GB2230498A

GB2230498A - Twin gearbox four wheel drive

Info

Publication number: GB2230498A
Application number: GB8906538A
Authority: GB
Inventors: Christopher William Hend Ellis
Original assignee: Individual
Current assignee: Individual
Priority date: 1989-03-21
Filing date: 1989-03-21
Publication date: 1990-10-24
Also published as: GB8906538D0

Abstract

A motor vehicle has an engine which drives the rear wheels through a primary clutch 2, primary gearbox 3 and rear differential 4. In addition the front wheels are driven from the front of the engine through a torque converter 5, a secondary gearbox 6 and a front differential 7. The secondary gearbox 6 automatically disengages whenever the primary clutch 2 disengages or the primary gearbox 3 is in neutral. <IMAGE>

Description

TWIN GEARBOX FOUR WHEEL DRIVE The present invention relates to the use of two gearboxes to provide four wheel drive in a vehicle.

There is considerable interest in providing vehicles with drive to more than one axle. The usual method is to divide the output from a single gearbox and feed the power to front and rear axles through propeller shafts. This can produce difficulties in feeding the drive under or through the engine and past the driver or front passenger.

According to one aspect of the present invention, in a four wheel drive vehicle the engine drives two gearboxes, a primary gearbox at the rear of the engine driving the rear wheels and a secondary gearbox at the front of the engine driving the front wheels.

According to another aspect of the present invention, in a four wheel drive vehicle there is provided a torque converter between the engine and the secondary, front, gearbox which accommodates any minor variation between the rotational speeds of the front and rear wheels.

According to another aspect of the present invention, in a four wheel drive vehicle there are provided means at the primary gearbox to send control signals and at the secondary gearbox to receive those control signals, modified if necessary by a computer system receiving input from an electronic engine management system and/or an anti-lock braking system, so that the appropriate gear ratio in the secondary gearbox may be selected.

According to another aspect of the present invention, in a four wheel drive vehicle a suitable choice of ratios in the secondary gearbox will enable the division of torque between the front and rear wheels to vary depending upon the gear selected in the primary gearbox.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 shows in plan and elevation the layout of a two seater motor car, with a front engine driving the rear wheels.

Figure 2 illustrates a similar vehicle additionally fitted with a secondary, front, gearbox and front differential and drive shafts to provide four wheel drive.

Referring to the drawing, the engine 1 drives the primary, rear, gearbox 3 through a clutch 2. Power is transmitted via a propeller shaft to the rear differential 4.

The engine also drives the secondary gearbox 6 from the front of the crankshaft through a torque converter 5. The output from the secondary gearbox is fed directly to the front differential 7.

In this particular implementation, the primary gearbox is manually operated, with five forward gears. It might equally well be a four or six speed manual or a three, four or five speed automatic.

In this particular implementation, the secondary gearbox is a two-speed, electrically-controlled mechanism, without reverse. It might alternatively be a single- or multi-speed gearbox, and might alternatively be hydraulically controlled. A microswitch in the primary clutch causes the secondary gearbox to be de-coupled whenever the primary clutch is disengaged. A pair of microswitches in the primary gearbox sense when the the primary gearbox is engaged in second or third gear and close an electrical circuit which causes the secondary, front gearbox to engage its first(low) gear. Another pair of microswitches in the primary gearbox sense when the the primary gearbox is engaged in fourth or fifth(highest) gear and close an electrical circuit which causes the secondary, front gearbox to engage its second(high) gear.In this particular implementation, the secondary gearbox is not engaged when the primary gearbox is in neutral or reverse or first (lowest) gear.

The torque converter 5 not only provides smooth transmission of power to the secondary gearbox as the primary gearbox comes up through its gears, but also avoids the need for a differential between the front and rear axles. This implementation also splits the torque between front and rear axles by a proportion principally dependent on the gear engaged in the primary gearbox.

Note that there is some variation depending on engine speed.

When the primary, rear, gearbox is in first (lowest) gear, the secondary front gearbox is disengaged. Consequently, the full torque of the engine is transmitted to the rear wheels. When the primary gearbox is in second gear, the secondary gearbox is engaged in low gear, with some 30% of the total torque being delivered to the front wheels. When the primary gearbox is in third gear, the secondary gearbox is still engaged in low gear, but the torque flowing to the front wheels is now approximately 50% of the total.

When fourth gear is engaged in the primary gearbox, the secondary gearbox engages high gear and the torque split is approximately 50:50. When the primary gearbox is in fifth gear, the secondary gearbox is in high gear, giving a torque split favouring the front wheels with approaching 70% of the total available torque.

Note how the torque split moves from a ratio favouring the rear wheels with torque when weight transfer to the rear is high in the low gears to a ratio favouring the front wheels at high speed. This desirable characteristic is achieved without the need for additional equipment, saving weight and cost.

To move away from rest in conditions of ice or snow, full four wheel drive is engaged by selecting second gear in the primary gearbox, whether manual or automatic. This is good practice with an ordinary two wheel drive vehicle and is similarly good practice in this implementation.

If an anti-lock braking system is fitted to the vehicle, it may be desirable to restrict drive to just the rear wheels, under adverse braking conditions. This implementation, with its electrical control of the front gearbox, makes such a requirement simple to meet.

Claims

1. A secondary gearbox at the front of the engine of a motor vehicle driving the vehicles front wheels and working in conjunction with the primary gearbox at the rear of the engine to provide four-wheel drive.

2. A secondary gearbox as claimed in Claim 1 fitted with a torque converter, allowing minor variations in the relative rotational speed of the front and rear wheels and removing the need for a differential between the front and rear axles.

3. A secondary gearbox as claimed in Claim 1 or Claim 2 fitted with a set of ratios so that the torque split between front and rear axles is varied automatically as the primary gearbox moves up through it's gears.

4. A primary gearbox and a secondary gearbox as claimed in Claim 1 or Claim 2 or Claim 3 linked by asignalling system whereby the gear ratio engaged in the primary gearbox is the principal but not necessarily only determinant of the gear ratio engaged in the secondary gearbox.