A

REPORT

ON

AIR (PNUEMATIC) BRAKE SYSTEM

BY

Submitted by:

Sudeep Singh (ME Final Year)

Satendra Bharti (ME Final Year)

Pramod Sonker (ME Final Year)

Ahsaan Ullah (ME Final Year)

Abstract:

An air braking unit is used in an air braking system. The air braking unit is arranged to be positioned, in use, at a vehicle wheel, and comprises an inlet for receiving, in use, compressed air from a central source. At least one first valve is arranged to selectively allow compressed air from the inlet to enter a wheel brake chamber in use.

At least one second valve is arranged to selectively allow air from the brake chamber to be released via an outlet to the atmosphere in use and control means controls the first and second valves to operate to selectively control the air pressure in the brake chamber in use.

1.1

CONTENTS

1. Introduction

2. History and development

3. Principles of Operation and Construction

4. Components

5. Advantages and Disadvantages

6. Literature Survey

7. Scope of Air Brake System

8. Future Aspects

9. Summary

10. References

1.

1.1 INTRODUCTION

1.1.1 Air Brake

A friction type of energy-conversion mechanism used to retard, stop, or hold a vehicle or other moving element. The activating force is applied by a difference in air pressure. With an air brake, a slight effort by the operator can quickly apply full braking force.

The air brake, operated by compressed air, is used in buses; heavy-duty trucks, tractors, and trailers; and off-road equipment. The air brake is required by law on locomotives and railroad cars. The wheel-brake mechanism is usually either a drum or a disk brake. The choice of an air brake instead of a mechanical, hydraulic, or electrical brake depends partly on the availability of an air supply and the method of brake control.

In a motor vehicle, the air-brake system consists of three subsystems: the air-supply, air-delivery, and parking/emergency systems. The air-supply system includes the compressor, reservoirs, governor, pressure gage, low-pressure indicator, and safety valve. The engine-driven compressor takes in air and compresses it for use by the brakes and other air-operated components. The compressor is controlled by a governor that maintains air compression within a preselected range. The compressed air is stored in reservoirs.

The air-delivery system includes a foot-operated brake valve, one or more relay valves, the quick-release valve, and the brake chambers. The system delivers compressed air from the air reservoirs to the brake chambers, while controlling the pressure of the air. The amount of braking is thereby regulated. In the brake chambers, the air pressure is converted into a mechanical force to apply the brakes.

As the pressure increases in each brake chamber, movement of the diaphragm pushrod forces the friction element against the rotating surface to provide braking. When the driver releases the brake valve, the quick-release valve and the relay valve release the compressed air from the brake chambers. The parking/emergency system includes a parking-brake control valve and spring brake chambers. These chambers contain a strong spring to mechanically apply the brakes (if the brakes are properly adjusted) when air pressure is not available.

During normal vehicle operation, the spring is held compressed by system air pressure acting on a diaphragm. For emergency stopping, the air-brake system is split into a front brake system and a rear brake system. If air pressure is lost in the front brake system, the rear brake system will continue to operate. However, the supply air will be depleted after several brake applications. Loss of air pressure in the rear brake system makes the front brake system responsible for stopping the vehicle, until the supply air is depleted.

Air Brake continued…

Either of two kinds of braking systems, the first, used by trains, trucks, and buses, operates by a piston driven by compressed air from reservoirs connected to brake cylinders. When air pressure in the brake pipe is reduced, air is automatically admitted into the brake cylinder. The first practical air brake for railroads was invented in the 1860s by George Westinghouse. The second type, used by aircraft and race cars, consists of a flap or surface that can be mechanically projected into the airstream to increase the resistance of the vehicle to air and lower its speed.

For more information on air brake, visit Britannica.com. Britannica Concise Encyclopedia. Copyright © 1994-2008 Encyclopædia Britannica, Inc.

1.1.2 Air Brake Implementations

1. Brake operated by compressed air, esp. in heavy vehicles and trains

2.  An articulated flap or small parachute for reducing the speed of an aircraft

3. Rotary fan or propeller connected to a shaft to reduce its speed

Collins Discovery Encyclopedia, 1st ion © HarperCollins Publishers 2005

Air brake  (Mechanical Engineering)

An energy-conversion mechanism activated by air pressure and used to retard, stop, or hold a vehicle or, generally, any moving element.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

1.2 History and Development

1.2.1 The straight air brake

The first form of the air brake consisted of an air pump, a main reservoir, and an engineer's valve on the locomotive, and of a train pipe and brake cylinder on each car. One problem with this first form of the air brake was that braking was applied to the first cars in a train much sooner than to the rear cars, resulting in shocks and damages when the rear cars bunted against the cars ahead of them. The main objection however was that it was not an automatic brake, i.e. even a minor mishap like a broken coupling left the entire train without any brake power at all.

