长安大学交通安全工程汽车新技术课件

Lecture 1IntroductionLecture 22.1 ABSWithout 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.4WDWhen 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 lossIn 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 cond itions the tyre’s ability to maintain full grip deteriorates and therefore the power loss increases at a very fast rate.●Maximum speedIf 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。