Each year, Earth Day - April 22 - marks the anniversary of the modern environmental movement. First celebrated in 1970, it was a key turning point in raising American awareness of environmental problems and within only a few months, the United States had enacted the Clean Air Act (CAA) and established the U.S. Environmental Protection Agency (EPA) to reduce pollutants expelled into the air by fossil fuel engines.
Since that time, EPA has set several standards for non-road diesel engines to decrease emissions levels of non-methane hydrocarbons (NMHC), nitrogen oxides (NOx) and particulate matter (PM). (See chart.) Signed in 2004, the next – and final – stage of emission standards, Tier 4, will push emission limits toward zero and require extensive use of new technologies and control for diesel engines by 2015.
As the final stages of Tier 4 standards are implemented; the EPA, through the CAA, mandates control of air pollution from mobile sources by regulating both the composition of fuels and emission levels of motor vehicles and non-road engines. Vehicle fuel standards require Ultra Low Sulfur Diesel Fuel (ULSD, 15PPM sulfur) and are met by the refiners. The regulation of diesel vehicle emission limits primarily targeting nitrogen oxides and particulates are met by the engine manufactures. These limits apply to on-road vehicles, off-road vehicles, and non-road sources (e.g., marine engines, locomotives, and stationary power generation).
Methods to control emissions
To meet the final tier 4 requirements, engine manufacturers are attacking the problem through three common paths; incoming fuel quality, efficient and controlled burn technologies and exhaust after-treatment systems. These challenges have driven the development and use of five primary technologies.
1) High Pressure Common Fuel Rail (HPCR) systems:
HPCR systems are advanced fuel injection and combustion designs that regulates fuel pressure (22,000-34,000 psi) and injection timing. This system stores the high pressure fuel in a common rail while precisely controlled injectors can deliver fuel several times each combustion cycle. The high pressure transforms the fuel into a fine mist as it leaves the injector, which, combined with the multiple injection delivery, improves combustion and efficiency, resulting in lower exhaust emissions. More efficient, cleaner combustion results in cleaner exhaust and, subsequently, less after-treatment, which equates to lower overall costs.
2) Cooled Exhaust Gas Recirculation (CEGR):
The CEGR system is an extension of common EGR systems widely used today. Exhaust gas recirculation reduces the amount of oxygen in the combustion process, reducing the peak temperature of combustion and the formation of NOx. The cooler exhaust gas allows more volume of gas to be used in combustion than previous uncooled versions. The increased use of exhaust gas continues to drive down NOx in the exhaust stream.
3) Selective Catalyst Reduction (SCR):
SCR transforms NOx in the exhaust gas into harmless nitrogen and water vapor. The SCR system injects a liquid urea (ammonia or other water-based fluid know as Diesel Exhaust Fluid or DEF) into the exhaust stream of the diesel engine prior to entering a special catalyst. The DEF initiates a chemical reaction with the diesel exhaust and catalyst, turning harmful NOx into nitrogen, water, and CO2. SCR technology can achieve up to a 90% reduction in NOx.
4) Diesel Oxidation Catalyst (DOC):
DOC is another exhaust after-treatment device for diesel engines. The DOC is a flow through device that contains palladium/platinum with aluminum oxide which serve as catalysts to oxidize hydrocarbons, carbon monoxide, diesel particulates and other pollutants into CO2 and water. The DOC is capable of reducing particulates by 20% to 30%.
5) Diesel Particulate Filter (DPF):
DPFs works to reduce particulate matter in the exhaust gas by physically trapping the particulates. The DPF will filter exhaust gas of solid waste products generated during the combustion process. The differential pressure is monitored across the filter for blockage until a predetermined differential is reached. The DPF is capable of reducing particulates by more than 95%.
Implications of Tier 4 for the engine and equipment OEM
In recent years, engine manufacturers have been faced with many concurrent challenges, including the increased requirements for control of the entire combustion cycle, packaging additional components required to comply with Tier 4 standards while trying to maintain the same compartment space, rising temperatures under hood due to increased heat rejection and decreased airflow resulting from overcrowding and ongoing compliance to the emission standards. Today, engine and equipment manufacturers also face the challenge of utilizing the appropriate available technologies to meet emissions requirements based on an engine's characteristics and the application - while maintaining reasonable costs. Equipment manufacturers now need to select appropriate treatment components to meet EPA requirements. For example, an excavator will typically run at full-rated rpm and load, which utilizes CEGR, DOC and DPF technologies to comply with Tier 4 requirements, while a dump truck will run a varied engine rpm with idle time and will utilize only SCR to comply. Selected engine and after-treatment systems must then be tested and certified as meeting the NOx and PM level requirements.
In order to meet and ensure compliance with Tier 4 standards, engines will now require more electronics and larger electronic control modules (ECM) also may be required to control and monitor the additional engine requirements. Engine management systems will need to be significantly upgraded, utilizing faster processing power, increased memory and unique programing and algorithms, to control power generation from air-intake through emission exhaust as a single system.
The sensors being developed and utilized today are, out of necessity, more advanced than traditional sensors. Out of the box and over the life of the sensor, the accuracy requirements of today’s sensors can be twice that of a standard sensor (0.5%FS vs. 1%FS) and ambient temperatures can exceed the typical threshold of 125ºC. Meeting regulated exhaust emissions levels now demands sensor integration, dictating the output and the accuracy of the sensor become part of the system. Sensors also have taken on a more critical role in engine control and exhaust management. Previously, being out of tolerance may have gone unnoticed and caused little or no problem, but now being out of tolerance can now lead to emission violations.
