用液体热等静压压制消除铝铸件的气孔和缩松

铸造厂降低费用和减轻铸件重量的努力每天都在进行.采购方要求制造方能够达到这样的技术水平,即其所提供的工件在具备必要的机械性能和物理性能的同时,减轻质量和节约成本.     

Uncertain demand,modal competition and optimal price-capacity adjustments in air transportation

A new microeconomic model for the operation of an airline facing modal competition with uncertain total demand is developed to analyze optimal price capacity combinations. The novelty is the treatment of the capacity restriction, which is not viewed as affecting negatively individual preferences (e.g. probability of a full flight), but does influence aggregate utility. A mode choice model is used to represent unrestricted individual preferences assuming full availability (phone call demand); air capacity is treated as a variable that acts on the actual choice set. Restricted choices and total demand stochasticity are integrated in welfare calculations (users' benefits and profits). Numerical examples are given and results are analyzed in terms of load factors fare levels, and sensitivity to the stochasticity of requests.This research was partially funded by FONDECYT, Chile, Direction Génerale de l'Aviation Civile, France, the Andes Foundation and the Fulbright Commission.     

Caribbean crushing: Competition among US homeports

The article explores the competitive aspects of US ports which provide cruise service to the Caribbean region. Via a shift-share analysis, growth at individual ports is apportioned into three components allowing for the suggestion of both long-and short-term planning strategis.     

Integrating congestion pricing,transit subsidies and mode choice

We model and analyze optimal (welfare maximizing) prices and design of transport services in a bimodal context. Car congestion and transit design are simultaneously introduced and consumers choose based on the full price they perceive. The optimization variables are the congestion toll, the transit fare (and hence the level of subsidies) and transit frequency. We obtain six main results: (i) the optimal car-transit split is generally different from the total cost minimizing one; (ii) optimal congestion and transit price are interdependent and have an optimal frequency attached; (iii) the optimal money price difference together with the optimal frequency yield the optimal modal split; (iv) if this modal split is used in traditional stand-alone formulations – where each mode is priced independently–resulting congestion tolls and transit subsidies and fares are consistent with the optimal money price difference; (v) self-financing of the transport sector is feasible; and (vi) investment in car infrastructure induces an increase in generalized cost for all public transport users.     

On the goods-activities technical relations in the time allocation theory

In areas like household production and travel choice, time assigned to the different activities plays a key role in addition to consumption as the main variables in utility within the consumer behaviour framework. However, a comprehensive conceptual structure to understand the technological relations between goods consumption and the assignment of time to activities is still lacking. In this paper the problem is reviewed and all possible relations between goods and time are re-formulated. Two general functions are defined and proposed to account for all these relations, forming a new taxonomy for the technical constraints. The resulting consumer behaviour model is used to obtain general expressions for both the value of saving time in constrained activities like travel, and the value of leisure.     

Patchwork in an interconnected world: the challenges of transport networks in Sub-Saharan Africa*

Sub-Saharan Africa has recently undergone, or still do in many countries, a period of transport infrastructure expansion. Current policies are centred on the development of international links, which require large capital-intensive projects and are sometimes economically dubious. This paper reviews the past policies and transport functions since colonial times by placing them in their economic and political context. We find that present strategies have similarities to the ones prevailing in previous periods, where expansion phases dominated by transport-led economic growth theories were followed by a stagnation of Africa’s infrastructure development. In view of the challenges in translating findings from empirical research into right policies, we identify the potential of more balanced and sustainable strategic investments, notably by reinforcing the existing secondary transport networks converging into urban centres.     

Optimal distribution of financial incentives to foster off-hour deliveries in urban areas

The main objective of this paper is to develop mathematical formulations to gain insight into the best way to distribute financial incentives to receivers of urban deliveries to maximize participation in off-hour deliveries. The paper considers two different types of incentive budgets: exogenous, and endogenous. The exogenous case represents the condition in which an external decision maker determines the incentive budget that is to be distributed among potential participants in off-hour deliveries. In the case of an endogenous incentive budget, the entity distributing the incentives must raise the necessary funds using revenue generation mechanisms such as tolls and fines. The optimal incentives are obtained from the Karush–Kuhn–Tucker conditions of a mathematical program that maximizes the number of truck trips shifted to the off-hours as a function of the incentives. The mathematical models developed in this paper provide guidelines about how to optimally distribute financial incentives to foster off-hour deliveries.     

The impact of a financial constraint on the spatial structure of public transport services

Using a single line model, it has been shown recently that the presence of a stringent financial constraint induces a less than optimal bus frequency and larger than optimal bus size. This occurs because the constraint induces a reduction of the importance of users’ costs (their time); in the extreme, users’ costs disappear from the design problem. In this paper we show that such a constraint also has an impact on the spatial structure of transit lines. This is done departing from the single line model using an illustrative urban network that could be served either with direct services (no transfers) or with corridors (transfers are needed). First, the optimal structure of lines is investigated along with frequencies and vehicle sizes when the full costs for users and operators are minimized (unconstrained case); the optimal lines structure is shown to depend upon the patronage level, the values of time and the cost of providing bus capacity. Then the same problem is solved for the extreme case of a stringent financial constraint, in which case users’ costs have relatively little or no effect in determining the solution; in this case the preferred outcome would be direct services under all circumstances, with lower frequencies and larger bus sizes. The impact of the financial constraint on the spatial structure of transit lines is shown to be caused by the reduction in cycle time under direct services; the introduction of users’ costs in the objective function makes waiting times reverse this result under some circumstances.     

Multiobjective pricing of bus-subway services in Santiago,Chile

Presently, the surface transit system in Santiago, Chile is private and atomized. It operates in competition with the subway, covering long distances and causing congestion in the central business district. As the subway operates below capacity, integrated services with a common fare have been identified as both operatively and financially possible, making better use of resources and avoiding negative externalities. A multi-objective approach is formulated and applied to determine the price of the combined service, accounting for both users' benefits and operators' profit. The role of different parameters in the financial and operative constraints is analyzed in particular, as well as the conflicting nature of the objective functions. The non-inferior set of solutions is constructed and analyzed both in the decision variables space and in the objectives space, and solutions are compared against the present conditions.