An overdrive (OD) is a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise, and lower wear.[1]
Use of the term is confused, as it is applied to several different, but related, meanings.[1] The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is now over-geared and can no longer reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning more quickly. In some cases this implies a second definition is also true, that the gearbox output driveshaft is rotating faster than the original engine RPM. This later definition was common in the past, but many cars no longer have a driveshaft and it is not technically correct.[1]
In the era of front-engine, rear-wheel drive layouts, the device for achieving an overdrive transmission was usually a small separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever or electrical actuation button. These were often an optional extra on some models of the same car. As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the 1973 oil crisis, the use of 5-speed gearboxes became more common in mass-market cars, with the 5th gear being an overdrive, eliminating the need for a separate gearbox.[1]
With the popularity of front wheel drive cars, the separate gearbox and final drive have merged into a single transaxle. However the fundamental meaning, that of an overall ratio higher than the ratio for maximum speed, still applies.[1] Although the deliberate labelling of an overdrive is now rare, the underlying feature is now found across all ca[IMG]http://en.wikipedia.org/wiki/File:Fairey_overdrive.jpg[/IMG[/COLOR]
How an overdrive unit works
The overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit. It can either couple the input driveshaft directly to the output shaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclic gearsets which are integrated in the transmission. In these cases, there is no separately identifiable "overdrive" unit. In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load.
Overdrive
By definition, an overdrive has a faster output speed than input speed. It's a speed increase -- the opposite of a reduction. In this transmission, engaging the overdrive accomplishes two things at once. If you read How Torque Converters Work, you learned about lockup torque converters. In order to improve efficiency, some cars have a mechanism that locks up the torque converter so that the output of the engine goes straight to the transmission.
In this transmission, when overdrive is engaged, a shaft that is attached to the housing of the torque converter (which is bolted to the flywheel of the engine) is connected by clutch to the planet carrier. The small sun gear freewheels, and the larger sun gear is held by the overdrive band. Nothing is connected to the turbine; the only input comes from the converter housing. Let's go back to our chart again, this time with the planet carrier for input, the sun gear fixed and the ring gear for output.
Ratio = 1 / (1 + S/R) = 1 / ( 1 + 36/72) = 0.67:1
So the output spins once for every two-thirds of a rotation of the engine. If the engine is turning at 2000 rotations per minute (RPM), the output speed is 3000 RPM. This allows cars to drive at freeway speed while the engine speed stays nice and slow.
How Overdrive in an Automobile Works
Manually - Activated Control
Most automobiles, especially newer models, utilize overdrive transmissions that are push-button operated. These push-button mechanisms are normally located on or near the transmission gear shift mechanisms, which on most newer automobiles are located centrally between the two front car seats. Many older model cars equipped with overdrive transmissions have their overdrive activation mechanisms located directly on the transmission gear shift dial, normally as an extra gear option along with the normal drive, park, reverse and lower gear options.To select overdrive in these older model cars, the transmission gear shift lever is simply dropped into the "O" slot on the transmission gear shift dial.
Electronic Overdrive Activation
When the overdrive activation button is engaged on a car equipped with an overdrive transmission, an electrical signal is sent directly to an electrical switch located in the transmission that engages the overdrive transmission gear. When the overdrive gear is engaged, the engine revolutions per minute (RPM) are reduced, which translates into a slower-revving engine, which ultimately produces increased gas mileage. The overdrive gear is a separate transmission gear. Although the overdrive gear can be engaged at any engine speed, it is intended for use at highway speeds, speeds that typically burn more gas and would benefit from the reduced engine work rate provided by the transmission overdrive gear.
Overdrive as a Gas-Saving Mechanism
The intended purpose of an overdrive transmission gear is to save gas. On cars not equipped with an overdrive transmission, gas mileage is reduced due to the lack of an extra, lower-revving transmission gear. If a car has a 4-speed transmission with no overdrive, highway speeds will be driven in the fourth gear. On a car with a 4-speed transmission with overdrive, the car can be driven in gear number 4 and then switched into overdrive, which is essentially an extra gear, basically a 5th gear, a gear that allows the engine to turn slower and thus save extra gas.
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ومنها الشرح المبسط للااوفر دريف انة جهاز ملحق والله العظيم وليس ترس وبيتم التحكم فية بواسطة الزر المثبت بعصا الجير ووظيفتة الاولي والاخيرة تقليل لفات المحرك بغرض الحصول علي سرعة مناسبة بعدد لفات اقل للموتور تسمح باستهلاك اقل للوقود ومعدل تاكل اقل للموتور
وياسيدي النطة اللي بتتكلم عنها ساعة فصل الاوفر دي ناتجة من برمجة العلبة نفسها ان يتم تسقيط نقلة لنقلة بنسب تخفيض اعلي للحصول علي تسارع مناسب لدورات المجرك دون الوصول للحد الحرج (فوق ال7000/لفة ) وبالتالي البرمجة لكنترول العلبة اللي بتحكم نزول نقلة او نقلتين طبقا لنوع الحمل وسرعة المحرك وسرعة تحرك السيارة
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