长安大学交通安全工程汽车新技术课件

Lecture 1

Introduction

Lecture 2

2.1 ABS

Without ABS:

The vehicle skids, the wheels lock and driving stability is lost so the vehicle cannot besteered; If a trailer or caravan is being towed it may jack-knife;

The braking distance increases due to skidding;

The tyres may burst due to the excessive friction and forces being concentrated at the points where the locked wheels are in contact with the road surface.

With conventional brake systems one of the road wheels will always tend to lock sooner than the other, due to the continuously varying tyre to road grip conditions for all the road wheels. To prevent individual wheels locking when braking, the pedal should not be steadily applied but it should take the form of a series of impulses caused by rapidly depressing and releasing the pedal. This technique of pumping and releasing the brake pedal on slippery roads is not acquired by every driver, and in any case is subjected to human error in anticipating the pattern of brake pedal application to suit the road conditions. An antilock brake system does not rely on the skill of the driver to control wheel lock, instead it senses individual wheel slippage and automatically superimposes a brake pipe line pressure rise and fall which counteracts any wheel skid tendency and at the same time provides the necessary line pressure to retard the vehicle effectively.

When the wheels stop rotating with the vehicle continuing to move forward the slip is 100%, that is, the wheel has locked.

To attain optimum brake retardation of the vehicle, a small amount of tyre to ground slip is necessary to provide the greatest tyre tread to road surface interaction. For peak longitudinal braking depends upon a maximum sideways tyre to ground resistance which is achieved only with the minimum of slip. Thus there is conflict between an increasing braking force and a decreasing sideways resistance as the percentage of wheel slip rises initially. As a compromise, most anti-skid systems are designed to operate within an 8-3=% wheel slip range. 4WD

When employing two-wheel drive, the power thrust at the wheels will be shared between two wheels only and so may exceed the limiting traction for the tyre and condition of the road surface. With four wheel drive, the engine’s power will be divided by four so that each wheel will only have to cope with a quarter of the power available, so that each individual wheel will be far below the point of transmitting its limiting traction force before breakaway (skid) is likely to occur.

During cornering, body roll will cause a certain amount of weight transfer from the inner wheels to the outer ones. Instead of most of the tractive effort being concentrated on just one driving wheel, both front and rear outer wheels will share the vertical load and driving thrust in proportion to the weight distribution between front and rear axles. Thus a four wheel drive when compared to a 2 wheel drive vehicle has a much greater margin of safety before tyre to ground traction is lost.

● Power loss

In general, overall transmission losses with four wheel drive will depend upon the transmission configuration and may range from 13% to 15%.

Tyre losses become greater with increasing tractive force caused partially by tyre to surface slippage. This means that if the total propulsion power is shared out with more driving wheels less tractive force will be generated per wheel and therefore less overall power will be consumed. The tractive force per wheel generated for a four wheel drive compared to a two wheel drive vehicle will only be half as great for each wheel, so that the overall tyre to road slippage will be far less. It has been found that the power consumed is least for the front wheel drive and greatest for the rear wheel drive, while the four wheel drive loss is somewhere in between the other 2extremes. The general relationship between the limiting tractive power delivered per wheel with either propulsion or

retardation and the power loss at the wheels is shown to be a rapidly increasing loss as the power delivered to each wheel approaches the limiting adhesion condition of the road surface. Thus with a dry road the power loss is

relatively small with increasing tractive power because the tyre grip on the road is nowhere near its limiting value. With semiwet or wet road surface conditions the tyre’s ability to maintain full grip deteriorates and therefore the power loss increases at a very fast rate.

● Maximum speed

If friction between the tyre and road sets the limit to the maximum stable speed of a car on a bend, then the increasing centrifugal force will raise the cornering force and reduce the effective tractive effort which can be applied with rising speed. The maximum stable speed a vehicle is capable of on a curved track is hightes with four wheel drive followed in order by the front wheel drive and rear wheel drive.