1.2.2 The plain automatic air brake

In 1872, George Westinghouse invented the automatic air brake by inventing the triple valve and by equipping each car with its own air cylinder. Air pressure is maintained in the auxiliary reservoirs and in the train pipe at all times when the brakes are not applied. Equilibrium of air pressure is maintained in the train pipe and in the auxiliary air cylinders.

To apply the brakes to all of the cars at about the same time, pressure is released from the train pipe, causing the triple valve on each car to apply the brakes. To release the brakes on each car, pressure is increased in the train pipe until an excess pressure above that of the pressure in each auxiliary cylinder is reached, which throws the triple valve so as to close the inlet to the brake cylinder and open the inlet to the auxiliary reservoir from the train pipe, thus allowing the equilibrium of the two pressures to be reached.

1.2.3 The quick action triple valve

Although the plain automatic air brake was a great improvement over the straight air brake, in an emergency the system still applied the brakes to the last cars in a train later than to the first cars in a train. To remedy that condition, George Westinghouse invented the quick action triple valve in 1887. It automatically vents air from the brake pipe locally on each car, which applies the brakes more quickly.

1.2.4 Electric railways

For the air brake to be employed on electric railways requires an air compressor that is powered by electricity. Powerful electric locomotives were produced by the Westinghouse Electric & Manufacturing Company and by other companies.

1.3 BASIC PRINCIPLE OF OPERATION

1.3.1 Basic Principles

Pneumatics can be described as the way of transferring mechanical energy from one point to another using air pressure. Gas laws describe the volume-temperature-pressure relationships for gases under a variety of conditions. Boyle's Law states that the absolute pressure of a contained gas is inversely proportional to its volume provided its temperature remains constant. Gay-Lussac's Law states that the absolute pressure of a confined gas is proportional to its temperature provided its volume remains constant. Charle's Law states that the volume of a confined gas is proportional to its temperature provided its pressure remains constant.( Principles of Pneumatics | eHow.com )

Compressed air systems have many uses. They can inflate tires, operate blowers used to clean out electronic systems and power tools like pneumatic nail drivers. Some air compressors are so big they must be hauled around with a truck, while some are so small they fit on a work bench. They all have similar parts and operate the same way.

1.3.2 The Tank

The tank holds the compressed air. In one sense the whole function of an air ` system is to put more air into the tank. These usually consist of a strong cylinder (with rounded ends) with three ports. The place where the air comes in usually has a valve to insure the flow of air moves one way, a place where the air goes out, and a place where the pressure gauge attaches so you can tell how much pressure the tank is experiencing.

1.3.3 The Pump

The pump is forcing the air into the tank. Typically the air compressor system automatically controls the pump--turning it on when the pressure in the tank is low and turning it off when the pressure in the tank is high. In more expensive systems, the pump speed can be changed depending on the pressure in the tank. This ensures the pressure in the tank is more consistent. The pump can be electric or gasoline powered. The gasoline powered pumps are used in outdoor locations where electricity is not available.

1.3.4 Valves

There is a one-way valve (called a check valve) between the pump and the tank. This enables the air to go from the pump to the tank but not the other way. There is usually a check valve on the output from the tank as well, in addition to an emergency shutoff valve to save the compressed air if something goes wrong with the system. This speeds up recovery time once the problem is fixed, so you do not have to waste time compressing more air.

1.3.5 Gauges

There is a gauge showing how much pressure is currently in the tank. If this is too much, the pump must be shut off. It is also handy to see when you have enough pressure to operate the pneumatic tools that the air compressor system is driving. Some of the gauges are not visible--such as those which turn the pump on and off and the gauge that controls the emergency relief pressure valve

1.4 Components

A basic air brake system capable of stopping a vehicle has five main components:

1. A compressor to pump air with a governor to control it.

2. A reservoir or tank to store the compressed air.

3. A foot valve to regulate the flow of compressed air from the reservoir when it is needed for braking.

4. Brake chambers and slack adjusters to transfer the force exerted by the compressed air to Mechanical linkages.

5. Brake linings and drums or rotors to create the friction required to stop the wheels.

It is necessary to understand how each of these components works before studying their functions in the air brake system.

