A large number of heavy-duty trucks idle a significant amount. Heavy-duty line-haul truck engines idle about 20-40% of the time the engine is running, depending on season and operation. Drivers idle engines to power climate control devices (e.g., heaters and air conditioners) and sleeper compartment accessories (e.g., refrigerators, microwave ovens, and televisions) and to avoid start-up problems in cold weather. Idling increases air pollution and energy use, as well as wear and tear on engines. Efforts to reduce truck idling in the US have been sporadic, in part because it is widely viewed in the trucking industry that further idling restrictions would unduly compromise driver comfort and truck operations. The auxiliary power units (APUs) available to replace the idling of the diesel traction engine all have had limited trucking industry acceptance. Fuel cells are a promising APU technology. Fuel cell APUs have the potential to greatly reduce emissions and energy use and save money. In this paper, we estimate costs and benefits of fuel cell APUs. We calculate the payback period for fuel cell APUs to be about 2.6-4.5 years. This estimate is uncertain since future fuel cell costs are unknown and cost savings from idling vary greatly across the truck fleet. The payback period is particularly sensitive to diesel fuel consumption at idle. Given the large potential environmental and economic benefits of fuel cell APUs, the first major commercial application of fuel cells may be as truck APUs. Ó

A nontrivial portion of heavy-duty vehicle emissions of NOx and particulate matter (PM) occurs during idling. Regulators and the environmental community are interested in curtailing truck idling emissions, but current emissions models do not characterize them accurately, and little quantitative data exist to evaluate the relative effectiveness of various policies. The objectives of this study were to quantify the effect of accessory loading and engine speed on idling emissions from a properly functioning, modern, heavy-duty diesel truck and to compare these results with data from earlier model year vehicles. It was found that emissions during idling varied greatly as a function of engine model year, engine speed, and accessory load conditions. For the 1999 model year Class 8 truck tested, raising the engine speed from 600 to 1050 rpm and turning on the air conditioning resulted in a 2.5-fold increase in NOx emissions in grams per hour, a 2-fold increase in CO2 emissions, and a 5-fold increase in CO emissions while idling. On a grams per gallon fuel basis, NOx emissions while idling were approximately twice as high as those at 55 mph. The CO2 emissions at the two conditions were closer. The NOx emissions from the 1999 truck while idling with air conditioning running were slightly more than those of two 1990 model year trucks under equivalent conditions, and the hydrocarbon (HC) and CO emissions were significantly lower. It was found that the NOx emissions used in the California Air Resources Board's (CARB) EMFAC2000 and the U.S. Environmental Protection Agency's (EPA) MOBILE5b emissions inventory models were lower than those measured in all of the idling conditions tested on the 1999 truck.

The objective was to determine the effect of several variables—vehicle operation, weight, and accessory use—on emissions production during common on-road heavy-duty vehicle operations. Oxides of nitrogen (NOX), hydrocarbon, and carbon monoxide emissions from a heavy-duty diesel tractor equipped with a 1999 engine were measured continuously during on-road tests. The vehicle was operated at predetermined steady-state modes of 25, 55, and 65 mph as well as full-throttle accelerations from 0 to 25 mph and 0 to 55 mph and decelerations from 65 to 0 mph. Vehicle weight (payload) and accessory use (air-conditioning) were varied. In general, increases in gross vehicle weight from 52,000 Ib to 80,000 Ib resulted in approximately 40% or greater increases in NOX grams per mile (g/mi) emissions during the accelerations and higher-speed steady-state operations. These results were consistent with simulation model results from the National Renewable Energy Laboratory's ADVISOR model. Analysis of variance (ANOVA) and regression models were used to identify relationships between the variables and emissions. With the use of ANOVA, it was found that mode explained most of the variation in emissions and had a significant impact on all species of emissions tested. With the use of regression, a strong relationship was confirmed between steady-state modal NOX emissions per hour and horsepower ( r2 = 0.89, P <.0001). When mode and weight were added as factors in the regression model, the overall precision of NOX emissions prediction was increased and horsepower became nonsignificant.

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The steady, buoyancy-driven, laminar motion induced in the annulus of two horizontal, concentric, circular cylinders by a difference in the boundary temperatures is studied analytically in the large Rayleigh number limit. The flowfield is divided into five physically distinct regions: (1) an inner free convection boundary layer near the inner cylinder, (2) an outer free convection boundary layer near the outer cylinder, (3) a vertical plume above the inner cylinder, (4) a stagnant region below the inner cylinder, and (5) a core region surrounded by the other four regions. Zeroth-order solutions which account for the coupling of those five regions are obtained in the high Prandtl number limit using a boundary-layer approximation and integral methods. Comparisons of the calculated heat transfer and temperature fields with experiment and numerical finite-difference results are favorable.