● 4wd 的缺点：传动效率低，消耗更多燃料。

⏹ 驱动力的传动效率低：发动机的动力消耗在了驱动系统，使汽车不能在充分发挥发动机的动力的条件下行驶。特别对动力不是很充裕的轻型汽车表现更加明显。

● 4wd车制动现象：在转向时，由于前后轮转弯半径不同，前后轮出现转速差，其中有车轮会出现滑动现象，需要装中部差速器（原理和轮间差速器同）

● 越野性能强：即使在2WD驱动轮打滑空转不能行驶的条件下，4WD车的轮胎也可紧贴地面向前行驶。 4WS

Lecture 3

TCS

Traction control systems (TCS) are designed to prevent the drive wheels from wheel spinning during starting off or accelerating on a wet or icy surface. Vehicles with powerful engines are particularly susceptible to this

phenomenon and results in reduction of either steering response on front-wheel-drive (FWD) or vehicle stability on rear-wheel-drive (RWD) vehicles. TCS operates to maximize adhesion to the road surface during acceleration whilst ABS has the same objective during braking. The two systems share common sensor information and may well be incorporated together in an ECU. The actuation part of the TCS varies for different systems and may be a permutation of fuel, ignition and driven wheel braking action to achieve reduction in driven wheel torque during wheel spin.

It is interesting to investigate TCS control strategies to achieve the apparently simple objective described above. Basically, the system should attempt to maintain the acceleration slip of the driven wheels at a value equal to the mean rotational velocity of the non-driven wheels plus a specified speed difference known as the slip threshold point. This means simply that if a vehicle’s driven wheels are constantly at a faster speed than the non-driven wheels then the vehicle must be accelerating at a constant rate proportional to the difference in the two speeds. This control objective must be qualified with a reference to road surface conditions or adhesion coefficient. For instance on dry road surfaces, maximum accelerative force is available at slip rates of 10 to 30%. On glare ice, maximum traction is achieved at levels between 2 and 5 percent so to cover these extremes TCS systems must respond to changes in adhesion coefficient and systems are designed for a slip rate range of between 2 and 20%. However, on loose sand or gravel and in deep snow the coefficient of adhesion increases continually with the slip rate. For this reason, TCS systems incorporate slip-threshold switches to allow the driver to select a higher slip threshold or switch off the TCS.

ESP

Background

The Electronic Stability Control, ESC or ESP, is an on-board car safety system, which enables the stability of a car to be maintained during critical manoeuvring and to correct potential under steering or over steering (1). In a general sense the equipment should eliminate loss of control. Since 1998, when the first mass-produced car with ESC standard equipment was launched, the market for cars with ESC has grown quickly. In Sweden, the

proportion of new car sales equipped with ESC has grown from 15% in March 2003, to 69% in Dec 2004. ESC operates normally with both brakes and engine management. If the car loses control, defined as when one wheel or more is moving faster or more slowly than calculated from the steering input and turning angle, braking is applied to one or more of the wheels, and the engine power might be reduced. [

ESP will go down in history as the active safety system to transform the fastest from an optional feature at extra cost to an undeniable element of standard fitment. Thanks to its interactive design, ESP stabilizes the vehicle in all driving situations. No matter whether the car's driving off from a standstill, driving straight, or braking, ESP undeniably helps the driver evade a potential accident. Thanks to ESP, the driver retains control of the vehicle even in critical situations. [“Since 1991 the number of injuries or fatalities in car accidents in Germany has remained at an almost constant level between 300,000 and 350,000. The number of occupant fatalities as a result of an accident has steadily been reduced from 7,000 in 1991 to 4,700 in 1998. Based on a study of approx. 17,000 car accidents, Langwieder showed that 20% - 25% of all car accidents with injuries or fatalities were the result of In approximately 60% of the accidents with spinning cars only a single car was involved. While inexperienced

drivers tend to correct the spinning motion with a single steering wheel correction, experienced drivers perform a sequence of corrections to gain control of their car. Often the vehicle motion reaches the physical limit of adhesion between the tires and the road because of the panic reactions of the driver in dangerous traffic situations. ” [cited from Bosch ESP systems 5 years of experience @ SAE 2000-01-1633]

It is rare for drivers with average driving experience to know when they are driving a car at the physical limit, i.e. at the limit of adhesion between the tires and the road. At this limit the tire behavior is extremely nonlinear and the linearized tire-wheel-brake system is unstable. As a result, the vehicle may suddenly spin and the driver is caught by surprise. Usually in these situations the driver tends to automatically steer too much and thus worsen the

situation. In both cases the vehicle dynamics control system ESP helps the driver keep his car under control. Since the average driver has no idea of the frictional stability margin between the tire and the road he may panic if the physical limit is reached and if the car starts to spin. He cannot be expected to react in a thoughtful manner. On the contrary, his reaction is often wrong and he will usually steer too much. ESP must therefore also be designed from the point of view of preventing panic situations. [cited from Bosch ESP systems 5 years of experience @ SAE 2000-01-1633]

