Shock absorbers, wheel bearings, brakes, electric motor - future sources of heat for an electric vehicle?
The development of a system that has reached its limit can be continued at the super-system level. Having exhausted the resources of its development, the system is combined with another system, forming a new, more complex system.
Fundamentals of the theory of the development of technical systems.
Electric cars differ from traditional cars not only in their simplicity of design. According to the law of development of the system, the transition to a new source of energy automatically means a revision of the design of the entire system. At the moment, electric cars have not lost their resemblance to conventional cars. There are not only design reasons for this, but also quite psychological ones.
Would you buy a car radically different from the usual? And most importantly - how to get used to the rest of the dynamics and control for the future as a horse lag behind the first cars?
The initial stage of the "restructuring" of the system-electric vehicle can be judged on the change in attitudes towards aerodynamics and heat preservation-production.
In previous articles Liquid brake cooling system and Liquid brake cooling. I talked about the development of the system mainly about the use of liquid cooling systems for a regular car and a hybrid electric car. This article will be about the development of a similar system in an electric vehicle in which motor-wheels will be used.
An electric car with motor-wheels, this is exactly the future of this type of transport, which many experts predict. The “classic” arrangement of the motor outside the wheel does not allow full use of the recovery, and arbitrarily changing the shape of the car. In this case, even in pure theory, such “wheel and engine crossing” gives a higher efficiency, compared to the scheme with an intermediate transmission element.
Why for all the advantages of motor-wheels do not use them? It's all about the inevitable flaws of this design.
The first, and probably obvious to all, is the thermal load of the structure. Three potentially heavily loaded, and producing heat element, are very close to each other!
1. Wheel bearing (since the wheel has a greater mass than conventional heating during operation is higher).
2. The brake mechanism, which in fact its work is subject to strong heating, and with such a dense placement of the cooling air passes in the worst conditions.
3. Electric motor - potentially, when overloaded, it heats not only itself, but also “neighbors”.
... And what is the way out in the event of such problems?
The first is to separate the heat-loaded elements. The
second is to be cooled with liquid.
The diagram in general terms shows the location of such cooling.
Next will be an explanation with the formulas, which I hope made the most understandable to a wide range of readers.
Shock absorber- a device for damping vibrations (damping) and absorbing jolts and impacts of moving elements (suspension, wheels), as well as the hull of the vehicle itself, by converting the mechanical energy of motion (vibrations) to HEAT.
Based on the definition it is not difficult to guess what is the main problem of shock absorbers in the calculation. On average, the maximum temperature for most shock absorbers is set at no more than 100 degrees. Work with exceeding this limit or close to this boundary largely determines the resource of this node (it is worth remembering that this temperature is largely determined by the durability of the weakest link of the suspension - rubber rod seals, which are responsible for the tightness of the system).
Based on the calculation, it is easy to understand that, having no other alternative than air cooling, it is necessary to lay certain reserves for the strength and heat capacity of the material. This extra weight is a guarantee of stable operation in the limit modes. But even this “stock” as a rule does not save in conditions of frost and extreme loads, heat. The damping characteristic changes due to a change in the viscosity of the fluid, and when the calculated parameters are exceeded, the shock absorber simply fails. Moreover, the cold weather is characterized by increased wear, if the shock absorbers did not “warm up” by starting at low speeds before the trip (this is a common recommendation for drivers when starting to move in winter, because, in addition to increased wear, the shock absorber characteristics are reduced, reducing its effectiveness before warming up) .
All this leads us to a certain “fork” of solutions.
The first (which is implemented at the moment), the increase in temperature resource stability of the shock absorber due to new materials, and the use of buffer tanks (shock absorbers with "gas" and others).
The second solution, which proceeds from the logic of the article, is the creation of a water "shirt" around the shock absorber, with its simultaneous decrease on the basis of a more compact heat release. Thus, not only the weight problem will be solved, but also the warm-up and cooling time of this suspension unit.
