Boost Control Operation

23-04 Investigate the boost control features on a turbocharged engine.

Boost is controlled by using a waste gate and/or a BPV. The expansion of exhaust gases produces boost, increasing exhaust air temperature and pressure that generates an excess of exhaust airflow, further increasing boost. This requires the need to control the increased airflow, regulating boost. Excessive turbo boost can lead to engine or turbocharger damage and detonation if left uncontrolled. Waste gates are used to control maximum boost and BOVs/BPVs are used to relieve boost pressure on deceleration. Most current late-model waste gate actuators and blow-off or BPVs are controlled by an electronic solenoid that receives its input from an ECM.

Waste Gates

A traditional waste gate is a mechanical pneumatic device that bypasses some exhaust flow around the turbine wheel to control maximum boost once a preset pressure has been reached. When a turbocharger’s exhaust driven turbine wheel spins fast enough to reach maximum boost, a waste gate is used to bypass exhaust pressure around the turbine wheel downstream into the exhaust. A waste gate is basically a rather simple valve controlled by a pressure actuator that turns boost pressure into a mechanical action to open the waste gate. The actuator is connected to the boost pressure of the turbocharger, which is usually sensed either from the compressor housing of the turbocharger or from the intake manifold. Bypassing exhaust gases slows the turbine speed, preventing the turbocharger from over-revving and limiting boost, which prevents engine and turbocharger damage.

Similar to other pumps on a vehicle (oil pumps and fuel pumps), turbochargers are designed to create more boost than is needed. This allows for maximum boost (and maximum airflow) below maximum engine rpm. Likewise, creating more boost at lower rpm results in excessive boost output at higher rpm, which is why a waste gate is used. The waste gate valve position is continuously varied based on engine load; it is not just open or closed. Left unregulated, the turbocharger would continue to build speed as exhaust flow increased, eventually damaging the engine.

Waste gates are located on the exhaust (hot) side of the turbocharger generally between the turbine wheel inlet and exhaust manifold, and they contain an inlet and an outlet to mechanically control the exhaust (hot) side of the turbocharger. The waste gate consists of two parts: the body, which houses the moveable valve/plunger and its stationary seat, and the actuator, which contains the diaphragm and a spring (FIGURE 23-31).

FIGURE 23-31 The waste gate controls the pressure inside the exhaust side of the turbocharger by directing excess pressure into the exhaust system.

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Waste gate operation involves a spring that works in the closing direction while pressure in the diaphragm works in the opening direction. The ECM supplies a pulse-width/duty-cycle-controlled solenoid that allows boost or vacuum pressure, depending on the manufacture, to overcome the spring force moving the actuator rod, opening the waste gate valve. The waste gate opening is variable and controlled by varying the duty-cycle/pulse-width–modulated signal to regulate turbine speed.

At rest (light loads), the waste gate is held closed by an internal spring in the actuator that routes all the exhaust gases to drive the turbine, creating boost pressure. At high loads, the exhaust gas volume is increased and may reach the maximum safe limit of the engine. When boost pressure exceeds the preset maximum, it compresses the spring, progressively or completely opening the waste gate valve. This allows some of the exhaust gases to bypass the turbocharger, regulating its speed and boost output. Not all vehicles use a waste gate. Some late-model OEM vehicles are fitted with a VGT, using adjustable vanes in the turbine to regulate boost instead of using a waste gate.

Waste gates can be internal (plunger style), commonly found on OEM turbochargers, or external to the turbocharger and located in the exhaust manifold. Both are controlled by actuators and operate in the same way to bypass the exhaust around the turbine to control boost. Location and physical design mark the differences between the two.

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Ensure that the control for any waste gate is properly routed to pressure, not manifold vacuum. Failure to properly control the turbocharger can result in damage to the engine or turbocharger. Later-model systems may use vacuum from an ECM-controlled solenoid.

In an internal waste gate setup, a mechanical “flapper door” blocks a hole on the turbocharger’s exhaust housing. The flapper door is connected to a linkage that is attached to an external pneumatic actuator. The actuator is a separate part that bolts onto the turbo with an arm that opens and closes the door. The actuator houses a diaphragm in a sealed chamber that is referenced to boost pressure and a spring that is used to keep the door closed. The boost pressure reference is usually found on the compressor outlet cover of the turbocharger. When boost pressure overcomes spring pressure, the valve opens and boost is controlled. The discharge air pressure is routed back into the exhaust system due to emissions in OEM systems. Internal waste gates are typically smaller and less expensive to the manufacturer than external versions are.

