Meeting the next T4 challenges
This article outlines the changes to the allowable emissions under the Environmental Protection Agency standards.
In the U.S., diesel engine emissions have been regulated for almost 40 years. For most of that time, the regulations applied primarily to on-highway engines in trucks and buses. Today, though, nonroad engines are also regulated, including those used in mobile equipment such as farm tractors, construction earthmovers, mobile generator sets on trailers, and other portable industrial engines. Figure 1 shows the dramatic reductions in emissions that have been mandated by the Environmental Protection Agency (EPA) for these engines, including those of Tier 4, which begins to take effect in January.
Exhaust substances covered in Tier 4 Interim
As the chart indicates, the EPA has set emission requirements in phases, or tiers. Tier 4 is a two-part tier; the next tier to take effect is Tier 4 Interim, or Tier 4i, which will be followed by Tier 4 Final.
In Tier 4i, four constituents in diesel exhaust are controlled:
- Nitrogen oxides (NOx): NOx is a combustion by-product that combines in the atmosphere to create ozone and smog. It is controlled by reducing the combustion temperature inside the cylinder.
- Particulate matter (PM): PM is made up of soot particles in diesel exhaust from unburned carbon. It is controlled by optimizing the combustion temperature and improving combustion efficiency.
- Hydrocarbons (HC) are essentially unburned fuel, which contributes to ozone and smog production. HC is controlled by improving combustion efficiency.
- Carbon monoxide (CO), which is also controlled by improving combustion efficiency.
To meet the Tier 4i target for 2011, NOx and PM levels need to drop 40% from the previous tier. These two reductions are especially challenging because most engine technologies that decrease NOx tend to increase PM and vice-versa. Figure 2 details the allowable limits for each of these four pollutants in Tiers 2 to 4.
The Transition Program for Engine Manufacturers (TPEM) allows manufacturers to use a mix of emission engines (current tier plus prior tier) in a given power band as long as the aggregate shipments from the company meet the EPA requirements then in force.
Technologies for meeting Tier 4i requirements
Engine makers have been busy working on product innovations and enhancements to satisfy the EPA’s ever-stricter requirements. Manufacturers are approaching the task in two primary ways: engine technologies and after-treatment solutions.
Cooled exhaust gas recirculation (EGR) recycles and cools a portion of the inert gases of the exhaust stream as they mix with incoming engine air. Combustion temperatures are thereby reduced, and so is NOx. The cooling also increases the density of the charge air, boosting power.
Variable geometry turbocharger allows the effective aspect ratio of the turbo to be altered as conditions change. The optimal aspect ratio at low engine speeds is very different from that at higher engine speeds. Varying the angle of the vanes reduces the turbo lag at low speeds without compromising boost at higher speeds.
Direct flow air filter allows the integration of a mass airflow sensor into the housing of the filter for Tier 4 Final, which has a pre-cleaner, a primary, secondary filter elements, and a dust ejector valve.
A redesigned and more durable electronic control module (ECM) has sensors and microprocessor-based controls that improve fuel efficiency and power output while decreasing both NOx and PM emissions. These controls manage fuel quantity, injection timing, and turbo boost pressure, while taking into account load, temperature, barometric pressure, fuel energy content, even engine wear. The result is optimal combustion efficiency.
The high-pressure common rail fuel system plays a big role in meeting the new requirements. Increasing fuel pressures, along with the retarding of injection timing, reduces NOx without increasing PM or HC. Alterations to nozzle design and injection systems themselves can have an impact as well. For example, multiple injection events per cycle improve fuel atomization and penetration of the combustion chamber, boosting fuel efficiency while reducing PM.
In-cylinder techniques can only do so much, however, because of the mechanical limits of current engines. After-treatment of exhaust is also required. Here there are also multiple strategies that can be used. Depending on the requirement, some or all of the following can be implemented:
- DOC, or a diesel oxidation catalyst, a flow-through device where exhaust gases are brought in contact with materials that oxidize unburned hydrocarbons and reduce emissions
- DPF, or diesel particulate filter, a device designed to physically capture particulate matter from the exhaust stream
- SCR, or selective catalytic reduction unit, which includes a “reducer” that is added to exhaust flow to create the reactions in a catalytic chamber.
Concerns with after-treatment technology
Other than the downside of additional costs, after-treatments raise some concerns that need to be carefully addressed. Cooling the exhaust gas before recirculating it, for example, is an effective method for reducing in-cylinder temperatures, but the cooling system then has to deal with an additional cooling circuit and up to 25% higher heat rejections.
Also, after-treatment solutions can raise concerns about packaging and space limitations, as well as thermal management and substance-level constraints like the handling and storage of urea, a material used in DPF and SCR technologies, as well as sulfur tolerance.
In addition, some after-treatment devices can add a significant amount of backpressure, requiring precise, duty-cycle-based control of temperatures and dosing frequency for regeneration. Such devices also represent additional items to be serviced.
Finally, most NOx after-treatment devices reduce emissions when operating at high temperatures. However, an emergency standby generator that is lightly loaded may not even reach such temperatures. A minimum load of 30% of capacity is recommended.
Natekar has an MS in automotive engineering from Lawrence Technological University (Southfield, Mich.) and a BS in mechanical engineering from the University of Pune (India). He has held various positions in research and development, market research, engineering, product development, and product marketing with a number of automotive companies prior to joining Cummins Power Generation and during his tenure with Cummins.