Transmission Gear Ratio vs Fuel Consumption: Retrospective Analysis for Future Terrain Vehicle Applications
Authors: Vantsevich Vladimir V. , Joshi Bhargav H. , Bortolin Gianantonio 
Published: September 2013

For decades, the technical problem of selecting optimal transmission gear ratio has been researched for various ground vehicles based on different selection criteria depending on a particular vehicle application; criteria of terrain mobility, traction and acceleration performance, engine power utilization and fuel efficiency have been widely in use. Innumerable analytical and experimental research results and data were implemented in thousand different transmission designs. Today, this unique information about the number of gears in transmission, and value of gear ratios make a field for (i) a research analysis of engineering efficiency of different transmission designs, which were in operation for decades, and (ii) developing more efficient analytical methodologies to select the number of gears and transmission gear ratios and, furthermore, continuously, in-real time control power transfer from the vehicle energy source to the wheels.

This paper considers the first, of the above-listed problems in detail with applications to various terrain vehicles and then specifically to off-road wheeled vehicles. The analysis presented in the paper, envelops simple random samples of up to 50 vehicles. It starts from WW2 military vehicles, goes to off-road trucks of 1980s and finally compares modern dump trucks and other terrain vehicles of several major world OEMs. The paper presents an analytical method, computational algorithm and results of a study in which, the efficiency of conventional analytical methodologies are evaluated using actual data on fuel consumption and characteristics of transmissions, vehicle engines, driveline and running gear systems and payloads. To serve this purpose, actual data of each vehicle is compared with analytical data of the vehicle, computed using the conventional methods, with focus on gear/velocity ratios and average fuel consumption at each transmission gear. The fuel consumption analysis was carried out by computing vehicle transport capacity as a function of the average velocity and mass of the payload for each vehicle.

The result shows a distinct change of behavior in gear design methodology between post war and present day vehicles. It was a determined divergence from the initial trends, which were based on either the geometrical progression method or arithmetic method for selecting the number of transmission speeds and the values of gear ratios. This resulted in not only having a wide range of speed characteristics of automatic transmission over a few manual gears, but also, as discovered in this study, lead to increased fuel consumption of some vehicles in all range of speeds.

The WW2 vehicles designed with manual transmission have gear ratios are closely aligned to analytically calculated geometric progression. Same behavior is observed in the off-road vehicles of 1980’s. Here, with a manual transmission, the trend is more towards less number of gears and with large interval between speed ratios. This of course gives a better fuel efficiency, but leads to trade off in lower average vehicle velocity.

The transmission design for modern day dump trucks is also very close to the geometric progression approach. The other modern off-road trucks, as discovered in the analysis, follow an arithmetic progression. Although this results in smooth transmission, but fuel efficiency is compromised significantly, compared to dump trucks.

It is important to note that a design based on geometric progression, would result in same speed distribution with less number of gears and better fuel efficiency. For a modern day terrain trucks, to have an optimum combination of both characteristics, it is important to consider all the parameters affecting velocity ratios and fuel consumption and incorporate an efficient analytical methodology to stay competitive, in the rapidly evolving market of all terrain vehicles.