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Conversely, some manufacturers use a normally open waste gate, which operates opposite to the turbocharger control systems used by other manufacturers. The default in this system (open waste gate) bypasses the turbine, reducing the chance for mechanical failure to cause an overboost condition. Applying vacuum to the actuator from a control solenoid referenced to boost pressure closes the actuator, allowing exhaust gases to drive the impeller, creating boost. Understanding normal system operation is critical to correctly diagnosing faults in any system.

External waste gates are located outside the turbo housing, are integrated into the exhaust manifold or header, and are typically used for high-boost aftermarket applications (FIGURE 23-32). Instead of using a “door” to block the exhaust, a valve is used. The valve and actuator control are self-contained in one piece. The valve blocks exhaust flow when closed, sealing against its seat. When boost pressure overcomes spring pressure, the valve also progressively opens to bypass exhaust gases. External waste gates can reintroduce exhaust gases farther downstream of the turbine, increasing performance. In a race application, excess exhaust flow can be directed to the atmosphere. External waste gates are not limited by size due to location, offering performance benefits. By increasing the size of the diaphragm and the inlet and outlet ports and by increasing spring pressure, external waste gates are ideal for use in systems designed to operate higher boost pressures.

FIGURE 23-32 An external waste gate is a high-pressure calibrated spring that will allow the valve to open up once the pressure inside the exhaust system reaches a certain pressure, measured in psi or kPa. This is directly calibrated to the operation of the engine and how it is engineered.

Waste gate operation is limited to 8–15 psi (55–103.4 kPa) on most production vehicles. OEM systems are typically not adjustable. If a locknut is used on the linkage, there may be an antitamper paint dot that covers the nut and linkage to indicate whether the adjustment has been tampered with. Some vehicle manufacturers do have a procedure to reset the waste gate to original specifications; refer to service information to perform this procedure.

Aftermarket systems can be fully adjustable, either by changing the actuating rod length; by venting an adjustable amount of control pressure, which increases boost pressure; by turning and adjusting a screw; or through a programmer. Electronically controlled waste gates, be they OEM or aftermarket, offer more accurate control under a variety of operating conditions, such as poor fuel quality, atmospheric pressure changes, and engine load and rpm based on inputs from various engine sensors (FIGURE 23-33).

FIGURE 23-33 In a performance application, it may be necessary to increase the boost pressure inside the intake, which will require an aftermarket adjustable waste gate to change when the waste gate will limit how much boost the turbocharger generates.

Waste gate failures are rare but can still occur. The most common problem is that the spring becomes weak and loses tension due to the extreme heat generated by a turbocharger. When the spring becomes weak, the waste gate can open prematurely, resulting in a loss of boost pressure. Other failures that can occur are a ruptured or split diaphragm and the linkage or spring can fracture sporadically. Waste gates can also stick or bind, resulting in sluggish operation or a failure to operate.

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To increase boost levels beyond the intent of the original design, an aftermarket waste gate actuator is usually required. OEM waste gates are engineered to be just enough to handle stock boost levels. When boost is increased, the stock actuator is unable to keep the waste gate closed. Boost pressure in the exhaust, turbine inlet pressure, can build against the bottom of the waste gate valve, overpowering the spring in the actuator that is holding the valve shut. When this happens, the waste gate can open at lower-than-designed boost levels, reducing performance. To solve this problem, aftermarket actuators are built with a larger spring to keep tension on the waste gate, holding it closed at higher boost levels. A bigger diaphragm is needed to overcome spring pressure when activated, increasing the actuator housing size. This allows the turbo to rapidly reach peak boost levels and maintain boost pressure throughout the engine’s entire rpm range.

