Both engine modifications and specific control technologies have been used to control motor vehicle emissions. These have included engine operation and design factors, engine-based control systems and exhaust gas control systems. Early control efforts were directed to reducing blow-by gases, evaporative emissions and HCs and CO from exhaust systems.
1. Engine Operation and Design:
Significant reductions in emissions of CO, HCs and NOx have been achieved in SI light-duty motor vehicles by engine operation and engineering design changes. These include use of lean-burn combustion and electronic ignition; changes in spark timing, compression ratios, combustion chamber shape and fuelling systems; and engine temperature control.
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Lean-Burn Combustion:
When NMHC and CO exhaust emission standards were first required on 1966 model cars in California and 1968 model cars sold nationwide, most automobile manufacturers complied by employing improved combustion systems. The most important combustion modification was to use lean air:fuel mixtures (above the stoichiometric ratio).
Carbon monoxide emissions were reduced significantly as A:F ratios were increased from ∼12:1 to 16:1. NMHCs were also reduced, but less effectively than CO. Although lean A:F ratios increase combustion efficiency, they reduce engine performance and vehicle drive ability and increase emissions of NMHCs (because of ignition failures).
To achieve optimal engine power production, the A:F or equivalence ratio should be set at ∼10 per cent rich. Since vehicle power was a major consumer concern in the 1950s and 1960s, vehicles were designed to operate at rich equivalence or low A:F, ratios. Lean-burn modifications resulted in significant reductions in engine power. Though lean-burn combustion significantly reduces CO and HC emissions, the associated increase in peak combustion temperatures results in optimum conditions for NOx production (Fig. 14.2).
Electronic Ignition:
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One of the early operating changes was to use electronic ignition. Previous ignition systems had significant maintenance problems that resulted in excessive HC emissions and poor engine performance.
Spark Timing:
Emission reductions can be achieved by changing ignition timing, an important performance variable. Optimally, it is set to maximise efficiency to what is described as MBT (minimum advance for best torque or maximum brake torque).
Optimum efficiency results when peak engine pressure occurs at 10 to 15° of top dead center of the compression stroke. Though actual spark timing varies considerably under different operating conditions, it may be retarded from the optimum to minimise emissions. It was one of the earliest engine operation adjustments used for emission control because of its ease of adjustment.
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When the spark is retarded, exhaust temperatures are hotter and both CO and HCs continue to be oxidised in the exhaust system. Once the piston commences its descent (expansion or power stroke), the increased volume in the combustion chamber reduces the proportion of the A:F mixture in quench zones.
Unfortunately, spark retardation reduces engine efficiency. It was a major cause of fuel consumption increases associated with emission controls before the use of catalytic converters. After catalytic converters were introduced, ignition timing was adjusted to optimum settings.
Compression Ratios:
In light-duty motor vehicles, compression ratios as high as 9 or 10:1 were once used to maximise power production and performance. However, they were decreased in new motor vehicles to reduce emissions of unburned HCs.
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As compression ratios decrease, significant reductions (as much as 50 per cent) in HC emissions are achieved. It is believed that these reductions are in part due to the decrease in the ratio of crevice zone:combustion zone volume. In addition, the lower expansion ratio (expansion volume:total combustion chamber volume) maintains exhaust gases at higher temperatures and thus promotes more complete HC oxidation. Lower compression ratios decrease engine efficiency and power output. They also increase fuel consumption.
Combustion Chamber Shape:
Combustion chamber surface area is a major HC emissions factor. A spherical shape has the smallest surface:volume ratio and would, in theory, be an ideal combustion chamber shape. It is approximated by using a hemispherical head and recessed bowl in the piston. This combustion chamber shape is used to achieve high performance with minimal knock. As a consequence, engine efficiency and burning rate improve, quench zone thickness decreases and HC emissions decrease.
Fuelling Systems:
Throughout much of the history of light-duty motor vehicle manufacture, a suction-type carburetor was used to mix air and fuel and distribute the mixture to each cylinder (combustion chamber). This carburetor design has been largely replaced by single (SPI) and multiport (MPI) fuel injection systems.
Fuel injection systems are readily adaptable to feedback control from measurements of exhaust parameters. An SPI works relatively well to control emissions but has problems maintaining correct A:F mixtures under transient conditions.
The MPI system improves fuel distribution between cylinders and its time wise distribution to each cylinder. Therefore, three-way catalytic converter systems are a viable option for emissions control.
The MPI systems have a number of advantages:
(i) They increase engine power and performance; and
(ii) Since the intake manifold can be better designed (because it is for air alone), less heating of induction air is required and less performance loss occurs due to abnormal A:F ratios in cylinder-to-cylinder fuel distribution.
