species, some of which were not present in the original fuel but were created in the combustion reaction and leave the tailpipe without being fully oxidized. Evaporative VOC emissions, on the other hand, result from vapor escaping the fuel storage and transfer system, as well as from fuel leakage, and are thus independent of combustion. NOx and CO emissions are generated during the combustion process and these only occur in the exhaust.
Tailpipe emissions of VOCs, CO, and NOx are measured for emissions certification by means of the Federal Test Procedure (FTP), during which a test vehicle is driven on a chassis dynamometer over a prescribed driving schedule. The car is first stored with the engine off ("soaked") at room temperature for at least 12 hr. Then it is started with a cold engine, run over an 18-cycle urban-like driving pattern, stopped for a 10-min hot soak, restarted, and rerun over the first 5 of those 18 cycles. This 18-cycle driving pattern, known as the LA-4 schedule, was developed in the late 1960s to represent a commute to work in the typical Los Angeles traffic of the time. Following some minor modification, it became the basis of the federal U.S. Environmental Protection Agency (EPA)-mandated certification testing procedure for LDVs in 1975 (and is therefore also called the FTP75).
As illustrated in Figure 4-1, the entire LA-4 schedule covers 7.45 miles (mi) at an average speed of 19.6 miles per hour (mph). After adding the 5 repeat cycles following the hot soak, the entire FTP urban driving schedule covers 11.1 miles of driving in 31 mill, excluding the 10-min hot soak.
During the FTP, tailpipe exhaust is collected in three bags: the so-called cold bag for the first 5 cycles of driving, the stabilized bag for the next 13, and the hot bag for the 5 repeat cycles following the hot soak. For regulatory purposes, the measured mass emissions from each bag are substituted in a prescribed equation to determine the emission rate per unit of travel (in this case, grams per mile) of each regulated emission.
Evaporative emissions, including those resulting from leaks of liquid fuel, are measured separately using a variable-temperature SHED (sealed-housing-for-evaporative-determination) facility; i.e., an instrumented temperature-controlled room in which the test vehicle is housed. The fuel system of each car includes an evaporative canister containing a bed of activated carbon particles that adsorb most of the fuel vapor that might otherwise escape to the environment. The canister is connected to both the fuel-tank headspace and the engine intake. During