6-Speed Manual Transmission
Mechanical Transmissions Technology is an optional Advanced Applications module taken as part of the master year of the mechanical engineering degree at Imperial College London. Different topics are covered including gear geometry specifications, contact and bending stresses in gears, shaft design, bearing selection, lubricant specification and film thickness calculations, among many others.
Three main projects are carried which conform the coursework element of this module. One of these is the detailed design and specification of a transmission system for a given application.
I was a member of a group of four students who selected to design the transmission systems for three engine configurations to be used for small city cars. These consisted of a 1.1 litre petrol engine, a 1.3 litre petrol engine and a 1.6 litre diesel engine.
My initial contributions to the project consisted of researching the torque-rpm characteristics of existing equivalent engines from which we could then select appropriate operating points for each gear pair. This was then followed by the development of a MATLAB script (shown below) which took into account the operating point of the engine, the road surface properties, the aerodynamic properties of the vehicles and the steepness of the road in order to select the most appropriate gear ratios for each of the six gears for each transmission.
Gear Ratio Calculator (MATLAB script):
clc clear close all %Chosen engine operating point TEngine=270; %in Nm rpmEngine=2200; %in rpm #Parameters gradient=0; %road gradient in percent rho=1.2; %air density in kgm-3 Cd=0.321; %coefficient of drag A=2.15; %frontal area of car in m2 R=0.301; %dynamic radius of wheel in m m=1640; %gross vehicle mass (i.e. loaded car) in kg mu=0.035; %coefficient of rolling resistance g=9.81; %gravity in ms-2 %Calculations alpha=atan(gradient/100); omegaEngine=rpmEngine*2*pi/60; %Solving the cubic from Favailable@wheels-Fdemand@wheels=0. a=TEngine/R; %G^3 coefficient b=-m*g*(mu*cos(alpha)+sin(alpha)); %G^2 coefficient c=-0.5*rho*(omegaEngine*R)^2*Cd*A; %G^0 coefficient G=roots([a b 0 c]); %G will be the only real value given in this vector (the rest are complex results).
The team then implemented preliminary and iterative detailed design processes to arrive at the final geometry of each of the gears for this application. Other aspects were then defined based on this geometry, for example the definition of shaft diameters and material, the selection of appropriate bearings for the life of the product and the selection of appropriate lubricants to provide adequate film thickness at the gear mesh.
I have shown below the graphs displaying the engine characteristics and chosen gear ratios for each engine configuration as well as a diagram created by the team to represent the general design that was selected for all transmission systems. This is followed by the detailed report that was submitted at the end of the design process.
