Blow off Valves

Background

In a turbocharged engine, the Blow-Off Valve (BOV) serves the crucial role of alleviating pressure from the turbocharger when the throttle is closed. These valves are specifically employed in engines equipped with a blow-through turbo setup.

When the engine is operating under boost, the BOV remains sealed, allowing the turbocharger to supply air to the engine in a standard manner. Without the BOV, closing the throttle would abruptly subject the turbocharger to the challenge of forcing air against a closed throttle plate. This results in a sudden increase in pressure within the turbocharger output hose, generating a pressure "wave" that oscillates between the throttle plate and the turbo compressor blades. This pressure spike can significantly decelerate the turbocharger and, in some cases, inflict damage.

Upon reopening the throttle, the turbocharger needs to regain its rotational speed, causing what is commonly referred to as turbo lag. When a BOV is installed, it promptly opens when the throttle is closed, allowing the pressure spike to vent into the air intake, effectively preventing the pressure wave phenomenon.

The objective of this study was to compare the various types of blow-off valves available in the market, with a focus on optimizing performance for turbocharged engines. The study encompasses product definition, engineering development, concept and idea generation, ultimately culminating in the creation of a unique blow-off valve design.

Problem Statement

When enthusiasts discuss race cars or high-performance sports cars, the conversation invariably turns to turbochargers. Turbos have the remarkable ability to boost an engine's horsepower significantly without adding substantial weight. However, they do pose certain challenges.

As the throttle body closes, the pressurized air generated by the turbocharger finds itself cut off from the inlet manifold. The only escape route available is to flow backward into the intake stream, surging back toward the turbo compressor. This reversal of the intake charge pulse places added stress on the turbo components and reduces the rotational speed of the compressor wheel. Consequently, the turbo takes longer to spool up when the throttle is reopened.

To address this issue, a valve is strategically positioned before the throttle body. This valve allows the pressurized charge to exit the intake system, keeping the compressor spinning and mitigating turbo lag. This phenomenon produces a distinct sound that many turbocharged sports car enthusiasts appreciate. Some blow-off valves are even designed with trumpet-shaped exits, amplifying the satisfying "Psshhhh" sound; such designs are typically marketed to the performance-minded.

Given the diverse range of blow-off valves available in the market, there exists a compelling need to delve into the concepts, designs, and components of the pressurized release system within turbocharged engines. Study 3 will further explore this subject through reverse engineering, analyzing various blow-off valve types available in the market. CAD modeling and Flow Simulation will be integral components of this study's methodology.

Objective Of The Product

The product's objectives are as follows:

1. Investigate and analyze existing blow-off valve designs and their components.
2. Develop a pressure relief valve tailored for turbocharged engine applications.

Scope of Product

The scope of this product encompasses three primary components:

1. Exploration of the concepts, designs, and components comprising the pressure release system within turbocharged engines.
2. Conducting reverse engineering on a variety of distinct blow-off valve types currently available in the market.

iii. Employing CAD modeling and Flow simulation tools, specifically SolidWorks 2005 and COSMOS Flo Works 2005/PE.

Literature Review

A turbocharger is a dynamic compressor that operates by using the mechanical action of impellers or vane rotors. These components are spun through the kinetic energy generated by air movement, resulting in the compression of air or gas and the consequent increase in velocity and pressure within the flowing medium.

In essence, a turbocharger serves as a device that harnesses exhaust gases produced by the engine to reintroduce compressed air back into the engine, as illustrated in Figure 2.1. The additional air is combined with fuel by the Engine Control Unit (ECU). This combination significantly enhances the engine's power output, as it receives a greater supply of air and fuel than it would otherwise be capable of. In this configuration, the maximum air pressure entering the engine's combustion chamber is slightly lower than the current atmospheric pressure.

However, with the inclusion of a turbocharger, air is forced into the chamber under positive pressure, allowing a substantially larger amount of air and fuel to enter. In a typical turbocharged engine, the generated positive pressure, often referred to as "boost," typically ranges from 7 to 10 psi (pounds per square inch).

The turbocharger, often referred to as a "turbo," is directly affixed to the exhaust manifold. Within this assembly, exhaust gases flow over a turbine impeller attached to a short shaft. On the opposite side of this shaft lies a compressor turbine, responsible for drawing external air through the air filter and directing it into the intake manifold. You can observe the components of a turbocharger.

