Gearbox

8.2.1 Gearbox Comparison

Specifying the gearbox was carried

out at an early stage in the project

as its attributes strongly affect other

parts of the car. The internal

combustion engine has a peak power point and a good gearbox should allow the engine to operate close to this point throughout the events, whilst not being too inefficient or heavy itself. Most Formula Student teams choose either an Epicyclic Gearbox or a Continuously Variable

Transmission (CVT). Both these options were considered as well as a Helical Sliding gearbox, as found in most road cars.

The three types of the gearbox were compared based on the following five parameters: mechanical efficiency, the ability of the gearbox to allow the engine to operate close to its peak power and

efficiency, gear change speed, mass and cost. These parameters were weighted (allocated weights are shown in brackets) and the gearbox types ranked in Table 8.1. The purpose of this table was to offer an initial basis for comparison.

Mechanical

Since the outcome of the gearbox comparison table is not conclusive, the simulation was used to compare the acceleration of the less efficient CVT with a more efficient four speed gearbox. Helical Sliding and Epicyclic gearboxes were considered similar in contrast to the CVT, so the four speed gearbox simulation is used to represent both Helical Sliding and Epicyclic. From Figure 8.2 it can be seen that the CVT offers a generally better acceleration than the more efficient helical gearbox, by virtue of the engine operating at peak power continuously. Performance of the. The time

required for the four speed gearbox to change gear was not accounted for here and would only favour the CVT further. The CVT is likely to require higher maintenance and not last as long as a helical geared gearbox but neither of these factors were considered to be important for Formula Student as the car will not be used often and there will be ample time for maintenance. Worth bearing in mind also is Section T8.4.1 of the FSAE rules, which requires CVTs to have a scatter guard (see Section 8.2.4) as their moving parts are not necessarily contained otherwise. The minimal extra mass and cost this would entail was not considered significant enough to alter this comparison. Therefore a CVT was shown to be the best choice of the three gearbox types considered.

Figure 8.2. CVT and fixed speed gearbox acceleration comparison

8.2.2 CVT Comparison

There are various CVT technologies that could be used, by far the most common is that invented by Van Doorne and known as a Variomatic CVT (Figure 8.3) which consists of a belt running between two double cone pulleys. The pulleys (also known as clutches) are each made from two cones and by varying the separation of the cones the diameter in contact with the belt can be altered. This allows for the ratio created between the pulleys to be varied continuously. The

separation is controlled by springs and masses which exploit the change in centripetal force (as the car accelerates) to shift the clutch cones. This project will not touch further on the mechanics of how a Van Doorne CVT is controlled but Section 9.3.3 describes how the CVT was simulated and how it is possible to tune the CVT for optimal lap times. A toroidal CVT was also investigated but was not considered viable to purchase due to poor variety and availability outside of the passenger car and truck markets, which are not as mass sensitive as in motorsport. Designing one’s own CVT was not considered worthwhile, especially when there are adequate belt drive CVTs available at low cost.

Figure 8.3. Example of a Variomatic CVT in a low gear and in a high gear^[1]

8.2.3 CVT Selection

There are two specifications of the CVT which must be met: maximum power handling capability and maximum rotational speed (this is especially important given that the engine chosen is capable of reaching 14000 rpm). Additionally, it is desirable that the CVT is lightweight and has a large ratio range so that the engine can be operating near peak power for as long as possible. A significant problem faced when selecting a CVT was the lack of suitable options on the market; CVTs for the passenger vehicle market are too heavy and those designed for lighter vehicles, such as go karts, do not have the power capacity necessary for Formula Student. These were still considered on the basis that two or more could be used in parallel. However when investigating the specifications of three CVTs, it was clear that using more than one would be too heavy [2] [3] [4] (see Figure 8.4). The only CVTs identified with sufficient power handling (and not designed for heavier passenger vehicles) were those made by TEAM Industries. The company offers designs suitable for 11 hp to 160 hp applications and supplies the input and output clutches separately, allowing for a selection suited to individual needs. An input and output clutch were selected based on the aforementioned criteria, see Table 8.2 for the individual specifications. The appropriate belt to use with these clutches is part #29C3596 manufactured by Gates. This set up will offer a generous ratio range of 0.71–6.3:1, allowing the engine to operate at peak power at speeds between 4.5 m/s and 40m/s.

Since the car’s speed will be above 4.5 m/s for all but the very start of any of the events, this allows for reasonable approximations to be made in the simulation as to how the car will be driven at very low speeds (see Section 9.3.4).

0 20 40 60 80 100 120

CVT comparison of power handling against mass

Max Power (hp) Mass (kg)

Figure 8.4. Comparison of how CVT mass varies with power handling capability[2] [3] [4]

Table 8.2. Chosen CVT Specifications

Part Description Max Speed Max Power Mass

TEAM TP-185-F^[4] Primary Clutch 11000 rpm 100 hp 3.0kg

TEAM TS-241-R^[4] Secondary Clutch 8000 rpm 100 hp 5.0kg

Gates #29C3596^[5] CVT Belt (sufficient) >100 hp 0.5kg

Self manufactured CVT Guard n/a n/a 3.0kg

8.2.4 CVT Guard

As per section T8.4.1 of the FSAE rules, belts must be covered by a scatter shield in case of failure. This can be easily manufactured although must be made from 3.0 mm Aluminium Alloy 6061-T6

according to section T8.4.3 of the FSAE rules. The mass of this part is 3.0kg when manufactured according to the design show in Figure 8.5.

Figure 8.5 CVT guard designed in SolidWorks