Why can't we build a 100% perfect car-on the "contradiction theory" in the car

I wonder if you have noticed that no matter how expensive a car is, it is not 100% perfect. It has some shortcomings in one way or another. This view applies to tens of thousands of dollars of family cars, tall BBA, top luxury cars such as Rolls-Royce and Bentley, and even the super sports car Bugatti regardless of cost. Some people will ask, then why not design a perfect car with 100%?

In fact, a 100% perfect car does not exist, because many functions or performances on the car are "contradictory". Often you can't choose B if you choose A, and you can't choose C if you choose B, such as the contradiction between cost and market, the contradiction between power and fuel consumption, the contradiction between comfort and maneuverability, the contradiction between space and maneuverability, and the contradiction between technological advancement and reliability. It can be said that automobile is a complex of contradictions, and automobile engineers are swinging, coordinating and balancing between these contradictions, and finally designing and manufacturing automobiles suitable for different users and different needs. Let's talk about the contradiction in the car and how the automobile engineer solved it.

1, the contradiction between automobile design and manufacturing cost and market terminal price.

I believe everyone has this feeling that the price of cars is getting cheaper and cheaper, and cars are becoming more and more "not equipped". In fact, this is the contradiction between the cost and price of the car. For an automobile engineer, he wants to install the best things in the car, use the best materials and technology, and make the car as perfect as possible. This is the so-called "engineer culture". But in fact, this is impossible. Now cars have become mass consumer goods, so there must be affordable prices. If you try hard to build a "good car" with real materials, the final result may be high and low, and the sales volume is low, which ordinary people simply can't afford. So cars can only reduce costs to meet the needs of the general public.

In the early days, car configurations were generally reduced, and some unimportant configurations that increased comfort and luxury were reduced. It turned out to be too sensitive. This is what China people value most. You cut all these things instead of killing them! Gradually, car companies have learned the world, making you luxurious where ordinary people can see it, and secretly reducing the configuration where ordinary people can't see it. The most common ones are reducing the anti-collision steel beam and roof reinforcement, reducing the strength of steel plate and lowering the suspension level (double wishbone to McPherson, multi-link to torsion beam). Actually, you can think for yourself. Cars of the same size used to be much heavier and more expensive than they are now. Labor and prices have gone up, but cars are cheaper. What's so fishy about this? I think you can understand!

2, the contradiction between engine power and economy.

Automobile engines require both good power and appropriate economy. How to solve the contradiction between them is the unremitting pursuit of automobile engineers. In fact, the structure and parameters of each part of the engine are the result of mutual balance. Here are a few simple ones.

(1) Compression ratio and thermal efficiency: Generally, the higher the compression ratio of an engine, the higher the thermal efficiency, so the engine has been invented for more than one hundred years, and now the compression ratio is only about11. The highest is Mazda's Chuangchi Blue Sky Engine, which claims to reach 14: 1, but it is also commonly used. The maximum thermal efficiency of the engine is about 40%. Then why not greatly increase the compression ratio and improve the thermal efficiency of the engine?

Under the existing technical conditions, if the compression ratio is raised too high, there will be many problems that cannot be handled. On the one hand, if gasoline is excessively compressed, it is likely to deflagrate, which is an uncontrollable combustion state and needs to be avoided; On the other hand, if the compression ratio is too high, it is necessary to improve the strength of the component. We know that the compression ratio of diesel engine is much larger than that of gasoline engine, but the diesel engine with the same displacement is much larger and heavier than gasoline engine, because the internal components need to be strengthened. In addition, gasoline engines can use aluminum alloy, and diesel engines can only use cast iron.

In addition, the engine has different requirements for compression ratio under different working conditions, so it is best to change the compression ratio with the engine working conditions at any time. Such an engine has been used in Nissan, but its reliability is still unpredictable. Variable valve timing and lift, which are widely used in automobiles, can actually be regarded as variable compression ratio in disguise.

(2) Cylinder diameter and piston stroke: In an engine, the ratio of cylinder diameter to piston stroke is called cylinder diameter stroke ratio, and different cylinder diameter stroke ratios will affect the power output characteristics of the engine. For example, our ordinary civil automobile engine generally adopts a large cylinder bore stroke ratio, which can produce relatively large torque at low speed, but its high-speed performance is not good; However, the racing engine generally adopts a smaller cylinder diameter stroke ratio, which makes the engine get higher speed and effectively improves the engine power. Automotive engineers need to choose engines with different power output characteristics according to the different positioning of automobiles. Generally speaking, it is difficult to give consideration to high and low speed performance at the same time, and only one of them can be biased.

