efficiency. However, the combustion process also plays the key role in the engine-out emissions. As a result, optimizing combustion to minimize FC and emissions simultaneously requires careful analysis of the interactions between fuel spray dynamics, in-cylinder fluid motions resulting from the interactions of the intake flow with the piston bowl shape (i.e., combustion chamber), gas temperature history, and chemical reactions of the fuel. As fuel composition evolves from entirely petroleum based to a mixture of petroleum and bio-sourced components in the next decade to reduce petroleum dependence and increase sustainability, it is critical that understanding of combustion be increased. It is believed that advanced combustion research with tools such as three-dimensional computational fluid dynamic computer codes, including spray and combustion as well as coordinated experiments in highly instrumented engines with optical access for advanced laser-based tools, will improve understanding of combustion in the longer term. This improved understanding is critical to reducing exhaust emissions without compromising engine efficiency and along with new technologies discussed later should enable reductions in FC.
Friction sources in engines are journal bearing friction, valve-train friction, and piston assembly friction. In the past 10 to 15 years, all significant sliding interfaces in valve trains have been replaced by rolling interfaces, which minimize friction. Connecting rod, camshaft, and main bearing friction is hydrodynamic, thus coming primarily from lubricating oil shear processes. This friction has been reduced by the use of lower viscosity lubricants. Therefore, the largest remaining friction sources in both CI and SI engines is that due to the piston assembly. Friction from this assembly comes from both piston skirt-to-wall interactions as well as piston ring-to-wall interactions. Both skirt and ring friction can be decreased by improved cylinder-bore roundness, which depends on both cylinder block design and associated thermal distortions as well as bore distortion due to mechanical loading by the preloaded cylinder head attachment bolts. Rounder bores under hot and loaded conditions allow lower ring tension, which in turn decrease ring-to-wall friction. Coatings to reduce ring friction are also being developed, although it is not yet clear whether such coatings can be both friction reducing and sufficiently durable. Piston skirt friction can be reduced by improved skirt surface coatings. Most current pistons have proprietary skirt coatings, but new materials are continuously being studied to further reduce skirt-to-wall friction.
Engine loads to drive accessories include those for coolant pump, oil pump, alternator, air-conditioning compressor, power-steering pump, etc. Electric-motor-driven coolant pumps are being considered because they can be turned off or run slowly during engine warm-up and at other conditions when coolant flow can be reduced without engine damage, thus reducing fuel use to drive the electrical alternator. Two-mode mechanical water pumps are also being developed that require less power to drive at part-load engine conditions but still provide more coolant flow at high-load conditions. Oil pumps, like coolant pumps, are sized for maximum engine power conditions and are hence oversized for part-load, low-speed conditions. Two-mode oil pumps are being developed and becoming available.
The most critical aspect of increasing the use of CI diesel engines in the United States to take advantage of their excellent efficiency is the development and production of technologies that can enable these engines to meet the 2010 and post-2010 exhaust emissions standards. As noted above, CI diesel engines without emission controls have very low