Hệ thống dẫn động 4 bánh trên ô tô tiếng anh

Ưu nhược điểm của các hệ dẫn động 4 bánh Với những kiểu như 4 bánh toàn thời gian hay bán thời gian, việc lựa chọn hoàn toàn phụ thuộc vào phong cách lái và điều kiện sử dụng bởi chúng có ưu, nhược điểm riêng. Động cơ sinh ra công suất và mômen xoắn. Để truyền năng lượng tới các bánh khiến chúng quay, chiếc xe của bạn cần phải có cơ cấu dẫn động. Tuy nhiên, không phải tất cả các bánh đều trực tiếp nhận công suất và mômen xoắn từ động cơ. Tùy thuộc yêu cầu kỹ thuật giữa các loại xe và tại từng thời điềm mà người ta có những phương pháp truyền động khác nhau như một cầu (cho 2 bánh) và 2 cầu (cho 4 bánh). Dưới đây là ưu nhược điểm của các loại dẫn động 4 bánh. Dẫn động 4 bánh bán thời gian (Parttime 4WD) Đây hệ dẫn động phổ biến nhất trên các mẫu xe địa hình. Nó hoạt động sau khi tài xế nhấn nút hoặc chuyển chế độ từ cabin. Lúc đó, mômen xoắn được truyền tới cả 4 bánh. Còn khi chưa được kích hoạt, năng lượng từ động cơ được truyền tới hai bánh trước hoặc sau.

Four-Wheel-Drive

Why Four-Wheel-Drive?

❚ Advantages

❙ Improved traction and handling

❙ Able to transmit torque to all four wheels

❚ Disadvantages

❙ Higher initial cost and weight

❙ Requires special service and maintenance

❚ 4X4 = Four wheeled vehicle, four wheels can receive torque.

❚ 4X2 = Four wheeled vehicle, two wheels can receive torque.

Definitions

4WD Designs

❚ FWD vehicles with an added transfer case, a rear drive shaft, and a rear axle

4WD Components

❚ Transfer case

❙ Transmits power from the transmission to the front and rear drive shafts

4WD Components

❚ Transfer case

❙ Will typically be a gear or chain type

Types of Transfer Cases

❚ Drive chain design

❙ Has less weight to

improve fuel economy

❙ Is usually used with

planetary gearsets

❙ Is the most common

design

Types of Transfer Cases

❚ Gear-type

❙ Uses only gear-sets to transfer power

❙ Stronger & more durable

4WD Components

❚ Drive shafts

❙ Connect to the front and rear differentials

4WD Components

❚ Universal joints or CV-joints

❙ Are used to connect front axles to the wheel hubs

❚ Part-time 4WD - Typically systems

that operate in two-wheel drive until the driver commands four-wheel

drive operation Torque is then

divided (50/50 fixed) between the

front and rear drive-shaft.

❙ They do not corner well on dry pavement.

❙ Selection of two- or four-wheel-drive is done by a shifter, electric switch and/or locking hubs.

4WD Definitions

Part-Time 4WD Shift

Controls

❚ Electric switch or shift lever

❙ Allows the driver to select

which axles receive power

❙ Power can be directed to:

❘ all four wheels

❘ two wheels

❘ none (neutral)

❙ Some vehicles have a

low-speed range that can

be selected

❚ Older designs may have different

drive-axle gear ratios.

❙ This will result in a push-pull type action if the vehicle is driven on dry pavement

❙ This phenomenon is called drive-line windup

Part-Time 4WD

Part-Time 4WD

❚ Locking Hubs

❙ Manual or automatic operation

Part-Time 4WD

❚ Locking Hubs

❙ manual operation

Part-Time 4WD

❚ Locking Hubs

❙ automatic operation - vacuum or electro-magnetically operated.

Part-Time 4WD

❚ Locking Axles - final differential will be of an

“open” design When one axle is disconnected from the wheel the spider gears rotate freely, neither axle receives torque

❚ Used with an open differential

Part-Time 4WD

❚ Locking Axles - Toyota’s Automatic Disconnecting Differential (ADD)

Transfer Case Modes of

Operation

❚ Ranges available with a part-time transfer case:

❙ Neutral

Transfer Case Modes of

Operation

❚ Ranges available with a part-time transfer case:

❙ Two-wheel-drive—high

Transfer Case Modes of

Operation

❚ Ranges available with a part-time transfer case:

❙ Four-wheel-drive—high

Transfer Case Modes of

Operation

❚ Ranges available with a part-time transfer case:

❙ Four-wheel-drive—low

❚ Full-time four-wheel drive (4wd) -

Constantly provides power to all four wheels These systems typically do NOT have a provision for “low”

operating range.

❙ Axles and differentials are constantly locked.

❙ Usually equipped with a center “differential” instead of

a transfer-case.

❚ All-wheel drive - see above

4WD Definitions

Full-time 4WD

❚ Open Center Differential - If the

wheels on either final differential

loose traction, maximum torque is delivered to the axle with the LEAST traction

❚ For this reason, limited-slip center

differentials are far more common today.

Full-time 4WD

allow for different operational modes.

receive power as required.

gear-reduction.

for slow, high torque driving

❚ On-demand four-wheel drive -

Automatically controlled 4WD One axle is driven until slip is detected or anticipated

❙ Computer controlled

❙ uses wheel speed sensors and/or axle speed sensors to detect slippage.

❙ When slippage is detected, torque is sent to the

opposite axle The amount sent depends on the amount

of slippage

4WD Definitions

❚ On-demand four-wheel drive

4WD Definitions

❚ Viscous coupling in a center differential

4WD Definitions

❚ Viscous coupling in a center differential

4WD Definitions

On-demand four-wheel drive

❚ Ford’s control-trac system

❙ normal operation - 96% of torque is sent to the rear wheels.

❙ Upon slippage a computer energizes an

electro-magnetic clutch in the transfer case Up to 96% of torque can be sent to the front wheels

4WD Systems

❚ Four-wheel drive systems without a center differential.

❘ Tire sizes must all be the same (this holds

true for any 4WD application)

❘ Tread wear must be similar

❘ Tire changes on an all-wheel drive must be done in sets of four

❘ Gear ratios should be the same in the front and rear differentials

Front Axle and Hub Service

Guidelines

❚ Most front axles are

serviced like those

❚ Some hubs are not

serviceable and must

be replaced

Effects of Vehicle

Modifications

❚ Installing larger tires and lift kits

❙ Alters vehicle handling by changing unsprung weight

❙ Increases steering effort

❙ Causes a rough ride

❙ May cause vehicle to wander and vibrate

❙ Possible driveshaft working angle issues

❙ Possible Vehicle Speed sensor issues

❘ May necessitate computer programming

Maintenance Tips

❚ Check the fluid level often

❚ Check for fluid contamination any time the vehicle has been driven through deep water

❚ Use only the recommended lubricant

❚ Always follow

the proper

towing

procedures