1.4.1 Compressor and Governor

Compressed air is used to transmit force in an air brake system. The source of the compressed air is a compressor. A compressor is designed to pump air into a reservoir which results in pressurized air.

The compressor is driven by the vehicle’s engine, either by belts and pulleys or shafts and gears. In vehicles where the compressor is driven by belts, they should be checked regularly for cracks and tension. Also, check the compressor for broken mounting brackets or loose bolts.

The compressor is in constant drive with the engine. Whenever the engine is running, so is the compressor. When pressure in the system is adequate, anywhere from a low of 80 psi to a high of 135 psi it is not necessary for the compressor to pump air. A governor controls the minimum and maximum air pressure in the system by controlling when the compressor pumps air. This is known as “loading” and unloading” stage respectively. Most compressors have two cylinders similar to an engine’s cylinders. When the system pressure reaches its maximum, which is between 115 and 135 psi, the governor places the compressor in the “unloading” stage.

The compressor must be able to build reservoir air pressure from 50 to 90 psi within three minutes. If unable to do so the compressor requires servicing. A compressor may not be able to build air pressure from 50 to 90 psi within three minutes if the air filter is plugged or if the belt is slipping. If these were not at fault the compressor could be faulty.

The governor must place the compressor in the “loading” stage at no lower than 80 psi. During the “unloading” stage, the compressor is able to cool.

Usually compressors are lubricated from the engine lubrication system, although some compressors are self-lubricating and require regular checks of the lubricant level.

It is very important the air that enters the system be kept as clean as possible. The air must first pass through a filter to remove any dust particles. The air filter must be cleaned regularly. A dirty filter will restrict the flow of air into the compressor, reducing its efficiency. Some vehicles have the inlet port of the compressor connected to the intake manifold and receive air that has been filtered by the engine air cleaner.

A piston type compressor operates on the same principle as the intake and compression strokes of an engine.

Intake stroke: The downward stroke of the piston creates a vacuum within the cylinder which causes the inlet valve to open. This causes atmospheric air to flow past the inlet valve into the cylinder.

Usually compressors are lubricated from the engine lubrication system, although some compressors are self-lubricating and require regular checks of the lubricant level.

It is very important the air that enters the system be kept as clean as possible. The air must first pass through a filter to remove any dust particles. The air filter must be cleaned regularly. A dirty filter will restrict the flow of air into the compressor, reducing its efficiency. Some vehicles have the inlet port of the compressor connected to the intake manifold and receive air that has been filtered by the engine air cleaner.

A piston type compressor operates on the same principle as the intake and compression strokes of an engine.

· Intake stroke: The downward stroke of the piston creates a vacuum within the cylinder which causes the inlet valve to open. This causes atmospheric air to flow past the inlet valve into the cylinder.

Compression stroke: The upward motion of the piston compresses the air in the cylinder. The rising pressure cannot escape past the inlet valve (which the compressed air has closed). As the piston nears the top of the stroke, the pressurized air is forced past the discharge valve and into the discharge line leading to the reservoir.

1.4.2 Reservoirs

Reservoirs or tanks hold a supply of compressed air. The number and size of the reservoirs on a vehicle will depend on the number of brake chambers and their size, along with the parking brake configuration. Most vehicles are equipped with more than one reservoir. This gives the system a larger volume of main reservoir air. The first reservoir after the compressor is referred to as the supply or wet reservoir. The other reservoirs are known as primary and secondary or dry reservoirs. When air is compressed, it becomes hot. The heated air cools in the reservoir, forming condensation. It is in this reservoir that most of the water is condensed from the incoming air.

1.4.3 Air Dryer

An air dryer may be installed between the compressor and the wet reservoir to help remove moisture from the compressed air. It may be partially filled with a high moisture-absorbent desiccant and an oil filter, or it may be hollow with baffles designed to assist in separating the moisture from the air.

Both types of air dryers use air pressure to purge or eject the accumulated contaminants from their desiccant bed. The purge valve has a heater element, which prevents the moisture from freezing in cold climate operation. The wiring connected to the heater should be inspected for loose or disconnected wires. They are also equipped with a safety valve.