The progress of crash energy absorbing car body design and the standard fitting of airbags significantly improved the passive safety especially combined with the use of seat belts. But many of the serious accidents happen through loss of control in critical driving situations. When the vehicle goes into a skid, a side accident is the frequent result. With a reduced protection zone for the occupants compared to front crashes, these accidents show an amplified severity.

Especially with vehicles of an elevated center of gravity like sport utility vehicles (SUV) and light trucks (LT) the loss of control with subsequent skidding may even lead to a rollover. Most of the rollovers are caused either by tripping at an obstacle or in the soil. The severity of rollover accidents is extremely high. Accounting for only 2% of the total crashes, they contributed in 2002 with 10.656 fatalities to one third of all occupant fatalities (Fig. 3) in the US.

In critical driving situations most drivers are overburdened with the stabilizing task. According to Foerster [4] the average driver can neither judge the friction coefficient of the road nor the grip reserves of the tires. The drivers are typically startled by the altered vehicle behavior in in-stable driving situations; as a result, a well-considered and thought-out reaction of the driver cannot be expected. For that reason the ESP has to be designed to stabilize the vehicle even in situations with panic reactions and driving failures like exaggerated steering.

ESP utilizes known components [The electronic stability program contains the subfunctions of anti-lock braking (ABS) and traction control (TCS) as well as electronic brake-force distribution (EBD), engine-drag torque control (EDC发动机扭矩控制) and active yaw-moment compensation (AYC). The first four systems concentrate solely on rotational wheel slip and vehicle dynamics in longitudinal direction. In this way, ABS reduces wheel lock during braking, TCS reduces the engine output and, in combination with EDC, brakes the wheels as they start to spin when the car drives off and accelerates. Interventions in the braking system like this are based on the physical principle that locked or spinning wheels are incapable of transmitting any cornering forces. And yet it's those forces that are needed to keep the vehicle steerable and controllable.

EBD and EDC serve primarily to control longitudinal slip. By means of the ABS sensors EBD determines the slip between wheel speeds at the front axle and at the rear axle, and can then make differentiated adjustments to the braking pressures. Once the degree of slip exceeds a certain amount, EBD keeps braking pressure at the rear axle constant or reduces it accordingly. EBD uses the computed values to provide maximum braking efficiency, even without ABS control. Engine-drag torque control is engaged during ABS braking. Once braking pressure is reduced, the speed of the drive wheels is not increased fast enough because the engine has a braking effect and creates a long-lasting wheel slip. This leads to reduced steerability in front-wheel-drive vehicles, and less stability in rear-wheel-drive vehicles. Under EDC, the engine management gets a command from ABS to quickly increase engine speed. The drive wheels are then released from disturbing drag torques and are able to transmit higher cornering forces.

Stabilization of a vehicle's transverse dynamics is only one of the areas intensively monitored by ESP and its affiliated subfunctions. ESP also senses and analyzes the rotary motion of a vehicle around its vertical axis. The driver is assisted in all his driving maneuvers, particularly in the avoidance of obstacles. Stabilization is achieved by decelerating one or more wheels before a dangerous swerving motion arises. Rotary motion of a vehicle around its vertical axis is known among engineers as 'yawing'.

Active yaw control (AYC) is made possible by means of a complicated sensor-device infrastructure. At the heart of the system is a yaw rate sensor found so far only in aviation and aerospace engineering. In the ESP from Continental Teves, the sensor element is shaped like two tuning forks joined together at their base. Emerging vibrations are converted piezo-electronically. During vehicle yaw motion, the tuning forks are deflected

perpendicularly to the plane of vibration. The strength of the yaw rate corresponds with the deflection. In addition, the ESP's system electronics determines the car's current driving condition via two central processing units, and a sensor measures the lateral acceleration within a spectrum of +1.7 to –1.7 g (1 g = gravitational acceleration of

9.81 m/s2).