Engine Temperature Control:
Engine temperature in the cylinders and induction system is an important parameter since a high proportion of emissions are produced when the engine is cold or only partially warmed. As a consequence, it is desirable to bring engines up to normal operating temperatures as quickly as possible. This is achieved by careful adjustment of cooling system thermostats and directing the flow of intake air across the engine. It is now common to use heat from the exhaust manifold to warm cool or cold intake air using a control valve.
2. Engine-Based Control Systems:
Motor vehicle emissions can be treated or prevented from being formed by engine-based control systems.
These include:
(i) Crankcase ventilation to control HC-rich blow-by gases;
(ii) Evaporative controls; and
(iii) Exhaust gas recirculation (EGR) to control NOx emissions.
(i) Crankcase Ventilation:
Blow-by or crankcase, emissions are controlled by a relatively simple and inexpensive technology known as positive crankcase ventilation (PCV). Gases slipping by the piston rings into the crankcase are returned to intake airflow by negative pressure in the intake manifold.
There they are mixed with the incoming air-fuel charge and drawn into a combustion chamber and combusted. A one-way PCV valve regulates this flow, ensuring that it only moves from the crankcase and is not excessive. In modern emission control systems, ambient air is also routed to the crankcase so that it is continually purged.
(ii) Evaporative Emissions:
Evaporative emissions from the carburetor and fuel tank are controlled by sorbing fuel vapours onto an activated carbon bed in a small canister connected to the fuel system. On engine operation, collected vapours are stripped from the sorbent and directed into the combustion chamber.
The efficiency of HC collection varies with fuel components. High-volatility, low-molecular-weight substances are less likely to be sorbed and retained than lower-volatility, higher-molecular-weight substances.
(iii) Exhaust Gas Recirculation:
Emissions of NOx can be reduced to varying degrees by introducing a diluent that absorbs heat. Diluents must be readily available and neutral relative to other engine emissions; they should not decrease engine performance. Exhaust gases meet these requirements.
Heat sorption by CO2 and H2O reduces peak combustion temperatures and consequently, NOx production. Recirculation of a portion of exhaust gases by means of a control valve is used to control emissions of NOx in many motor vehicles. On modern systems, an engine management microprocessor couples the flow of exhaust gases with other variables such as engine speed.
The effectiveness of EGR in reducing NOx emissions depends on the quantity of exhaust gases used. An EGR value of 10 per cent may reduce NOx emissions by 30 to 50 per cent. Since combustion instability occurs above 15 to 20 per cent EGR, this range represents its practical limit.
3. Exhaust Gas Control Systems:
Exhaust gas emissions can be controlled by devices located downstream of the combustion chamber. These include thermal reactors (afterburners) and catalytic converters.
Thermal Reactors:
Thermal reactors were used by some automobile manufacturers in the late 1960s to control emissions of HC and CO. Thermal reactors are devices that under optimal temperature conditions, oxidise CO and HC to CO2 and H2O in the exhaust system.
Reactors require relatively high temperatures 580°C (1076°F) to oxidise 80 per cent of HC; they require 750°C (1382°F) to oxidise 80 per cent of CO. To reach these temperatures, rich A:F mixtures are required so that the extra fuel can be burned in the reactor.
Since exhaust gases include only small quantities of O2, an air pump is needed to supply additional air. Due to the use of fuel-rich mixtures and spark retardation, thermal reactors have decreased thermal efficiency and increased fuel consumption.
Oxidising Catalytic Systems:
Oxidising catalytic converters control emissions of HC and CO. Catalytic systems have the advantage of being able to operate efficiently at moderately low exhaust temperatures and optimal compression ratios and spark timing. Oxidation efficiencies of >90 per cent can be achieved at temperatures of 300°C (572°F) for CO and 350 to 370°C (662 to 698°F) for HC. Catalysts promote desired oxidation reactions at temperatures much lower than would otherwise be required.
Noble metals such as platinum (Pt), palladium (Pd) and rhodium (Rh) are preferred catalysts for catalytic converters. Most catalytic converter systems use some combination of these metals applied to the surface of small, 3 mm diameter pellets, a ceramic honeycomb structure or a metal matrix.
Pellet- type systems have been largely replaced by honeycomb or matrix types (because of high pressure drops). Catalyst thickness on substrate surfaces is on the order of 20 to 60 um. Gas flow through the converter is usually axial, but in some cases it is radial.
Oxidation of CO and HC occurs best using lean A:F fuel mixtures since more O2 is available for oxidation. Air pumps must be used to provide additional air with richer A:F mixtures. Control efficiencies on the order of 90 per cent for HC and 95 per cent for CO can be achieved with ϕ values that are stoichiometric or on the lean side of stoichiometric.
The same catalytic materials promote the reduction of NOx to nitrogen (N2) and O2. Reduction of NOx is facilitated by removing O2 (from NOx) by reaction with CO and hydrogen (H2). Rhodium is particularly effective in reducing NOx, but Pt and Pd also work relatively well.
