local hot spots in the cooling system since much of the waste heat enters the cooling system via the exhaust ports.

Further discussion on the parasitic losses associated with these types of engine components is provided in Chapter 7 of this report.

THERMODYNAMIC FACTORS

Fast-Burn Combustion Systems

Fast-burn combustion systems are used to increase the thermodynamic efficiency of an SI engine by reducing the burn interval. This is generally achieved either by inducing increased turbulent flow in the combustion chamber or by adding multiple spark plugs to achieve rapid combustion.

Fluid-mechanical manipulation is used to increase turbulence through the creation of large-scale in-cylinder flows (swirl or tumble) during the intake stroke. The in-cylinder flows are then forced to undergo fluid-motion length-scale reduction near the end of the compression stroke due to the reduced clearance between the piston and the cylinder head. This reduction cascades the large-scale fluid motion into smaller scale motions, which increases turbulence. Increased turbulence increases the turbulent flame speed, which thereby increases the thermodynamic efficiency by allowing for reduced burn intervals and by enabling an increase in knock-limited compression ratio by 0.5 to 1.0. This decrease in burn interval increases dilution tolerance of the combustion system. Dilution tolerance is a measure of the ability of the combustion system to absorb gaseous diluents like exhaust gas. Exhaust gas is introduced by means of an exhaust-gas-recirculation (EGR) system or by a variable-valve-timing scheme that modulates exhaust-gas retention without incurring unacceptable increases in combustion variability on a cycle-by-cycle basis. Combustion variability must be controlled to yield acceptable drivability and exhaust emissions performance.

Multiple spark plugs are sometimes used to achieve rapid combustion where fluid-mechanical means are impractical. Here, multiple flame fronts shorten the flame propagation distance and thus reduce the burn interval. High dilution-tolerant combustion systems can accept large dosages of EGR, thereby reducing pumping losses while maintaining thermodynamic efficiency at acceptable levels.

Fuel Consumption Benefit and Cost of Fast-Burn Combustion Systems

Combining fast-burn and strategic EGR usage typically decreases fuel consumption by 2 to 3 percent, based on manufacturer’s input. The implementation of this technology is essentially cost neutral. Variable mixture-motion devices, which may throttle one inlet port in a four-valve engine to increase inlet swirl and in-cylinder mixture momentum, may add another 1 to 2 percent benefit at a cost of $50, $80, and $100 for I4, V6, and V8 engines, respectively, based on manufacturer’s input. As of 2007 the implementation of this technology has become common; therefore, fast burn and strategic EGR is considered to be included in the baseline of this analysis.

Variable Compression Ratio

If an engine’s compression ratio could be adjusted to near the knock-limited value over the operating range, significant fuel economy gains could be realized. Many mechanisms to realize variable compression ratios have been proposed in the literature and many have been tested. However, to date all these attempts add too much weight, friction, and parasitic load as well as significant cost and have therefore not been implemented into production designs (Wirbeleit et al., 1990; Pischinger et al., 2001; Tanaka et al., 2007). It should be recalled that alterations to the effective compression ratio via intake-valve closing (IVC) timing adjustments with higher-than-normal geometric compression ratios achieves some of this benefit.

VALVE-EVENT MODULATION OF GAS-EXCHANGE PROCESSES

Alteration of valve timing can have a major impact on volumetric efficiency over an engine’s speed range, and thus peak torque and power are affected by this. IVC timing is the main determinant of this effect (Tuttle, 1980). Early IVC (compression stroke) favors torque, and later IVC favors power. Implementations of valve-event modulation (VEM) typically are referred to as specific technologies such as variable valve timing, variable valve timing and lift, two-step cam phasing, three-step cam phasing, and intake-valve throttling. VEM aids fuel consumption reduction by means of reducing pumping loss. Pumping loss is reduced by either allowing a portion of the fresh charge to be pushed back into the intake system (late IVC during the compression stroke) or by allowing only a small amount of the mixture to enter the cylinder (early IVC during the intake stroke).

It should be noted that any of the VEM schemes that reduce or eliminate the pumping loss also reduce or eliminate intake-manifold vacuum. Alternative means to operate power brakes, fuel vapor canister purge, and positive crankcase ventilation (PCV) systems, normally driven by intake-manifold vacuum, must then be considered. To overcome this issue, an electrically operated pump may need to be added. It should also be noted that while the implementation of VEM techniques can boost torque output of a given engine, this report assumes that constant torque will be maintained, leading to engine downsizing. The fuel consumption benefits listed in the following section consider a constant-torque engine.



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