Diesel engines offer good fuel economy and low emissions of hydrocarbons (HC) and carbon monoxide (CO). The air-fuel mixture in the combustion chamber is compressed to an extremely high pressure, causing the temperature to increase until the fuel's auto-ignition temperature is reached. The air-to-fuel ratio for diesel engines is much leaner (more air per unit of fuel) than for gasoline engines, and the larger amount of air promotes more complete fuel combustion and better fuel efficiency. As a result, emissions of HCs and CO are lower for diesel engines than for gasoline engines. However, with the higher pressures and temperatures in the diesel engine, emissions of nitrogen oxides (e.g., NO or NO2), which are known collectively as NOx, tend to be higher because the high combustion chamber temperatures cause the oxygen and nitrogen in the intake air to combine as nitrogen oxides. Hence, emissions of nitrogen oxides are a result of very high thermal efficiencies of the diesel engine.
Additionally, as a further disadvantage, diesel engines produce an amount of exhaust particulate matter (PM), or soot, that is comparatively larger than that of gasoline engines. PM is a complex emission that includes elemental carbon, heavy hydrocarbons derived from the fuel and lubricating oil, and hydrated sulfuric acid derived from the fuel sulfur. Diesel particulates include small nuclei mode particles having diameters below 0.4 μm and their agglomerates of diameters up to 1 μm. PM is formed when insufficient air or low combustion temperature prohibits the complete combustion of free carbon. As such, PM is partially unburned fuel or lube oil and is often seen as black smoke.
NOx emissions from diesel engines pose a number of health and environmental concerns. Once in the atmosphere, NOx reacts with volatile organic compounds or hydrocarbons in the presence of sunlight to form ozone, leading to smog formation. Ozone is corrosive and contributes to many pulmonary function problems, for instance.
Moreover, the fine particles that make up PM in diesel exhaust can penetrate deep into the lungs and pose serious health risks including aggravated asthma, lung damage, and other serious health problems. PM from diesel engines also contributes to haze, which restricts visibility.
Due to their damaging effects, governmental agencies have imposed increasingly stringent restrictions for NOx as well as PM emissions. Two mechanisms can be implemented to comply with emission control regulations: manipulation of engine operating characteristics and implementation of after-treatment control technologies.
In general, manipulating engine operating characteristics to lower NOx emissions can be accomplished by lowering the intake temperature, reducing power output, retarding the injector timing, reducing the coolant temperature, and/or reducing the combustion temperature.
For example, cooled exhaust gas recirculation (EGR) is well known and is the method that most engine manufacturers are using to meet environmental regulations. When an engine uses EGR, a percentage of the exhaust gases is drawn or forced back into the intake and mixed with the fresh air and fuel that enters the combustion chamber. The air from the EGR lowers the peak flame temperatures inside the combustion chamber. Intake air dilution causes most of the NOx reduction by decreasing the O2 concentration in the combustion process. To a smaller degree, the air also absorbs some heat, further cooling the process. The use of EGR increases fuel consumption.
In addition to EGR, designing electronic controls and improving fuel injectors to deliver fuel at the best combination of injection pressure, injection timing, and spray location allows the engine to burn fuel efficiently without causing temperature spikes that increase NOx emissions. For instance, controlling the timing of the start of injection of fuel into the cylinders impacts emissions as well as fuel efficiency. Advancing the start of injection, so that fuel is injected when the piston is further away from top dead center (TDC), results in higher in-cylinder pressure and higher fuel efficiency, but also results in higher NOx emissions. On the other hand, retarding the start of injection delays combustion, but lowers NOx emissions. Due to the delayed injection, most of the fuel is combusted at lower peak temperatures, reducing NOx formation.
With EGR engines, one of the key components to emissions control is the turbocharger. Most manufacturers using EGR technology have developed versions of variable geometry turbochargers (VGT), which are designed to regulate the flow of cooled exhaust air back into the combustion chamber, depending on the engine's speed.
The precise amount of exhaust gas that must be metered into the intake manifold varies with engine load. High EGR flow is generally necessary during cruising and mid-range acceleration, when combustion temperatures are typically very high. On the other hand, low EGR flow is needed during low speed and light load conditions. No EGR flow should occur during conditions when EGR could negatively or significantly impact engine operating efficiency or vehicle driveability, e.g., during engine warm up, idle, or wide open throttle.
Reducing NOx by manipulating engine operation generally reduces fuel efficiency. Moreover, mere manipulation of engine operation may not sufficiently reduce the amount of NOx to mandated levels. As a result, after-treatment systems also need to be implemented. In general, catalysts are used to treat engine exhaust and convert pollutants, such as carbon monoxide, hydrocarbons, as well as NOx, into harmless gases. In particular, to reduce NOx emissions, diesel engines on automotive vehicles can employ a catalytic system known as a urea-based selective catalytic reduction (SCR) system. Fuel efficiency benefits of 3 to 10% can result from using SCR systems that reduce NOx through chemical reduction rather than manipulating engine operation for NOx reduction which negatively impacts fuel efficiency. SCR catalysts (sometimes referred to herein as “SCR”) currently are used in diesel after-treatment systems. The SCR is typically fluidly connected to, and positioned downstream a diesel oxidation catalyst (DOC) with a particulate filter (e.g., a diesel particulate filter (DPF)) provided between the SCR and DOC. The SCR requires a reductant dosing system, such as a diesel exhaust fluid (DEF) dosing system, which is provided upstream of the SCR to inject a reductant such as anhydrous NH3, aqueous NH3, or most often a precursor that is convertible to NH3 such as urea ammonia or urea, into the exhaust flow.
