Most advances in thermal efficiency will be achieved through continued improvements in combustion, air handling, fuel injection equipment, and other subsystems. In addition, an effective exhaust heat recovery system may be necessary for achieving 50 percent BTE. However, the design of a waste heat recovery (WHR) system must take into account the temperature requirements of exhaust emission control devices as well as considerations such as weight, space, cost, reliability, and durability. In order to be commercially viable, the WHR system needs to last for the life of the engine, which carries an emissions warranty of 435,000 miles, but typically has a design life of 600,000 to 1 million miles. The 55 percent BTE stretch goal will require the research and development (R&D) of technologies discussed below in this chapter and should include comparable BTE improvements over the entire engine operating map, especially for those conditions used in a duty-cycle-weighted BTE.
Exhaust emissions of diesel engines have been regulated since 1973 by the California Air Resources Board (CARB) and since 1974 by the U.S. Environmental Protection Agency (EPA). After 1974, diesel engine manufacturers achieved remarkable reductions in oxides of nitrogen (NOx) (~99 percent) and particulate matter (PM) (99 percent) emissions by modifying their engines and adding aftertreatment devices. Through 2006 heavy-duty diesel engines were certified at 2.5 g/bhp-h of NOx + HC and 0.10 g/bhp-h PM (<0.05 g/bhp-h for transit buses). In 2007 the regulations allowed a phase-in of sales-averaged NOx at approximately 1.2 g/bhp-h2 and PM at 0.01 g/bhp-h (DOE, 2006).
Compliance with the 2007-2010 federal emissions standards is perhaps the strongest example of progress by diesel engine manufacturers since the National Research Council (NRC) Phase 1 review of the 21CTP in 2007 (NRC, 2008). Until 2007, exhaust aftertreatment had not been required or utilized to meet emissions standards for heavy-duty diesels (except for limited use of oxidation catalysts on buses and medium-sized trucks). The 2007-2010 regulations were intended by the EPA to be “aftertreatment-forcing.” Aftertreatment technologies for PM were necessary in 2007, and all new truck heavy-duty diesel engines were equipped with diesel particulate filters (DPFs). Catalyst-based DPFs used with ultra-low-sulfur diesel fuel (<15 parts per million [ppm]) achieve PM reductions in excess of 90 percent from 2006 levels. In October 2006, ultra-low sulfur diesel fuel became the mandatory on-highway fuel, thus enabling the use of DPFs and other types of exhaust aftertreatment (NRC, 2008).
For 2010, NOx emissions standards were lowered another 83 percent to 0.20 g/bhp-h NOx + HC, along with 0.01 g/bhp-h PM. These standards have been met by most engine original equipment manufacturers (OEMs) by a combination of cooled exhaust gas recirculation (EGR) and selective catalytic reduction (SCR) for NOx control and an actively regenerated DPF for particulate control. Meeting the requirements of 2010 exhaust emissions regulations required significant in-cylinder control, high-performing aftertreatment for NOx and PM systems, and engine thermal management that includes a degree of control over exhaust mass flow rate, exhaust temperature, and exhaust oxygen. Thermal management, an essential element in the integration of engine and aftertreatment, allows both the DPF and the SCR to operate at peak efficiency over a wide range of duty cycles. In contrast to most manufacturers, which use SCR for NOx control, Navistar is planning to achieve the NOx standard with an EGR-only system and the PM standard with a DPF (Navistar, 2010).
Substantial effort across the industry went into the design of systems for storing and metering urea on the vehicle; these systems are required to support SCR systems. Considerations of freeze protection, contamination, labeling, and stability had to be accounted for. In addition, the infrastructure for distributing and dispensing urea at refueling outlets had to be developed. The industry adopted the name of Diesel Exhaust Fluid (DEF) for the aqueous urea solution. It was found that there is an optimum balance between in-cylinder control of NOx and PM and aftertreatment control of NOx and PM. The primary parameter determining this optimum balance is the operating cost, driven by both fuel consumption and DEF consumption. Fuel consumption has been affected by some of these emission control strategies, such as fuel used to regenerate particulate filters and DEF usage for the SCR system.
Another key enabling system technology is high-pressure common rail fuel systems with high-pressure capabilities exceeding 2,400 bar and allowing multiple injections per cycle. In addition, advancements in turbomachinery have resulted in variable-geometry turbochargers, the use of multiple turbochargers (in series and parallel) with aftercooling and intercooling, and turbocompounding. The turbomachinery serves several purposes in engine performance and emissions control, including airflow for high BMEP and transient response, EGR delivery and control, enhanced engine braking, and exhaust thermal management. EGR systems were introduced in 2002 and have mainly been high-pressure loop with cooling by means of the engine coolant. Some low-pressure loop EGR systems have also been introduced to the market. The first stage of implementation of on-board diagnostics (OBD) was completed on heavy-duty 2010 engines with a second stage in 2013.
As discussed in DOE (2006), engine controls deserve mention here, because historically, controls requirements for diesel engines have lagged those for gasoline engines in passenger cars. For the truck diesel engine, controls were primarily limited to one or two degrees of freedom (i.e.,
2 The NOx and nonmethane hydrocarbon (NMHC) standards were phased in for diesel engines between 2007 and 2010. The phase-in was on a percent-of-sales basis: 50 percent from 2007 to 2009 and 100 percent in 2010. In 2007, most manufacturers opted to meet a Family Emission Limit (FEL) around 1.2 to 1.5 g/bhp-hr NOx for most of their engines (average of 0.2 g/bhp-h NOx standard for 2010 and about 2.2 g/bhp-h NOx portion of the 2.5 g/bhp-h NMHC + NOx standard for 2006).