152 Computer Control Of Evaporative Emissions

Motor fuels give off vapors that contain harmful hydrocarbons, such as benzene. In order to restrict emissions of hydrocarbons from the fuel tank, vehicle systems are equipped with a carbon canister. This canister contains activated charcoal which has the ability to bind toxic substances into hydrocarbon molecules. In the evaporative emission control system the carbon canister is connected by valve and pipe to the fuel tank, as shown in Fig. 1.21.

The evaporative purge solenoid valve connects the carbon canister to the induction system, under the control of the ECM, so that the hydrocarbon vapors can be drawn into the combustion chambers to be burnt with the main fuel-air mixture. The control valve is operated by duty cycle electrical signals from the computer which determine the period of time for which the valve is open. When the engine is not running the vapor from the fuel in the tank passes into the carbon canister. When the engine is started up the ECM switches on the solenoid valve so that the vapor can pass into the induction system. The frequency of operation of the solenoid valve after this is dependent on operating conditions.

Evaporative emissions control is part of the emissions control system of the vehicle and it must be maintained in good order.

1.6 Anti-lock braking (ABS)

Anti-lock braking is another form of a computer controlled system that is commonly used. Figure 1.22 shows a relatively modern system that uses individual wheel control for ABS and is known as a four-channel system. The braking system shown here uses a diagonal split of the hydraulic circuits: the brakes on the front left and rear right are fed by one part of the tandem master cylinder, and the brakes on the front right and rear left are fed from the other part of the tandem master cylinder.

The wheel sensors operate on the Hall principle and give an electric current output which is considered to have advantages over the more usual voltage signal from wheel sensors. The ABS control computer is incorporated into the ABS modulator and, with the aid of sensor inputs, provides the controlling actions that are designed to allow safe braking in emergency stops.

Starting at the top left corner of Fig. 1.23 there are two hydraulic accumulators (A1 and A2) which act as pressure reservoirs for hydraulic fluid. Below these is the modulator pump which is under computer control. At the bottom of the diagram are the four wheel brakes and above these are the inlet and outlet valves (labelled

Fig. 1.21 Evaporative emissions control system

C and D, respectfully) which, under computer control, determine how braking is applied when the ABS system is in operation.

ABS is not active below 7 km/h and normal braking only is available at lower speeds. When ABS is not operating, the inlet valves rest in the open position (to permit normal braking) and the outlet valves rest in the closed position. At each inlet valve there is a pressure sensitive return valve that permits rapid release of pressure when the brake pedal is released and this prevents any dragging of the brakes.

Fig. 1.22 Elements of a modern ABS system
Fig. 1.23 Details of the ABS system

1.6.1 OPERATION OF ABS

Depressing the brake pedal operates the brakes in the normal way. For example, should the wheel sensors indicate to the computer that the front right wheel is about to lock, the computer will start up the modulator pump and close the inlet valve C4. This prevents any further pressure from reaching the right front brake. This is known as the 'pressure retention phase'. If the wheel locks up, the computer will register the fact and send a signal that will open the outlet valve D4 so that pressure is released. This will result in some rotation of the right front wheel. This is known as the 'pressure reduction phase'. If the sensors indicate that the wheel is accelerating, the computer will signal the outlet valve D4 to close and the inlet valve C4 to open and further hydraulic pressure will be applied. This is known as the 'pressure increase phase'. These three phases of ABS braking, i.e. pressure retention, pressure release and pressure increase, will continue until the threat of wheel lock has ceased or until the brake pedal is released.

1.6.2 SOME GENERAL POINTS ABOUT ABS

The system shown in Fig. 1.23 illustrates one mode of ABS operation. The front right and rear right brakes are in the pressure retention phase, the front left brake is in the pressure increase phase, and the rear left brake is in the pressure reduction phase. This is indicated by the open and closed positions of the inlet valves C1-C4 and the outlet valves D1- D4.

During ABS operation the brake fluid returns to the master cylinder and the driver will feel pulsations at the brake pedal which help to indicate that ABS is in operation. When ABS operation stops the modulator pump continues to run for approximately 1 s in order to ensure that the hydraulic accumulators are empty.

1.7 Traction control

The differential gear in the driving axles of a vehicle permits the wheel on the inside of a corner to rotate more slowly than the wheel on the outside of the corner. For example, when the vehicle is turning sharply to the right, the right-hand wheel of the driving axle will rotate very slowly and the wheel on the left-hand side of the same axle will rotate faster. Figure 1.24 illustrates the need for the differential gear.

