Locating and Identifying Emission Control Devices

Words and Photos By: Mark Yanochko

The use of emission control devices on automobile engines began in earnest in the early 1970s with the passage of the Clean Air Act. What started out with crankcase ventilation and heated air intake systems has evolved into an electronic maze of sensors and inputs that control all aspects of combustion. The net result is a significant reduction in emissions that was probably not thought possible when the environmental movement began nearly 40-years-ago.

Many of these devices and sensors are visible under the hood or chassis and can be replaced if damaged, broken or worn out. We’ll look at some of the more common devices and briefly discuss how each operates.

Before we begin our review of emission control devices, let’s check out the VECI (Vehicle Emission Control Information) label, A, shown in Figure 1 (2007 and earlier label format). This underhood sticker shows the basic vacuum line routing, lists general engine information (such as spark plug and engine size) and has a simple engine diagram which shows the relative location of some of the emission control devices. In this case, the location of the purge solenoid, EVAP canister, PCV valve and MAP sensor are shown.

The first control device we are going to discuss is the Evaporation Control System (EVAP). This system prevents the emission of fuel tank vapors. Years ago, fuel systems were vented to the atmosphere and caused a considerable amount of hydrocarbon (HC) emissions, but not anymore. This system includes a non-vented fuel cap, B (Figure 2), a vapor canister, C (Figure 3), to collect fuel vapors and a purge solenoid, D (Figure 4), that allows the vapors to be drawn out of the canister and into the throttle body to be mixed with the air-fuel mixture.

Another one of the first emission control devices was the Positive Crankcase Ventilation (PCV) valve. For decades, the blowby gases that accumulated in the crankcase were vented straight to the atmosphere causing a much more severe pollution problem than the hydrocarbons vented from the gas tank. The first solution was to route the vent hose directly to the base of the carburetor mounting plate, mixing these gases with the air-fuel mixture. The PVC valve used today, E, shown in Figure 5, is a much more complicated device, incorporating a spring-loaded valve to calibrate the flow of blowby gases into the air-fuel stream.

It was found that controlling a significant amount of the HC and carbon monoxide (CO) emissions was not that difficult. The methods used to control one, controlled the other or, at least, did not increase the other. That was not the case with oxides of nitrogen (NOx). Controlling this pollutant often hindered the efforts to control HC and CO. One of the first devices used to specifically control NOx was the Exhaust Gas Recirculation (EGR) valve. This valve directs a small amount of exhaust back into the intake flow to cool the combustion temperature, thus lowering the amount of NOx emissions. The EGR valve, F, used on many 3.3/3.8L V-6 engines is shown in Figure 6.

The emission control device that was a breakthrough in emissions control was the catalytic converter, G (Figure 7). The modern version is a 3-way device that controls all three of the major pollutants, HC, CO and NOx. Noble metals, such as platinum and rhodium, are used as catalysts to complete the combustion process.

Used in conjunction with the catalytic converter is the oxygen (O2) sensor, H (Figure 7). This device measures the oxygen content in the exhaust stream to determine if the air-fuel ratio is rich or lean. There are several other sensors that are used to gather information that helps the onboard computer determine the optimum air/fuel ratio. These include the MAP sensor (measures intake manifold pressure), J (Figure 8), the coolant temperature sensor and the Intake Air Temperature (IAT) sensor.

Over the last 35 years, these different devices and sensors have enabled engineers to design the most efficient engines ever built. Not only are today’s engines fuel efficient, these powerplants emit very low amounts of harmful pollutants into the atmosphere. This has resulted in a significant increase in air quality across the entire country.