Thermodynamical limits for models of car-sharing systems: the Autolib example

TL;DR

This paper analyzes the thermodynamical limit of mean-field models for a large car-sharing system called Autolib, proving unique solutions and exponential convergence to equilibrium, with explicit formulas for system intensities.

Contribution

It introduces a new mean-field model for Autolib, proves the existence and uniqueness of solutions, and characterizes the equilibrium distribution explicitly.

Findings

Unique solution to the nonlinear differential system

Exponential convergence to equilibrium

Explicit formulas for queue intensities

Abstract

We analyze mean-field equations obtained for models motivated by a large station-based car-sharing system in France called Autolib. The main focus is on a version where users reserve a parking space when they take a car. In a first model, the reservation of parking spaces is effective for all users (see [4]) and capacity constraints are ignored. The model is carried out in thermodynamical limit, that is when the number $N$ of stations and the number of cars $M_N$ tend to infinity, with $U = \lim_{N\to\infty} M_N/N$. This limit is described by Kolmogorov equations of a two-dimensional time-inhomogeneous Markov process depicting the numbers of reservations and cars at a station. It satisfies a non-linear differential system. We prove analytically that this system has a unique solution, which converges, as $t\to\infty$, to an equilibrium point exponentially fast. Moreover, this equilibrium point corresponds to the stationary distribution of a two queue tandem (reservations, cars), which is here always ergodic. The intensity factor of each queue has an explicit form obtained from an intrinsic mass conservation relationship. Two related models with capacity constraints are briefly presented in the last section: the simplest one with no reservation leads to a one-dimensional problem; the second one corresponds to our first model with finite total capacity $K$.