1.4.4 Safety Valve

A safety valve protects reservoirs from becoming over pressurized and bursting if the governor malfunctioned and did not place the compressor in the unloading stage. The valve consists of a spring loaded ball that will allow air to exhaust from the reservoir into the atmosphere. The valve’s pressure setting is determined by the force of the spring. A safety valve is normally set at 150 psi. If the pressure in the system rises to approximately 150 psi, the pressure would force the ball off its seat, allowing the pressure to exhaust through the exhaust port in the spring cage. When reservoir pressure is sufficiently reduced to approximately 135 psi, the spring will force the ball back onto its seat, sealing off the reservoir pressure. Not all safety valves have a manual release feature.

If the safety valve has to relieve pressure, the governor or compressor requires adjustment, service or repair. This should be done by a qualified mechanic.

1.4.5 Foot Valve

The foot-operated valve is the means of applying air to operate the brakes. The distance the treadle of the foot valve is depressed by the driver determines the air pressure that will be applied, but the maximum application will not exceed the pressure in the reservoir. Releasing the foot valve treadle releases the brakes.

When the driver applies the brakes, depressing the treadle part way, the foot valve will automatically maintain the application air pressure without the driver having to adjust the pressure of his foot on the treadle.

Releasing the treadle allows the application air to be released through the exhaust ports into the atmosphere. Air treadles are spring loaded, producing a different “feel” from hydraulic brake applications.

1.4.6 Brake Chambers, Slack Adjusters and Brake Lining

A brake chamber is a circular container divided in the middle by a flexible diaphragm. Air pressure pushing against the diaphragm causes it to move away from the pressure, forcing the push rod outward against the slack adjuster. The force exerted by this motion depends on air pressure and diaphragm size. If a leak occurs in the diaphragm, air is allowed to escape, reducing the effectiveness of the brake chamber. If the diaphragm is completely ruptured, brakes become ineffective.

Front brake chambers are usually smaller than those in the rear because front axles carry less weight.

A brake chamber is usually mounted on the axle, near the wheel that is to be equipped for braking. Air pressure is fed through an inlet port. The air pushes against the diaphragm and the push rod. The push rod is connected by a clevis and pin to a crank arm type lever called a “slack adjuster.” This converts the pushing motion of the push rod from the brake chamber to a twisting motion of the brake camshaft and S-cams. When the air is exhausted, the return spring in the brake chamber returns the diaphragm and push rod to the released position.

As indicated by its name, the slack adjuster adjusts the “slack” or free play in the linkage between the push rod and the brake shoes. This slack occurs as the brake linings wear. If the slack adjusters are not adjusted within the limitations, effective braking is reduced and brake lag time is increased. If too much slack develops, the diaphragm will eventually “bottom” in the brake chamber, and the brakes will not be effective.

1.4.7 Brake Assembly

Brake lining material is attached to the shoes. The material used depends on the braking requirements of the vehicle. Brake lining must give uniform output of brake effort with minimum fade at high temperatures.

Fading or reduction in braking effort occurs when the heated drums expand away from the brake linings. The brake linings also lose their effectiveness with overheating.

The twisting action of the brake cam shaft and Scam forces the brake shoes and linings against the drums. The brake linings generate heat from friction with the brake drum surface.

1.5 The Advantages and Disadvantages of Air Brakes

The Advantages

In a vehicle, heat energy from the engine is converted into kinetic energy to enable its motion. When it comes to stopping motion, the same needs to be done in reverse--the kinetic energy needs to be converted into heat energy, bringing the vehicle to a halt. This is achieved using frictional devices called brakes. Most light vehicles today use hydraulic fluid brakes while in most heavy vehicles, brakes operating on compressed air are used.

1.5.1 Safety

A hydraulic brake contains special braking fluid in cylinders which is compressed when the brake pedal is pressed, delivering the pressure to the frictional components near the wheels which then press against each other to stop the vehicle. This process has a serious drawback: if there is a leak in the braking system which results in partial or complete loss of the braking fluid, the efficiency of the braking system is significantly reduced or even completely lost. Air brakes remedy this issue by using air instead of any special braking fluid to deliver pressure to the braking components. Since air is readily available for free everywhere on the surface of earth, this significantly reduces the chance of brake failure due to leaks in the braking system. This is the primary reason for the use of air brakes being mandatory under Government regulations for vehicles exceeding a certain size or carrying passengers for commercial purposes.