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Standard Requirements | New Requirements |
| Accuracy (including non-linearity, hysteresis and repeatability) | ± 1% FS Max | ± 0.5% FS Max |
| Non-linearity (best fit straight line) | ≤± 0.2% FS | ≤± 0.2% FS |
| Hysteresis and repeatability | ≤± 0.1% FS Max | ≤± 0.1% FS |
| Thermal error (within compensated temperature range) |
≤± 1% FS Max 0 ---> +100°C |
≤± 1% FS -20 ---> +100°C |
At the same time, sensors are experiencing higher operating temperatures as a result of the added heat trapped due to the additional after-treatment equipment added in the same dimensional footprint. However, sensitive components in these sensors often cannot meet these new temperature requirements, which can lead to being out of tolerance or even failure. Inaccurate sensor readings can cause noncompliance with Tier 4 standards, and failures can lead to expensive equipment breakdowns, maintenance repairs or in some cases fines.
What sensors were added?
In order to meet Tier 4 emission rules, equipment manufacturers have added dozens of sensors and actuators to their equipment, further crowding precious free room under the hood. But, depending on the final solution to meeting Tier 4 emission requirements (SCR and DOC, CEGR and DPF, or other combinations of systems), sensor requirements will differ. Any combination of systems, however, likely requires a NOx sensor to ensure the NOx emission level remains under EPA-regulated levels.
HPCR systems are likely to be used with any final Tier 4 solution, as the control provided offers an advanced level of combustion technology that benefits the diesel engine. Pressure sensors capable of monitoring and maintaining the 28,000 psi fuel pressure will be required with the potential addition of in-cylinder pressure and temperature sensors further aiding in control of clean burn technology.
CEGR systems also are likely to be used with any final Tier 4 solution, due to the ability of the EGR system to reduce oxygen levels of the intake air and the temperature of combustion, as well as ultimately reduce NOx emission. The addition of the EGR cooler is a challenge of space constraints and heat rejection, ultimately raising under-hood temperatures and the temperature requirements of under-hood sensors.
Systems using SCR technology will add temperature sensors to the SCR plenum and level, pressure and temperature sensors will be needed in the DEF tank. With the SCR system lowering and controlling the NOx, it will need to be paired with a system that will reduce exhaust particulate matter.
Systems designed with DOC use precious metals to convert exhaust gas to CO2 and water. The DOC is an exhaust flow through device that includes a honey-comb formed substrate having a large surface area coated with a catalyst layer. The catalyst layer is comprised of the precious metal. As the exhaust flows over the catalyst layer, carbon monoxide, gaseous hydrocarbons and liquid hydrocarbon particles are oxidized to reduce emissions. The DOC may be adversely affected when the temperature of the exhaust exceeds a threshold. Inlet and outlet temperature sensors are typically associated with the DOC to monitor exhaust temperatures. Proper functioning of the temperature sensors is required to enable the vehicle control system to monitor exhaust temperature. Because the temperature sensors work over a large operating range (e.g., −40° C. to 800° C.), it has traditionally been difficult to ensure accuracy over the entire range.
Working to physically reduce exhaust particulate matter, the DPF’s large honeycomb, ceramic filters are coated with precious metal catalysts that trap PM in the exhaust stream. When this filter becomes full enough to affect backpressure, the ECM will inject diesel fuel into the DPF, raising the temperatures and burning off accumulated PM - a process known as the regeneration cycle. The DPF may contain an exhaust back pressure sensor and/or a differential pressure sensor in addition to temperature sensors that monitor the regeneration cycle.
Improvements realized
These emission reductions are equivalent to taking 35 million passenger cars off the road.
Annual emission reductions are estimated at 738000 tons of NOx and 129000 tons of PM.
According to the EPA, with final implementation and conversion of all inventory of current non-road and off-road diesel engines, the program will prevent up to 12,000 premature deaths, one million lost work days, 15,000 heart attacks and 6,000 children's asthma-related emergency room visits. These benefits are estimated to outweigh the costs by a 40 to 1 ratio.
Driven by the EPA’s emission regulations, engine manufacturers have developed cleaner, more efficient diesel engines for non-road and off-road applications.
Further/Future requirements
Now that solutions are available to meet the Tier 4 requirements, efforts are underway further improve combustion technology and reduce or eliminate the after-treatment technologies presented to provide the most cost effective means to meet Tier 4 requirements. This attention to the combustion process alone can reduce or eliminate the need for the costly additions. A recent study conducted and presented reported that multiple fuel injections at pressures up to 43,500 psi used in conjunction with a CEGR system decrease both PM and NOx to levels meeting Tier 4 requirements. Alternately, using this combustion technology without the CEGR system requires only the addition of a modest SCR system to meet Tier 4 requirements.
Is there a Tier 5 on the horizon? Based on history and finalization of Tier 4, the industry should anticipate future regulations and possibly a Tier 5 standard. Future regulations will likely include efficiency standards (fuel consumption and idle times), as well as particle count in addition to particulate mass and CO2 emissions. While no proposals have been made to date, the off-road and non-road industry historically follows the highway truck market, which will likely introduce new standards in the coming year.
Pressure transmitters
Our pressure transmitters are used in a variety of industrial engines such as trucks, generator sets and smaller power stations. Our product range offers high flexibility.
The input from the sensor allows the control system to adjust fuel injection pressure and other parameters on the engines. Furthermore, we supply sensors for a number of measuring points, such as those which measure oil pressure.
The optimum choice for these applications are;
- MBS1200 series
- MBS2100 series
- MBS9200 series
Temperature sensors
The flexible temperature sensor MBT 3270 can be used in many industrial applications such as: Air Compressors, Mobile Hydraulics and Exhaust gas return systems. In other words applications where robustness, size and performance are essentials.
The optimum choice for these applications are:
- MBT3270-series