Design of Advanced Power Transmissions

During the last five years, significant progress has been made in the development of automotive transmissions, and especially in increasing the gear ratios and improving the efficiency. With the help of software products and programs, new transmission design schemes have been constantly developed, which are characterized by improved operational parameters. Different optimization methods have been considered about the implementations of main objectives: power transmissions’ component design and efficiency improvement. Conclusions have been made concerning the opportunities for reducing the energy consumption by engineering tools inside the power transmissions.


Introduction
The largest concentration of carbon monoxide, nitrogen oxides, ozone and other chemical compounds has been registered in the central part of the cities.In [1], the authors' team creates an extremely significant mathematical model of four analyzed situations in order to suggest important recommendations for reducing pollution in an urban environment.The author teams of [2, 3, 4 & 5] investigate and propose several significant measures that can be implemented aiming to reduce energy consumption and deleterious emissions, the most important of which is related to the renewal of automotive vehicles and innovation of their components.
Besides, using new automobiles and new technologies to design gearboxes can contribute to reduce noise pollution in urban residential areas.In [6, 7 & 8] the authors evaluate noise pollution from car traffic.The authors identify opportunities for solving the problems of traffic and pollution in the central part of the cities by applying extremely interesting and relevant traffic simulations.
As summarized briefly and unambiguously in [9], sources of air pollution are motorcycles, cars, trucks, airplanes, ships, locomotives, etc.It is emphasized that technologies which are applied in engines such as computer control and direct gasoline injection can improve fuel economy and reduce harmful emissions.
Transmission technologies such as the application of seven or more speeds, dual-clutch transmissions (DCTs) and continuously variable transmissions (CVTs) also improve fuel economy and reduce emissions.As it has been mentioned, better transportation planning for passengers and cargo also leads to similar beneficial results, Figure 1.

Theoretical background
The level of harmful emissions and the energy efficiency of cars are highly dependent on the operational parameters and the quality of the engines.The authors of [10 & 11] conducted an up-to-date experimental study on the function of air temperature, the formation of the air/fuel mixture, the modification of the cooling system and several design parameters of the engine.In order to increase the combustion quality of the engine, as a result of the research implemented, measures of particular importance have been suggested: application of different injection strategies; optimization of combustion components, etc. [12].
Along with the development of new technologies in the area of design and production of engines, the application of new methods and approaches concerning the layout of automotive transmissions is also growing extremely rapidly.According to [10,11] over the past few years, continuously variable transmissions (CVT) and dual-clutch transmissions (DCT) have also increased their market share significantly.
Over the past few years, transmission design has evolved significantly, impacting both performance and fuel economy.Modern, well-performing transmissions have been shown to be one of the most costeffective technologies for improving automotive fuel economy, [13].
According to the authors' team, the determination of the transmission ratios takes into account the decisions on the number of planetary gears, the number of clutches and the number of kinematic combinations of different transmission elements.A transmission design procedure is described in [14] in which planetary gears, joints, clutches, brakes and motors have been represented with appropriate mathematical constraints.Suitable analytical equations for these constraints have been solved in order to determine several possible design solutions.
An interesting solution for improving the quality of the automotive industry has been proposed in [15].The authors determine the time required for each operation and the corresponding components in order to realize a change in the production process.Innovative methods based on time, visual control and visual inspection have been applied.
Based upon the results of the theoretical analysis implemented, the authors consider that it is possible to make significant changes in the design and calculation procedures for transmissions and gearboxes in order to improve their functional parameters and their energy efficiency.

Methods and prerequisites for solving the problem
Based on the experience of the authors' team in the field of design theory, systems approach and new product development, a methodology for improving the operational parameters of transmission and gearbox components is proposed.
The requirements for clarity in the layout of a new design version of the gears correspond to the necessity for unambiguously executable manufacturing and assembly processes.
Improved and advanced procedures for evaluation and comparative analysis of different design solutions for a certain problem, involving several main stages, have been described in details in [16][17][18].Methodology suggested in [18] includes: analysis of elements in the power flow of the vehicle and the possibilities to reduce mechanical losses in these components; new and modified generally valid assessment criteria; a new approach for calculating the cardan drive in order to reduce its internal dynamic load; a method for calculating internal meshing with increased efficiency; specific criteria for the analysis and evaluation of design solutions that have a relatively equal degree of significance.
The main results of these studies described the prerequisites created for conducting simulation and experimental research of gears and other power transmissions.
Besides, in a previous study [19], one of the authors considered the strength calculations of gearbox components by several design methods -conventional standard calculations, automated design with a CAD system, and strength simulation with an appropriate CAD system.
Due to these reasons, the author's team considers to introduce two new stages to the methodology presented in [18]: improvement of the algorithm for determining the transmission ratio, which will enable the improvement of the transmission elements.This new stage of the methodology will be part of the refinement of the algorithm for switching gears and the selection of optimal gear ratios for future transmissions.The second new methodology stage is to be a precise calculation of the meshing according to the contact strength criterion based upon a comparative analysis of a database of accumulated calculations.

Results and Discussion
Calculations of geometric, kinematic parameters and strength parameters of an exemplary five-speed automobile transmission (Figure 2) have been performed.The modules of the gear sets are: m = 2.25 mm or and m=1.75 mm.The number of teeth and gear ratio of each stage are indicated in Table 1.
The results of the calculations lead to the following conclusions: the values for contact stresses obtained are about 5% lower when working with SolidWorks in comparison with the calculations implemented according to ISO 6336, [20].The corresponding coefficients and assumptions when working with the CAD system have been compared with the analytical results implemented.It has been found that the system accepts a greatest possible meshing width depending on the module compared to the recommendations of the international standard.

Conclusions
A theoretical investigation in the area of advanced power transmissions is presented.Based upon the results of the theoretical analysis done, the authors consider that it is possible to make significant changes in the design and calculation procedures for transmissions and gearboxes in order to improve their functional parameters and their energy efficiency.The author's team introduces two new stages to the transmission design methodology: improvement of the algorithm for determining the transmission ratio and more precise calculation of the meshing according to the contact strength criterion.
The results obtained can be used at an early stage of design process of transmissions, allowing the designer to choose the optimal parameters in terms of maximum efficiency.

Figure 2 .
Figure 2. General view of the structure of a manual gearbox 02T [19]

Table 1 .
Teeth numbers and gear ratios