As more stringent diesel engine emissions standards are introduced, they present significant challenges to engine manufacturers. The initiative to lower diesel engine emissions began with on-highway engines back in the early 1970s and now extends to non-road mobile equipment and stationary engines.
In the USA, for example, the challenges will continue as the Environmental Protection Agency (EPA) phases in lower diesel engine emissions limits. Since 1 January this year, manufacturers now have to certify that diesel engines built for stationary applications, which include electric power generation, must meet the new EPA NSPS (New Source Performance Standards) regulations for emissions.
Since the introduction of Tier 1 regulations in 1996, the EPA has lowered allowable emissions from diesel-powered, non-road mobile equipment, and manufacturers have responded by utilizing different technologies to achieve compliance. This not only affected mobile equipment (e.g. construction machines and agricultural tractors), but also industrial equipment, such as diesel-powered generator sets used in electric power generation. Regulated diesel exhaust emissions include oxides of nitrogen (NOx), carbon monoxide, hydrocarbons and particulate matter.
The EPA adopted the first set of emissions standards (Tier 1) for all new non-road mobile diesel engines greater than 50 horsepower. These non-road mobile regulations were phased in by engine horsepower range between 1996 and 2000, reducing NOx emissions by 30 per cent.
Stricter Tier 2 emissions standards were then phased in from 2001 to 2006 for all engine sizes, and Tier 3 standards for engines from 50 to 750 horsepower are being implemented from 2005/2006 through 2008. The EPA’s Tier 4 will phase in from the beginning of 2008 and continue through to 2015.
However, exhaust after-treatment, which is expected to be required to meet Tier 4 emissions levels, will not be required for emergency standby applications where the generator set is used during the failure of the normal power source, such as the grid.
In addition, the EPA has stated that these emergency standby generator sets may be certified under the previous Tier levels, such as Tier 2 or 3, depending on power range. The table below summarizes the four tiers of EPA regulations for non-road mobile and stationary applications.
Stationary diesel engines
An immediate issue for electric power generator sets is the EPA’s New Source Performance Standards (NSPS) affecting stationary diesel engines, including those used in generator sets. These standards closely match EPA non-road mobile regulations in terms of the regulated exhaust pollutants and associated tier levels.
The EPA phased in the stationary rules in two stages, which began in April 2006. Between 1 April and 31 December 2006, engines built and shipped for stationary use in the USA had to be able to meet Tier 1 emissions levels. No factory certification was required during this interim period, and it was up to end-users to verify that new engines were Tier 1 compliant.
The second stage took effect for engines built on or after 1 January 2007, and does involve factory certification. Engines must be certified to the same EPA non-road mobile tiers in effect at the time the engine is built. Thus, for 2007 through to 2010, stationary diesel engines must meet Tier 2, 3 or 4, depending on power range.
With the phase-in of Tier 4 in 2011 for non-emergency engines greater than 175 horsepower, stationary diesel engines will have to meet Tier 4. Emergency standby generator sets will again be allowed to maintain the previous tiers instead of having to meet Tier 4. Engines rated at 3000 horsepower and above will be certified to Tier 1 non-road mobile standards until 1 January 2011, at which time they will have to be certified to Tier 4 for non-emergency engines.
The EPA has also specified instructions for existing engines. If an existing engine or generator set is to be rebuilt, replaced or overhauled, and the cost is more than 50 per cent of the price of an equivalent new engine, or if there is an increase in the “potential to emit,” then the new regulation applies. More specifically, if the engine was built before 1 January 2007, and the above conditions are met, that unit must meet Tier 1 emissions after rebuild, repair or overhaul. If the engine was built on or after that date, and the conditions are met, the engine must meet the applicable emissions tier level in effect in the year it was manufactured.
It is important to note that the EPA NSPS stationary regulation sets minimum standards for diesel exhaust emissions levels in the USA. Some regions, states, and localities have set exhaust emission limits that are more stringent than the EPA regulation and thus take jurisdiction.
The situation in Europe is similar to that in the USA. Diesel generator sets sold into mobile applications in Europe have to meet minimum exhaust emissions regulations published by the European Union (EU). EU Stage II limits for mobile generator sets operating between 19 bkw and 560 bkw (approximately 20 kVA to 625 kVA) became effective 1 January 2007. Stationary generator sets sold in Europe are not currently regulated by the EU but are, in most cases, regulated by domestic regulations.
Reducing engine emissions
The EPA regulatory programme puts the responsibility on engine manufacturers to offer certified engines that meet the applicable regulations. The primary concern is NOx because it contributes to ground-level ozone.
