The first emissions controls implemented in the mid 1960s consisted of engine modifications and add on pollution control devices. These pollution control systems moderately lowered emissions, but they also caused a reduction in engine performance and increased fuel consumption.
In 1972 the federal government initiated a standardized test procedure for measuring vehicle compliance with federal emissions standards. This test incorporates a chassis dynamometer to provide a consistent and accurate way to measure the amount of HC, NOx , CO, and CO2 that a vehicle produces. This Federal Test Procedure (FTP) applies to both Light-Duty Vehicles (LDV) and Light-Duty Trucks (LDT). Over the years it has been tailored to the support the amended federal emissions requirements.
The FTP is designed to simulate typical driving conditions in urban areas. This FTP certification is a requirement of all pre-production vehicles that are to be sold in the U.S. Being compliant includes being able to pass the FTP throughout the vehicle's useful life. Because of this, the FTP is also used on in-use vehicles.
In 1975, catalytic converters were introduced to help lower the emissions of HC and CO. Catalytic converter efficiency of this era was poor and required cumbersome air management systems. By cleaning-up the exhaust (post-combustion) instead of reducing engine emissions (pre-combustion), auto manufacturers were able to address the Federal Government's concerns regarding pollution contributed by automobiles, however this did nothing for fuel efficiency.
The Federal Government's emissions standards combined with the price and availability of fuel at the time created a demand for the auto manufacturers to produce vehicles that were more fuel efficient and had lower emissions. At the time, the easiest way to reduce both fuel consumption and vehicle emissions was to lower the engine displacement and lean the air/fuel ratio out. Though emissions were lowered, the smaller, leaner engines created additional hurdles including a lack of performance.
One hurdle caused by the smaller engines pulling heavy vehicle weights was compounded by the leaner air fuel mixtures, which resulted in increased cylinder temperatures. These increased cylinder temperatures caused a dramatic rise in the production of oxides of nitrogen (NOX ).
The air that enters the combustion chamber consists of approximately 20.8 percent oxygen, 78 percent nitrogen and 1.2 percent mixed gases. As the air/fuel mixture enters the combustion chamber and ignition occurs, the nitrogen forms various compounds with the oxygen. These compounds have varying amounts of oxygen and are know as NOX -oxides of nitrogen.
Oxides of nitrogen are present during all phases of combustion; however, they are developed in high quantities when combustion chamber temperatures reach 2500° F. To help control the high production of oxides of nitrogen, manufacturers began equipping more of their vehicles with Exhaust Gas Recirculation (EGR). In addition to the EGR system, manufacturers adjusted camshaft profiles, retarded ignition timing, and lowered compression ratios in their attempt to lower NOX emissions