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Vehicle Dynamics


Published on Jan 28, 2016

Abstract

Vehicles used for transport of loads have their efforts on the axles very close to the allowed or critical limits mainly during its travel on a bumpy surface or during cornering. In such cases the use of conventional suspension systems can increase the axle's overload phenomena. Hydropneumatic suspension leads to an even distribution of load per axle, thereby decreasing the overload problem and simultaneously increasing the efficiency and comfort levels.

About the Suspension System:

This suspension system was invented in the late 50's by Citroen® and has been fitted to many of their cars since. As its name suggests, its core technology and mainstay of its functionality is hydraulics. Superbly smooth suspension is provided by the fluid's interaction with a pressurized gas. This system is powered by a large hydraulic pump operated directly by the engine in much the same way as an alternator or an air- conditioner is, and provides fluid toan accumulator at a pressure, where it is stored ready to be delivered to servo a system.

The spheres are like the springs on your cars, and the struts and the hydraulic components that make the fluid act like a spring. There is a hydraulic component called an accumulator, which is gas under pressure in a bottle contained within a diaphragm, effectively a balloon which allows pressurized fluid to compress the gas, and then as pressure drops the gas pushes the fluid back to keep the system's pressure up. As you can see in the drawing, the pink gas(Nitrogen) is compressed when the pressure in the green fluid overcomes the gas pressure and pushes back the diaphragm, which compresses the gas. Then as the pressure of the fluid decreases, the gas pushes back the diaphragm and as the gas overcomes the fluid, it expels the fluid from the sphere, returning gas and fluid to equilibrium.

The difference, comparing with conventional suspension system is the gas spring instead of a mechanical spring, and the hydraulic fluid passing through the valve, where the energy is dissipated without using additional dampers, achieves the damping.

The most important item of this system is the gas chamber, therefore the stiffness will be defined basically, by the pressure and volume contained within the chamber. In some vehicle (like the Citroen® CX series), the chamber is built from two different parts that then join to take the shape of a sphere, and the gas is separated hydraulic fluid by a flexible diaphragm.

The hydraulic fluid that drains out of the system is stored in a reservoir and thereby returned to the system using a hydraulic pump, keeping the vehicle height constant throughout. A semi active control can be done through an adjustable valve that increases or decreases the damping. Controlling the level of the hydraulic fluid either manually or automatically may do an active suspension.

One or two of the more obvious ones are that since the system is hydraulic, the ride height can easily be altered, a trend low riders are now following on with in California, nearly fifty years later. Also, they could link the four corners together to make a system that prepared the car for the bump to keep it even and offer the passengers a smoother ride. Basically they put fancy valves called height correctors on the anti-roll bar. These were mounted in such a way that as the suspension twisted, this operated the valves that controlled the transfer of fluid to the struts

Theoretical determination of hydropneumatic spring characteristics

In the following development the gas is considered to be inert so that it would not react with the sides and thereby change its characteristics during working. The ideal gas model is considered and an isothermal process as well. This assumption does not represent exactly the realistic conditions due to the heating of gas during the process. If the system is running at a very high frequency, there isn’t time to allow
heat exchange with the environment, and a hysterises phenomenon will occur. This effect will not be considered once the energy starts to dissipate from the system.

According to the hypothesis mentioned above,

PV=nRT & …(1)
PV=constant …(2)
Where P is the pressure, V is the volume and T is the temperature of the gas.

To find the spring stiffness function which represents a force (F) done by the hydraulic cylinder as a function of displacement (x) must be developed. The initial state is where the force is zero for null displacement. This condition only occurs when the gas pressure inside the chamber is equal to the atmospheric pressure. The figures (a) and (b) show the forces and pressures for the initial condition and generic condition respectively.

Recent trends in Hydropneumatic Suspension Systems:

Recent developments - which have seen a shift toward active, individually adaptable hydropneumatic suspension systems - have extended the range of available solutions.

In order to be able to use the diaphragm accumulator at both maximum and minimum axle loads, a hydraulic opposite load is built up in the cylinder ring chamber and generally controlled at a constant level.

The Simrit system uses a pressure control valve to keep the ring chamber pressure at a constantly low level and regulate it with every new control procedure. This allows for two different strategies.

The first generation hydro pneumatic suspension system works as follows: the pressure level in the ring chamber is adapted by the structure in such a way that the components and accumulator design are harmonized for completely different types of vehicles. This reduces the number of part designs and increases quantities in an economical way. The size of the suspension cylinder is adapted in such a way that the reduced difference in pressure at the cylinder seals is beneficial for the suspension function as it creates low frictional behaviour. Moreover, this variant is not dependent on maximum pump pressure.

The second-generation hydro pneumatic system was developed for suspension systems whose axles are actually subjected to some really very high load ratios.
Here, the pressure level in the cylinder ring chamber is automatically controlled by the axle load.

Once the minimum axle load falls below a structurally fixed level, the pressure is automatically raised to a higher level. Similarly, once the minimum axle load is exceeded, the pressure is reduced to a lower level.

The fact that the pressure in the ring chamber can be adjusted means that load ratios of up to 1:20 can easily be managed very easily by changing the ring chamber pressure

Changing the ring chamber pressure influences the axle's elastic spring rate so that the functions can accurately be adapted to suit the operating conditions.
This innovative suspension system's variety of adaptation possibilities allows more effective use to be made of the functional advantages of a hydropneumatic suspension system no matter what the operating conditions.

In short, Simrit offers two different types of hydropneumatic suspension system, both of which allow different types of vehicles to fine tune differently at variable pressure ranges

The standardization of components also allows samples to be produced very quickly.

The self-adapting settings of these intelligent, active systems open up completely new opportunities.

Consequently, planned developments such as combined front axle, cab and rear axle suspension will exploit the full safety potential that is of such vital importance for tractors that are travelling at ever-increasing speeds.









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