Urea-based SCR systems can be viewed according to four major subsystems: the injection subsystem that introduces urea into the exhaust stream, the urea vaporization and mixing subsystem, the exhaust pipe subsystem, and the catalyst subsystem. Several SCR catalysts are available for diesel engines, including platinum, vanadium, and zeolite.
A diesel vehicle, stationary engine system, portable engine system must carry a supply of urea solution, or DEF, for the SCR system. The DEF is typically 32.5% urea in water by weight and is stored in a container, such as a tank or removable and/or refillable cartridge. The DEF is pumped from the container and sprayed through an atomizing nozzle into the exhaust gas stream, although other types and forms of reductant dosing systems can be used such as solid ammonia stored in a refillable cartridge. Complete mixing of urea or ammonia with exhaust gases and uniform flow distribution are critical in achieving high NOx reductions.
Urea-based SCR systems use gaseous ammonia to reduce NOx. During thermolysis, the heat of the gas breaks the urea (CO(NH2)2) down into ammonia (NH3) and hydrocyanic acid (HCNO). The ammonia and the HCNO then meet the SCR catalyst where the ammonia is absorbed and the HCNO is further decomposed through hydrolysis into ammonia. When the ammonia is absorbed, it reacts with the NOx to produce water, oxygen gas (O2), and nitrogen gas (N2). The amount of ammonia injected into the exhaust stream is a critical operating parameter. The required ratio of ammonia to NOx is typically stoichiometric. The ratio of ammonia to NOx must be maintained to assure high levels of NOx reduction. However, the SCR system can never achieve 100% NOx reduction due to imperfect mixing, etc.
A common problem with all SCR systems is ammonia slip. Ammonia slip refers to tailpipe emissions of ammonia that occur when: i) exhaust gas temperatures are too cold for the SCR reaction to occur, or ii) the amount of ammonia introduced into the exhaust stream is more than is required for the amount of NOx present Ammonia that is not reacted will slip through the SCR catalyst bed and exhaust to the atmosphere. Ammonia slip is a regulated emissions parameter which may not exceed a fixed concentration in the SCR exhaust.
Although the approaches described hereinabove may be effective in reducing NOx emissions, it is generally difficult to reduce both NOx emissions and PM emissions simultaneously. Conventionally, efforts to reduce NOx through various aspects of engine design tend to increase PM, or vice versa. In particular, very high temperatures in the combustion chamber help reduce PM emissions, but produce higher levels of NOx. On the other hand, lowering the peak temperatures in the combustion chamber reduces the amount of NOx as described previously, but increases the likelihood of PM formation. For example, advancing injection timing creates higher peak cylinder temperatures which burn off PM but produce NOx. Meanwhile, retarding timing reduces temperatures to minimize NOx emissions, but the reduced temperatures result in less complete combustion and increases PM. For similar reasons, the use of EGR to cool lower combustion temperatures increases PM emissions.
Among various engine operating characteristics, PM emissions can be reduced by advancing injection timing, increasing fuel injection pressures, increasing the power output, reducing engine speed, and reducing oil consumption. Additionally, a turbocharger can be employed to increase the charge pressure which allows the engine to operate on a leaner mixture resulting in lower particulate emissions.
After-treatment devices, such as a particulate filter (e.g., DPF), also exist to reduce or remove PM in diesel exhaust. Such after-treatment devices are often required in order to meet both NOx and PM emissions requirements, due to the difficulty of simultaneously reducing NOx and PM emissions by altering engine parameters, such as fuel injection timing.
For example, the DOC and DPF combination can provide effective approaches to purify PM emissions from a diesel engine. A DOC is a catalytic device that is used in the abatement of HC, CO, and the soluble organic fraction (SOF) of PM in diesel exhaust. A DPF has a filter with very small pores which are designed to remove PM, or soot, from diesel exhaust. Efficiencies for a DPF can be 85%, and even over 90%.
Through a process known as regeneration, many DPF's burn off PM that accumulates on the filter. Regeneration may be accomplished passively by adding a catalyst to the filter. Alternatively, regeneration may be accomplished actively by increasing the exhaust temperature through a variety of approaches, e.g. engine management, a fuel burner, or resistive heating coils. Active systems use extra fuel to cause burning that heats the DPF or to provide extra power to the DPF's electrical system. Running the cycle too often while keeping the back pressure in the exhaust system low, results in extra fuel use.
A DOC may also be used as a heating-device in active regeneration of a DPF. The accumulated PM in the DPF is continuously oxidized by NO2 which is generated by oxidizing NO in a DOC that is upstream of the DPF. Such a system requires accurate control to maintain the mass ratio of NO/PM in engine-out exhaust gas over a critical value. In-cylinder dosing is employed in active regeneration, where fuel injectors add a dose of fuel into the cylinder after the primary combustion has taken place. Unburned fuel is exhausted out of the cylinder downstream to the DOC where it burns and generates additional heat for the DPF. The additional heat in the DPF helps to convert the accumulated PM into ash, which has lower volume.
Engine control modules (ECM's), which are also known as engine control units (ECU's), control the engine and other functions in the vehicle. ECM's can receive a variety of inputs to determine how to control the engine and other functions in the vehicle. With regard to NOx and PM reduction, the ECM can manipulate the parameters of engine operation, such as EGR and fuel injection.
ECM's can also control the operating parameters of exhaust after-treatment devices, such as a urea-based SCR system, a DOC system, or a DPF system. For instance, an ECM can meter urea solution into the exhaust stream at a rate calculated from an algorithm which estimates the amount of NOx present in the exhaust stream as a function of engine operating conditions, e.g. vehicle speed and load. As a further example, an ECM can monitor one or more sensors that measure back pressure and/or temperature, and based on pre-programmed set points, the ECM activates the regeneration cycle.
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