(3) intake manifold length and engine performance: a long intake manifold is beneficial to the low-speed torque of the engine, but the high-speed performance is not good; The shorter the intake manifold, the better the high-speed performance, but the lower the low-speed torque. Therefore, it is necessary for automotive engineers to find the balance point of intake manifold length, so that the engine can deliver higher torque at normal speed as much as possible. A more balanced way is to use a variable length intake manifold, use a long intake manifold at low speed, and open a valve at high speed to shorten the intake passage. However, the length of this intake manifold is abrupt and cannot be adjusted step by step.

(4) Valve timing and lift and engine performance: When the engine is working, the working conditions change at any time, and the requirements for intake and exhaust are also different, such as intake advance angle, valve overlap angle and valve lift. For example, a smaller intake advance angle can be used at low speed, but a larger intake advance angle must be used in order to intake more air at high speed, and so on. For ordinary engines, valve timing and valve lift are fixed, so they are generally designed according to the requirements of common speeds, and it is impossible to give consideration to high and low speed performance. In the past, this was an insoluble contradiction. Now the technology of variable valve timing and lift is adopted, and this contradiction has been well solved.

3. The contradiction between the number of gearbox gears and the power and economy of the car.

Theoretically speaking, the more gears in the gearbox, the finer the power distribution to the engine, and the better the power and economy of the car. However, due to various factors such as the size, complexity and reliability of the gearbox, it is impossible for the gearbox to do too many gears. At present, there are at most eleven-speed gearboxes, and seven-speed or eight-speed gearboxes are commonly used. Manual gearboxes are usually five-speed or six-speed, with too many gears and complicated operation consoles. The maximum number of gears for a large truck is sixteen, which is a special case. Not all gears can be used in daily use. In fact, CVT continuously variable gearbox can realize countless gears in principle, which can distribute engine power most finely and achieve better economy. However, at this stage, CVT gearbox still has insurmountable defects and cannot be applied to all models.

4. The contradiction between car comfort and sportiness.

If you drive a lot of cars, you will find that the suspension of some cars is very comfortable and soft, but when cornering at high speed, the body roll is large and the car handling is poor; There are also some models whose suspension is very hard. If the road surface is slightly uneven, it will feel very bumpy and the ride comfort is not good. However, this kind of car has good body support, small body roll and good handling when cornering at high speed, which is what we commonly call a car with certain sports. These two different personalities depend on the adjustment of the chassis, which is generally difficult to take into account. The most common method is to adjust the stiffness of the balance bar to achieve the effect of "soft at the beginning and hard at the back". In high-end vehicles, the variable suspension system can perfectly achieve the effect of "vertical flexibility and horizontal rigidity", which can give consideration to comfort and sportiness, but its cost is relatively high and it is difficult to popularize in ordinary family cars.

5. The contradiction between automobile power and economy.

It is impossible for a horse to run fast without eating grass. The same is true of cars. If the thermal efficiency of the engine is not obviously improved, more fuel must be consumed to output more power. The so-called "strong power and low fuel consumption" is actually just a publicity stunt. If there is such a model, it will inevitably give up some aspects of performance. If you look closely, most models on the market have little difference in fuel consumption on the premise of similar grade, displacement and power. Some models with excessive fuel consumption also belong to the poor technical level of the main engine factory, and the overall adjustment of the car is not good.

6. The contradiction between automobile modeling and wind resistance coefficient.

In fact, in theory, the drag coefficient of a car can be extremely low, but such a car shape may not be straight, and it is far from people's aesthetic angle. Therefore, automotive engineers need to design the shape that best conforms to human aesthetic point of view, and at the same time, have the smallest wind resistance coefficient as possible. Generally speaking, this is very contradictory. The tall and powerful off-road vehicle looks like a square box, so the drag coefficient will not be very low; Ordinary cars are too streamlined and feel too soft. In fact, the drag coefficient is low. Automotive engineers will design different shapes of car bodies according to different car positioning. However, the lower the drag coefficient, the better. We should also consider the influence of air resistance on driving stability when the car is driving at high speed. Excessive pursuit of low resistance may lead to insufficient grip when the car is at high speed and the car floats out of control. At present, the model with the lowest drag coefficient is CLA of Mercedes-Benz, which is only 0.24.

In fact, there are still many contradictions in the car, such as the contradiction between tires and comfort and fuel consumption, the contradiction between the hardness and safety of car body steel plates, the contradiction between the performance of the car itself and national policies, and the contradiction between the advanced technology and reliability. The contradiction between them, and so on. In a word, everything has two sides. There is a good side and a bad side. There is no absolute good or bad. Automobile is such a contradictory synthesis, it needs to balance all aspects of performance, and finally make the most suitable car for us to use.

This article comes from car home, the author of the car manufacturer, and does not represent car home's position.