Boost Pressure Release Valve

The BPV or BOV releases excess boost pressure on the inlet side (cold) of the turbo, between the turbocharger outlet and throttle body, when required. Releasing inlet pressure reduces compressor surge and extends the longevity of turbochargers and centrifugal superchargers. A BOV/BPV is installed in the pressure side (cold) of the intake tract, which is normally located between the turbocharger and throttle body. Excess boost pressure in the compressor results from an abrupt change in engine speed during a rapid closing of the throttle plate under a boost condition (FIGURE 23-34). If an engine is equipped with BPVs/BOVs, each turbocharger will have one. The position and control of the valve may vary from manufacturer to manufacturer.

FIGURE 23-34 A BOV is used on the “cold” side of the intercooler system. This side comes from the intercooler to the intake manifold. The valve allows pressure to escape when the throttle is suddenly closed because of the vehicle being stopped because of the excess pressure that is built up in the system.

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Verify that the control for any waste gate is properly routed to pressure, not manifold vacuum. Failure to properly control the turbocharger can result in damage to the engine or turbocharger. Later-model systems may use vacuum from an ECM-controlled solenoid.

A normally closed valve located in the cold side (pressurized) of the turbo piping uses manifold pressure (low vacuum) and internal actuator spring pressure to keep the valve closed. The main housing includes a vacuum chamber with a diaphragm and spring that is connected to a valve. The diaphragm reacts to pressure changes and at a preset vacuum (in the intake manifold); it pulls the diaphragm open, toward the vacuum source, with the help of boost pressure working on the underside of the valve. When the spring is compressed, the valve is lifted off its seat to vent undesired pressure, preventing compressor surge. Venting the pressure prevents a rush of compressed air from hitting the closed throttle plate and reversing into the turbocharger, inhibiting compressor surge or stalling the engine. Pressure can be released to the atmosphere or a BOV, or it can be bypassed back to the pre-turbo piping. On acceleration, the spring forces the valve closed against its seat, closing the valve and building boost. The valve basically opens every time the throttle is opened to build boost, and it then closes rapidly during a shift or a return to idle.

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Some (pressure-controlled) turbocharger waste gates may have a default that allows a minimum amount of boost (3–5 psi, or 20.7–34.5 kPa) if a failure occurs in the system. This can cause confusion during diagnosis if it’s not known. Always read and understand system operation information supplied in the workshop manual.

Engine load changes exhaust flow, resulting in a turbocharger with a dynamic operating speed range. When transitioning from idle, the turbocharger is spinning very slowly, approximately 15,000 rpm; as the vehicle accelerates to cruising speeds, or when it is under acceleration, turbo speeds can exceed 200,000 rpm. Rapidly closing the throttle to shift, or coming to a stop, reduces the need for boost, causing a buildup of compressed air pressure between the outlet of the turbocharger and the closed throttle plate, increasing intake pressure. The turbocharger’s rotating speed carries inertia that does not slow instantly. This creates a pressure spike when the throttle is closed, attempting to force its way past the closed throttle plate. The inlet airflow pressure reverses to the compressor of the turbocharger after hitting the closed throttle, causing compressor surge. This may result in an audible, abnormal noise. The compressor wheel loses momentum rapidly as the backpressure from a boosted intake opposes normal compressor wheel rotation. When the throttle is opened again, compressor speed will have to be recovered to create sufficient boost, causing “turbo lag.” Returning boost to the compressor wheel also places additional stress on the shaft and bearings. This can not only damage the turbocharger but also effect drivability.

BOVs are referred to by various names: dump valves, pop-off valves, or antisurge valves. No one term is more correct than the other; the names change by manufacturer, country, and region. Air pressure is released to the atmosphere (BOV/BPV), creating a distinctive hiss noise. The BOV is used on speed-density systems (MAP) and is coveted by aftermarket enthusiasts due to the distinguishable hiss sound created when releasing pressure. Incorrect sizing or failure to use a BOV/BPV can result in compressor surge.

