Advancing engine performance with Cummins' two-stage turbocharging innovation
Cummins Turbo Technologies develops and tests Holset turbochargers, delivering durable, reliable solutions for medium- to heavy-duty engines over decades. www.cummins.com Commercial engines need advanced technology to be efficient and meet strict emissions standards. Therefore, Cummins Inc. offers various turbo technologies, like fixed, variable, and two-stage turbos, all designed to improve efficiency and reduce costs. As we continue to work on the latest advancements for the next generation of engine technology, let us take a closer look at two-stage turbocharger technology that uses two turbochargers in a series, to improve performance and efficiency. What is a two-stage turbocharging system? Turbochargers give engines more power by pushing extra air into the cylinders, making the fuel burn more efficiently. They do this by using energy from the engine's exhaust gas to spin a turbine. The turbine is connected to a compressor by a shaft and as the turbine spins, it also spins the compressor, which forces more fresh air into the engine. A single-stage turbocharger uses one turbine and one compressor. If the turbocharger is large, it provides higher torque at high engine speeds, giving good peak power performance. However, it may not deliver as much power at low speeds. On the other hand, a smaller turbocharger is better at providing power at low speeds but may struggle to give enough boost for peak power at high speeds. A two-stage turbocharger uses both a small and a large turbo. The smaller turbo spins up quickly to deliver an instant boost, improving torque and responsiveness at low speeds or in high-altitude conditions. The larger turbo then kicks in to provide smooth, consistent power at higher speeds. This setup allows for better performance across a range of conditions, not just at low or high speeds. In a two-stage system, exhaust gases are managed by bypass systems called ‘wastegates’. These wastegates adjust based on engine speed to optimize the performance of the turbocharger. With rising fuel prices and stricter emissions standards, engines need to be more efficient. Two-stage turbochargers provide a strong solution by balancing low-end torque and peak power, offering better performance at higher altitudes, and enhancing overall fuel efficiency. How does two-stage turbocharging system work? A two-stage turbocharger system uses two turbochargers positioned along the exhaust path. The one closer to the engine is the high-pressure ("HP") turbo and the one farther away is the low-pressure ("LP") turbo. Each turbocharger has a compressor and a turbine. The system's efficiency and performance come from the balance of hot exhaust gases and fresh air moving through both the HP and LP turbochargers. Hot exhaust gases from the engine enter the HP turbine first, where energy from the high-temperature gas is extracted, causing the turbine blades to spin. The spinning of the turbine blades powers the HP compressor, which further compresses air that has already been pressurized by the LP compressor. This compressed air allows for higher pressure ratios and more air in the engine's combustion chamber. The remaining energy in the exhaust gases then drives the LP turbine, which in turn powers the LP compressor to pressurize more air for the HP stage. Bypass valves regulate the exhaust gases. At low speeds, the valves stay closed to direct more energy to the HP turbo. As speed increases, some exhaust gas bypasses the HP turbine, redirecting more energy to the LP turbo. Two-stage turbochargers offer better "turbo matching," which improves transient response, peak torque and peak power, making them highly efficient across the engine's torque curve. This makes them ideal for high-power engines, engines with high air demands (e.g., hydrogen internal combustion engines) and applications that operate at high altitudes. Types of two-stage turbocharger systems At Cummins, our turbo technology has advanced alongside innovations in engine performance. Fixed Geometry Turbochargers, for instance, channel all exhaust gas through a chamber in the turbine housing to the turbine wheel, improving performance based on chamber design and turbine size. Over time, we introduced Wastegate and Variable Geometry Turbochargers. Wastegate Turbochargers include a valve (wastegate) that allows some exhaust gas to bypass the turbine wheel, reducing speed and pressure in the exhaust manifold and improving airflow control. Variable Geometry Turbochargers adjust the exhaust passage before the turbine wheel to optimize pressure and gas velocity for better performance. Two-stage turbochargers can be used in different configurations, including sequential, compound, parallel and variable geometry twin-turbocharging. Sequential turbocharging: In sequential turbocharger configurations, the smaller turbo handles the lower engine speeds to reduce turbo lag and the larger one kicks in at higher speeds to provide additional boost. Com
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Cummins Turbo Technologies develops and tests Holset turbochargers, delivering durable, reliable solutions for medium- to heavy-duty engines over decades.
www.cummins.com
Commercial engines need advanced technology to be efficient and meet strict emissions standards. Therefore, Cummins Inc. offers various turbo technologies, like fixed, variable, and two-stage turbos, all designed to improve efficiency and reduce costs.