In essence, the energy carried by the expelled exhaust gases, which would typically go to waste, is harnessed to reintroduce air into the engine. A bearing housing supports the shaft and is lubricated and cooled through an oil line connected to the engine. It's worth noting that due to the high temperatures associated with engine exhaust, the exhaust side of the turbocharger can reach temperatures in the thousands of degrees Fahrenheit. This underscores the critical importance of regular engine oil changes, as aged oil can lead to the formation of deposits in oil lines and housings, a phenomenon known as "coking." The use of synthetic oil and frequent oil changes can largely prevent coking.

Furthermore, the turbocharger employs a coolant passage to manage temperatures, particularly after the engine is turned off. Without this coolant circulation, the housing surrounding the turbocharger would reach extremely high temperatures due to the heat transferred from the exhaust manifold. This, in turn, would adversely affect the lifespan of the bearings. The presence of the coolant passage helps maintain a lower temperature for the housing.

When the engine idles or operates at low speeds for an extended period, the turbocharger either ceases to spin or does so very slowly due to the limited exhaust exiting the engine. As soon as the throttle is opened, the engine generates a greater volume of exhaust, causing the turbocharger to spin at an accelerated rate. A turbocharger spinning faster translates to a larger quantity of air and fuel being forced into the engine, consequently leading to the production of even more exhaust. This, in turn, further accelerates the turbo's rotation. It provides a visual representation of the turbocharger's operational principle.

This recurring process is referred to as "turbo spool-up," which is experienced as a sudden surge in engine power and is evident on a boost gauge as a rapid increase in pressure. The duration leading up to this surge, when the turbo is spooling up but the engine hasn't yet reached its peak power output, is known as "turbo lag." Larger turbochargers can deliver greater airflow and pressure but tend to exhibit more lag due to their increased size. In contrast, smaller turbochargers have less lag but are limited in their ability to move as much air.

Comparisons between Turbocharging and Supercharging

The term "supercharging" is a technical reference to any device that pushes air into an engine. However, in everyday language, it typically describes devices directly driven by the engine, as opposed to turbochargers, which derive their power from exhaust gas pressure.

Positive displacement superchargers can consume up to a third of the total power generated by the engine's crankshaft. In many applications, they exhibit lower efficiency compared to turbochargers. However, in scenarios where engine responsiveness and power take precedence over other factors, like in top-fuel dragsters and vehicles used in tractor pulling competitions, positive displacement superchargers are quite common. Superchargers are often the source of the distinctive high-pitched whine heard during acceleration in tuned engines.There are three main styles of supercharger for automotive use:

• Centrifugal turbochargers - driven from exhaust gasses.

• Centrifugal superchargers - driven directly by the engine via a beIt-drive.

• Positive displacement pumps (such as the Roots and the Lysholm (Whipple) blowers).

The thermal efficiency, which represents the portion of energy from the fuel/air mixture converted into output power, tends to be lower in engines equipped with mechanically driven superchargers compared to turbochargers. This difference arises because turbochargers harness energy from exhaust gases that would otherwise go to waste. Consequently, turbocharged engines often exhibit superior fuel economy and power output when compared to supercharged ones.

However, mechanically driven superchargers offer a notable advantage in terms of throttle response. Modern turbocharging technology has significantly improved throttle response in turbocharged vehicles, approaching the level of mechanical superchargers. It's worth noting that the majority of mechanically driven superchargers now feature clutched pulleys, similar to those used in air compressors.

Efficiently managing the temperature of the air entering the engine is a critical aspect of both supercharger and turbocharger design. Compressing air tends to raise its temperature, making the use of an intercooler, a small radiator placed between the pump and the engine, common practice to reduce air temperature.

Turbochargers can experience a phenomenon known as "turbo lag," which limits initial acceleration at low revolutions per minute (RPM) due to insufficient exhaust gas pressure. Once the engine RPM reaches a point where it can spin the turbo, there is a rapid increase in power as higher turbo boost results in more exhaust gas production, spinning the turbo even faster and leading to a delayed "surge" in acceleration. This can make maintaining a smooth and gradual increase in RPM more challenging with turbochargers compared to belt-driven superchargers, which provide boost in direct proportion to engine RPM.

It's important to differentiate between "turbo lag" and "turbo spool." Turbo lag refers to the time it takes to spool the turbo when there is enough engine speed to generate boost. This is significantly influenced by the specifications of the turbocharger. Using a turbocharger that is too large for the desired power band can lead to unnecessary delays in spooling the turbocharger.

Blow off Valve

A blow-off valve, also known as a compressor bypass valve or diverter valve, is a valve, typically of the piston type, positioned between the turbo compressor and the throttle. Its purpose is to divert pressurized air when the throttle is closed, either by redirecting it back into the turbo inlet for a quieter operation or releasing it into the atmosphere to create the distinctive "whoosh" sound. This valve is commonly used in turbocharged systems.

