1. Introduction

The first occurrence of ground-level ozone, or smog, in Los Angeles was in 1943 and concerned, apart from vehicular internal combustion engines, domestic and industrial combustion processes. From the transportation point of view, the major smog culprits are hydrocarbons (HC), NO_x, CO, sulfur dioxide (SO₂), and VOCs. These react with sunlight, heat and various other compounds to form the noxious vapors, ground level ozone, and particles that are constituents of smog.

A subsequent key event in the air-pollution history of the United States was the deadly smog in Donora Pennsylvania in 1948. The town’s heavy industrial production and a temperature inversion resulted in high air-pollutant concentrations for a period of one week [4].

European regulations use the term positive ignition (PI); U.S. regulations refer to Otto-cycle (heavy-duty) engines.

These three terms, test, transient and driving/drive cycle, are used interchangeably in the literature; nonetheless, not all the cycles are transient and not all of them are exactly driving.

The World Forum for Harmonization of Vehicle Regulations is a working party (WP.29) of the Inland Transport Division of the United Nations Economic Commission for Europe (UNECE—established in 1947—see also Fig. 1.14). WP.29 was created on 6 June 1952 as ‘Working party of experts on technical requirement of vehicles’; the current name was adopted in 2000. The forum works on regulations aiming at improving vehicle safety, protecting the environment, promoting energy efficiency and increasing anti-theft performance. It is tasked with creating a uniform system of regulations for vehicle design to facilitate international trade [22].

The ‘Agreement concerning the Establishing of Global Technical Regulations for Wheeled Vehicles, Equipment and Parts which can be fitted and/or be used on Wheeled Vehicles’, or 1998 Agreement, is a subsequent to the 1958 Agreement. Following its mission to harmonize vehicle regulations, the UNECE solved the main issues preventing non-signatory countries to the 1958 Agreement to fully participate to its activities. The 1998 Agreement was born to produce meta-regulations called Global Technical Regulations (GTRs) without administrative procedures for type approval and so, without the principle of mutual recognition of type approvals. The 1998 Agreement stipulates that contracting parties will establish, by consensus vote, United Nations Global Technical Regulations (UN GTRs) in a UN Global Registry. The UN GTRs contain globally harmonized performance requirements and test procedures. Each UN GTR contains extensive notes on its development. The text includes a record of the technical rationale, the research sources used, cost and benefit considerations, and references to data consulted. The contracting parties use their nationally established rulemaking processes when transposing UN GTRs into their national legislation [22].

The Working Party on Pollution and Energy (GRPE—Group de Rapporteurs sur la Pollution et l' Énergie) is the subsidiary body of the WP.29 of the UNECE that prepares regulatory proposals on pollution and energy efficiency to WP.29. This group of experts, whose meetings are attended by representatives from governments, manufacturers etc., conducts research and analysis to develop emission and energy requirements for vehicles [22].

UNECE WP.29 regulations can be accessed online through the following link http://​www.​unece.​org/​trans/​main/​welcwp29.​html.

This term is also used for the steady-state cycles of heavy-duty vehicles (Sect. 1.3.2) that are defined by a series of steady-state points or modes (in terms of engine speed and torque).

A fundamental difference between the modal and the transient cycle is that the former is defined in a tabular form containing specific driving modes, while the latter as a sequence of vehicle speed at each time point (second).

Particularly for soot, which notoriously peaks during transients more than NO_x, the legislation often included a special soot or smoke test even when steady-state cycles were the only applicable option for the other exhaust pollutants.

Caterpillar Inc., Cummins Engine Company, Detroit Diesel Corp., Mack Trucks Inc., Navistar International Transportation Corp., Renault Vehicles Ind. S.A. and Volvo Truck Corp; these seven companies together manufactured more than 95 % of U.S. heavy-duty diesel engines at the time [11].

Interestingly, for the U.S. EPA SFTP data collection in Baltimore in 1992 (one of the biggest in history), the sub-contractor did not explicitly discuss the monitoring of the vehicle speed with the car owners involved in the survey [49].

A snippet is a change in traffic density for a given freeway or a change in the physical link to or from an area transportation network [51].