Peak conversion of NOx (90 per cent) can be achieved at ϕ values that are ∼0.5 per cent richer than stoichiometric. Under leaner conditions, conversion efficiency declines significantly. For A:F mixtures 1.5 per cent lean of stoichiometric, conversion efficiency drops to 20 per cent. Therefore, ϕ values that are optimal for oxidation reactions are less than optimal for NOx reduction.
For most of their history of use, catalytic converters have been oxidising systems.
This reflects the fact that:
(i) Two of the three major pollutants in auto exhaust are subject to oxidation; and
(ii) Oxidation catalysts obviate the detrimental effects of emission controls on fuel consumption and vehicle operability. Consequently, NOx emission reduction had to be achieved by other means. EGR was the control technique of choice when oxidising catalytic systems were used.
Reducing Catalytic Systems:
Separate catalytic systems to reduce NOx to N2 and O2 can be used in conjunction with oxidising systems. A reducing catalyst must be placed upstream of the oxidation catalyst since exhaust gases passing through it need to be fuel-rich and contain CO as a reductant. Since the system is fuel-rich, additional air must be pumped into the exhaust gas stream. Platinum has been the catalyst of choice for reduction systems. Copper ion-exchange zeolites can be used to reduce NOx under lean conditions.
Three-Way Catalytic Systems:
In three-way catalytic systems, oxidation and reduction occur in a single catalytic unit. The three-way catalyst is now the most widely used exhaust gas control system, as all three major pollutants are controlled in one bed.
For oxidising catalysis, an A:F ratio on the lean side of the stoichiometric ratio provides the highest efficiency, whereas for reduction catalysis of NOx, an A:F ratio that is stoichiometric to rich provides the highest efficiency. However, at ϕ values of 0.995 to 1.008, conversion efficiencies of >80 per cent are Possible for each of the three major pollutants. The effect of A:F ratio on catalyst efficiency in reducing emissions can be seen in fig 14.7.
Moderately high temperatures >300°C (572°F) must be attained and maintained for efficient catalytic system performance. Such temperatures are only attained several minutes after engine operation begins.
Under cold-start conditions, catalytic converter performance is relatively poor, resulting in higher exhaust emissions. Catalytic systems must therefore be constructed of materials that have a low specific heat but high thermal conductivity to quickly warm to operating conditions. Catalytic materials are mixed with aluminium oxide (Al2O3) to facilitate rapid warming.
A feedback system is required to maintain ϕ values within the optimum range. Oxygen sensors are used to monitor residual O2 in exhaust gases. These sensors detect O2 partial pressure and thus provide direct feedback to fuel injection processes. These sensors work well when ϕ > 1.0.
Deterioration of Catalyst Performance:
Catalytic system performance can be easily degraded. Degradation results from overheating and contamination of catalytic surface by substances in fuel (e.g., sulphur (S), phosphorous (P) and lead (Pb)).
Overheating occurs when engine malfunctions allow excessive fuel to pass into the exhaust system. Ignition failure for as little as 20 second may produce sufficiently elevated temperatures to totally destroy the catalytic system.
Gasoline has to be formulated to minimise concentrations of P, S and Pb to prevent contamination of catalytic materials. Phosphorous and S levels can be reduced or nearly eliminated at refineries. Lead was a significant concern, as it was added to gasoline to increase octane ratings and engine performance. Motor vehicles equipped with catalytic systems must therefore be operated on Pb-free gasoline.
Lead, S, P and possibly other substances are said to ‘poison’ catalysts. In fact, they simply coat the surface of catalytic materials, preventing contact with exhaust pollutants. Catalytic converters can convert S to sulphur dioxide (SO2) and hydrogen sulphide (H2S), thereby producing malodours.
Emission Controls on Currently Marketed Vehicles:
Most, if not all, currently manufactured vehicles are designed with the following systems to control emissions of CO, HC and NOx– (i) PCV; (ii) evaporative emission control systems; (iii) electronic ignition; (iv) a compact combustion chamber that minimises wall effects; (v) a throttle position sensor to allow precise control of idling speeds; and (vi) an engine management system.
In addition, special design features are incorporated for SPI and MPI system engines. In SPI engines, a catalytic system is used to oxidise CO and HC. It is placed close to the exhaust manifold. A secondary air supply to the exhaust manifold is provided by an air pump. Intake air is heated by transferring heat from exhaust gases.
Intake temperatures are regulated by use of a thermostat and a flap valve. An EGR system is used to limit emissions of NOx. Temperature control from the engine coolant is used to shut off secondary air to the exhaust during engine warm-up. It also limits purging of fuel tank vent canisters and EGR during cold conditions.
In MPI engines, a microprocessor-type engine management system is utilised. A three-way catalytic converter is placed immediately downstream of the exhaust manifold. The management system controls spark timing as determined by throttle position, engine speed, knock sensor, manifold vacuum and engine temperature. Purging from the fuel tank vent canister is subject to temperature control from the engine coolant.