As we continue to work on the latest advancements for the next generation of engine technology, let us take a closer look at two-stage turbocharger technology that uses two turbochargers in a series, to improve performance and efficiency.
What is a two-stage turbocharging system?
Turbochargers give engines more power by pushing extra air into the cylinders, making the fuel burn more efficiently. They do this by using energy from the engine's exhaust gas to spin a turbine. The turbine is connected to a compressor by a shaft and as the turbine spins, it also spins the compressor, which forces more fresh air into the engine.
A single-stage turbocharger uses one turbine and one compressor. If the turbocharger is large, it provides higher torque at high engine speeds, giving good peak power performance. However, it may not deliver as much power at low speeds. On the other hand, a smaller turbocharger is better at providing power at low speeds but may struggle to give enough boost for peak power at high speeds.
A two-stage turbocharger uses both a small and a large turbo. The smaller turbo spins up quickly to deliver an instant boost, improving torque and responsiveness at low speeds or in high-altitude conditions. The larger turbo then kicks in to provide smooth, consistent power at higher speeds. This setup allows for better performance across a range of conditions, not just at low or high speeds.
In a two-stage system, exhaust gases are managed by bypass systems called ‘wastegates’. These wastegates adjust based on engine speed to optimize the performance of the turbocharger. With rising fuel prices and stricter emissions standards, engines need to be more efficient. Two-stage turbochargers provide a strong solution by balancing low-end torque and peak power, offering better performance at higher altitudes, and enhancing overall fuel efficiency.
How does two-stage turbocharging system work?
A two-stage turbocharger system uses two turbochargers positioned along the exhaust path. The one closer to the engine is the high-pressure ("HP") turbo and the one farther away is the low-pressure ("LP") turbo. Each turbocharger has a compressor and a turbine.
The system's efficiency and performance come from the balance of hot exhaust gases and fresh air moving through both the HP and LP turbochargers.
Hot exhaust gases from the engine enter the HP turbine first, where energy from the high-temperature gas is extracted, causing the turbine blades to spin. The spinning of the turbine blades powers the HP compressor, which further compresses air that has already been pressurized by the LP compressor. This compressed air allows for higher pressure ratios and more air in the engine's combustion chamber. The remaining energy in the exhaust gases then drives the LP turbine, which in turn powers the LP compressor to pressurize more air for the HP stage.
Bypass valves regulate the exhaust gases. At low speeds, the valves stay closed to direct more energy to the HP turbo. As speed increases, some exhaust gas bypasses the HP turbine, redirecting more energy to the LP turbo.
Two-stage turbochargers offer better "turbo matching," which improves transient response, peak torque and peak power, making them highly efficient across the engine's torque curve. This makes them ideal for high-power engines, engines with high air demands (e.g., hydrogen internal combustion engines) and applications that operate at high altitudes.
Types of two-stage turbocharger systems
At Cummins, our turbo technology has advanced alongside innovations in engine performance. Fixed Geometry Turbochargers, for instance, channel all exhaust gas through a chamber in the turbine housing to the turbine wheel, improving performance based on chamber design and turbine size. Over time, we introduced Wastegate and Variable Geometry Turbochargers.
Wastegate Turbochargers include a valve (wastegate) that allows some exhaust gas to bypass the turbine wheel, reducing speed and pressure in the exhaust manifold and improving airflow control. Variable Geometry Turbochargers adjust the exhaust passage before the turbine wheel to optimize pressure and gas velocity for better performance.
Two-stage turbochargers can be used in different configurations, including sequential, compound, parallel and variable geometry twin-turbocharging.
- Sequential turbocharging: In sequential turbocharger configurations, the smaller turbo handles the lower engine speeds to reduce turbo lag and the larger one kicks in at higher speeds to provide additional boost.
- Compound turbocharging: This configuration, used in heavy-duty diesel engines like trucks, boats and industry, has two turbos arranged in a series. Exhaust gases pass through the high-pressure and larger turbo, allowing for high boost pressures and improved efficiency.
- Parallel turbocharging: In V-8 or V-12 engines, parallel turbocharging, two identical turbochargers receive exhaust gases from separate sets of cylinders, providing compressed air to the intake system to balance power output and improve response times.
- Variable geometry turbochargers in two-stage systems: Some systems use a combination of a fixed turbocharger and a variable geometry turbocharger, where the Variable Geometry Turbocharger’s ability to change the aspect ratio and optimize across a wide range of engine speeds is especially useful.
