Application of High Temperature Mechanical Sealing Device in Exhaust Assist Braking System

Fluid sealing under high temperature and high pressure conditions has always been a difficult problem in engineering. Elastomeric seals and mechanical seals are the two main forms of current technology. Ordinary engineering plastics, synthetic rubber and other sealing components, although excellent sealing performance, it is difficult to adapt to high temperature conditions. The mechanical sealing components are made of a metal material and have excellent high temperature resistance. This paper introduces a high temperature resistant mechanical seal for use in automotive exhaust auxiliary brake systems. The device is installed near the engine, and its exhaust gas discharge temperature is higher than 700 °C, the environment is bad, and the brake effect is satisfactory after practice.

Mechanical seal device main structure

The structure of the mechanical seal is shown in Figure 1. The valve seat 5 is fixed to the exhaust manifold outlet of the engine by a three screw hole, and the other end of the valve seat is connected with the exhaust pipe flange. Figure 2 is a partial enlarged view of the sealing portion of the brake system.


Figure 2 Partial magnification of the mechanical seal structure M


Brake function realization

The vehicle exhaust is discharged into the atmosphere through the exhaust manifold, the exhaust auxiliary brake, the exhaust pipe and the muffler under high temperature and high pressure. The rotating shaft is driven by the solenoid-controlled cylinder to open and close the butterfly valve plate fixed by the force-transmitting fork to control the exhaust of the vehicle from the exhaust manifold, affecting the compression ratio of the two ends of the engine piston. The back pressure is formed, the crankshaft is reduced in speed by the connecting rod, and energy is consumed, thereby achieving an auxiliary braking effect. Because the device is resistant to high temperatures, it can be directly connected to the manifold outlet and adjacent to the engine cylinder, so its back pressure can react quickly and in real time.

It can be seen from Fig. 1 and Fig. 2 that the rotating shaft 4 and the sleeves 1, 6 have a certain movement gap, and the outer circumference of the sleeve 1 and the valve seat 5 are interference fit, which are blind holes, and there should be no gas leakage problem. The outer diameter of the sleeve 6 and the valve seat 5 also have an interference fit, and the inner hole has a moving relationship with the rotating shaft, and the gap is unavoidable, and the gap is the only passage for the high temperature and high pressure exhaust gas to escape. High temperature and high pressure gas will have a certain amount of exhaust gas leakage when the butterfly valve 3 is closed (brake) and opened (non-braking). In severe cases, whistling will occur and the surrounding environment will be deteriorated. The movable washer rotates together with the rotating shaft, and the static washer and the valve seat are fixed, and the gap between them forms a cavity.

Reasonable selection of the number of gaskets, separated into a number of cavities, forming a step labyrinth path, can extend the gas escape path, as shown by the arrow in Figure 2, increase the resistance along the path. Reasonably increasing the number of moving and static gaskets can also reduce the volume of the cavity, making the upstream and downstream throttling effects more obvious, increasing the pressure loss of the fluid, and making the energy consumption in the step-by-step throttling process. Within the engineering tolerance, the sealing requirements are met.

Tolerance matching

In the actual assembly process, if the inner hole of the moving washer and the outer diameter of the shaft, the outer circle of the static washer and the seat hole are too loose (the gap is too large), the gasket will generate axial turbulence, and the airflow will take a shortcut. Axial escape; if the fit is too tight (interference fit), the assembly difficulty is increased, and the interference is large, which will deform the gasket, affect the formation of the axial cavity, and hinder the rotation of the rotating shaft, resulting in brake control. Failure.

When selecting the appropriate type of compounding, it is necessary to consider the two states of high temperature and normal temperature, as well as constraints such as assembly processability.

The principle of matching is as follows: first, the gap between the axial spacers is as small as possible; second, the radial matching micro-gap or no gap; third, the cavity formed by the elements is as small as possible.

In principle, the material of the parts is made of stainless steel, and the axial thermal expansion properties are similar. The axial clearance of the gasket is controlled at 0.05 to 0.10 mm, and it has been proved by experiments that this clearance range is feasible. The radial fit of the gasket to the hole and the shaft is made by a transition fit K7/h6 (base shaft system), H7/K6 (base hole system) or micro clearance fit H7/h6, which have been compared and verified. The difference between the two is that the mating interference ratio of the transitional K7/h6 and H7/K6 is 42% to 45%, which is more important than the improvement of the sealing property, while the micro gap matching H7/h6 tends to be assembly processability.

After the axial dimension of the gasket is determined, the selection of the outer diameter of the movable gasket and the inner diameter of the static gasket determines the volume of the cavity, and the volumetric cavity of the upper and lower sides can improve the interchangeability of the parts, which is advantageous for tissue production and cost reduction.

How to ensure the uniform axial gap between the dynamic and static gaskets is the assembly point, and special tooling needs to be designed to ensure.

Conclusion

The high-temperature automobile exhaust auxiliary brake assembly has been verified by the Jiangling N800 project and has been put into mass production and has applied for a patent.

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