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Understanding and maintaining wastegate devices on engines

If there were no means to control boost pressure, it is possible through a series of events that the cylinder pressure could exceed safe limits for the engine design.

In the industry, we like to say, “Turbos make small engines think they are big ones!” Regardless of the size of the engine and whether it is gasoline or diesel, for every 14.68 psi of boost pressure, the engine thinks it doubled in size.

This is rooted in the fact that atmospheric pressure is nominally considered to be 14.68 psi. So, if the engine sees that same value in boost pressure, then it has the potential to produce the power of one twice its displacement. 

Though most manufacturers monitor or rate turbocharger boost in psi, some will use the abbreviation atm, which stands for atmosphere. The metric equivalent of this measurement is bar. To convert atm to psi, multiply by 14.68.

For example, if the engine specification sheet states that maximum boost pressure is 2.1 atm, it would be 14.68 times 2.1, which equals 30.83 psi. One bar is equal to 0.9869 atm, or 14.5 psi. Thus, a pressure of 3 bar would be equal to 43.5 psi. 

For a quick calculation, most people use 15 psi as the nominal atmospheric reading.

With this established, turbocharger boost as read in the intake manifold is the pressure above atmospheric. On a normally aspirated engine at wide-open throttle, the pressure in the induction system is considered to be atmospheric. It is slightly less than that due to flow losses. In contrast, vacuum is any pressure less than atmospheric.

A turbocharger has two main components: a turbine and a compressor. Colloquially, these are the hot and cold sides, respectively. The turbine is connected to the exhaust of the engine and is linked to the compressor via a shaft. 

The fins on this shaft are angled the opposite way. As hot exhaust gas exits the cylinder head, it expands and rotates the turbine wheel in the same fashion that a river operates a water wheel. Since the compressor is on the same shaft, as the turbine rotates, so does the compressor wheel. The turbine is the drive member, while the compressor is the driven member. 

The action of the compressor sends air into the induction path of the engine, which has two important effects. It raises the pressure that enters the cylinder to above atmospheric pressure, and it increases the volume of airflow into the engine measured in cfm (cubic feet per minute). 

The cumulative effect of the increase in pressure and the mass of the incoming air result in the boost pressure. The speed of the turbine and, in turn, the boost pressure is controlled by the exhaust gas flow and temperature. A need to regulate the pressure is required and is accomplished in most applications with a wastegate.

The wastegate

If there were no means to control boost pressure, it is possible through a series of events that the cylinder pressure could exceed safe limits for the engine design.

The wastegate is employed to control boost pressure via bypassing a controlled amount of exhaust gas from interacting with the turbine wheel. It consists of nothing more than a disk that closes against a passageway that redirects a portion of the exhaust flow.

When the passage is open, boost pressure is limited. When it is closed, the full potential of the turbocharger can be realized.

It must be recognized that every turbocharger is a sophisticated piece of engineering since there is a dedicated science to the shape and size of both the turbine and compressor wheels. The airflow and pressure potential are created by the design of the two wheels and, as with every aspect of engineering, there are compromises.

Preventing overboosting

The wastegate allows the engineer to create a turbocharger that can deliver the desired performance at low- to mid range engine speeds while not overboosting under full engine load. It can also allow the turbine wheel to accelerate quicker at low exhaust flow and temperatures, bringing in boost pressure sooner and making the engine more tractable when being lugged. 

An additional benefit of turbocharging beyond engine power is a general reduction in engine emissions and improved efficiency. 

The increased air pressure and flow into the cylinder creates more turbulence in the bore and, in turn, improves flame speed and mixes the fuel and air more thoroughly. The wastegate allows an engineer to use the mixture motion to decrease emissions while also keeping the combustion pressure in check. Yet, the wastegate cannot accomplish this all by itself.

A turbocharger, when equipped with a wastegate, also uses an actuator. The actuator resembles a cannister with a rod and is attached to the turbocharger.

This unit connects to the wastegate with a rod and is responsible for moving the disk away from the passage for exhaust flow control. Inside the cannister, there are a bellows and a spring along with a port that connects a rubber hose to sense boost. The internal spring positions the rod to keep the wastegate port closed. Now all of the exhaust will go to the turbine wheel.

On the other side of the bellows manifold (boost), pressure works in opposition to the spring, wanting to move the rod and open the wastegate. Depending on the tension of the spring at a desired boost pressure, the bellows will take over and open the bypass passage, thus, limiting turbine speed and pressure in the cylinder.

Most engines with electronic controls also employ a solenoid to signal to the bellows in the actuator. This allows for quicker spool-up of the turbo at low speeds while adding more finite control to maximum boost. Some older diesel engines do not use a wastegate. In those applications, the turbo is designed to produce the desired maximum boost so no safety is employed. A turbo without a wastegate is not as efficient since it does not compensate for weather and combustion conditions. It is also lazy to spool up, making the engine less responsive.

Potential problems

Though the wastegate system is very reliable, the following are common and easily corrected conditions.

  • Low boost. The cause of the problem in this case is the disk in the wastegate housing is not sealing because of carbon and bypassing exhaust. The actuator spring is either weak or failed.
  • Overboost. The cause here is a rubber cracked, failed boost-sensing line, failed bellows or a bellows that is leaking internally. If so equipped, another cause is the boost solenoid has failed or has lost its electrical signal. 
  • Fluttering boost. The spring in the actuator is weak.

For any of these problems, the turbocharger does not need to be replaced or rebuilt. These are all external parts and often can be serviced with the unit on the engine. If the wastegate rod is threaded with a lock nut, when you shorten the rod the boost pressure will increase before the exhaust is bypassed. If you lengthen the rod, the boost will be lower.

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