Advantages of two-stage turbocharger systems:
A two-stage turbocharger system is often the best choice for optimizing both cost and performance. For example, when a single-stage system cannot achieve a high compression ratio efficiently, a two-stage turbocharger can provide the necessary performance boost.
Two-stage turbochargers are also ideal for light-duty engines with a wide speed range or when higher torque is needed at lower RPMs. They excel in providing a quick response during low ramp-up cycles for torque.
Two-stage configurations can be paired with either a Wastegate or VGT turbo. A two-stage setup with a VGT offers a wider flow range, higher efficiency at low flows and low inertia, which is beneficial for transient response. Alternatively, an Electric Wastegate (eWG) can be a good option, where the Wastegate is electronically controlled, similar to a VGT.
Challenges of two-stage turbocharger systems:
Two-stage turbochargers pose several engineering challenges in modern engine design. One common issue is packaging constraints, which can make it difficult to integrate these systems into different engine configurations, limiting their use across various vehicle platforms. Another key challenge is thermo-mechanical fatigue (TMF). The dual turbocharger setup requires strong support structures and interconnections to handle significant temperature changes and mechanical stresses. To address this, Cummins uses extensive TMF validation, both analytically and empirically, drawing on years of experience to mitigate these concerns.
The inclusion of wastegate bypass functionality adds further complexity to two-stage turbocharger systems. The high-pressure (HP) turbine needs larger wastegate ports to manage increased exhaust flow, causing the wastegate valve to operate more frequently in the open position compared to single-stage systems. Additionally, sequential turbocharging systems require an extra compressor bypass mechanism, increasing the system's complexity and control challenges. Cummins tackles these issues by validating specific customer duty cycles and carefully selecting materials based on the fuel type—whether Diesel, Natural Gas, or H2ICE.
Cummins Inc. has long led in advanced turbocharging technologies, leveraging our engineering expertise to overcome the challenges of two-stage systems. Our team consistently pushes the limits of design, addressing packaging constraints, reducing thermo-mechanical fatigue, and optimizing wastegate functionality.
Through rigorous testing and refinement, Cummins has successfully implemented two-stage turbocharging across various engine platforms, enhancing performance, fuel efficiency, and emissions compliance. Our advanced materials, precision manufacturing, and sophisticated controls ensure we maximize the benefits of this technology.
www.cummins.com
A two-stage turbocharger system is often the best choice for optimizing both cost and performance. For example, when a single-stage system cannot achieve a high compression ratio efficiently, a two-stage turbocharger can provide the necessary performance boost.
Two-stage turbochargers are also ideal for light-duty engines with a wide speed range or when higher torque is needed at lower RPMs. They excel in providing a quick response during low ramp-up cycles for torque.
Two-stage configurations can be paired with either a Wastegate or VGT turbo. A two-stage setup with a VGT offers a wider flow range, higher efficiency at low flows and low inertia, which is beneficial for transient response. Alternatively, an Electric Wastegate (eWG) can be a good option, where the Wastegate is electronically controlled, similar to a VGT.
Challenges of two-stage turbocharger systems:
Two-stage turbochargers pose several engineering challenges in modern engine design. One common issue is packaging constraints, which can make it difficult to integrate these systems into different engine configurations, limiting their use across various vehicle platforms. Another key challenge is thermo-mechanical fatigue (TMF). The dual turbocharger setup requires strong support structures and interconnections to handle significant temperature changes and mechanical stresses. To address this, Cummins uses extensive TMF validation, both analytically and empirically, drawing on years of experience to mitigate these concerns.
The inclusion of wastegate bypass functionality adds further complexity to two-stage turbocharger systems. The high-pressure (HP) turbine needs larger wastegate ports to manage increased exhaust flow, causing the wastegate valve to operate more frequently in the open position compared to single-stage systems. Additionally, sequential turbocharging systems require an extra compressor bypass mechanism, increasing the system's complexity and control challenges. Cummins tackles these issues by validating specific customer duty cycles and carefully selecting materials based on the fuel type—whether Diesel, Natural Gas, or H2ICE.
Cummins Inc. has long led in advanced turbocharging technologies, leveraging our engineering expertise to overcome the challenges of two-stage systems. Our team consistently pushes the limits of design, addressing packaging constraints, reducing thermo-mechanical fatigue, and optimizing wastegate functionality.
Through rigorous testing and refinement, Cummins has successfully implemented two-stage turbocharging across various engine platforms, enhancing performance, fuel efficiency, and emissions compliance. Our advanced materials, precision manufacturing, and sophisticated controls ensure we maximize the benefits of this technology.
www.cummins.com
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