When we go to understand a car, we can't live without it from the parameter list to understand its various aspects. How to correctly interpret the contents of the parameter table, how to correctly convert a parameter to the corresponding actual performance, there are a lot of knowledge. A lot of parameters, in fact, it is not directly related to the actual performance of a car. For example, "the length of the wheelbase does not represent the size of the space," "the maximum power does not represent the actual dynamic feeling" is the most typical example, and " Such as the number of transmission gears, the fuel consumption of the Ministry of Industry for plug-in hybrid vehicles, etc. We are also often zealously introducing a variety of "alien and eggs" articles. In fact, we want to continue to emphasize that superstition parameters are better than parameters.
The "reef" of the parameter list is not limited to the above-mentioned several common examples. There are also some parameters that are similarly common and have strong obfuscation. It seems that the science has not been done in the recent past. (The popular science is a process that requires constant repetition.) Projects, whim, to explain to everyone how to correctly understand them.
Drag coefficient
At the just-concluded Frankfurt Motor Show, the IAA concept car released by Mercedes-Benz had a drag coefficient of 0.19, which is an astonishing figure. It should be noted that the lowest drag coefficient in the natural world is 0.05 for raindrops, and for common cars, the drag coefficient is around 0.3 (it can be as low as 0.26, as high as 0.39). The Mercedes-Benz IAA discussed how a car with a basic loading capacity (two rows of seats for four people) can be optimized for air resistance.
When we look at the model introduction, the aerodynamic effect of a car is usually reflected in a number, the drag coefficient. Then, the higher the wind resistance coefficient, the greater the wind resistance of the car? Actually not. Because the drag coefficient is really just a “coefficientâ€, you can think of it as the optimization of the air styling to reduce the air resistance, but it does not directly represent the actual air resistance that a car should withstand.
To calculate the actual air resistance, multiply the area of ​​the cross-section of the windward surface after Cd, and multiply it by (the wind speed squared and the air density is finally divided by 2). The number in parentheses can be said to be a variable, but when we compare the wind resistance of two cars, it is usually said that they are traveling at the same speed. Therefore, in the case of the same vehicle speed, the set of parameters in parentheses is equivalent to quantification, then the product that determines the air resistance is the product of the drag coefficient and the cross-sectional area. Right, there is a parameter called cross-sectional area, many people will ignore this. Some SUVs with good drag coefficients may have similar drag coefficients to cars. For example, the BMW X5 has reached 0.31, which is the same as Fox and even lower than that of Magotan and Corolla. But don't forget that the higher SUV of the car body, the windward area is generally 1.2~1.3 times that of the car. So multiply the Cd value with the windward area and get the result. The SUVs smiled less brilliantly.
In order to convert air into down pressure or to cool various equipment, the drag coefficient of Formula One is almost 0.4, and the drag coefficient of the sports car is also more than 0.3 minutes per minute. However, due to their low body and relatively small crosswind area, the total The air resistance is no bigger than a car.
Therefore, the figure of the drag coefficient, alone, does not indicate how large the wind resistance of a car is, but also counts the cross-sectional area of ​​the wind. Since the width of the body width of all cars is small, the change in cross-sectional area is more directly reflected in the height of the vehicle body. Therefore, the wind resistance of MPV and SUV is generally higher than that of cars and hatchbacks, even though they have a lot of Cd. Very low value.
High compression ratio
For conventional engines, the compression ratio is literally the ratio between the maximum volume and the minimum volume of the cylinder, that is, the degree to which the air is compressed. But since the Atkinson cycle engine has entered our line of sight, it has been accompanied by the concept of ultra-high compression ratio. Engines with Atkinson cycle properties, such as the Mazda Chuangchi Blue Sky 2.0/2.5, and Toyota's D4-S 2.0/Hybrid, have compression ratios of more than 12.0 and even 12.5 or more. How high is the demand for oil products? Wouldn’t it knock on the cylinder with such a compression ratio?
In fact, such a high compression ratio is not the actual compression ratio of the air inside the cylinder. For the sake of understanding, I have a name, mechanical compression ratio, which means that the compression ratio is an inherent property of the engine. It is indeed calculated using the algorithm of "the ratio of the maximum cylinder volume to the minimum volume", but it is actually difficult to achieve.