Air can also be recirculated to the air inlet tubing by a BPV or recirculation (recirc) valve. The BPV is used on MAF-equipped vehicles and serves the same function as a BOV. During positive loads (high pressure, low vacuum), the valve is closed, sealing pressurized air into the intake path between the turbocharger and intercooler or throttle body. The valve is opened during rapid closing of the throttle to release boost pressure to the un-boosted side of the turbocharger. The BPV creates a loop in the intake system that continues to dump bypassed air back to the intake side of the compressor—rather than to the atmosphere, as a BOV would. This loop maintains intake air velocity when the throttle is opened and the ECM commands the valve closed. OEM systems recirculate the pressurized air after the MAF and before the compressor inlet to prevent false readings and a rich running condition as the throttle is rapidly closed. Air that passes over the MAF has already been accounted for by the ECM in its fuel control strategy and is expected to be burned in the cylinders. Simply venting this air to the atmosphere would upset that control and result in multiple drivability issues, including stalling, hesitation on acceleration, and increasing emissions (FIGURE 23-35).

FIGURE 23-35 A BOV is a very simple device, like an oil pressure–relief valve. A calibrated spring is used that can be overcome with the increased pressure within the intake tract.

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To prevent pressure spikes from occurring in the intake manifold and unloading or overrunning the turbo, the ECM opens the BPV by means of a control solenoid. The compressed air on the pressure (cold) side of the turbo is sent to the intake manifold by the open valve. This allows the pressure to drop but maintains the turbine speed proportionately, preventing compressor surge. Maintaining turbine rpm also aids in reducing turbo lag on acceleration after a decel event.

Electronic Boost Control

The waste gate solenoid simply varies the boost allowed to control the waste gate diaphragm. Electronic control of boost and bypass controls by the ECM are used on late-model OEM systems to increase turbocharger efficiency, reduce turbo lag, control CO2, and increase fuel economy. Aftermarket electronic controls offer custom tailoring to achieve desired performance, affecting how long the valve is held open by using the valve in a boost-limiting function. They are also adaptable for sensitivity to throttle movement, making them much more efficient than adjustable screws and/or altering the actuator spring or control lines on mechanical systems.

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OEM (factory) valves are designed to open slightly under high boost pressures by creating unequal pressure on the top and bottom of the boost valve, overcoming spring pressure at a preset boost level. Opening the valve protects the engine and turbocharger from boost pressure spikes.

Non-electronic controlled systems rely on spring pressure to keep the waste gate closed. Increasing boost pressure can overcome spring pressure, allowing the waste gate to open prematurely, reducing performance. Precise control by the ECM prevents the waste gate from cracking and thus opening too soon, providing ideal boost without the risk of engine damage. The ECM uses one of two options for monitoring turbocharger operation and waste gate control: It uses direct measuring sensors or data from other engine sensors that infer turbocharger operation and performance.

Modern boost control systems use an ECM to control the solenoid(s). The waste gate solenoid is usually duty cycled and offers infinite positioning of the waste gate valve between being fully open or closed, increasing performance (FIGURE 23-36). When the solenoid is off, boost pressure is available only to the mechanical limit of the actuator’s internal spring. Generally, the boost solenoid’s function is to bleed off boost pressure from the hose that is attached to the waste gate actuator when activated. The amount of time the solenoid is held open—duty cycle is high—determines the amount of boost pressure that is bled off. Controlling the bleed-off pressure regulates boost pressure. There are two- or three-port designs of duty-cycle-controlled solenoids. Other systems use a solenoid that controls vacuum, either blocking it or allowing vacuum to reach the waste gate. In these applications, the boost control solenoid has two ports: an in and an out.

FIGURE 23-36 Sometimes a waste gate solenoid is controlled by a PCM in a duty-cycle method, which pulses the waste gate to bleed off pressure.

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A pressure-relief valve may also be installed so that if the waste gate should fail, it can prevent an abnormal rise in manifold pressure.

Various sensor inputs to the ECM are needed to properly control boost, which may include all or some of the following:

Three methods of electronic waste gate control are currently being used. Depending on manufacturer design, pressure or vacuum is supplied to the pneumatic actuator. When compressor outlet pressure increases to a predetermined level, the solenoid-controlled actuator opens the waste gate to limit turbocharger boost pressure.

