Introduction:
The emission controls on today’s cars and trucks are not merely add-on components; they are an integral part of the engine and its electronic control system. In many ways, the electronic control systems themselves are fundamentally emission control systems. The ongoing drive toward cleaner, more fuel efficient vehicles has directly led to the development of these sophisticated technologies. Importantly, these systems have also contributed to significant increases in power, reliability and overall driveability.
The Primary Pollutants and Their Formation:
For environmentalists, engineers, and technicians, the automotive emissions of greatest concern are hydrocarbons (HC), carbon monoxide (CO), oxides of nitrogen (NOx), and oxygen (O2). While O2 is not a pollutant, its presence in exhaust indicates the completeness of combustion and is monitored during inspections to detect exhaust leaks that would dilute the sample. Another gas, sulfur dioxide (SO2), is a colorless gas with a rotten egg smell, produced by high sulfur content in gasoline and the catalytic converter. SO2 is largely unmonitored in standard tests but poses serious health risks, including heart problems, asthma, and other respiratory conditions. The formation of HC, NOx, and CO is driven primarily by combustion chamber temperatures and the air fuel ratio. A fundamental challenge in emissions control is that the conditions which minimize HC often promote high NOx formation. Government regulations have progressively lowered the maximum allowable emissions from automobiles, and all vehicles for the past 40 plus years have been equipped with tailpipe emission reduction devices.Hydrocarbons (HC):
Hydrocarbon emissions are the result of incomplete combustion, consisting of unburned or partially burned fuel molecules. Even a well-maintained engine produces HC, and evaporative losses from the fuel system are a significant additional source. HC is generated through several specific mechanisms: air-fuel mixture trapped in sheltered crevices of the combustion chamber such as areas around the piston rings, head gasket, spark plug threads, and valve seats fuel absorption by cylinder wall oil or carbon deposits, flame quenching as it approaches the cooler cylinder wall, incomplete flame propagation, inadequate fuel-air mixing and fuel leakage past the exhaust valve. An excessively lean air fuel ratio causes misfire and high HC, while a very rich ratio also increases HC. Conversely, HC emissions are low at the stoichiometric air-fuel ratio.
Carbon Monoxide (CO):
Carbon monoxide is a colorless, odorless and highly poisonous compound that forms when there is insufficient oxygen to combine with carbon during combustion, resulting in CO rather than carbon dioxide (CO2). CO exposure can cause dizziness, headaches, impaired thinking, and death by oxygen starvation. While primarily found in exhaust, it can also appear in the crankcase. CO is a direct indicator of a rich air-fuel mixture as the mixture becomes richer, CO levels rise sharply. At stoichiometric or leaner ratios, CO emissions remain very low. It is important to note that CO will only be present if combustion occurs and it is not an accurate indicator of a lean mixture.Nitrogen Oxides (NOx):
NOx refers to various compounds of nitrogen and oxygen formed during combustion. They cause respiratory problems such as lung irritation, bronchitis, and pneumonia, and when combined with HC and sunlight, they form photochemical smog a brownish haze linked to chest pains, shortness of breath, and eye irritation. NOx also mixes with rainwater to create nitric acid, or acid rain. Formation occurs when combustion temperatures exceed 1,261°C, causing nitrogen and oxygen in the intake air to combine. Since the atmosphere is 78% nitrogen, the only control method is to manage combustion temperature. This creates a delicate balance: a slightly rich mixture lowers temperature and reduces NOx but increases CO and HC; a slightly lean mixture reduces CO and HC but increases NOx due to higher temperatures.The x in NOx is variable, representing different oxides like NO, NO2, and NO3. Engine emissions are mostly nitric oxide (NO), with less than 1% being the more toxic nitrogen dioxide (NO2). NO is unhealthy and contributes to the greenhouse effect, while NO2 contributes to smog, ozone, and acid rain. Notably, diesel engines produce a higher proportion of NO2 than gasoline engines, with about one third of their converted nitrogen becoming NO2.
