Divide text into logical parts and give each a suitable title.



Мы поможем в написании ваших работ!


Мы поможем в написании ваших работ!



Мы поможем в написании ваших работ!


ЗНАЕТЕ ЛИ ВЫ?

Divide text into logical parts and give each a suitable title.



 

5. Fill in the gaps with the words from the box:

Octane calibrated useless closed system air-fuel correct sacrifice distributor coolant

 

  The spark, no matter how adequate, is in itself 1)_______unless it is produced at the appropriate time. The appropriate time depends upon many variables such as engine design, engine speed and load, 2)_________mixture, atmospheric pressure, mixture turbulence, temperature and the 3)________rating of the fuel.

The manufacturer determines the 4)__________timing by running the engine on a dynamometer. The engine is run at both full load and road load through the speed range, and at fixed points in the range the 5)___________is moved by hand to obtain the timing point for maximum output. The production distributor is then 6)_________to give this timing. Exhaust emission controls require an even finer tailoring of spark timing, in some cases at the cost of a small 7)__________in power and economy. General Motors, for example, uses a control device outside the distributor with its Transmission Control Spark 8)___________ This switches off the vacuum advance at idle and during first and second gear operation, except when the 9)_________temperature is higher than 220 degrees. Other makers use a double diaphragm vacuum advance unit on the distributor. This device retards the timing at idle and during coast-down with a 10)___________ throttle, but retains normal vacuum advance when cruising with the throttle open.

 

Steering

Steering is the term applied to the collection of components and linkages which allow for a car to follow the desired course. The most conventional steering arrangement is to turn the front wheels using a hand-operated steering wheel which is positioned in front of the driver, via the steering column, which may contain universal joints to allow it to deviate somewhat from a straight line. You might be surprised to learn that when you turn your car, your front wheels are not pointing in the same direction. For a car to turn smoothly, each wheel must follow a different circle. Since the inside wheel is following a circle with a smaller radius, it is actually making a tighter turn than the outside wheel. If you draw a line perpendicular to each wheel, the lines will intersect at the center point of the turn. The geometry of the steering linkage makes the inside wheel turn more than the outside wheel. There are two most common types of steering gears: rack-and-pinion and recirculating ball-and-nut which is practically the same as worm-and-sector.

Many modern cars use rack and pinion steering mechanisms the major components of which constitute tie rod, steering arm and kingpin. When the steering wheel turns the pinion gear, the pinion moves the rack, which is a sort of linear gear which meshes with the pinion, from side to side. This motion applies steering torque to the kingpins of the steered wheels via tie rods and a short lever arm called the steering ann.

Older designs often use the recirculating ball mechanism, which is still found on trucks and utility vehicles. This is a variation of the worm-and- sector design, where the steering column turns a large screw (the ‘worm gear’) which meshes with a sector of a gear, causing it to rotate about its axis as the worm gear is turned; an arm attached to the axis of the sector moves the pitman arm, which is connected to the steering linkage and thus steers the wheels. The recirculating ball version of this apparatus reduces the considerable friction by placing large ball bearings between the teeth of the worm and those of the screw; at either end of the apparatus the balls exit from between the two pieces into a channel internal to the box which connects them with the other end of the apparatus, thus they are ‘recirculated’.

The rack and pinion design has the advantages of a large degree of feedback and direct steering ‘feel’, it also does not normally have any backlash, or slack. A disadvantage is that it is not adjustable, so that when it does wear and develop lash, the only cure is replacement.

The recirculating ball mechanism has the advantage of a much greater mechanical perfection, so that it was found on larger, heavier vehicles while the rack and pinion was originally limited to smaller and lighter ones. Due to the introduction of power steering, however, this is no longer an important advantage, leading to the increasing use of rack and pinion on newer cars. The recirculating ball design also has a perceptible lash, or ‘dead-spot’ on center, where a minute turn of the steering wheel in either direction does not move the steering apparatus; this is easily adjustable via a screw on the end of the steering box to account for wear, but it cannot be entirely eliminated or the mechanism begins to wear very rapidly.

This design is still in use in trucks and other large vehicles, where rapidity of steering and direct feel are less important than robustness, maintainability, and mechanical advantage. The much smaller degree of feedback with this design can also sometimes be an advantage; drivers of vehicles with rack and pinion steering can have their thumbs broken when a front wheel hits a bump, causing the steering wheel to kick to one side suddenly. These factors motivate the driving instructors telling students to keep their thumbs on the front of the steering wheel, rather than wrap around the inside of the rim. This effect is even stronger with a heavy vehicle like a truck; recirculating ball steering prevents this degree of feedback, just as it prevents desirable feedback under normal circumstances.

Various developments in steering systems like power steering, speed adjustable steering, four-wheel steering have appeared which are driven mostly by the need to increase the stability, safety and also the fuel efficiency of cars.

 



Последнее изменение этой страницы: 2021-04-04; Нарушение авторского права страницы; Мы поможем в написании вашей работы!

infopedia.su Все материалы представленные на сайте исключительно с целью ознакомления читателями и не преследуют коммерческих целей или нарушение авторских прав. Обратная связь - 3.239.58.199 (0.013 с.)