Activity Suspension

Background

Function of suspension

The potential benefits of controllable suspensions are not confined to improving just the

individual performance at each wheelstation, but offers also the possibility of controlling ride

height, roll, dive and squat, giving generally improving vehicle safety. At the present time only

a limited number of proposals have been implemented on production cars and these have tended

to be associated with luxury vehicles where the increased cost can be more easily absorbed.

Body roll control

When the car is negotiating a corner the body tends to tilt so that the inner and outer wheel loads are reduced and increased, respectively. This lateral load transfer compresses the outer springs and expands the inner springs thus causing the body to roll and to become uncomfortable for the driver and passengers. To compensate for the weight transfer, fluid is pumped or released into the outer strut actuators via the leveling control valve until it has lifted the body on the outside to the same height as the inside. Usually a small angular roll is deliberately allowed to provide the driver with a sense of caution.

Anti-dive control

If the car is braked rapidly there is a tendency for the body to pitch forwards, that is, the front of the body temperately dives downwards and the rear lifts; the dive experienced is due to the longitudinal weight transfer since the body mass wants to continue moving forwards but the road wheels and the unsprung suspension mass are being retarded by the action of the braking force. To overcome this inherent deficiency in the suspension design which occurs usually when soft springs are used, fluid is rapidly transferred into both of the front strut actuator cylinders, thereby correcting the front to rear tilt of the body over the braking period and then releasing the excess fluid from the front actuators when normal driving resumes.

Anti-squat control

If a car is accelerated rapidly, particularly when pulling away from a standstill, there is a proneness fro the body due to its inertia to hold back whereas the propelled wheels an dunsprung part of suspension tend to move ahead of the interlinked body. This results in the body tilting backwards so that it squats heavily on the rear axles and wheels. To correct this ungainly stance when the car is being accelerated, fluid is quickly displaced from the

accumulator and pump through the open leveling control valve into the rear strut actuator cylinders; this levels the body longitudinally. Once the acceleration sensor detects a reduction in acceleration, the electronic-control unit signals the leveling control valve to return the excess fluid trapped in the rear actuator cylinders back to the reservoir so that under steady driving conditions the body remains parallel to the road.

Lecture 4 engines

Variable Valve Timing

Variable valve timing opens the possibility to control the process of filling the cylinder solely by means of restricting the opening of the intake valve, i.e. without using the conventional throttle valve. It also offers the possibility of extending the expansion phase by opening the exhaust valve later. The best results are obtainable by providing free choice of closing the intake valve over the whole speed and load range of the engine. Only this system will be discussed in the following pages. Its advantages are:

1. Improved thermodynamics efficiency at part-load because there is no longer throttling loss. Throttling losses can be avoided if it becomes possible to meter the cylinder charge without throttling, solely by the opening duration of the intake valve. To this end, at part-load the intake valve must be closed earlier in order to limit the cylinder charge.

2. Increased effective efficiency at full load as a result of increased full load torque at low speeds. In this connection we should recall the well known problem of determining the best valve timing for a passenger car engine. At low speed, closing the intake valve after bottom dead center causes some back flow of fresh mixture into the intake manifold; here, the intake valve should close earlier. On the other hand, at high speed the intake valve should remain open as long as possible so that the inertia effect of the air column can be utilized for a

possible supercharging effect. It becomes obvious from the above that if control of the intake closing point can be made dependent on engine speed, beneficial effects with respect to the trapped cylinder charge can be realized, thereby improving the full load torque of the engine.

3. Shifting engine operating conditions to points of better effective efficiency. This can be done by adapting the gear ratios to any improvement in the engine torque curve. In this way the numerical transmission ratio can be made lower while retaining the same acceleration characteristics. This will make it possible to operate the engine more frequently at lower speed and therefore, at higher efficiency.

Lecture 5 engines

Disadvantage and advantage of diesel engine

The diesel engine is the internal combustion engine with the highest thermal efficiency developed so far. For

driving conditions typical for a passenger car, i.e., much city driving, the fuel consumption is reduced by one-third compared to a gasoline engine. From the point of view of conserving oil reserves and reducing carbon dioxide in the atmosphere, the diesel engine is a very desirable power plant for an automobile. In addition to the fact that the gaseous emissions of the diesel engine are as low as the emissions of a gasoline powered car with a three way catalyst, it also should be appreciated that these low emission quantities already are attained in the combustion

process, therefore, no aging of any after-treatment device has to be taken into account. Another advantage of the diesel combustion principle might be its tolerance for very different fuel qualities. For instance, after some modification, bio-alcohols or vegetable oils can be used as fuel.