Carbon Dioxide (CO2) and the Greenhouse Effect:
Though not a pollutant, CO2 is a greenhouse gas linked to global warming. Transportation accounts for 27% of North American CO2 emissions, with automobiles contributing 14%. From an efficiency perspective, high CO2 indicates complete combustion, with levels increasing from 6% to 13.5% as the air-fuel ratio moves from 9:1 to 14.7:1, peaking when the mixture is slightly leaner than stoichiometric. CO2 production is directly tied to fuel consumption. Engineers are working to reduce CO2 by improving fuel efficiency, though this is challenging as some methods like lean mixtures can increase other emissions. This concern is a primary driver for alternative fuels and the government's CAFÉ standard hike to 35 mpg. Currently, there are about 700 million vehicles globally, producing 2.8 billion tons of CO2 annually roughly 20% of fossil fuel CO2 emissions. There are currently no CO2 standards in North America, though the EU has them.
Oxygen (O2) and Water (H2O):
O2 is not a pollutant, but its level indicates mixture condition. A rich ratio consumes all available oxygen, leaving low O2 in the exhaust; a lean ratio leaves higher O2. Therefore, O2 is a good indicator of a lean mixture and is unaffected by catalytic converter operation. Water (H2O) is a normal and desirable exhaust byproduct, resulting from the oxidation of HC in the converter to form CO2 and H2O. The amount varies with converter efficiency and exhaust composition. Steam is often visible during cold starts as the exhaust system warms and condensation forms.
Diesel Engine Emissions:
Diesel engines are the most efficient internal combustion engines, with low fuel consumption and low greenhouse gas output. However, they emit particulates, NOx, water, CO, SO2, and various hydrocarbon compounds. Soot is the most visible emission, comprised of carbon-based substances that absorb other contaminants. Particulate emissions are the primary obstacle to wider diesel use in passenger and light trucks, and California and other states have set increasingly stringent standards. These emissions are heavily influenced by fuel quality and engine design.Evaporative Emission Control (EVAP) Systems:
Fuel vapors from the gasoline tank and carburetor are controlled by EVAP systems. Modern systems are computer controlled and monitored by OBD-II. They typically include a domed fuel tank, canister vent solenoid, special filler neck, fuel tank pressure sensor, vapor lines, vapor separator, charcoal canister, purge lines, and a purge solenoid valve. The system stores vapors in a charcoal canister until they are purged into the intake manifold to be burned. It also allows controlled atmospheric pressure into the tank to prevent collapse. Vapors from the tank pass through a vapor separator which returns liquid fuel to the canister. Purge strategies vary: older systems used a fixed orifice and thermal vacuum valve, often causing driveability issues due to uncontrolled enrichment regardless of load. PCM controlled systems use a duty cycled purge solenoid that operates only in closed loop and under specific conditions, preventing driveability problems.Enhanced EVAP systems:
It can detect leaks as small as 0.5 mm. They use a fuel tank pressure (FTP) sensor, canister vent (CV) solenoid, and sometimes a leak detection pump (LDP). The system creates a vacuum and monitors decay; if vacuum cannot be built or held, a leak is identified. A fuel level input sensor determines the appropriate vacuum build time, and monitors will not run if fuel level is too high or low. In LDP systems, the PCM pressurizes the system if pressure fails to build, the pump runs until the test cycle completes. The purge monitor checks flow by comparing MAF/MAP data to FTP/fuel trim changes as the solenoid duty cycle is increased, or via a purge flow sensor. If a large leak like a loose gas cap is detected, the MIL illuminates and the leak test is aborted.Precombustion Emission Control Systems:
Precombustion systems prevent pollutant formation. Engine design changes include better-sealing, low-friction piston rings; combustion chamber redesign to reduce quench areas, center spark plugs, and induce controlled turbulence, optimized compression ratios, reduced friction through improved oils and materials, equalized intake manifold air distribution enhanced by port fuel injection advanced cooling systems that run hot but not too hot to limit NOx and PCM controlled spark timing.Positive Crankcase Ventilation (PCV):
