Modern methods of Turbocharging:

(a) Pulse Converter System:

In medium-speed engines, pulse systems are commonly used. Their main advantage is rapid response to load changes, but this comes at the cost of reduced efficiency due to long intervals between gas inlet and partial gas loss (inherent in multi-inlet turbine designs). To improve performance, a pulse converter system partially converts pulse energy into kinetic energy before the gas reaches the turbine, while still retaining some pulse energy. Exhaust gas enters a pre-set manifold, connected via a carefully designed pipe to a two-branched manifold feeding a single turbine. This pipe design minimizes pressure pulse transmission between exhaust pipes, preventing one cylinder's exhaust from interfering with another's scavenging process. Pulse converters also simplify exhaust piping by eliminating the need for complex multi-entry turbocharger casings.

This technique uses multiple turbochargers (T/Cs) sequentially. A small turbocharger operates at lower engine speeds, while larger, high-flow turbochargers are engaged at higher speeds. A bypass valve controls gas flow between the T/Cs based on engine speed. At low engine speeds, with minimal exhaust energy, only the smaller T/C is active, receiving all the engine's exhaust energy. This provides higher scavenge pressure, minimises turbo lag, and increases power output at lower engine speeds. Once a preset engine speed or boost pressure is reached, the bypass valve opens fully, shutting off the smaller T/C. Sequential turbocharging improves fuel economy and transient response and reduces low-speed smoke emissions by optimising T/C matching to the engine, thereby significantly increasing overall efficiency.

Two-stage turbocharging is a significant advancement in large diesel engines and is crucial for meeting emission regulations. It uses two turbochargers of different sizes connected in series. Exhaust gas first drives a smaller, high-pressure (HP) turbocharger. This HP Turbocharger's turbine then drives the larger, low-pressure (LP) turbocharger's turbine. The LP T/C's compressor draws in ambient air and sends it through an intercooler to the HP T/C's compressor, which further compresses the already-cooled air. This air then passes through an air cooler before reaching the engine. At low engine speeds, exhaust gas is diverted entirely to the smaller HP T/C for rapid boost pressure build-up. At high engine speeds, a bypass valve diverts exhaust to the larger LP T/C. This system increases charge air pressure, resulting in higher air mass flow, more efficient combustion (and therefore better fuel efficiency), and reduced exhaust emissions despite increased engine output.

A Variable Turbine Inlet (VTI) turbocharger, commonly known as a Variable Geometry Turbocharger (VGT) or Variable Nozzle Turbocharger (VNT), optimizes engine performance by adjusting the geometry of the turbine inlet to regulate exhaust gas flow.

• The turbine housing contains movable vanes that surround the turbine wheel. These vanes can change their angle to vary the cross-sectional area of the turbine inlet, controlling the speed and pressure of exhaust gases impacting the turbine.
• At low engine speeds, the vanes close to create a narrower passage. This accelerates exhaust gases onto the turbine wheel, increasing its speed and generating higher boost pressure quickly, which reduces turbo lag and improves low-end torque.
• At higher engine speeds, the vanes open to create a wider passage. This allows a greater volume of exhaust gases to pass through, preventing excessive boost and maintaining optimal performance.