However, this same differential action can lead to loss of traction (wheel spin). If for some reason one driving wheel is on a slippery surface when an attempt is made to drive the vehicle away, this wheel will spin whilst the wheel on the other side of the axle will stand still. This will prevent the vehicle from moving. The loss of traction (propelling force) arises from the fact that the differential gear only permits transmission of torque equal to that on the weakest side of the axle. It takes very little torque to make a wheel spin on a slippery surface, so the small amount of torque that does reach the non-spinning wheel is not enough to cause the vehicle to move.

Differential Rear-wheel drive Front-wheel drive

Differential Rear-wheel drive Front-wheel drive

The wheel on the inside does not travel so far

Fig. 1.24 The need for a differential gear

The wheel on the inside does not travel so far

Fig. 1.24 The need for a differential gear

Traction control enables the brake to be applied to the wheel on the slippery surface. This prevents the wheel from spinning and allows the drive to be transmitted to the other wheel. As soon as motion is achieved, the brake can be released and normal driving can be continued. The traction control system may also include a facility to close down a secondary throttle to reduce engine power and thus eliminate wheel spin. This action is normally achieved by the use of a secondary throttle which is operated electrically. This requires the engine management system computer and the ABS computer to communicate with each other, and a controller area network (CAN) system may be used to achieve this.

Figure 1.25 gives an indication of the method of operation of the throttle.

The ABS system described in section 1.6 contains most of the elements necessary for automatic application of the brakes, but it is necessary to provide additional valves and other components to permit individual wheel brakes to be applied. Figure 1.26 shows the layout of a traction control system that is used on some Volvo vehicles.

In the traction control system, shown in Fig. 1.26, the ABS modulator contains extra hydraulic valves (labelled 1), solenoid valves (labelled 2) and by-pass valves (labelled 3). The figure relates to a front-wheel drive vehicle and for this reason we need to concentrate on the front right (FR) brake and the front left (FL) brake. In this instance wheel spin is detected at the FR wheel which means that application of the FR brake is required.

The solenoid valves (2) are closed and this blocks the connection between the pressure side of the pump (M) and the brake master cylinder. The inlet valve (C1) for the FL brake is closed to prevent that brake from being applied.

Fig. 1.25 The electrically-operated throttle used with the traction control system
Fig. 1.26 A traction control system

The modulator pump starts and runs continuously during transmission control operation and takes fluid from the master cylinder, through the hydraulic valve 1, and pumps it to the FR brake through the inlet valve (C4).

When the speed of the FR wheel is equal to that of the FL wheel, the FR brake can be released, by computer operation of the valves, and then re-applied until such time as the vehicle is proceeding normally without wheel spin. In the case here, of spin at the FR wheel, the controlling action takes place by opening and closing the inlet valve (C4) and the outlet valve (D4).

When the computer detects that wheel spin has ceased and normal drive is taking place, the modulator pump is switched off, the solenoid valves (2) open and the valves (C4) and (D4) return to their positions for normal brake operation. Because the modulator pump is designed to provide more brake fluid than is normally required for operation of the brakes, the by-pass valves (3) are designed to open at a certain pressure so that excess brake fluid can be released back through the master cylinder to the brake fluid reservoir. The system is designed so that traction control is stopped if:

1. the wheel spin stops;

2. there is a risk of brakes overheating;

3. the brakes are applied for any reason;

4. traction control is not selected.

1.8 Stability control

The capabilities of traction control can be extended to include actions that improve the handling characteristics of a vehicle, particularly when a vehicle is being driven round a corner. The resulting system is often referred to as 'stability control'.

Figure 1.27 shows two scenarios. In Fig. 1.27(a) the vehicle is understeering. In effect it is trying to continue straight ahead and the driver needs to apply more steering effect in order to get round the bend. Stability control can assist here by applying some braking at the rear of the vehicle, to the wheel on the inside of the bend. This produces a correcting action that assists in 'swinging' the vehicle, in a smooth action, back to the intended direction of travel.

In Fig. 1.27(b) oversteer is occurring. The rear of the vehicle tends to move outwards and effectively reduce the radius of turn. It is a condition that worsens as oversteer continues. In order to counter oversteer, the wheel brakes on the outside of the turn can be applied and/or the engine power reduced, via the secondary throttle, by the computer. In order to achieve the additional actions required for stability control it is necessary to equip the vehicle with additional sensors, such as a steering wheel angle sensor, and a lateral acceleration sensor that has the ability to provide the control computer with information about the amount of understeer or oversteer.

To achieve stability control it is necessary for the engine control computer, the ABS computer and the traction control computer to communicate, and

Steered path

Steered path

With stability control

Brake_ force

- Without stability ^ control

With stability control

Brake_ force

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