Even in the construction of shock absorbers, as a rule, there are compression and recoil valves that help dampen high-frequency oscillations, and also serve to dampen oscillations in the event of an increase in the viscosity of the damping fluid at a low ambient temperature (i.e., perform a similar function with a thermostat - serve to quickly warm up fluid and maintain the stability of the mechanism).
I understand that such a proposal can cause great indignation among colleagues in the car industry, and this is not surprising. The cost of thinking here is primarily a dependence on the pace of development of the industry, i.e. the automobile industry cannot be compared with the development of processors for computers (otherwise, as stated in one well-known comparison, “cars would consume a minimum of fuel and have already flown”).
We now turn towheel bearing.
Let's start with the "horror stories"
Unfortunately, this statement is not an attempt to intimidate, but a banal truth of life. Despite the fact that the quality of bearings has increased many times the possibility of a “wedge” at speed, no one has canceled (and given what accuracy classes are used now in the manufacture of bearings, the sharpness of actuation and shimming of the wheel is even higher than before).
The imaging of the wheel imager points directly to the hottest spot on the wheel (if you don't count the temperature of the brake discs).
Thermal calculation of wheel bearing looks like this:
It follows from the above that the weak points of the bearing are precisely the temperature in working condition. The problem of wear resistance is now well solved by protecting the bearing from contamination, and more wear-resistant alloys in the bearing itself (manufacturing accuracy also affects, but not so much, and nobody puts precision bearings in the hub, although in theory this could still increase efficiency).
Especially well at the moment solved the problem of lubrication. Modern lubricants for heat resistance and durability have long been superior to analogues of the times of the USSR.
Why still no one solved the problem of cooling the bearing? And because in fact it was almost nonexistent, and it did not stand so sharply.
The heat of the bearing went perfectly in different directions along the metallic elements of the suspension. Here you can give an example from the internal combustion engine, where there were also bearings that were not cooled with oil, but they worked perfectly. These examples are clear, but in the case of a motor-wheel, an attempt to ignore the risk of overheating can be very expensive! In an accident, you lose not only the wheel, but also the engine (which is several times higher than the other components)!
How to cool the bearing, and it does not increase the mass of the motor-wheel?
Here it is worth remembering that in our time, various composites have successfully made up the competition for metals in the suspension. Therefore, you just need to take the wheel bearing, or rather its place of attachment (housing), and make it out of a composite with internal coolant channels. This not only solves the problem of bearing overheating, but also stabilizes the temperature mode of the composite itself, since the weakest point of plastics is heating to high temperatures.
The next bidder, and perhaps the main “supplier” of high-temperature liquid, will be the usual drum brake. Its design is simple and clear, and the appearance of its liquid cooling system was described in a previous article. Now, after what has been described above, I hope it’s clear where another heat source will come from in the drum.
Calculation of heat for brake mechanisms will look like this:
The main element of the motor-wheel - the electric motor with liquid cooling will also give a certain heat. On Habré there is also a very interesting material about domestic developments in this direction. From myself I would just note that with the inevitable increase in the power of the motors, the heat release will also increase.
All of the above can carry another useful function - diagnostic. Measuring the performance of important components and assemblies in real time is practically the same as monitoring the operation of the internal combustion engine, through a series of sensors on the engine, for maximum efficiency of the system.
Obviously, the question arises - why do we need cooling-heating of a hub bearing and shock absorbers on an electric car? Why do these weak heat inflows, even if we consider that the electric car needs heat?
The answer is complicated, but quite logical. At the moment, an ordinary electric car has a drive to the wheel through the CV JOINT to the wheel. The ideal option for an electric car in the future, many experts call the motor-wheel. When the drive is located directly in the wheel, there are additional opportunities for recovery, and in general, the reliability of the system increases.
By analogy with bicycles and electric motorcycles it is easy to understand that the motor-wheel scheme is not the distant future, but the nearest present.
