Manual and automatic transmissions for use in wheeled vehicles have been well known for many years. The need to control the power reaching the wheels of a vehicle generated by an engine having a limited range of rotational energy occurs in almost every vehicle having wheels. But how is this engine power actually delivered to the wheels? The engine’s rotational speed is too high to be directly transferred to the wheels, and hence there is a need of an intermediate system to reduce that speed before it reaches the wheels. And this is accomplished by the use of a gearbox with various gear ratios adapted to achieve the desired vehicle speed. But we aren’t going to delve deeper into the gearbox designs of normal road cars, instead we will see if there’s any alternative design that has the potential to outperform a traditional gearbox.
The current FS car setup uses a motorbike engine with the standard 5-speed gearbox associated with that engine. Often only 3 or 4 of these 5 ratios are utilized during a race, therefore this system is inefficient and is an area which could be improved. In this transmission types, discrete gears are used with each gear providing a preselected gear ratio that cannot be changed. These discrete gearing arrangements require that the engine speed be controllable so that the speed of the vehicle can be more precisely controlled. It has been found that each engine has a particular rotational speed range within which it operates most efficiently,i.e., its power output and fuel efficiency are both high at this particular engine rotational speed range. While it would be desirable to constantly operate the engine within this most efficient engine speed range such technique is not possible with present discrete geared transmissions. Because of the relatively large separation between adjacent gears, engine speed can vary by large amounts during the gear shifts.
But on a regular road car, these are really not issues because they aren’t designed to race and hence, certain benefits can be compromised with, benefits that otherwise if compromised in a racecar, will cost a lot of points in the events because on track, performance is everything. Also, there’s a need to get rid of the redundant gears that aren’t used during the track events and add unwanted weight to the car.
A CVT(Continuously Variable Transmission) does not have many of the problems mentioned above. They enable the engine to run at the most preferable speed, may that be for economy, torque or power, and then the transmission itself will adjust the gear ratio to accelerate the car. This would mean, for use in a Formula Student car, the engine could be run at maximum power to gain maximum acceleration potential, a potential that is higher than a normal gearbox. There is no period of zero power as the ratio change is continuous. This is as opposed to the many different ratios in a gearbox. Clearly, this will lead to faster acceleration and is a big advantage. But, the driving experience with a CVT is an issue that cannot be overlooked as it is technically different to driving a car with a gearbox. This is one of the main reasons CVTs are not popular in the road car market, people simply do not like or are not comfortable with this different driving experience when compared to a normal car. How the driver of the car feels due to the CVT, and then how that feeling influences his/her driving can have a big impact on performance. With many disadvantages or new experiences related to them, CVTs are by no means perfect.
There are a few teams known to use a CVT in FSAE. Aston University has been using one for around 5 years, and though their cars have not performed at the top level in the competition so far, due to various reasons, they have shown some of the potentials of a CVT and more importantly they have done much of the investigative and practical work needed to refine their designs. This enables onlookers to see what has not worked, and from this others can move in the correct design direction by correcting the flaws. CVTs are most widely used in scooters, quad bikes and snowmobiles but the design parameters vary in all these and may not be suitable to directly implement in a FS car.
Coming to the different types of CVT systems, there are mainly 3 such types- Variable diameter pulley based, Toroidal and Hydrostatic. Not much but a brief overview of these systems will be discussed below.
VARIABLE DIAMETER PULLEY
This system has two pulleys connected by a V-belt. One pulley(primary) is connected to the engine crankshaft and the other(secondary) to the rear axle. The basic working principle behind this is that as the engine speed changes, the primary and secondary pulleys’ V-belts expand or contract, changing the effective radii of the path on which they ride. This is similar to changing the sprocket sizes and hence, the gear ratios. The mechanism behind their working is quite interesting and creatively thought out but discussing the same here would consume a lot of time(and you might get bored too :P). One amazing feature to highlight is that this system works as an autonomous traction control system that would prevent the wheels from slipping in case it loses traction on the road by automatically changing the pulley radius on which the belts ride. Audi’s ‘Multitronic’ CVT is an example of this type.
