an overhaul before severe engine damages occurs.
In Europe, a large project to elaborate remote sensing as a tool for measurement of emissions from vehicles in use (REVEAL) was recently finished. The main objective of the project was to verify if it could be used as part of an I/M program. Findings from the work within the project showed that it was possible to conclude that remote sensing is suitable to monitor overall "car fleet emissions", and that remote sensing can identify "gross polluters" but the method seems less suitable to precisely quantify emissions from individual vehicles.
From this document references are made to reports presenting results from tests of heavy duty buses and trucks. Most of the tests are carried out on chassis dynamometers, but some reports from On-board measurements are also included as sources. The objective with the references is to include reports presenting "field tests".
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