1.5.2 Reliability

Air brakes are much more reliable than hydraulic brakes. Firstly, most modern air brakes operate on a principle known as the triple valve system, as explained on SDRM.org's Train Air Brake Description and History article, which is designed in a manner opposite to hydraulic brakes or even previously used types of air brakes. In a conventional braking system, the brake is in its released position by default and is activated only when the braking fluid is compressed. Triple-valve system air brakes however, are in the activated state by default and are released only with compressed air pressure. When the vehicle is started, the compression begins and the brakes are released when the vehicle is put to motion. Thus, if there is a leak or even if the compression mechanism completely fails, the brakes revert back to their default, activated position and the vehicle is brought to rest.

1.5.3 Cost Effectiveness

The special braking fluid used in hydraulic brakes is quite expensive. Air on the other hand, is freely available. Although this doesn't make much of a difference in small vehicles which require little hydraulic braking fluid, it does matter significantly in larger vehicles like heavy duty trucks and locomotives which would require high quantities of the fluid if hydraulic brakes were used.

The Disadvantages

Among the disadvantages are the costs of the equipment, which may be greater, compared to other power systems. Also, the high pressure air might be dangerous in the event of a failure.

Read more: Principles of Pneumatics | eHow.com http://www.ehow.com/about_6775965_principles-pneumatics.html#ixzz13jLacVHk

1.6 Literature Survey

1.6.1 Air brake system monitoring for pre-trip inspection

United States Patent 7363127

Inventors: Fogelstrom, Kenneth A. (Fort Wayne, IN, US)

Application Number: 10/862836

Publication Date: 04/22/2008

Filing Date: 06/07/2004

SUMMARY OF THE INVENTION

According to the invention monitoring of the operating variables of an air compression and storage system for a motor vehicle air brake system is provided. The air compression and storage system include a compressed air storage tank and air distribution system, an air compressor coupled to the compressed air storage tank for charging the air storage tank, and a governor responsive to pressure in the compressed air storage tank and air distribution system for controlling cut-in and cut-out of the air compressor. An air pressure sensor is disposed in communication with the compressed air storage tank and air distribution system and is responsive to air storage tank air pressure for generating an air pressure signal which is applied to a vehicle body computer. The body computer operates on the air pressure signal for determining leakage and charging rates for the compressed air storage tank and air distribution system. The determined rates are compared to threshold limits for generating warning indications. Averages over various periods of time may be kept for comparison to the thresholds. The system is provided for vehicles equipped with an air brake system connected to utilize compressed air from the compressed air storage tank through the air distribution system. The system may further monitor position of a brake pedal for actuating the air brake system as modifying monitoring leakage. Results of the comparison are used to alert the driver as to the need of a follow on manual inspection, with such inspection being indicated if thresholds are broken or in case of a lack of data.

1.6.2 Tractor-trailer air-brake system

United States Patent 4210368

Inventors: Sontheimer, Georg (Ulm, DE)

Application Number:05/929955

Publication Date:07/01/1980

Filing Date:

08/01/1978

SUMMARY OF THE INVENTION

These objects are attained in a tractor-trailer air-brake system according to the invention which is provided with a timer connected to the electrically operated valve that is opened for pressurizing and energizing the trailer wheel brakes on energization of the tractor engine brake. This timer periodically closes the valve for the trailer wheel brakes even while the tractor brake is energized, giving these trailer wheel brakes a brief period of time to cool off. During such shutting-down of the trailer wheel brakes when the continuous-duty brake is actuated only the trailer continuous-duty brake system is temporarily shut down.

The system according to the instant invention has been found extremely effective in that brake fade is minimized even during very long downhill runs. In fact the braking force remains effectively constant during the entire downhill run so that the driver is completely freed of the necessity of either augmenting the continuous-duty brake by operating the service brake, or periodically switching the continuous-duty brake off so that it can cool somewhat.

In accordance with a particular feature of this invention the timer means is set up so that it maintains the valve closed for substantially longer than it maintains the valve open. In fact the ratio between the length of time closed and the length of time open for the valve is established by the timer to be at least 5:1 and under normal circumstances is set at about 10:1, with the open time generally at 3 seconds.

According to the instant invention the timer means is in fact constituted as a simple sealed unit which is connected in the hot line that is energized when the continuous-duty brake is turned on. Thus when the continuous-dJohn uty brake is turned on electricity is fed to the timer means which then establishes the on and off pahses of the valve that is connected in the air line going to the wheel brakes of the trailer.

