In light- and medium-duty diesel engines, piston bowl shape influences thermal efficiency, either due to changes in wall heat loss or to changes in the heat release rate. The relative contributions of these two factors are not clearly described in the literature. In this work, two production piston bowls are adapted for use in a single cylinder research engine: a conventional, re-entrant piston, and a stepped-lip piston. An injection timing sweep is performed at constant load with each piston, and heat release analyses provide information about thermal efficiency, wall heat loss, and the degree of constant volume combustion. Zero-dimensional thermodynamic simulations provide further insight and support for the experimental results. The effect of bowl geometry on wall heat loss depends on injection timing, but changes in wall heat loss cannot explain changes in efficiency. Late cycle heat release is faster with the stepped-lip bowl than with the conventional re-entrant bowl, which leads to a higher degree of constant volume combustion and therefore higher thermal efficiency. This effect also depends on injection timing. In general, increasing the degree of constant volume combustion is significantly more effective at improving thermal efficiency than decreasing wall heat loss. Maximizing thermal efficiency will require a deeper understanding of how bowl geometry impacts flow structure, turbulent mixing, and mixing-controlled combustion.
Busch, Stephen B.; Zha, Kan; Perini, Federico; Reitz, Rolf; Kurtz, Eric; Warey, Alok; Peterson, Richard
Diesel piston bowl geometry can affect turbulent mixing and therefore it impacts heat-release rates, thermal efficiency, and soot emissions. The focus of this work is on the effects of bowl geometry and injection timing on turbulent flow structure. This computational study compares engine behavior with two pistons representing competing approaches to combustion chamber design: a conventional, re-entrant piston bowl and a stepped-lip piston bowl. Three-dimensional computational fluid dynamics (CFD) simulations are performed for a part-load, conventional diesel combustion operating point with a pilot-main injection strategy under non-combusting conditions. Two injection timings are simulated based on experimental findings: an injection timing for which the stepped-lip piston enables significant efficiency and emissions benefits, and an injection timing with diminished benefits compared to the conventional, re-entrant piston. While the flow structure in the conventional, re-entrant combustion chamber is dominated by a single toroidal vortex, the turbulent flow evolution in the stepped-lip combustion chamber depends more strongly on main injection timing. For the injection timing at which faster mixing controlled heat release and reduced soot emissions have been observed experimentally, the simulation predicts the formation of two additional recirculation zones created by interactions with the stepped-lip. Analysis of the CFD results reveals the mechanisms responsible for these recirculating flow structures. Vertical convection of outward radial momentum drives the formation of the recirculation zone in the squish region, while adverse pressure gradients drive flow inward near the cylinder head, thereby contributing to the formation of the second recirculation zone above the step. Bulk gas density is higher for the near-TDC injection timing than for the later injection timing. This leads to increased air entrainment into the sprays and slower spray velocities, so the sprays take longer to interact with the step, and beneficial recirculating flow structures are not obseved.
Despite compliance issues in previous years, automakers have demonstrated that the newest generation of diesel power trains are capable of meeting all federal and state regulations (EPA, 2016). Diesels continue to be a cost-effective, efficient, powerful propulsion source for many light- and medium-duty vehicle applications (Martec, 2016). Even modest reductions in the fuel consumption of light- and medium duty diesel vehicles in the U.S. will eliminate millions of tons of CO2 emissions per year. Continued improvement of diesel combustion systems will play an important role in reducing fleet fuel consumption, but these improvements will require an unprecedented scientific understanding of how changes in engine design and calibration affect the mixture preparation, combustion, and pollutant formation processes that take place inside the cylinder. The focus of this year’s research is to provide insight into the physical mechanisms responsible for improved thermal efficiency observed with a stepped-lip piston. Understanding how piston design can influence efficiency will help engineers develop and optimize new diesel combustion systems.
For a pilot-main injection strategy in a single-cylinder light-duty diesel engine, the dwell between the pilot- and main-injection events can significantly impact combustion noise. As the solenoid energizing dwell decreases below 200 ls, combustion noise decreases by approximately 3 dB and then increases again at shorter dwells. A zero-dimensional thermodynamic model has been developed to capture the combustion noise reduction mechanism; heat release (HR) profiles are the primary simulation input and approximating them as top-hat shapes preserves the noise reduction effect. A decomposition of the terms of the underlying thermodynamic equation reveals that the direct influence of HR on the temporal variation of cylinder pressure is primarily responsible for the trend in combustion noise. Fourier analyses reveal the mechanism responsible for the reduction in combustion noise as a destructive interference in the frequency range between approximately 1 kHz and 3 kHz. This interference is dependent on the timing of increases in cylinder pressure during pilot HR relative to those during main HR. The mechanism by which combustion noise is attenuated is fundamentally different from the traditional noise reduction that occurs with the use of long-dwell pilot injections, for which noise is reduced primarily by shortening the ignition delay of the main injection. Band-pass filtering of measured cylinder pressure traces provides evidence of this noise reduction mechanism in the real engine. When this close-coupled pilot noise reduction mechanism is active, metrics derived from cylinder pressure such as the location of 50% HR, peak HR rates, and peak rates of pressure rise cannot be used reliably to predict trends in combustion noise. The quantity and peak value of the pilot HR affect the combustion noise reduction mechanism, and maximum noise reduction is achieved when the height and steepness of the pilot HR profile are similar to the initial rise of the main HR event. A variation of the initial rise rate of the main HR event reveals trends in combustion noise that are the opposite of what would happen in the absence of a close-coupled pilot. The noise reduction mechanism shown in this work may be a powerful tool to improve the tradeoffs among fuel efficiency, pollutant emissions, and combustion noise.
In-cylinder flow measurements are necessary to gain a fundamental understanding of swirl-supported, light-duty Diesel engine processes for high thermal efficiency and low emissions. Planar particle image velocimetry (PIV) can be used for non-intrusive, in situ measurement of swirl-plane velocity fields through a transparent piston. In order to keep the flow unchanged from all-metal engine operation, the geometry of the transparent piston must adapt the production-intent metal piston geometry. As a result, a temporally- and spatially-variant optical distortion is introduced to the particle images. To ensure reliable measurement of particle displacements, this work documents a systematic exploration of optical distortion quantification and a hybrid back-projection procedure that combines ray-tracing-based geometric and in situ manual back-projection approaches. The proposed hybrid back-projection method for the first time provides a time-efficient and robust way to process planar PIV measurements conducted in an optical research engine with temporally- and spatially-varying optical distortion. This method is based upon geometric ray tracing and serves as a universal tool for the correction of optical distortion with an arbitrary but axisymmetric piston crown window geometry. Analytical analysis demonstrates that the ignorance of optical distortion change during the PIV laser temporal interval may induce a significant error in instantaneous velocity measurements. With the proposed digital dewarping method, this piston-motion-induced error can be eliminated. Uncertainty analysis with simulated particle images provides guidance on whether to back-project particle images or back-project velocity fields in order to minimize dewarping-induced uncertainties. The optimal implementation is piston-geometry-dependent. For regions with significant change in nominal magnification factor, it is recommended to apply the proposed back-projection approach to particle images prior to PIV interrogation. For regions with significant dewarping-induced particle elongation (Ep > 3), it is recommended to apply the proposed dewarping method to the vector fields resulting from PIV interrogation of raw particle image pairs.