Three Methods of Controlling the Waste Gate

  1. Pressure-operated control: The waste gate is controlled by the pressure applied to the waste gate actuator. The amount of pressure vented from the pneumatically powered actuator allows the ECM to control boost pressure. For maximum boost, the ECM controls the waste gate regulating valve solenoid, completely venting the pressure being applied to the waste gate actuator, allowing exhaust gas flow to drive the turbine, creating boost. While driving under normal conditions, the waste gate will be controlled variably (progressively) to maximize boost pressure via a duty- cycle signal from the ECM. If a mechanical or electrical concern is present, spring pressure internal to the waste gate actuator will default the waste gate to the fully closed position, directing exhaust gas flow to bypass the turbine, eliminating boost (FIGURE 23-37).
  2. Vacuum-actuated control: This is typical waste gate operation. The waste gate regulating valve solenoid controls vacuum applied to the turbocharger waste gate actuator. The amount of vacuum applied to the waste gate allows the ECM to control boost. To generate maximum boost pressure, the ECM controls the turbocharger waste gate solenoid that applies vacuum to the turbocharger waste gate actuator, allowing exhaust gas flow to drive the turbine. If a mechanical or electrical concern is present, spring pressure internal to the waste gate actuator defaults the waste gate to a fully open position, with no vacuum applied to the actuator, directing exhaust gas flow to bypass the turbine, eliminating boost (FIGURE 23-38).
  3. Electric motor control: This type of control directly drives the waste gate actuator rod. This allows the waste gate to open without boost pressure, providing very high actuation speeds and precise boost regulation. The waste gate is capable of rapidly reaching any ECM-desired position regardless of negative or positive system pressures. Due to very high clamping forces, minimal leakage past the waste gate is present, resulting in rapid boost buildup. The waste gate can also be opened fully after a cold start. Full exhaust gas flow to the catalytic converter (CAT) shortens CAT light off time, reducing emissions and fuel consumption (FIGURE 23-39).

FIGURE 23-37 Using the pressure created by the turbocharger, the waste gate is controlled to release the excess pressure that the turbocharger is creating.

FIGURE 23-38 The ability of the PCM to control the waste gate can increase engine performance because everything can be working in sync to become efficient as possible.

FIGURE 23-39 An electrically controlled waste gate allows for precise control of when and how much the waste gate controls pressure within the turbocharger system. Each waste gate has a fail-safe feature that can override the waste gate if the electrical portion fails.

Additional Benefits of Electronic Waste Gate Valve Control Options
  • The waste gate can be used to speed up the catalytic converter warm-up period. Bypassing the turbocharger allows high-temperature exhaust gases to the catalyst, reducing the warm-up period.
  • During light loads, when boost is not required, the waste gate diverts exhaust gases from the turbine decreasing exhaust backpressure. Fuel economy increases as pumping losses are reduced and combustion is improved.
  • Heavy load or acceleration requires turbocharger boost. The waste gate closes, forcing exhaust gases to drive the turbine, creating boost, which is regulated by the ECM.
  • Some OEMs momentarily allow a higher boost pressure than normal under sudden acceleration to improve performance.

Electronic Bypass Valve Solenoid

The turbocharger BPV prevents revision (backflow) through the turbo during rapid deceleration and closed throttle to prevent unwanted noise and/or slowing the turbocharger’s inertia. The high-pressure air after the turbocharger is vented back to the air inlet system or atmosphere. An ECM-operated solenoid is used to electronically control BPV operation (surge valve). Basic operation of the valve is similar to the purely pneumatic version with electronic control, offering more precise operation, improving drivability and turbo efficiency. The ECM can operate the solenoid by simply being turned on and off or via a duty-cycle control, regulating manifold vacuum to the BPV. The length of time the solenoid is held open determines the amount of pressurized inlet air that is bled off. The BPV remains closed until engine vacuum is applied to the diaphragm of the valve, allowing compressed air to be released.

On cold start-up, the ECM may cycle the BPV as part of the ECM diagnostic strategy of checking for proper operation. Cycling of the valve may produce a popping noise, which should be considered normal. Again, properly understanding system operation prevents unneeded and time-consuming repairs.

Types of Bypass Valves

There are three types of BPVs:

  1. a remote-mounted, solenoid-controlled system located between the turbocharger intake and the turbocharger boost pressure output to either the intercooler/CAC or the intake manifold
  2. a solenoid-controlled system integrated into the turbocharger housing
  3. an electro-pneumatic controlled system located in the turbocharger housing—consisting of a pneumatically controlled BPV, a vacuum pump to supply vacuum, an ECM-controlled solenoid for both pressure and vacuum, and the necessary tubing.