Engine Technology Evolution: Heavy-Duty Diesels

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Abstract: In recent decades, the diesel engine has evolved significantly to meet regulatory emission standards and, more recently, greenhouse gas and fuel economy requirements. Initially, emissions were reduced by various combustion approaches, such as reduced intake manifold temperatures, retarded timing and exhaust gas recirculation. Since the late 2000s, to meet even more stringent emission limits, diesel engines have been integrated with exhaust gas aftertreatment technologies, such as particulate filters and NOx reduction catalysts. The reductions in emissions have been accompanied by increased engine efficiency and improved fuel economy.

Introduction

The diesel engine has advanced technologically since the 1990s, adopting many new efficiency and emission technologies at a very fast pace. This technology development process has been driven by three main groups of factors:

Regulation of exhaust emissions from heavy-duty on-road engines began in the early 1970s in North America and Japan and in the 1980s in Europe. In North America, California had limits in place for MY 1973 while US federal limits applied for MY 1974. Prior to the 1988 MY, the focus of the US regulations was NOx, HC, CO and smoke. In 1988, limits for PM were introduced.

Japan started regulating emissions from heavy-duty diesel engines in 1974 with a focus on NOx, CO and HC. Limits for PM were introduced in 1994.

In Europe, limits on emissions of NOx, CO and HC for heavy-duty diesel engines were introduced in 1988 (often referred to as Euro 0). Limits for PM were introduced in 1992 with the Euro 1 stage.

Figure 1 provides a compact illustration of the technologies implemented by the US heavy-duty diesel engine industry for on road applications starting in the 1980s. This figure is an update of a similar figure by Flynn [681].

Starting in the mid-2010s, the focus for heavy-duty engine designers was the reduction of greenhouse gas emissions. This primarily entailed reducing fuel consumption to limit the emission of CO₂. In addition to reducing engine friction and other parasitic losses, high efficiency urea SCR aftertreatment systems for NOx allowed engines to be tuned for higher engine out NOx emissions and lower fuel consumption.

In heavy-duty trucks, a plethora of engine/transmission/driveline choices has made the job of optimizing the engine and driveline combination to minimize fuel consumption challenging. With the focus on reducing fuel consumption, manufacturers are increasingly offering engine/transmission packages that could make this task less daunting. By optimizing the engine and transmission package, manufacturers can better take advantage of the fuel consumption benefits of engine downspeeding.

For engines to be released in the mid- to late 2020s, the focus will again shift to further reductions in NOx emissions while also achieving further greenhouse gas emission reductions and ensuring that PM levels achieved with the introduction of DPFs in 2007 do increase. Much of the focus will be to reduce NOx during cold start and low load operation when maintaining catalyst activity is more challenging.