The use phase rather than the manufacturing phase of most consumer products dominates the environmental impacts [ASHB09]. The case study of an automotive manual transmission drivetrain exemplifies how higher manufacturing efforts can reduce the overall environmental impact [HELU11].
The automotive powertrain consists of the engine, transmission, and drivetrain (drive shaft, differentials and drive wheels). Gears are functional elements in the transmission. Several abrasive processes exist for gear finishing [KARP08]. In this
study, general grinding processes are applied [HELU11]. The empirical equation (8.2) relates the average surface roughness, Ra, to the specific material removal rate and wheel speed [MALK08, HELU11].
(8.2)
Ra average surface roughness R1 experimentally determined constant Qw specific material removal rate vs wheel speed
x experimentally determined constant (0.15 < x < 0.6)
The specific energy requirement is assumed as 200,000 J/cm3 for a process rate of about 10-3 cm2/s [GUTO06]. With this specific energy, a constant wheel speed, and a Michigan energy mix, the specific energy to decrease the surface roughness of the gears in the final drive reduction is calculated [HELU11]. The roughness, Ra, can be decreased to 20-60 % for less than 0.5 MMBtu of primary energy [HELU11].
In the gear use phase, the powertrain delivers power to accelerate the vehicle, overcome losses in the drivetrain and engine and to power accessories [HELU11]. With the frictional losses and all accessories constant, fuel power changes with drivetrain efficiency. For a helical gear pair modeled after a final drive reduction the gear mesh efficiency depends on the root mean square surface roughness, Rq, of the gear [XU07]. During vehicle usage, the power to overcome tractive losses and accelerate the vehicle depends on the vehicle velocity, climbing resistance, rolling resistance, and aerodynamic drag [HELU11]. Assuming a standard driving cycle from the U. S. EPA Federal Test Procedure 75 and a regular gasoline, the primary energy demand of the transmission can be calculated in dependence of a range of the root mean square surface roughness, Rq [HELU11]. Decreasing Rq lowers primary energy demand relative to a standard finished final drive reduction in a range of 2-5 MMBtu depending on the lubricant temperature in the final drive reduction [HELU11].
Comparing the manufacturing energy for decreasing surface roughness and gained reduction of use phase energy demand shows that manufacturing precision has a big impact. Since there are several gears in a vehicle in addition to the final drive reduction there is an even much bigger opportunity for manufacturers to improve efficiency [HELU11].