Turbochargers : What is Surging?

As we saw in our previous articles, a turbocharger uses the exhaust gas of the engine to drive the turbine and compressor. We have also seen the various parts of a turbocharger and their functions. The engine is subjected to a variation in load which results in the fluctuation of exhaust gas pressure. This fluctuation of in pressure makes the turbocharger run on varying speed, which leads to many unfavourable effects on the turbocharger. Let's define surging and take a close look at this effects of turbocharger surging on the performance of ship's engines.

Surging of turbocharger occurs when the air pressure after the compressor is higher than the pressure compressor can internally maintain. This means, when the pressure of the air delivered by the compressor is higher than the pressure inside the compressor a reverse flow of air is created towards the impeller and inlet of the compressor, which reduces the speed of the turbine shaft and creates noise and vibration.

Surging can better be understood by drawing a graph of pressure ratio against mass airflow of the system. From the graph it can be seen that surging is an unavoidable phenomena. The efficiency of compressor is highest near the surge line. This means that if high turbocharger efficiency is desired, a compromise between high efficiency or surging needs to be made.

Surging leads to a sharp fall in the flow and acceleration of air mainly because of the reversal pressure. This imbalance in the demand and supply also leads to heavy damage of the turbocharger

click to enlarge

click to enlarge

The main reason , as discussed above, is the deviation of the pressure and operating condition of the turbocharger from the set condition. Apart from that surging can also occur due to any or all of the following reasons.

• It can also happen due to sudden change in the engine load or speed.
• Imbalance in cylinder power or faulty injectors
• Un-cleaned turbine nozzle ring.
• Damaged blades
• Dirty or choked filter
• The capacity of turbocharger is larger than required.
• Increased back-pressure at the turbine side.

Sometimes a dirty hull that makes the ship run at full torque has also been shown as the reason for surging. Malfunction of engine's fuel system may also lead to surging.

There are mainly there things on which the the functioning of turbocharger depends. They are :

• Pressure ratio
• Air volume flow
• Speed

When the air enters the compressor it follows the direction of diffuser vanes. The radial velocity attained by the rotational motion of the impeller is converted into pressure by the diffuser. This increases pressure at the compressors outlet.

When surging occurs, due to the reverse air flow the velocity angles are disturbed which causes breakdown of the boundary layers. Turbulence is created near the boundary which reduces the air flow area, causing resistance. When the turbulence increases beyond a certain limit, the diffusion of air drastically reduces leading to reduced pressure. Thus the pressure downstream of the diffuser goes higher than the diffuser pressure, leading to increase in reversal of air flow.

Introduction to Marine Engineering 2nd Edition by D.A Taylor

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