At the most basic level, the key to reducing NOx emissions is to reduce combustion temperatures. This requires some combination of cooling charge air to the cylinders and controlling the combustion process. On a deeper level, critical challenges include: achieving lower NOx emissions without major trade-offs in performance, durability, reliability and operating costs, and without the use of after-treatment, meeting not just emissions standards in effect today, but the more stringent standards yet to come – specifically EPA Tier 4 and delivering results across a broad range of load and speed conditions and in multiple engine applications. Engine manufacturers take three basic approaches to reducing NOx. These are detailed below.
Exhaust gas recirculation
Exhaust gas recirculation (EGR) lowers combustion temperature by reducing oxygen concentration. In simple terms, exhaust gas lacking oxygen and essentially non-combustible is drawn back into the cylinders, where it lowers combustion temperatures.
In external cooled EGR, the exhaust gas passes through a cooling system before being reintroduced into the cylinders. A portion of exhaust gas is drawn off at the manifold, routed through a cooler and mixed with charge air on the way into the intake manifold. This technology can reduce emissions, but it requires an additional cooling system that adds cost and requires maintenance.
A variation is internal EGR, which does away with the external cooling system. However, because the exhaust gas is hot (i.e. less dense), internal EGR may not be as fuel efficient as cooled EGR. In addition, because the air/fuel ratio is reduced, load acceptance and starting capability may be compromised.
Concerns with EGR systems include how suppressing combustion by limiting oxygen concentration affects engine performance and fuel efficiency, and whether combustion products in exhaust gases affect operation/maintenance costs and the service life of components. However, in some markets, such as standby power generation, these issues may not be critical.
Some manufacturers have relied on a combination of steps to reduce emissions by optimizing fuel combustion. Design elements involved in this process include air handling, fuel delivery control systems and the cylinder itself. Optimized combustion involves computer modelling that enables engineers to study in a short time how changes in design parameters affect combustion activity in the cylinders.
This then leads to choosing design parameters that optimize results. Optimized combustion is said to enable compliance with Tier 3 NOx levels, without using EGR technology or emissions control devices such as oxidation catalysts.
This approach to emissions reduction, developed by Caterpillar Incorporated, uses a combination of technologies that are referred to as “building blocks” to regulate emissions at the point of combustion. These building blocks achieve advanced combustion and reduced emissions, while maintaining performance and efficiency.
The building blocks of ACERT Technology can be applied as necessary to meet different emissions requirements, depending on applications. These include:
- Integrated electronics that regulate fuel injection, ignition timing, valve timing and a host of other engine functions to increase responsiveness;
- Advanced fuel delivery that uses a series of micro bursts to allow precise control of the combustion cycle. Hydraulically and mechanically actuated, electronically controlled unit injectors to optimize combustion; and
- Advanced air management, which includes innovative turbo-charging, variable valve timing and cross-flow cylinder heads, can help lower combustion temperature.
Results to date indicate ACERT Technology will form a solid foundation for enabling diesel engines to meet future EPA Tier 4 emissions levels.
Out in the field
ACERT Technology’s ability to not only reduce engine emissions, but also enhance engine performance can be seen at OSF St Francis Medical Centre in Peoria, IL, USA. It recently upgraded its emergency power system from three 700 kW generator sets to four Cat C32 generator sets, with ACERT Technology, delivering 1000 kW.
Many features of the C32 generator sets made this equipment the right fit for the OSF St Francis Medical Centre. The reliability and performance is paramount: “This equipment is used to support life within the medical centre, including the emergency department, surgery department and the critical care units.”
The NFPA (National Fire Prevention Association) Standard 99 defines the role and specifications for emergency power in healthcare facilities. “We feel very comfortable with Caterpillar in meeting those standards.” Fuel consumption on the C32s also exceeded the customer’s requirements. “We are very happy with the testing. We have even realized cost savings as the generator sets have performed at fuel consumption levels lower than specified,” said Edward McKenzie, OSF corporate engineer.
The generator sets offer the latest in engine design technology to provide emissions compliance. As a part of the medical centre’s mission, this environmental benefit was also of paramount importance.
The C32 generator sets’ reliability, performance and emissions technology made for an ideal solution at the OSF St Francis Medical Centre, McKenzie concluded.
Lower diesel emissions through control technologies mean cleaner air and more choices for electric power generation customers. Manufacturers who produce diesel-powered equipment for the US market have invested heavily in research and development to meet EPA emissions tier levels.
The resulting technologies, first developed for the on-highway market, will now benefit power generation customers by enabling compliance with new EPA rules affecting stationary equipment.
In limited-duty applications, such as peak shaving and load management, customers will retain the option of evaluating diesel-fueled power generation based on a range of economic parameters (e.g. capital cost, power density, fuel price and availability).
In choosing among competitive diesel emission-control technologies, customers will evaluate equipment based on its capability to meet current and future tier-level emissions requirements.