Drivability, a word that is often mentioned but always misunderstood.
When cars enter the homes of ordinary people, when the car market enters the era of exchange, the driving experience begins to be paid more and more attention. "This car is easy to drive, the mechanical quality is solid, the drivability is very good..." Car enthusiasts and even the car media sometimes give such comments. As everyone knows, driveability is not equal to driving-related performance, but a relatively narrow concept. Drivability refers specifically to straight-line driving performance, that is, the performance of the powertrain.
The term "drivability" is an imported product. For a long time, "driveability" was habitually translated as "driving". But in fact, this is a beautiful misunderstanding. Doesn't "Drive" mean "drive"? Yes, but it can also mean "drive". Some people joked that because the English teacher had not studied vehicle engineering, the problem that could be summarized by a coordinate axis had become a three-dimensional coordinate system. Back to the point, remember what drivability actually means: drive.
Previously, the "Yan Must Have Something" column interpreted the handling and comfort, and readers can click on the link below to view the original text. Coupled with the drivability of this article' interpretation, we have put together the main elements related to dynamic performance. The relationship between the three is roughly as follows: longitudinal (X-axis) corresponds to drivability, transverse (Y-axis) corresponds to handling, and vertical (Z-axis) corresponds to comfort.
Drivability: The essence is "driving"
Although the translation of drivability is slightly off, it hits the point by mistake. If a car wants to drive smoothly, power matching is the basic factor. Only when the power calibration is perfect, people will feel that a car is easy to drive (driveable), which is commonly known as "easy to drive". On the basis of driving passing, excellent comfort can bring a sense of quality, and excellent handling can bring excitement. Conversely, if the drivability is bad, comfort and handling will not help. To paraphrase Maslow's hierarchy of needs, drivability is the most basic layer of dynamic performance.
To add here a piece of knowledge, the so-called tip-in/tip-out, refers to the rapid (urgent) pressing the throttle/closing the throttle. The question is, how is "fast" defined? There are specific standards in the field of engineering. Typically, engineers measure the speed of the accelerator pedal input by the rate of change (stroke change divided by time), which can be divided into three scenarios:
Change the throttle input in a very short time, the throttle change rate is more than 100%, that is, tip-in/tip-out. For example, if you press down 50% of the throttle stroke in 0.4 seconds, the throttle change rate is 125%.
Change the throttle input in a short period of time, and the throttle change rate is between 40% and 100%. For example, if you press down on 25% of the throttle stroke in 0.5 seconds, the throttle change rate is 50%, which is a more common situation in daily driving.
Change the throttle input over a longer period of time with a throttle change rate of less than 40%. For example, if you press down 25% of the throttle stroke in 1.0 seconds, the throttle change rate is 25%. In general, full-time drivers tend to use this delicate footwork to keep it as smooth as possible.
If you list and interpret the drivability of the second-level project and the third-level project, I am afraid that you will have to write tens of thousands of words. For ordinary consumers, it is of little significance to delve into the details of driving, so it is better to start from the scenes we often encounter around us to see what excellent drivability looks like and what factors affect drivability.
Frustrated: All the sins are in the gearbox?
On public roads, camouflage test vehicles appear from time to time, and a considerable part of them are used for drivability research and development. Artificial intelligence technology is so developed, can't computers replace heavy road test work? Indeed it is. In real traffic environments, the conditions/scenarios associated with the powertrain are difficult to count. Only by testing with real knives and guns can the "raw edges" be rounded and the excellent drivability can be created.
Talking about theory seems to be a bit empty, so it is better to give a few examples to illustrate the importance of "hair matching". On many BMW models equipped with a vertical 8AT, low-speed creep occasionally suffers from setbacks. The specific performance is that after starting with a small throttle and maintaining the speed at 15-20km/h for a period of time, the throttle is suddenly released (tip-out), and the car will have several high-frequency oscillations (jerks) in a row. This oscillation is fast and gradually converges, similar to the sudden oil collection of low speed in the manual model 1st gear. Some people say that the gearbox should be back for this. Is this really the case?
Under these conditions, tip-out setbacks are caused by the engine, the gearbox, and even the ancillary components of the powertrain (such as the suspension system) are also liable. On the one hand, the engine pursues rapid response, and the positive and negative torque changes rapidly. On the other hand, the torque converter locks up earlier, and the rigid transmission means that the setback is relatively direct, and the polishing of the oil is missing. To take care of NVH performance, the suspension sacrificed some support and limit capability to amplify the oscillation of the powertrain.
In similar tip-out conditions, Audi models equipped with ZF 8AT (e.g. Q7, S4, RS 4) tend to be very submissive, with gentle torque changes and rarely high-frequency oscillations. It's clear that Audi engineers focused on low-speed compliance. Interestingly, tip-in ride and responsiveness are the only ones that BMW wins. This set of cases shows that the key to how drivable a car is depends on what style each brand wants to create. Many times, drivability is the result of the reverse deduction based on the target customer's expectations.
Whether the power system is smooth or not, the impact on the quality of driving is self-evident. Behind the outstanding smoothness, it must be the joint efforts of the entire power system, not the transmission alone. Conversely, if there is a setback in the powertrain, the problem may not only lie in the gearbox, but also in any part of the engine and transmission chain, or even the result of multiple factors.
Response: Damn emissions regulations
If drivability calibration were only for smoothness, more than half of the powertrain engineers would probably lose their jobs. If the car wants to be easy to drive, there should be no obvious shortcomings in its drivability, and ride and responsiveness are equally important. Power response refers to the vehicle's ability to execute on the driver's intent (throttle input). The faster the response, the more "heeled" the car became. Today, most hybrid models and pure electric models can take care of both ends, but the performance of fuel vehicles is uneven.
What people say about dynamic response on a daily basis involves not only the acceleration timeliness mentioned above, but also the throttle characteristics. The so-called pedal map characteristic refers to the relationship between the acceleration pedal stroke and the power output. Every 1% of the pedal is pressed, and how strong the engine is, directly affecting the driver's perception of power. In addition, the gearbox is used as a deceleration and torque increase device to adjust the engine conditions. Depending on the throttle input (stroke/rate of change), brake input (stroke/rate of change), engine load level, etc., the gearbox automatically selects the appropriate gear to provide the acceleration force required by the driver.
Power calibration takes into account not only ride and responsiveness, but also fuel economy and emission limits. In the context of increasingly stringent emission regulations, the drivability of fuel vehicles has encountered huge challenges. At present, China and Europe are pushing for electrification technology, and many supportive policies have been introduced for this purpose. Fake emissions and fuel consumption hands, pushing fuel vehicles to a dead end, is part of the new energy strategy.
Editor's Note: It is often said that chassis tuning (comfort or sport) defines the character of a car, there is no absolute good or bad. In contrast, the power system plays a more basic and important role. The car can be inflexible to handle, and the filter can be slightly rough, but the power match should not drag its legs. Only by doing a good job of driving can the car drive smoothly and become an extension of the driver's body. Unfortunately, emissions regulations are eating away at drivability, and the glorious era of fuel vehicles is over. Don't worry, the electrification trend provides positive guidance and greatly improves drivability. In the next issue of "Yan Must Have Something", let's talk about various types of hybrid architectures and the research and development concepts behind them.