Overdrive is the term used to describe the operation of a cruising vehicle at a continuous speed with reduced engine speed per minute (RPM), which leads to better fuel consumption, lower noise, and wear lower. The use of the term is confusing, as it is applied to several different but related meanings.
The most basic meaning is the overall gear ratio between the engine and the wheel, so the car is over-geared, and can not reach its maximum speed, ie the car can travel faster if it is in the lower gear, with the engine spinning at a higher RPM.
The purpose of such equipment may not be immediately obvious. The power generated by the engine increases with the engine's RPM to its maximum, then falls. The maximum power point is somewhat lower than the machine's absolute maximum RPM, RPM "redline". The speed of the car is limited by the power required to drive it against the air resistance, which increases with speed. At the maximum possible speed, the engine runs at its maximum power point, or peak power , and the car runs at a speed where air resistance is equivalent to maximum power. Therefore there is a special dental ratio in which the car can reach its maximum speed: that matches the engine speed with that traveling speed. At speeds traveling below this maximum, there is a range of gear ratios that can match the engine power with air resistance, and the most efficient fuel is the one that produces the lowest engine speed. Therefore, the car requires one gear to reach the maximum speed but the other to achieve maximum fuel efficiency at a lower speed.
With the early development of the car and the almost universal rear-wheel layout, the final drive ratio (ie the rear axle) for fast cars is chosen to provide the ratio for maximum speed. The gearbox is designed in such a way that, for efficiency, the fastest ratio would be a 1: 1 direct-drive or "straight-through" ratio, avoiding friction losses in gears. Achieve overdrive ratio for roaming so that gearbox ratios are required even higher than this, ie the rotational output shaft of the gearbox rotates faster than the machine. The propeller shaft connecting the gearbox and the rear axle is thus overdrive, and a transmission capable of doing this is called an "overdrive" transmission.
The device to achieve overdrive transmission is usually a small separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever. This is often optional on some models of the same car.
Because popular cars are faster than legal limits and fuel costs are becoming more important, especially after the 1973 oil crisis, the use of the 5-speed gearbox is becoming more common in mass market cars. It has 4 straight gear (1: 1) with 5 gear teeth, replacing the need for a separate overdrive gearbox.
With the popularity of front-wheel drive cars, separate gearboxes and final drives have merged into a single transaxle. No more propeller shafts and so one meaning of "gear" can no longer be applied. But the basic meaning, that the overall ratio is higher than the ratio for maximum speed, is still valid. Although the deliberate labeling of overdrive is now scarce, its underlying features are now found in all cars.
Video Overdrive (mechanics)
Description
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The power required to drive the car at any set of conditions and speed is very easy to calculate, based mainly on the total weight and speed of the vehicle. This produces two major forces to slow down the car: rolling resistance and air resistance. The former varies roughly with the speed of the vehicle, while the latter varies with the square of the speed. Calculating this from the first principle is generally difficult due to various real-world factors, so this is often measured directly in wind tunnels and similar systems.
The power generated by the engine increases with the engine's RPM to its maximum, then falls. This is known as the maximum power point. Given a curve that illustrates the overall resistance of the vehicle, it is easy to find the speed at which the total drag force is equal to the machine's maximum strength. This determines the maximum speed a vehicle can achieve. The speed of wheel spin for the given forward speed is easy to calculate, it is just the tire circumference multiplied by RPM. Since RPM tires at maximum speed are not the same as the engine's RPM on that power, the transmission is used with the gear ratio to change one to the other.
At speeds slightly lower than the maximum, the total obstacles on the vehicle are much less, and the engine must transmit this much reduced amount of power. In this case the RPM of the machine has changed significantly while the RPM wheels have changed very little. Obviously these conditions require different gear ratios. If one is not provided, the machine is forced to run at a higher RPM than optimal. Since the engine needs more power to cope with internal friction at higher RPMs, this means more fuel is used just to keep the engine running at this speed. Each round of the engine leads to wear, so keeping the engine at a higher RPM is also not beneficial for engine life. In addition, engine sound is strongly related to RPM, so running on lower RPMs is generally quieter.
If one runs the same RPM transmission exercise described above for maximum speed, but sets "maximum speed" to the roaming highway, the output is a higher gear ratio that provides ideal fuel mileage. In an era when the car can not travel very quickly, the maximum power point may be close enough to the desired speed so additional gear is not required. But when more powerful cars emerged, especially during the 1960s, the difference between the maximum power point and the desired speed increased greatly. This means the car often operates away from their most efficient point. When the desire for a better fuel economy grew, especially after the 1973 oil crisis, the need for "roaming" became more urgent.
Gearbox vs. final drive
The obvious solution to this problem is to add more transmission to the transmission. Indeed, in a modern vehicle this is common. However, because of the peculiarities of history, this is not always practical.
