The fuel consumption of engine systems is driven by two major elements, the base engine (i.e., combustion subsystem, friction, accessories, etc.) and the exhaust aftertreatment subsystem. As a result, the fuel consumption of an engine system depends on both the base engine and the aftertreatment. Technologies affecting engine system fuel consumption through changes to the base engine and to the aftertreatment system are discussed below.
The strategies being pursued to improve base engine efficiency are the following:
Downsizing the engine while maintaining equal power,
Improving thermodynamic cycle efficiency (e.g., improved combustion),
Reducing engine friction (e.g., reduced piston skirt friction), and
Reducing accessory loads (e.g., electric water pump, reduced fuel pump loads by avoiding fuel recirculation, modulated oil pump).
Note that all these strategies apply as well to SI engines, although the gains may have different magnitudes due to process differences between CI and SI engines.
The most significant of these strategies is engine downsizing, which consists of using a smaller displacement engine for a given vehicle mass while still maintaining the same power to give equal vehicle performance.1 This approach requires higher cylinder pressures (i.e., higher engine brake mean effective pressure [BMEP], which is equivalent to torque) at any given point on the vehicle drive cycle, which reduces engine brake specific fuel consumption (BSFC). To downsize an engine while still maintaining the same vehicle performance, the torque and hence BMEP of the downsized engine must be raised at all speeds including the maximum-power speed. One of the key enablers to raising the BMEP is increasing the intake boost provided by the turbocharger system. The emerging approach to increase intake boost is two-stage turbocharging (Figure 5.1). Increased boosting is also used for downsizing SI engines.
Most current light-duty CI diesel engines use a single-stage, variable-geometry turbocharger (VGT). Two-stage turbocharger (turbo) systems are being actively developed for two reasons. First, they are a key enabler for engine downsizing. Second, they enable increased exhaust gas recirculation (EGR) rates. Cooled EGR is the principal method to reduce engine-out NOx emissions, as discussed later. With a two-stage turbo system, two separate turbos are combined with additional flow-control valves. The first-stage turbo is usually sized smaller than the normal single-stage VGT used currently, and the second-stage turbo is usually sized larger than the current single-stage VGT. Electronic flow control valves triggered by the engine controller are used to direct exhaust flows to the small turbo and/or to the large one. At lower engine speeds only the smaller turbo is used and a relatively high inlet pressure is generated, even for the low inlet air flow characteristic of operation at high EGR rates.
At higher engine speeds, when the air flow rates have increased and the smaller turbo does not have sufficient flow capacity, air flow rates are sufficient to generate high intake pressures when the exhaust flow is directed through the larger turbo. Therefore, with the use of a two-stage turbo system, the problem of insufficient inlet boost pressure at low speeds with high EGR flow rates is solved without losing engine power at high speeds. The ability of two-stage turbo systems to generate higher boost pressures at low engine speeds is the key characteristic of two-stage systems that makes them enablers for engine downsizing. By providing higher intake boost, two-stage systems provide more air in the cylinder, thus allowing increased BMEP and torque to compensate for the smaller engine displacement. Naturally, two-stage turbo systems are more expensive than single-stage systems.
To utilize the increased charge mass in the cylinder resulting from the higher boost, more fuel must be injected per unit of engine displacement. The resulting increased power output per unit of engine displacement then compensates for the downsized engine displacement. Increasing the fuel flow is generally accomplished by increasing the maximum injection pressure, which enables higher injection-pressures at all loads. To support the increased cylinder pressures, the engine structure, sealing (e.g., head gasket), and lubrication (e.g., connecting rod bearings must support higher cylinder pressures with the same bearing areas) must be improved. Cylinder pressures also increase piston/ring friction, and an additional challenge is to keep the increase to a minimum. These changes require careful engineering but increase engine cost only slightly.
The combustion process and its phasing relative to piston motion are important determinants of thermodynamic-cycle
Truly equal performance involves nearly equal values for a large number of measures such as acceleration (e.g., 0-60 mph, 30-45 mph, 40-70 mph, etc.), launch (e.g., 0-30 mph), gradability (steepness of slopes that can be climbed without transmission downshifting), maximum towing capability, and others. In the usage herein, equal performance means 0-60 mph times within 5 percent. This measure was chosen because it is generally available for all vehicles. The equal-performance constraint is important because vehicle FC can always be reduced by lowering vehicle performance. Thus objective comparisons of the cost-effectiveness of different technologies for reducing FC can be made only when vehicle performance remains equivalent.