Everyone knows that the Atkinson cycle engine saves fuel. The principle of specific fuel economy is that after the intake stroke is over, the intake valve is delayed and closed so that when the piston goes upward, it will “spit out†some air before closing the intake valve. . After closing the intake valve, the cylinder actually begins the actual compression stroke. The power stroke will be longer than the actual compression stroke. It can be understood as the thrust generated by the detonation of the gas, and the piston can be pushed a little further down, thereby improving the efficiency of mechanical energy conversion.
The Atkinson cycle engine is widely used for press heat efficiency. Toyota's 40% thermal efficiency engine uses the Atkinson cycle and the Mazda Skyactiv technology engine. It also uses the Atkinson cycle to save fuel.
Therefore, this compression ratio is a static compression ratio. When the Atkinson cycle engine is actually working, since the actual air intake of the single cylinder is smaller than the maximum capacity of the single cylinder, the actual compression ratio is Mechanical compression ratio, the former's value is still maintained at about 11. This can explain why the high compression ratio Atkinson cycle engine can use low-grade fuel, because the mechanical compression ratio is less than the actual compression ratio, and you are “deceived†by the data. (Voice has been specially treated in the tone: so manufacturers use ultra-high compression ratio as a selling point for "high-tech", in fact, hehehehe.)
Turbine hysteresis
The consumption tax of 1.6L or less has taken a 50% discount, and it seems that the future of turbocharged engines is a triumph. There is no mention of turbo hysteresis in any of the turbine engine's parameter tables. However, many people will delay the turbine engine to a late point and misunderstand it as "turbo lag." So, let's take it with you.
Many manufacturers’ propaganda materials now say that the maximum engine torque is around 1500rpm, but our vehicle evaluation team also discussed this and agreed that this parameter is inaccurate. Our actual experience is that most of the torque should be at the nominal maximum torque. The minimum rotational speed is pushed back to about 1000 rpm, which is about 2300~3000 rpm, and the true maximum torque occurs. And this is called 1500rpm of the maximum torque, according to my personal guess is likely to be "the maximum boost value" rather than the maximum torque.
When the maximum boost value is accumulated into the maximum torque, the power of the engine is suddenly increased. It has been said before that this process requires about 1000 rpm, and many people will misunderstand that this process is called turbo lag, but it is not. Turbine hysteresis does not appear in steady-state conditions, that is, to limit an accelerator opening, such as full throttle acceleration, there is no way to feel the turbo lag. This is about the mechanism of turbo lag.
Turbocharged engines rely on exhaust gas to propel the turbine's exhaust blades. The exhaust blades drive the intake blades, causing the intake blades to “blow†air into the engine's cylinders, producing pressurized air above normal atmospheric pressure. If the throttle is released at this time, the exhaust gas supply will decrease, the turbine speed will slow down, and the air pressure inside the intake pipe will drop. Then, after a period of time and then pressing the gas pedal again, it is necessary to go through such a process: the turbine is driven by the exhaust gas, resuming a speed that can generate pressurized air, and then generating high-pressure air, where the high-pressure air fills the air intake pipeline ( Including intercooler, these pressurized air began to enter the cylinder combustion. There is a time lag from the depression of the throttle to the participation of pressurized air. Turbine from idle to normal operation, not a moment, but slightly slower than the throttle action, it is called turbo lag.
When we drive at ordinary times, for example, after releasing the throttle, we can step on the accelerator again and sometimes feel this kind of lag. But in many cases, the time required for the automatic gearbox to downshift will obscure the time for the turbo lag, thus making the turbo lag less noticeable. However, if we are driving a manual gearbox, or using a manual gearshift mode to fix it in a certain gear, releasing the throttle for a few seconds and then pressing it on, it will feel that the turbo is retarded.
In general, the larger the turbine body, the heavier the blades, the longer the intake pipe, and the larger the intercooler, the easier it is to produce turbo lag. So the main means to reduce the turbo lag now is the low-inertia turbine, water-cooled heat exchange intercooler and so on.
People, ah, learn more knowledge to prevent self-defense, not the same as not broken, to prevent being fooled by advertising, but also to understand the car emperor. With sister to 4S shop to buy a car, sales are blablabla introduce what selling point of technology, cold or not to be Tucao you can only return to the lounge to drink water to avoid the limelight, think about it is done beautifully.
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