So looking at the conclusions here , it is clear how they see the future of such wheels.
The real prototype that combines the maximum of the characteristics of controllability and dynamics was shown back in 2008!
The engine is electric and fluid cooled, and the brakes are all electric ! Those. everything is exactly as I described in the previous article.
A test drive of a car with such wheels is in this article and in more detail here.
As we see, from this it follows that the cooling of the electric motor will be exactly liquid. The system due to the large number of elements becomes complicated, and conventional air cooling simply cannot cope with the powerful heat dissipation of all heat generation points. The most obvious drawback here is already the low reliability of some elements, and specifically the brakes, wheel bearing and shock absorbers. The loads in the wheel will increase, and the heat balance will need to be stabilized in operation to increase the durability of all elements.
There is another motor-wheel problem - the weight of the whole set in the wheel. This lack of high-tech methods leveled only partially. The most reasonable thing here will be to remove from the unsprung mass of the system the most “flexible” element - shock absorbers with springs. The “removal” method is a transfer from the wheel to the side, using the system of this element.
It is possible both to do on 2CV ...
and it is possible as in some sports cars.
An additional factor in weight savings is possible when using composites.
So at the moment the composite can be:
Springs
Spiral suspension springs made of fiberglass (GFRP) .
Since 2015, they are installed on a modification of the Audi A6 Avant ultra with a 190-horsepower diesel engine.
Subframe
Composite subframe for Ford (Currently undergoing tests).
Levers
Composite lever "Lift". Experimental technology that involves replacing 3 suspension elements at once (shock absorber, lever, springs).
Williams Platform for Electric Vehicles - FX-EXV contains the technology for the production of plastic suspension arms.
Brake discs (and in our case it would be good to apply the “carbon” technology to drum brakes)
"Carbon" brake discs. The main drawback of such brakes is the need to warm up, and this problem is sometimes even partially solved by electric heating of the disc! In a drum, a similar process would be much more economical.
Composite
Discs Audi plastic discs. 2007 prototype. Passed the test 250,000 km. (Uncoated aluminum). A similar design tests VW. The technology is already at the stage of introduction into mass production.
The use of composite technology in the suspension simplifies the task of installing a motor-wheel, but does not solve the problem of heating. Cooling - heating the bearings, brakes and shock absorbers with liquid will increase the service life, and most importantly, the reliability of all these elements is independent of external temperature conditions.
Such a cooling-heating system can seem really complicated at first glance. The heat generated from auxiliary sources may be very small, but in an electric vehicle I see great opportunities for heat saving (for more details, see the next articles).
Therefore, what may not be significant for a car with an internal combustion engine, for electric vehicles can give significant savings.
Improving the reliability of the suspension is very useful for vehicles operating in car sharing or long-term rental, as well as robotic vehicles operating in difficult conditions.
Critical Moment
- Why complicate the suspension so much?
The complication of cooling is easy to understand by analogy in history. At the time of the transition from air to liquid cooling of the internal combustion engine, the same thought that this would create only additional problems. As a result of the transition, does anyone want to throw out the radiator and the rest of the cooling system from its motor? There is no such thing ... but there are advantages in the form of increasing the reliability of cooled parts.
You can certainly think of why all these improvements for the sake of "eternity", and here I would like to give an example of the Porsche FLA Concept car .
If you transfer the idea to electric cars, then the question of durability can be increased multiple. Moreover, the solution to the problem of increasing the service life of consumables, against the background of the service life of electrical components, seems quite logical.
PS - As a result of all the described transformations, you can get a lightweight suspension with motor-wheels, using the full thermal potential of the moving mechanisms. In this case, the level of simplicity and reliability of the electric drive will be able to be compared with the durability of the suspension required for the future. There is already a pure psychology, namely, the desire to have a car is even better and more durable than the previous model, as well as an environmental factor (the longer the mechanism is operated, the less waste it will create, and the expenses mainly on the disposal of liquids and the replacement of small parts).