However, this unique sound can come with certain consequences, particularly in cars equipped with a mass airflow sensor. In such systems, the blow-off valve's action can confuse the engine control unit (ECU). The ECU relies on precise measurements of air entering the intake system to determine the fuel injection rate. When the blow-off valve releases air that the ECU hasn't accounted for, the engine can run rich for a period, affecting performance and fuel efficiency.

When a turbocharged car is operating under boost, the entire intake system is filled with pressurized air, starting from the turbo compressor, passing through the throttle body, and entering the combustion chambers. Upon closing the throttle, the air can no longer enter the engine, leading it to attempt to flow back in the reverse direction, through the turbo compressor against its normal flow. This creates a fluttering noise as the air affects the still-spinning turbo compressor blades.

Aside from the unwanted noise, this reverse airflow can potentially place stress on the turbocharger and cause premature wear or damage. While directly attributing turbo failure to the lack of a blow-off valve is challenging, it can lead to issues such as engine hesitation, stalling, and damage to spark plugs and the catalytic converter, especially at higher boost pressures.

Blow-off valves are primarily employed to prevent compressor surge. Compressor surge occurs when a turbocharged engine with no or a faulty bypass valve experiences a sudden closure of the throttle plate while under boost. This results in high-pressure air within the intake system having no outlet and returning to the turbocharger as a pressure wave. This can cause the turbo's compressor wheel to rapidly decelerate and stall, resulting in a fluttering air sound. In extreme cases, the compressor wheel may even stop or reverse direction. Compressor surge can significantly reduce the turbocharger's lifespan due to added stress on its bearings and lead to turbo lag, which refers to the time it takes to spool the turbocharger.

The choice of a blow-off valve is also influenced by various factors such as emissions, fuel economy, and drivability in factory applications. However, in aftermarket settings, blow-off valves are often installed to handle higher-than-standard boost levels, enhance throttle response by remaining closed when not venting, and, of course, to produce the desired noise.

How a Blow off Valve Works

A blow-off valve, often referred to as a BOV, is a piston-type valve that operates through vacuum and pressure signals. It relies on these signals to determine when to open and close.

During idle, there is engine vacuum on top of the BOV piston, attempting to pull it open, while the bottom of the piston experiences no vacuum or pressure. To prevent air from being drawn in through the BOV at idle, a spring inside the BOV is responsible for keeping the piston closed. The adjustment of this spring preload accommodates variations in engine vacuum across different cars and changes in atmospheric pressure at varying elevations.

In cars equipped with airflow meters, allowing an open vent-to-atmosphere BOV at idle can confuse the Engine Control Unit (ECU), leading to issues like over-fueling and stalling. Additionally, unfiltered air is drawn in through the open valve.

Under cruise conditions (off boost), the BOV experiences a situation similar to that at idle, but with less vacuum on top of the piston because the throttle is partially open. If the BOV's spring has been adjusted to keep the piston closed at idle, it will also remain closed during cruising.

When the engine is under boost, both the top and bottom of the BOV experience boost pressure, which counteraces each other, effectively keeping the BOV closed.

Immediately after the throttle is closed while under boost, there is vacuum on the top of the piston and boost pressure on the bottom, resulting in a rapid opening of the BOV to release the pressure. Once the pressure is released, the BOV closes.

The spring's role is to maintain valve closure, but if sufficient pressure reaches the BOV inlet, it can force the valve open. When the throttle is closed and vacuum is applied to the front of the BOV diaphragm, it releases pressure from the turbo.

Conclusion

In conclusion, a blow-off valve (BOV) is a vital component in turbocharged systems, utilizing vacuum and pressure signals to control its operation. During engine idling, the BOV remains closed to prevent unwanted air intake, thanks to an internal spring mechanism adjusted to suit different engine vacuum levels and atmospheric pressures.

The use of open vent-to-atmosphere BOVs at idle can cause confusion in cars with airflow meters, leading to issues like over-fueling and stalling. Moreover, such open valves allow unfiltered air to enter the engine.

Under cruising conditions, the BOV typically remains closed, similar to idling, unless adjustments have been made to the spring preload. During boosted operation, both the top and bottom of the BOV experience boost pressure, which keeps the valve firmly shut.

However, when the throttle is abruptly closed while under boost, the BOV responds by quickly opening, allowing excess pressure to escape, and then promptly closing once the pressure has been released. The spring within the BOV serves to maintain valve closure, but it can be overridden if sufficient pressure reaches the BOV inlet.

In essence, the BOV plays a crucial role in safeguarding the engine from harmful pressure surges and ensuring proper air management in a turbocharged system, contributing to both performance and engine longevity.