Nissan has just released animation showing the inner workings of the 2-stage turbocharger on the upcoming 5.0-liter Cummins V8 that will power the next generation Titan XD pickup. This animation helps explain in more detail the original EngineLabs story announcing the M2 2-stage turbo designed by Holset.
The M2 2-stage turbo isn’t a compound grouping where a small turbo provides a boosted charge into a second larger turbo. Instead, this system is designed more of a sequential arrangement using different sized turbos designed to work more efficiently under both low and high engine speeds. Basically, the small turbo provides transient response due to its low inertia, and the large turbo maintains power at higher engine speeds. This helps eliminate turbo lag, providing a continuous delivery of peak torque through the rpm range.
The difficulty with a sequential system has always been the control and operation of the bypass valve that allows the individual turbos to be switched in and out. This relationship also relates packaging to ensure the engine fits into the required space.
The M2 incorporates a new patented rotary valve to open the ports that perform the bypass or wastegate function and provide exhaust aftertreatment thermal management in addition to serving as an engine brake. This single valve basically works as a high-pressure turbine bypass and a low-pressure turbine wastegate.
No sound but plenty of information
At low speeds, the rotary valve is positioned so that all the exhaust gasses pass through the small turbine. The exhaust then passes through the second, larger turbo, but the flow is small. Only a small amount of energy is extracted through the larger turbine.
As engine airflow increases with engine speed and load, the electrically actuated rotary valve modulates to allow an increasing amount of exhaust gas to flow directly to the larger turbine, thereby increasing its contribution to the power output. All the while, the work accomplished by the smaller turbo remains consistent. Then at high engine speeds, the rotary valve diverts most of the exhaust gas directly to the large turbo. The smaller turbo is out of the equation at this point so that it doesn’t become a restriction. In other words, the system is now working solely as a single-stage turbo.
While the goal of the system is to provide optimum efficiency and fuel economy, there are conditions where the engine is asked to work harder to raise EGT in order to regenerate the diesel particulate filter. Positioning the rotary valve to restrict the exhaust passage produces the necessary increase in exhaust back pressure to boost EGT. When the valve is completely closed, the exhaust is totally blocked to provide an engine brake.
Although there is no narration on the video, the video very patiently takes the viewer through the entire process in very clear 3D animation. It shows all the different modes with airflow simulation for the turbine and compressor sides. For an overall analysis of the engine, check out this story.