In the conventional rear wheel layout, the transmission system usually consists of two parts, a "gearbox" or "transmission" mounted on the back of the engine, and a "final drive" mounted on the rear axle at the rear of the car. The reason for the separation of tasks between the front and rear of the car is to allow the drive shaft to run at a lower torque, using a higher RPM. Since power is a product of RPM and torque, running the shaft at higher RPM allows more power to be transferred at lower torque. Doing so reduces the torque that the driveshaft should carry, and thus the strength and weight required.
Although the designer is theoretically free to choose any ratio for the gearbox and final drive, there is one additional consideration which means that the top gear of the gearbox is 1: 1 or "direct drive". These are selected for efficiency, because they do not require gears to transmit power and thus reduce the power lost by them. This was especially important in the early days of the car, because their straight lines were not finished well, noisy and inefficient. The last drive then takes this output and adjusts it in a fixed ratio transmission setting which is much easier to build. The final ratio of 4: 1 drives is common, which means that the wheels will spin at a quarter of the rate they would do if connected directly to the machine.
Overdrive
In an era when different car models with different wheel sizes can be accommodated by simply changing the final drive ratio, it makes sense for all transmissions to use the direct drive as the highest gear. However, as mentioned earlier, this will cause the machine to operate at too high a RPM for efficient cruising. Although adding cruising equipment to the main gearbox is possible, it's generally easier to add two separate gear overdrive systems to the existing gearbox. This not only means that it can be set for different vehicles, but has the added advantage that can be offered as an easy-to-install option.
With the use of the front wheel layout, the gearbox and final drive are combined into a single transaxle. There is no longer a driving shaft between them so the idea of ​​"direct mover" does not apply. Although "overdrive" is still called, it's now mostly marketing terms to refer to the extra-high ratios for efficient exploration, whether it's achieved through gearbox ratios, or by unusually high final drives.
Maps Overdrive (mechanics)
Usage
In general, overdrive is the highest gear in transmission. Overdrive allows the engine to operate at a lower RPM for the given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operations on the highway. When turned on, the automatic transmission can switch to overdrive mode after a certain speed is reached (typically 70 km/h [40-45 mph or more] depending on load). When turned off, the automatic transmission transmission is limited to the lower gear. Overdrive should normally be selected when the average speed is above 70 km/h (40-45 mph).
Automatic transmission automatically shifts from OD to direct drive when more load is present. When fewer loads are present, it shifts back to the OD. Under certain conditions, such as driving uphill, or pulling a trailer, the transmission can "hunt" between the OD and the next highest gear, shifting back and forth. In this case, turning it off can help the transmission to "decide". It may also be advantageous to turn it off if engine braking is desired, for example when driving is down. The vehicle owner's guide will often contain appropriate information and procedures regarding such situations, for each given vehicle.
Almost all vehicles (cars and trucks) have overdrive today whether manual or automatic transmissions. In the automotive aftermarket you can also retrofit overdrive to the existing initial transmission. Overdrive is widely used in European cars with manual transmissions in the 60s and 70s to improve mileage and sport driving as the bolt-on option but is becoming increasingly common for transmission later in life to have these gears built. If the vehicle is equipped with a bolt-ondrive overdrive (eg: GKN or Gear Vendor) as opposed to having overdrive built in one will usually have the option of using a gear in the gear more than just the top gear. In this case the tooth replacement can still be done in all gears, even with a disconnected overdrive. Overdrive only adds an effective range to the gears, so the third and fourth overdrive becomes valid "third and half" and five gears. In practice this gives the driver more ratios that are closer together giving greater flexibility especially in car performance.
How overdrive units work
A gear consists of electrically operated electric or hydraulic epicyclic gear bolted on the back of the transmission unit. It can pair the driveshaft input directly to the output shaft (or propeller shaft) (1: 1), or increase the output speed so that it is faster than the input shaft (1: 1 n ). So the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, overdrive speed is usually the result of a combination of planetary/epicyclic gearsets integrated in the transmission. In this case, no "overdrive" unit can be identified separately. In older vehicles, sometimes driven by buttons or buttons, often incorporated into the gear switch, and do not require clutch operation. The newer vehicles have an electronic overdrive in which the computer automatically adjusts to conditions of need and power load.
In Europe
Most of the overdrives in European cars were created and developed by a man named de Normanville and produced by a British company called Laycock Engineering (later GKN Laycock), on the Little London Road site in Sheffield. The system was designed by Captain Edgar J de Normanville (1882-1968), and was made by Laycock through an opportunity to meet a Laycock Products Technician. De Normanville overdrives are found in vehicles produced by Standard-Triumph, the first, followed by Ford, BMC and Leyland UK, Jaguar, Rootes Group and Volvo just a few names. Another British company, former Fairey aircraft builder, built a successful all-mechanical unit for Land Rover, which is still in production in America today.