PCV systems remove blowby gases unburned fuel and combustion products from the crankcase, preventing sludge, oil dilution, and seal damage. The system uses manifold vacuum to draw fresh air through the crankcase, mixing with blowby, and routing it into the intake to be burned. The PCV valve is spring loaded and tapered, it regulates flow based on vacuum. At idle, high vacuum closes the valve to a small opening, at part-throttle, lower vacuum allows a larger opening, at wide open throttle, the valve opens fully. Excessive blowby or a restricted valve forces gases back through the breather, causing oil in the air cleaner. A stuck open valve causes a rough idle; a backfire seats the valve, preventing an explosion. Some engines use a fixed orifice tube instead of a valve. Heated PCV systems (coolant or electrically heated) prevent freeze up in cold weather, with PCM or thermistor control. OBD-II PCV monitors use specialized valves with cam-lock threads to ensure a total seal.Exhaust Gas Recirculation (EGR):
EGR reduces NOx by recirculating inert exhaust gas into the intake, diluting the mixture and lowering peak combustion temperatures. Excessive EGR causes driveability problems, so flow is disabled during cold start, idle, and wide-open throttle, with maximum flow only at light-load cruising. Many modern engines use variable valve timing to retain exhaust, serving the same purpose. OBD-II monitors EGR operation. Vacuum controlled EGR valves used on older engines incorporate thermal or ported vacuum switches, delay timers, and WOT cut offs. Backpressure EGR uses a transducer to modulate flow based on exhaust pressure. Positive backpressure valves have a bleed held open by a spring; exhaust pressure closes it as speed increases, allowing vacuum to open the valve.Negative backpressure valves have a normally closed bleed that opens under negative pulses at higher speeds. PCM-controlled EGR (vacuum operated) uses a differential pressure feedback (DPFE) sensor across an orifice. The PCM calculates desired flow, commands a duty-cycled vacuum regulator, and receives feedback from the DPFE (0-5V proportional to pressure drop). Some systems include an exhaust gas temperature sensor (NTC thermistor) for feedback. Electric EGR (EEGR) uses a stepper motor to position a pintle valve, eliminating the need for vacuum components. Digital EGR valves use up to three solenoids (each with a different orifice size) operated by the PCM to achieve variable flow. Linear EGR valves use a single solenoid with a tapered pintle and a position sensor (linear potentiometer, ranging from 1V closed to 4.5V open) for precise PCM feedback and PWM control.
Intake Heat Control:
CO and HC are highest when cold. Intake heat systems warm the air-fuel mixture to improve vaporization on TBI engines. EFE heaters are resistance grids controlled by the PCM based on coolant temperature. Port injection engines do not require these, as fuel is injected directly into the intake ports or cylinders.Postcombustion Emission Control Systems:
These systems clean exhaust after combustion. The catalytic converter is the most effective for reducing HC, CO, and NOx. Secondary air injection also helps by oxidizing residual gases.Catalytic Converters:
Unleaded gasoline enabled the use of catalytic converters. Modern vehicles typically have two converters per exhaust stream a small, quick heating light up precat near the manifold and a main converter downstream. OBD II monitors catalyst efficiency using heated oxygen sensors (HO2S). Most vehicles have two HO2S per stream in front for fuel control, rear for monitoring, while PZEVs may have three in front, center, and rear for long term trim and efficiency. Converters are ceramic honeycomb or bead structures coated with precious metal catalysts like platinum, rhodium, palladium. The honeycomb design offers immense surface area. Older dual-bed converters had a reduction catalyst for NOx followed by an oxidation catalyst for CO and HC. Modern three way converters handle all three. Rhodium reduces NOx to N2 and O2; the O2 reacts with CO to form CO2. Platinum and palladium oxidize CO and HC to CO2 and H2O. Oxygen storage via cerium in the converter and PCM toggling of the air-fuel mixture between slightly lean and slightly rich provide the necessary O2 and CO for these reactions. Efficiency depends on temperature. This warm up delay is why precats are used. For lean burn engines, a NOx storage/adsorber converter is added. It uses barium to trap NOx; once full, the PCM enriches the mixture to regenerate the converter, converting stored NOx to nitrogen.Air Injection Systems:
AIR systems inject fresh air into exhaust manifolds to promote secondary combustion of HC and CO. They used an air pump or pulse air using exhaust pressure pulses. Over time, their role shifted to injecting air directly into the converter to improve its efficiency, but they are less common today due to improved combustion and catalysts. A typical system includes a belt-driven air pump, an air control valve i.e. vacuum-operated, routing air to the manifold when cold and to the converter when warm, and an air by-pass valve to vent air during deceleration to prevent backfires. One way check valves prevent exhaust backflow. Pulse systems rely on negative exhaust pulses to draw air from the air cleaner through check valves. Electronic secondary air systems use a PCM controlled electric pump and solenoid-operated by pass/diverter valves. In upstream mode, air is sent to the exhaust manifold to burn rich mixture HC and CO, heating the O2 sensor to enable closed loop operation sooner. This mode forces open loop fuel control as the added O2 makes the sensor read lean. In downstream mode, air is sent to the catalytic converter to aid in oxidation and the PCM operates in closed loop.Diesel Emission Controls:
Modern diesel engines are durable but heavy, with higher displacement and turbochargers to compensate. They produce substantial torque but are noisy. Cleaner fuels and technologies like PCM controlled injection, EGR, PCV, catalytic converters, particulate filters now allow diesel emission levels to match gasoline engines.Low-sulfur fuel (now 15 ppm vs. previous 500 ppm) is essential for enabling modern aftertreatment. Diesel fuel is denser and contains more energy per gallon, also offering better lubrication. Biodiesel fuels are also common.
Diesel fuel injection is a critical emission control system. Precise timing is key: early injection increases NOx; late injection increases HC, CO, and PM. The PCM controls pulse width and fuel pressure. Most light-duty diesels use common rail systems with solenoid or piezo injectors. Piezoelectric injectors expand quickly when current is applied, allowing multiple injections per stroke often three, sometimes five. Pilot injection (a small pre-injection) reduces combustion harshness, and a third injection at the end of the stroke lowers cylinder temperature.
Glow plugs are used for cold starts when coolant is below 48°F. The PCM energizes them and a warning lamp signals the driver when they are ready. They remain active during warm up to reduce emissions.
Diesel PCV differs due to low intake vacuum. Many systems use a multi-stage filter to collect and return oil or a hose connected to the turbo intake where airflow creates vacuum.
A Crankcase Depression Regulator (CDR) valve similar to a PCV valve but designed for low vacuum, maintains crankcase pressure.
Diesel EGR is similar to gasoline systems but often includes an EGR cooler that uses engine coolant to reduce the temperature of recirculated exhaust gas, further lowering NOx. The PCM operates a digital EGR valve and monitors its function.
Diesel catalytic converters are primarily oxidation catalysts (Pt) that oxidize CO and HC. For NOx, they may include a NOx adsorber using barium to store NOx and rhodium for reduction. A diesel particulate filter traps soot. The filter monitors backpressure; when it reaches a specified level, the PCM adds extra fuel to heat the oxidation catalyst and burn off the ash (the regeneration cycle). The use of low sulfur fuel is critical, as high sulfur fuel causes excessive ash buildup.
Selective Catalytic Reduction (SCR) is an alternative NOx control technology used on new diesels. A reductant i.e. ammonia or a urea water solution is injected into the exhaust over a catalyst where it strips oxygen from NOx to form water. Urea is an organic compound found in mammal urine and can eliminate over 90% of NOx. SCR systems are cheaper than NOx traps and do not affect engine performance. They require refillable tanks, and the EPA is considering enforcement mechanisms, including warning lamps, limp-home modes or preventing starts if the tank is empty. Honda has developed an alternative system that generates ammonia internally using a lean NOx (LNT) catalyst with two layers; one absorbs NOx and converts it to ammonia, while the other stores the ammonia. The PCM toggles the mixture between lean and rich to drive this reaction, allowing the ammonia to reduce NOx to N2 without an external reductant tank.
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