Functions:

During normal operation this valve permits a particular value of hold-off pressure to the spring brakes thereby keeping it released. This hold-off pressure can be varied from 4 bar to 7 bar.

During parking of the vehicle, this valve quickly exhausts air from the spring brakes thereby allowing a fast application of the spring brakes

Prevents compounding of service brake and spring brake

Modulates application of spring brakes in the event of a failure in the primary service circuit

Current layout and function

The piping diagram indicating the positioning of the inversion valve along with the double check valve and the relay valve is indicated in figure 1.

Figure 1 – Current layout

a) Providing hold-off pressure to release the spring brakes: With the park control valve in the ‘on’ condition, air pressure from the reservoir flows through the park control valve and through the double check valve to the control port of the inversion valve. The inversion valve upon receipt of this signal air pressure communicates supply air to delivery. This delivery air pressure is then used as the signal air pressure to the relay valve. The relay valve on receipt of this signal delivers hold off air pressure to the spring brakes thereby releasing the brakes.

b) Applying spring brakes: With the park control valve shifted to the ‘off’ condition, signal air pressures to the inversion valve and consequently to the relay valve are depleted. This causes air in the spring brakes to quickly exhaust through the relay valve thereby applying the spring brakes.

c) Preventing compounding of service brake and spring brake: With the spring brakes in applied condition, operation of service brakes will result in air from the primary delivery of the dual brake valve to flow through the double check valve and then on to the control port of the inversion valve. Spring brakes are then released as detailed in (a) thereby preventing the compounding of the service brake and the spring brake.

d) Modulation of the spring brakes: In the event of failure in the primary circuit, air from the secondary service is used to modulate the spring brakes with the help of the relay valve.

Inversion valve with anti-compounding and relay

In the inversion valve, all the four functions are achieved with one valve. The functions served by the double check valve and the relay valve are integrated with the inversion valve.

The major components of the inversion valve are the valve body, top cover, primary piston, relay piston, an integrated double check valve, inlet/exhaust valve and graduating spring (refer figure 2).

This valve has five modes of operation and they are detailed below:

1. Spring brakes released

When the system is being charged, air pressure enters the supply port and is present in the cavity beneath the inlet/exhaust valve. The inlet/exhaust valve is in contact with the inlet valve seat in the body thus preventing passage of air to delivery as well as through the exhaust. When the full system pressure is reached, the ‘park control valve’ can be operated to release the spring brakes. The park control valve is a manually operated on/off valve. Shifting the park control valve to the ‘on’ condition ensures continuous supply of air pressure at the spring brake control port of the inversion valve. Pressurised air at the spring brake control port then enters the double check valve, deflects the diaphragm, and flows into the cavity on top of the primary piston through the cross-hole provided in the top cover. This air pressure forces the primary piston to move downward and contact the relay piston. The relay piston then moves downward and contacts the inlet/exhaust valve. Further movement of the relay piston opens the inlet passage thereby allowing air to flow from the supply port to the delivery ports. Consequently the spring brakes are released. At the same time the exhaust valve seat in the relay piston prevents loss of air through exhaust. The valve in this mode of operation is illustrated in figure 4.

Figure 4 – spring brakes released

The limiting value of the hold-off pressure (or pressure at the delivery ports) to the spring brakes can be varied between 4 bar and 7 bar by changing the spring load on the relay piston with the help of the adjustment screw. The air pressure that is communicated to the delivery ports also acts on the underside of the relay piston. When the airhead load on the relay piston equals the load exerted by the graduating spring, the valve is in the ‘balanced’ condition. In this condition the inlet valve seat and the exhaust valve seat are in the same plane thereby preventing further flow of air to delivery ports and at the same time preventing loss of air through exhaust. This condition is illustrated in figure 5.

Figure 5 – Balanced condition

2. Spring brakes applied

The park control valve is switched to the ‘off’ position to apply the parking brakes. Consequently, air pressure at the spring brake control port and on top of the primary piston is exhausted through the park control valve. The primary piston then moves upward. Airhead load beneath the relay piston forces it to move upward thereby opening the exhaust passage and sealing the inlet. Air from the delivery ports is then exhausted thereby applying the spring brakes. This condition is illustrated in figure 6.

Figure 6 – Spring brakes applied

3. Simultaneous application of service brakes and spring brakes applied (Anti-compounding)

Application of service brakes provides air pressure at the primary service port P1 and secondary service port P2. There is no air pressure in the spring brake control port since the spring brakes are applied. Air entering the service port P1 deflects the diaphragm in the double check valve and seals the passage in the spring brake control port. Air then flows through the cross hole provided in the top cover to the top of the primary piston. This air pressure forces the primary piston to move downward and contact the relay piston. The relay piston then moves downward and contacts the inlet/exhaust valve. Further movement of the relay piston opens the inlet passage thereby allowing air to flow from the supply port to the delivery ports. Consequently the spring brakes are released thereby preventing the compounding of the service brake and the parking brake. This condition is illustrated in figure 6. Continuous application of the service brakes results in the ‘balanced’ condition explained earlier.

Figure 6 – Service brakes applied with spring brakes released

Air that enters the valve body through the ports P1 and P2 gets neutralised since the pressure and sealing areas on the top and bottom of the relay piston are essentially equal.

4. Application of spring brakes with primary in failed condition (Modulated application)

In the event of failure in the primary circuit, air pressure, upon application of service brakes will be available only in the secondary service port P2. However, air pressure will be available at the spring brake control port and consequently the delivery ports thereby keeping the spring brakes released. Air that enters the valve through the secondary service port P2 acts beneath the relay piston and forces the piston to move upward thereby exhausting air from the delivery ports and applying the spring brakes. The air released from the spring brakes will be proportional to the pressure at secondary service port P2 thus providing the driver with a modulated application of the spring brakes. This condition is illustrated in figure 7.

Figure 7 – Application of service brakes with primary in failed condition

Unique features of the valve

1. Integrated relay and inversion functions

In a typical air brake circuit that complies with FMVSS 121 regulations, all the above-mentioned functions are achieved by a combination of three different valves, namely, a double check valve, an inversion valve and a relay valve. However, in the inversion valve the functions accomplished by the combination of the above three valves have been integrated into one single valve. Therefore, the inversion valve can be directly replaced with the combination of the three valves.

2. Inversion, pressure-limiting and relay functions are achieved by the same valve

The same combination of relay piston and inlet/exhaust valve in the inversion valve is used to achieve all the three functions, namely, inversion, pressure limiting and relay. Normally the pressure limiting and inversion functions are achieved by the combination of piston and inlet/exhaust valve in the inversion valve, and the relay function is achieved by another similar piston/valve combination in the relay valve.

3. Pressure limiting is achieved by caging the spring

Limitation of hold-off pressure to the spring brakes is achieved by caging the graduating spring between the spring guide and the spring retainer. Normally in such applications one end of the graduating spring is always made to rest on a stationary surface. In ’s valve the unique feature is that the entire spring assembly is in a caged condition and allowed to move along with the relay piston even as it provides the necessary load to achieve the pressure-limiting function.

4. Split arrangement of relay piston

The split arrangement of relay piston makes the valve more stable and insensitive to pressure lead from primary service port. Also the split arrangement of relay piston makes the valve resistant to false actuation of the valve during a traction control situation.