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Technical Paper

The Limits of Drag Behavior for Two Bluff Bodies in Tandem

We demonstrate the interaction of two truck shapes in tandem. Both trucks experience a decreased drag coefficient from the interaction. The degree of drag saving depends strongly upon the drag coefficients of the model trucks in isolation, and upon how the two trucks are arranged. For the two simplest shapes-parallelepipeds with or without partial leading-edge rounding-the total drag saving can range from 10 percent to 40% at a spacing of 2√A (approximately 18 feet at full scale) depending upon whether the lead or the trail parallelepiped has rounding. These two shapes-blunt and rounded-have drag coefficients in isolation of 0.94 and 0.51 respectively, and probably bracket the savings to be obtained for all real truck geometries. Our realistic model trucks (with wheels and a gap between tractor and trailer serving to distribute the source of drag along the length of the truck) have drag coefficients in the range CD = 0.5-0.7, and the total drag saving is a more modest 15-20% at 2√A.
Technical Paper

Quantifying Platoon Fuel Savings: 1999 Field Experiments

The results reported are from tests on July 6-8, 1999, on a limited-access 12km section of I -15 in San Diego. The tests involved 2, 3 and 4-car platoons operated and maintained by PATH personnel under the auspices of CALTRANS and utilized Buick LeSabre sedans under fully automatic longitudinal and lateral control. Multiple sensor data was acquired, including the fuel injector pulse width. We demonstrate that the fuel injector pulse width, in combination with engine RPM and forward speed, can be used to determine accurate estimates of instantaneous fuel consumption. The repeatability for total fuel consumed over a 2.4 km portion of the test path is ±1% based upon multiple single car runs over the three day period, with the major portion of the uncertainty arising from changing wind conditions. Fuel savings for individual vehicles vary from 0-10% depending upon number of vehicles, vehicle spacing, and vehicle position within the platoon.
Technical Paper

Progress in Reducing Aerodynamic Drag for Higher Efficiency of Heavy Duty Trucks (Class 7-8)

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. In addition, greater use of newly developed computational tools holds promise for reducing the number of prototype tests, for cutting manufacturing costs, and for reducing overall time to market. Experimental verification and validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California. Companion computer simulations are being performed by Sandia National Laboratories, Lawrence Livermore National Laboratory, and California Institute of Technology using state-of- the-art techniques, with the intention of implementing more complex methods in the future.
Technical Paper

Modeling of Engine Cyclic Variation by a Thermodynamic Model

Kantor model showing that prior-cycle effects resulting from exhaust gas residuals are a significant factor in cyclic variability of combustion in IC engines is due to a number of model assumptions that misrepresent the thermodynamic process experienced by the mixture of fresh combustible gas plus exhaust residual in important ways. In particular we show that exhaust blowdown process and variable exhaust residual gas mass fraction, neglected in the Kantor model, significantly reduce cyclic variability. However, unburned fuel not considered in the Kantor model apparently aggravates cyclic variability. These three factors cancel each other resulting in cyclic variation appeased. Using modified Kantor models, we examine the effects of all major engine operating parameters on mean and fluctuating exhaust residual temperature and indicated work. No significant cyclic variability is predicted for realistic ranges of these parameters.
Technical Paper

Drag Forces Experienced by Two, Full-Scale Vehicles at Close Spacing

The present study aims to document the drag reduction for a two-vehicle platoon by operating two full-scale Ford Windstar vans in tandem on a desert lakebed. Drag forces are measured with the aid of a special tow bar force measuring system designed and manufactured at USC. The testing procedure consists of a smooth acceleration, followed by a smooth deceleration of the platoon. Data collected during acceleration allows the calculation of the drag force on the trail-vehicle, while data collected during deceleration is used to calculate the drag on the lead vehicle. Results from the full-scale tests show that the drag behaviors for the two vans are in general agreement with the earlier conclusions drawn from the wind tunnel testsænamely, both vans experience substantial drag savings at spacings of a fraction of a car length.
Technical Paper

