AIR SUSPENSION

Air suspension systems are becoming increasingly popular because of certain advantages they possess over the conventional metal springs. These are

1. A variable space for wheel deflection is put to optimum use by virtue of the automatic control devices.
2. Since the vehicle altitude remains constant, the changes in head lamp alignment due to varying load are avoided.
3. The spring rate varies much less between the laden and un-laden conditions, as compared that of that of conventional leaf springs, reducing the dynamic loading.
4. The improved standard of ride comfort and noise reduction attained by the use of air springs reduces both driver and passenger fatigue.

In the lay out shown the four air springs are mounted in the same positions where generally the coil springs are mounted. An air compressor takes the atmospheric air though an air filter and compresses it to a pressure of about 240 Mpa. The same pressure is maintained in the accumulator tank, which is provided by a safety relief valve. This high pressure air goes through the lift control valve and the leveling valves to the air springs as shown. The lift control valve is operated manually by means of a handle on the control panel through a cable running from the valve to the handle. The initial height is adjusted according to the loading conditions.

TORSION BAR SPRING

A torsion bar spring, usually called as a torsion bar is a spring steel rod that uses its torsional elasticity to resist twisting and takes only the shear stresses. One end of the torsion bar is anchored to the frame or other structural member of the body and the other end to a component that is subjected to torsional load.

The amount of energy stored per unit weight of material is nearly the same as that of the coil spring. Torsion bar is oftenly used with the independent suspension. As shown in the figure, the bar is fixed at one end to the frame, while the other end is fixed to the end of the wheel arm and supported in the bearing. The other end of the wheel arm is connected to the wheel hub. When the wheel strikes a bump, it start vibrating up and down, thus exerting torque on the torsion bar, which acts as a spring.

Torsion bar is lighter as compared to the leaf springs and so it occupies less space. As the torsion tubes are much stiffer than the bars, it is preferred. The main disadvantage of the torsion bar is that it does not take the braking or driving thrust so that additional linkages has to be provided for this purposes. The second disadvantage is that the absence of friction force to damp out the vibrations and hence additional dampers are to be provided.

LEAF SPRING


Leaf springs are made up of a number of curved bands of spring steel called leaves sticking together in order from shortest to longest. This stack of leaves is fastened together at the center with a center bolt or U- bolt to prevent the longitudinal movement. Similarly sometimes the leaves are made with pips or projections at the bottom and recess at the top surface. The leaves are arranged in such a way that the projection of the upper spring should mesh in the recess of the lower spring. Also to keep the leaves from slipping out of place, they are held at several places with the clips. Both ends of the longest or main leaves are bent to form spring eyes, used to attach the spring to the frame. 

To adjust the variations in length of the master leaf while the vehicle move across the road irregularities, one end of the spring is connected to the fame through a shackle and the other end is mounted directly on the frame with a pin. For the front suspension, it is a usual practice to provide the shackle in the front side of the spring to reduce the wheel wobble.

Generally, the longer a leaf spring, the softer it will be. Also the more leaves in a leaf spring, the greater the load they will withstand. But on the other hand as the spring will become firmer, the riding comfort will suffer.

The curvature of each leaf is called a nip. As the nip of the leaf is greater, shorter the leaf will be. Each leaf curves sharply than the one above the stack. When the center bolt is tightened, the leaves flatten somewhat and causing the ends of the leaves to press very lightly against one other.

The suitable steels that have been used for the manufacture of leaf springs are chrome-vanadium steel        (C-0.46%, Cr-1.4%, Va-0.18%), silico-manganese steel (C-0.52%, Si-1.95%, Mn-1.05%) and carbon steel (C-0.55%, Mn-0.6%, Si-0.2%).

Types of leaf spring:         

a)  Semi-elliptic type spring  

b)  Quarter elliptic spring   

c)  Transverse type       

d)  Helper springs

The semi-elliptic type leaf spring is the most common type in use where, the spring is attached to the frame at its middle to the axle. One end is connected through a shackle and the other end is connected to the frame through a pin.The quarter elliptic type spring is a cantilever type spring, which is pivoted at its one end and the other end is shackled or pivoted to the axle. The short leaves in this type of springs are arranged in at the top. This type is not in common use now.

Transverse type spring is arranged transversely to the vehicle or parallel to the axle. This spring is rigidly bolted to the frame at its center. Both the ends of the spring are connected to the axle through the shackles. The disadvantage in using this type of spring is that the vehicle tends to roll at the turns since the frame is clamped only to their centers.

The helper springs or auxiliary springs are provided in addition to the main leaf springs when the vehicle is meant to carry heavy loads. This will allow a wide range of loading. Helper springs are an additional set of leaf springs clamped with the same U– bolt on the top of the main spring. Generally the helper springs are used in the rear side only. When the vehicle is lightly loaded, these helper springs will not take any loads and will come in action and share the loads only after certain deflection of the main leaves.

COMMON RAIL DIRECT INJECTION   [CRDI]

In CRDI, we have sensors for engine speed, air mass flow, crank speed, cam shaft position, turbo air boost pressure, fuel rail pressure, air temperature, coolant temperature, accelerator pedal position etc.Thus it is clear that the functioning of all electronically controlled fuel control systems depends on the inputs from different types of sensors. The ECU (Electronic Control Unit) receives these signals from sensors and after manipulation and calculations sends outputs to vary the injection timing, fuel quantity, ignition timing etc and also to control systems like the fuel injection pump, idle speed control unit, particulate trap regenerator (in a diesel engine), coolant supply etc.