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Glossary: P

PART-TIME FOUR-WHEEL DRIVE

Part-time four-wheel drive is the most common type of all wheel drive. It usually operates only in two-wheel drive mode and it can be switched to all-wheel drive whenever the situation requires.

By employing a center differential or a transfer box, these types of all-wheel drive systems can transfer power to either the front only, rear or to all the drive wheels, depending on the driver's wishes.

On older designs, the all-wheel drive mode had to be manually engaged and the vehicle had to be stopped in order for the (usually front) wheel hubs to be locked, but it can now be done from the inside of the vehicle and on some vehicles even while they're moving.
Permanent Four-Wheel drive

PERMANENT FOUR-WHEEL DRIVE

Permanent four-wheel drive systems send power to all four wheels in a continuous manner. There is no need for the driver to engage two or four-wheels drive mode, since all of the wheels are always powered. Power curve (powerband)

POWER CURVE

The power output of an internal combustion engine forms a curve if charted on a graph, since the engine has different outputs at different rpms. This is called the power curve, or powerband. The power curve can be more abrupt, or relatively flat, depending on the power output along the rev range.
Power steering

POWER STEERING

Power steering was developed in order to reduce the effort needed to steer the vehicle. In other words, the driver can change the vehicle's direction with the help of an external power source that can assist this operation.

Most power steering systems employ hydraulic pressure and are operated using power from the engine, but in recent years, electro-hydraulic and even 100% electric systems have been introduced.

Some modern steering systems can provide a variable amount of assist, depending on the speeds at which the vehicle is moving, while others are even using the "drive-by-wire" technology, with no direct linkage between the steering wheel and the the wheels. Power Train Electronic Control

PTEC

The Power Train Electronic Control (PTEC) is the Aston Martin moniker for a system which controls the engine management, fuel-injection, ignition and other diagnostics. Employing a central ECU, the PTEC technology is capable of transmitting information between different electronic elements in the engine in microseconds. Push-Button Four-Wheel Drive

PUSH-BUTTON FOUR-WHEEL DRIVE

In some modern part-time four-wheel drive systems the "all-wheel drive mode" can be engaged electronically by simply pushing a button somewhere on the instrument panel, thus relieving the driver of locking the wheel hubs manually. Pushrods (or rods)

PUSHRODS

Pushrods are found only in OHV (Overhead Valve) engines. Essentially, they are metallic rods which are actuated by the camshaft in order to operate (open and close) via rocker arms the intake and exhaust valves.

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