Drag Forces Experienced by 2, 3 and 4-Vehicle Platoons at Close Spacings

Drag measurements are made on each of the members of 2, 3 & 4-vehicle platoons. One-eighth scale vehicle models are used in a wind tunnel equipped with a suction surface ground plane for boundary layer control. Strong interaction between vehicles takes place for spacings less than one vehicle length, leading to drag values substantially lower than for an isolated vehicle. All vehicles in the platoon experience lower drag. The average drag coefficient for a 4-vehicle platoon at a nominal spacing of 0.2 vehicle lengths is just 56 percent of the drag of the vehicle in isolation. It is also concluded that little additional benefit is achieved by forming platoons longer than 6-7 vehicles. Finally, the 2-vehicle platoons are operated in different orientations-front-to-front, back-to-back and reversed-to provide an estimate for drag reduction sensitivity to vehicle shape.
Technical Paper

DOE's Effort to Reduce Truck Aerodynamic Drag Through Joint Experiments and Computations

At 70 miles per hour, overcoming aerodynamic drag represents about 65% of the total energy expenditure for a typical heavy truck vehicle. The goal of this US Department of Energy supported consortium is to establish a clear understanding of the drag producing flow phenomena. This is being accomplished through joint experiments and computations, leading to the intelligent design of drag reducing devices. This paper will describe our objective and approach, provide an overview of our efforts and accomplishments related to drag reduction devices, and offer a brief discussion of our future direction.
Journal Article

Combined Effects of Multi-Pulse Transient Plasma Ignition and Intake Heating on Lean Limits of Well-Mixed E85 DISI Engine Operation

Well-mixed lean SI engine operation can provide improvements of the fuel economy relative to that of traditional well-mixed stoichiometric SI operation. This work examines the use of two methods for improving the stability of lean operation, namely multi-pulse transient plasma ignition and intake air preheating. These two methods are compared to standard SI operation using a conventional high-energy inductive ignition system without intake air preheating. E85 is the fuel chosen for this study. The multi-pulse transient plasma ignition system utilizes custom electronics to generate 10 kHz bursts of 10 ultra-short (12ns), high-amplitude pulses (200 A). These pulses were applied to a custom spark plug with a semi-open ignition cavity. High-speed imaging reveals that ignition in this cavity generates a turbulent jet-like early flame spread that speeds up the transition from ignition to the main combustion event.
Technical Paper

Aerodynamic Forces on Truck Models, Including Two Trucks in Tandem

The present wind tunnel experiment describes 6-component force and moment data measured for both the cab and the trailer of a simplified model truck. Forces and moments are presented in coefficient form. The cab is sufficiently smooth that no flow separation occurs at zero yaw. The trailer has rounded forward vertical edges and sharp upper and lower edges. Both cab and trailer have wheels. The test matrix includes variation of the cab-trailer gap, and the yaw angle between the model plane of symmetry and the axis of the wind tunnel. The yaw angle is meant to account for the presence of an over-the-road side-wind. Cab-trailer gap separation is varied from 0-1.55√A, where A is the frontal cross-sectional area of the trailer. Yaw angle is varied from 0-16 degrees. A second experiment provides drag information for two trucks in tandem with a variable spacing between the trucks.
Technical Paper

Aerodynamic Drag of Heavy Vehicles (Class 7-8): Simulation and Benchmarking

This paper describes research and development for reducing the aerodynamic drag of heavy vehicles by demonstrating new approaches for the numerical simulation and analysis of aerodynamic flow. Experimental validation of new computational fluid dynamics methods are also an important part of this approach. Experiments on a model of an integrated tractor-trailer are underway at NASA Ames Research Center and the University of Southern California (USC). Companion computer simulations are being performed by Sandia National Laboratories (SNL), Lawrence Livermore National Laboratory (LLNL), and California Institute of Technology (Caltech) using state-of-the-art techniques.