The first production vehicle that had the Laycock system was the 1948 Standard Vanguard Saloon. The first unit made was the A-type overdrive, which was fitted to many sports cars during the 1950s, and until the late 1960s. Some of the famous marques use A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC, Armstrong Siddeley and TRU TR sports cars, from TR2 to late 1972 year models from TR6.
In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was installed for various automobiles including Volvo 120 and 1800, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also 1962-1967 MGB (which has 3-synchro transmission).
From 1967, the LH type overdrive was introduced, and it was featured in various models, including 1968-1980 MGB, MGC, Ford Zephyr, early Reliant Scimitars, TVRs, and Gilberns.
The J-type overdrive was introduced in the late 1960s, and adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler cars, and Ford Transit vans.
P-type overdrive marks the last update and includes a US version of Gear Vendor and Volvo versions. The Volvo version stores the same packet size as the J-type but with update of 18 freewheel elements and stronger splines through the planet carrier. The US Vendors Gear Version uses a larger output outer diameter axis 1,375 for higher capacity and longer rear casing.
Over a period of 40 years, Laycock Engineering produced over three and a half million overdrive units, and over one million of them were installed for Volvo cars.
In 2008, Company Gear Vendor, Inc. from El Cajon, California buys all of GKN's overdrive assets to continue US production versions and all spare parts for J and P types worldwide.
The system is equipped with an oil pressure operated device mounted on the back of a standard gearbox that operates on the output shaft of the gearbox. Through the oil pressure, solenoid, and piston system, the gear will reduce the rotation of any tooth used by 22% (0.778). For example, the overdrive system applied to Triumph TR5 operates on 2nd, 3rd and top gear. When involved, overdrive will decrease revs from 3000 by 666 RPM, or from 3500 drop will be 777 RPM to 2723 net. This advantage reduces rpm in fuel consumption most often quite close to 22% decrease during highway driving.
In North America
In the days before automatic transmission was common, especially in the 1950s, many American rear-wheel drive cars were available with overdrive options. Borg Warner provides factory-installed boxes between transmission and driveshaft. Because overdrive function, if enabled, can be routed only by reducing the accelerator without pressing the clutch pedal, the action is semi-automatic. Also, an electrically operated solenoid will disable the unit through a switch under an accelerator pedal that gives the same result as an automatic kickdown. Keys connected to the bowden cable, similar to some emergency brake applications, are also provided to lock the unit mechanically. Using overdrive with a 3-speed main transmission in 2nd gear is equal in ratio to 3rd gear, and with the main transmission at 3, the overall ratio is fractional (ie, "true overdrive").
Such additional add-on boxes were available from the 1930s through the 1970s for cars and light trucks.
Currently, most petrol and diesel and truck cars come with overdrive transmissions as they are beneficial to fuel economy. Overdrive is included in automatic and manual transmissions as additional gears (or two in some cases).
Fuel economy and drivetrain wear
When using overdrive drive, engine speed decreases, reduces wear and saves fuel normally. Since 1981 the company's average fuel economy legislation (CAFE), almost all domestic vehicles include overdrive to save fuel. One should refer to the car owner's manual for the proper speed to run on the gear. All engines have a peak efficiency range and may be for the use of the gear to keep the engine out of this range for all or part of its usage time if used at improper speeds, thereby cutting to fuel savings from lower engine speeds.
The overall reduction of the drivetrain comes down to three basic factors: transmission gearing (including overdrive), differential gearing (in shaft), and tire size. The problem of rotational speed starts to apply when differential gearing is a high ratio and overdrive is used to compensate. This can create unpleasant vibrations at high speeds and possible destruction of driveshaft due to centripetal force or unbalanced balance.
Driveshaft is usually a hollow metal tube that requires balance to reduce vibration and has no internal support.
Higher speeds on driveshaft and associated parts can cause heat and wear problems if overdrive and high differential gearing (or even very small tires) are combined, and create unnecessary friction. This is especially important because the differential gear is bathed with heavy oil and rarely comes with cooling in addition to the airflow above the housing.
The impetus is to minimize the use of the gear and provide a higher first gear ratio, which means more gears between the first and last to keep the engine at the most efficient speed. This is part of the reason that modern cars tend to have larger gear counts in their transmissions. That's also why more than one over-hard gear is rarely seen in a vehicle except in special circumstances where high differential (numerical) gear is required to get a moving vehicle like in a truck or performance car even though the dual overdrive transmission is common in other vehicles, often with numbers small on the reduction of axle teeth, but usually only involved at speeds exceeding 100 kilometers per hour (62 mph).
References
- Notes
- Quote
External links
- "How Auto Transmission Works - Overdrive" (with Flash interactive animation)
Source of the article : Wikipedia
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