This one works on the same basic principle as the variable diameter pulley but differs in its construction and look. This system uses disks and rollers and though the number of parts used is quite less that makes it look simpler, the actual mechanism requires the implementation secondary components too to control the rollers on the disks. Again, detailed working would be irrelevant to discuss here.
Both pulley-based and toroidal systems have a limit to the amount of power they can transfer, due to belt strength and the limit to the friction possible between the disks and rollers. As both systems work through friction there will be increased slippage as the torque transmitted through the system is increased. Traction fluidsTraction fluids are used to increase grip between rollers and discs, and also between pulley and belt. These traction fluids are a key area for the improvement of CVT efficiency and torque handling abilities.
This uses hydraulic fluid as the power transmission medium and is a heavy setup to implement in a racecar. These are preferred mainly in heavy machinery that handle quite a lot of torque than a racecar. Hence, this is considered highly inappropriate for a FS car.
At this point, it would be good enough to take a look into Aston University’s CVT which they have been using for the past 5 years. Initially, this team used a Honda CBR600 engine with a Comet Duster CVT. However, the extremely powerful engine led to the downfall of this system due to the fact that it resulted in excessive belt slippage over the pulleys. Also, the final reduction gears were too small that made the chain to jump off them leading to disappointing results. So the next model in 2006 used a turbocharged engine with the same previous CVT. The problem that came up now was the underpowered engine. The entire system couldn’t perform to the desired level and hence, this model failed too despite a well thought out plan. The final successful model that they put forth was the Rotax 500cc engine coupled with a matching CVT and differential. The major con was the weight but this drawback could be alleviated by ruling out the redundant gears from the gearbox. Nevertheless, this team has been the one who dared to experiment this on a FS car and strived to make it successful despite a myriad of failures.
Regarding the driving experience, the Team Manager Phillip Scott said ”The CVT makes the car incredibly easy to drive, literally anyone can get in and drive it without having to worry about stalling it or trying to keep it in the power range – the CVT does all of that for you. After studying the dynamics of driving with a CVT this becomes more obvious. For instance, the driver has more time to think about corners and braking because he/she is not concentrating on downshifting and both hands can stay on the wheel. Also, downshifts can sometimes cause locking of the rear wheels if they are done too quickly, this will not happen with a CVT equipped car, this improves stability and therefore will give the driver more stability. The driver can also be more consistent as there is no chance of missing a gear change. All of these things show how a CVT car may be easier to drive than a car with a stepped gearbox.”
It can therefore be concluded that a good overall package is what is required to succeed in terms of speed and reliability, not very good individual parts such as the highly powerful CBR600 engine which was fast but in turn made the car unreliable or the underpowered engine. Finally, on the subject of the driving experience, it can only be assumed that a FS car with a CVT is actually much easier to drive than a car with a gearbox. Looking at all three of CVT systems and considering the formula student rules it may be concluded that a simple mechanical pulley based CVT with a belt is probably the best type of CVT to use due to its simplicity.
To verify whether using CVTs really made a difference to how well a car can perform in dynamic events, tests too were done by the Aston University team members. With a gearbox, it took 5.5 seconds for the car to go from 0-100kmph while it took just 3.5 seconds with a CVT in the car. This shows how advantageous it is during events like autocross and endurance. Also, in a car using CVT, the engine can operate at higher RPMs without much load or consuming much fuel, thus, improving its fuel efficiency as well.
Overall, a CVT car can be far easier to drive and an easy to drive car can allow the team to worry more about the car than driving, a fine card to hold in the competitive world of motorsport. There would be no retrofitting of components and this could lead to a better layout and more accessible components, all good attributes for the costing and design sections. Using a CVT in FS is somewhat of a rarity, it is innovative and shows flare, a desire to think outside the box and push the boundaries. Aston FS Team Manager Phillip Scott remarked that the FS judges greatly approved of the use of a CVT in their car. It could be assumed then that Aston was awarded extra points for the use of a CVT system. Overall, on balance, the CVT has shown itself to be a far better transmission than a traditional gearbox, on almost all levels and could well be the perfect match for an application such as Formula Student.