Unfortunately, connected with these very positive properties are also disadvantages. The output of a diesel engine for a given piston displacement is less than that for a gasoline engine; diesel engines are louder at cold start and idle, and their manufacturing costs are higher. Last but not least, its particulate emission will play a determining role in the future; give the expected tightening in exhaust gas legislation.

Electronic diesel engine

The task of fuel metering and mixture formation has always been much more difficult for diesel engines than for gasoline engines. The reason is that injection pressures of several hundred bars and, at the same time, injection quantities of only a few milligrams must be reliably controlled. This is also the reason why injection equipment for diesel engines has always been more complex and costlier than that of gasoline engines. Additional requirements regarding the precision of diesel injection equipment must be expected in the future as a result of stiffened exhaust gas legislation. Examples are: engines must be equipped with exhaust gas recirculation systems for even greater reduction of NOx emissions; injection timing must be synchronized even more precisely over the whole

operational range in order to reduce soot and particulate emissions; and the maximum quantities of injected fuel must be adjusted even better to the instantaneous operating conditions. As a result, the mechanical governors used so far have reached their limit and the step to electronic governors and electronic systems for control must be made in the field of diesel engines.

Direct injection

So far, diesel engines for passenger cars are still using divided combustion chambers. In this concept the fuel is injected into a prechamber or a swirl chamber and prevaporized, pre-mixed and ignited in a part of the combustion air. The burning mixture then streams into the main combustion chamber and completes combustion there. The mixture preparation in any prchamber slows down the otherwise rapid rise in cylinder pressure. This slow down results in quieter running. On the other hand, the heat and the flow losses of the gas on its way from the

prechamber to the main chamber causes a loss in efficiency. From a thermodynamic point of view, it would be more favorable to inject the fuel directly into the cylinder chamber. Fuel consumption improvements of up to 20% can be expected since heat and flow losses are omitted.

The principle of the combustion process for direct injection consists of two phases: in the first phase the injected fuel undergoes an ignition delay period followed by spontaneous ignition and burning. The second phase is the combustion phase, characterized by diffusion flames. During the period of ignition delay the injected fuel quantity should be a small as possible. In the diffusion phase, intensive mixture formation and rapid burning is desired. Therefore, it is important to create a high amount of turbulence in the combustion chamber air and to develop an injection system with high injection pressure and the smallest possible nozzle holes.

Supercharging

Supercharging will gain in importance in the future for several reasons. One of the reasons is that more emphasis on aerodynamically designed vehicles leaves less room for the engine, or expressed in a different way, future engines must be designed for a specific output that is as high as possible. Another reason is, as a result of

continuously increasing traffic density, a continually increasing requirement for still higher acceleration capability of passenger cars must be assumed.

Lecture 6

Definitions

Transmission – This term can be used to describe one unit within the driveline of a vehicle, often the main gearbox, or as a general term for a number of units.

Driveline – This includes all of the assembly(s) between the output of the engine and the road wheel hubs.

Automatic transmission – Automatic transmissions come in various forms but have the common ability to change the ratio at which they are operating with no intervention from the driver.

Manual transmission – As the name suggests, drivers have to change the gear ratio setting rather than the transmission doing the job for them.

Continuously Variable Transmission (CVT) – CVTs are able to vary the ratio between input and output in a stepless manner rather than having a number of discrete ratios.

Infinitely Variable Transmission (IVT) – Essentially a CVT which has the additional ability to operate with zero output speed, hence negating the need for a separate starting device. This chapter is going to look at the

transmission systems used in cars. The rest of the driveline will not be considered in any detail so there will be no detail on such things as axles or 4 4 transfer gearboxes.

Flexibility in driving, reduced fuel consumption and reduced noise call for a high number of gear steps and if we carry this thought further we arrive at the concept of a continuously variable transmission. Because of its wide spread between bottom-to-top ratio, such a transmission would be ideally suited for passenger cars. In this

connection we will have to examine the prospects of the adjustable tapered pulley transmissions where power is transmitted through a chain or a compressive belt. Such transmissions have recently appeared on the market.

Lecture 8 – lecture 12 新能源汽车