1.6.3 Crash Forensics

Inventors: John C. Glennon

SUMMARY OF THE INVENTION

Truck brakes are often blamed for causing crashes. Most commonly, this claim comes from the truck driver, who is trying to transfer blame from himself to a failure of the truck. When this claim is made, interested parties often assume that a catastrophic failure caused a complete loss of braking force. Stated differently, the assumption is that a defective component of the brake system spontaneously failed causing the brakes to no longer function. In reality, brake systems are designed so that a complete catastrophic failure is an extremely rare event. Therefore, alleged brake failures usually are not failures at all but performance problems stemming from deficient maintenance.

Truck braking systems can usually still provide low levels of braking force even with maintenance deficiencies. This low-level braking force will allow the truck driver to adequately stop the truck for normal operations such as slowing for a stop sign. However, when a high level of braking force is demanded in an emergency, these deficiencies will show themselves. Even though the driver is applying the brakes very hard, he will not get the expected result, which is a high level of deceleration. In this case, the brakes are slowing the truck, but not as quickly as the driver expects them to. Most likely the driver will perceive that the brakes are not working at all. In reality, the brakes are working, but not at the level of performance expected for an emergency application.

Brake ImbalanceAnother brake performance problem that results from poor maintenance is brake imbalance, which can be caused by deficiencies that affect some of the brakes in the system but not others. Brake imbalance can also happen by having mismatched brake system components that cause some brakes to work harder than others. Brake imbalances can lead to instability during braking, brake fade, and brake fires.

Good brake balance is a result of having properly matched, maintained, and adjusted brake system components, as well as a properly loaded trailer. There are two main types of brake balance; torque balance and pneumatic balance. Proper torque balance is created by having matched mechanical components, which are working properly and adjusted correctly.

1.7 SCOPE OF THE PROJECT

Today tractors and trailers fitted with air brakes in agricultural and forestry applications play an important part in the tractor industry. Many vehicle manufacturers are now fitting their tractors with these air compressing systems when they are created at the factory. As well, more tractors are being retro-fitted with air braking equipment therefore more workshops will need to install and support these systems now and in the future.  The trend today is to use tractors with more engine power to carry heavier loads.  Safety is enhanced by using either air or hydraulic braking systems. Responsible farm managers have recognized the advantages of air braking systems and fitted their tractor-trailer combinations accordingly.

1.7.1 Features

 Tractor Trailer

Compressor

Unloader

Air reservoir

Trailer control valve

Coupling head (two, single and control lines)

Line filter

Relay emergency valve with release valve 

Automatic load sensing valve

Brake cylinder

Coupling head (supply and brake)

1.7.2 Benefits

Permits a smooth gradual braking process for the tractor-trailer combinations

Advancement of the braking process for the trailer prevents under running

Buffing of the trailer is prevented

When the tractor-trailer combination separates, the trailer's air brake is automatically activated

The compressed air generating system on the tractor and the air braking system on the trailer can be easily retrofitted

Air braking systems are ecologically friendly for the agricultural environment

Air braking systems can reduce service costs and they are robust and durable

1.7.3 Applications

a) Agricultural tractor and trailer

b) Forestry vehicles

c) City Works vehicles

d) Tractors with self propelled power engines

e) Telehandlers

1.8 FUTURE ASPECTS OF AIR BRAKING

This system resembles the Westinghouse air brake. A locomotive employs a main

reservoir of compressed air which is distributed along a brake pipe running the

length of the train. For each car, a triple valve connects the brake pipe to an auxiliary

reservoir and a brake cylinder. If the pressure in the brake pipe is lower than that in

the auxiliary reservoir, the brakes are applied from compressed air inside the

auxiliary reservoir. If the pressure in the brake pipe is higher than that of the

auxiliary reservoir, the brakes are released and the auxiliary reservoir is refilled from

the brake pipe. Low pressure in the brake pipe (usually still higher than atmospheric

pressure) thus applies the brakes, while high pressure releases the brakes. Due to the

design of the triple valve, a partial brake application is possible while a partial release

is basically not. The driver has a non-self lapping handle consisting of release, lap and

service positions.

1.9 Project Summary

According to a first aspect, the control system having one or more inputs indicative of a vehicle operating state, and an output for determining whether a compressor is on-load or off-load, the system further including target means to calculate in real time a target pressure for a reservoir downstream of said compressor, said output being responsive to said target means.

Control system inputs may comprise one or more of the following variables; engine speed, vehicle speed, vehicle throttle opening, air pressure in a reservoir of the braking system, air temperature at the outlet of the air compressor, dryness of the desiccant of an air dryer downstream of the compressor relative humidity of ambient air, dryness of air downstream of the air dryer, etc.

In a preferred embodiment the control system comprises a first input for indicating vehicle engine speed, a second input for indicating vehicle speed, a third input for indicating vehicle throttle opening, and a fourth input for indicating air pressure in a reservoir downstream of the compressor, the target pressure being higher during throttle-off modes than during throttle-on modes.

In this specification the term ‘throttle’ is used in relation to the vehicle accelerator pedal or other means used to control admission of fuel to the vehicle engine.

Such a system requires a higher target pressure in throttle-off modes when the vehicle is likely to be coasting or slowing down. In such circumstances the fuel supply is normally closed off by the driver releasing the accelerator pedal and accordingly vehicle momentum drives the engine and thus the compressor. The energy to drive the compressor in this mode is ‘free’, at least to the extent that fuel is not being burnt. Additional slowing of the vehicle occurs as a result of the compressor being on-load, but this may be useful where the throttle-off mode is accompanied by or followed by a braking event. In order to take maximum advantage during the throttle-off mode, the target pressure in the air reservoir can be raised above the normal level, and as a result compressor on-time during throttle-on modes can be reduced.

The invention permits a small but significant, reduction in vehicle fuel consumption, and requires only minor adaptation of existing electronic control systems.

In a preferred embodiment the higher target pressure exceeds the normal target pressure by 8-10%. The system may include a third yet higher target pressure to meet high pressure requirements of associated air systems such as air suspension.

A particular advantage of the invention is that the higher target pressure exists during a braking (throttle-off) mode, and where this is the final braking event before the engine is stopped, the reservoir has an extra air charge to give a final purge of the usual air dryer. This is especially useful since the air braking system is left in a dry and clean state at the end of the working day. Also, the vehicle air system is clean and dry at the beginning of the next working day.

Preferably independent control of compressor and purge valve is provided. This ensures that the air line connecting the compressor and purge valve/reservoir is not exhausted each time the purge valve is actuated. Clearly if this air line is exhausted, as has hitherto been the case, the compressor is required to operate for a greater time when brought on load.

Conversely, the normal target pressure may be reduced. For example, if the output from the compressor is very hot, due for instance to hot ambient conditions and significant air demand (and hence significant compressor on-time), the compressed air may approach the temperature at which desiccant in the usual air dryer may be damaged. In such circumstances it may be desirable to reduce the normal target pressure, thereby reducing compressor on-time and permitting the compressor to cool down.

The control system preferably includes an override to ensure that lower target pressures are not imposed during conditions when maximum air volume is required, for example during an emergency braking event.

According to a second aspect there is provided a control system for the compressor of a vehicle air braking system, the compressor being capable of being taken offload at a predetermined target pressure, wherein the control system has an input indicative of vehicle throttle position and is adapted to increase said target pressure in real time at a zero throttle opening.

According to a third aspect there is provided a control system for the compressor of a vehicle air braking system, the control system having a first input for indicating vehicle engine speed, a second input or indicating vehicle speed, a third input for indicating vehicle throttle opening, a fourth input for indicating air pressure in a reservoir downstream of the compressor, and an output for determining whether the compressor is on-load or off-load, the system further including means to calculate in real time a target pressure for said reservoir, the target pressure being higher during throttle-off modes than throttle-on modes.

According to a fourth aspect there is provided a method of controlling a compressor of a vehicle air braking system, the method comprising the steps of:

Provide a control system for the compressor having one or more inputs indicative of a vehicles operating state,

Provide an output from the control system to place the compressor either on-load or off-load depending upon said vehicle operating state,

Provide target means to calculate a target pressure for a reservoir downstream of said compressor, wherein said output from the control system is responsive to said target means.

1.10 REFERENCES

1. WIKIPEDIA.CO.UK

2. HOWSTUFFWORKS.COM

3. EHOW.COM

4. FREE PATENTSONLINE.COM

5. TECHNOPEDIA.COM

6. ENCYCLOPEDIA.COM

7. CRASHFORENSIC.COM

8. www.gnb.ca/0276/vehicle/pdf/ab_manual-e.pdf