With reference to large medium speed diesel engine turbo chargers:
(a) explain what is meant by the term pulse system;(6)
(b) explain the advantage the pulse system has over the constant pressure system.(4)
Pulse System in Large Medium Speed Diesel Engines
A pulse system in a large medium-speed diesel engine turbocharger refers to a method of utilizing the pressure waves generated by the exhaust gases to drive the turbine.
How it works:
- Exhaust pulses: The exhaust gases from each cylinder are discharged into the exhaust manifold in discrete pulses.
- Manifold design: The exhaust manifold is designed to optimize the timing and strength of these pressure waves.
- Turbine inlet: The turbine inlet is connected to the exhaust manifold in a way that maximizes the impact of the pressure waves on the turbine blades.
Advantages of pulse system:
- Improved turbocharger response: The pulse system can provide quicker turbocharger spool-up due to the direct energy input from the exhaust pulses.
- Increased turbocharger efficiency: By effectively utilizing the energy in the exhaust pulses, overall turbocharger efficiency can be improved.
- Reduced turbo lag: The pulse system helps to minimize the delay in turbocharger response, enhancing engine performance.
It’s important to note that while pulse systems offer advantages, they can also introduce challenges such as backpressure and interference between exhaust pulses.
Disadvantages of Pulse Turbocharging Systems
While pulse turbocharging offers advantages in terms of turbocharger response and efficiency, it also presents certain challenges:
- Inefficient Turbine Operation: At low engine speeds or under light load conditions, the exhaust gas pulses may not be sufficient to effectively drive the turbine, leading to reduced efficiency.
- Exhaust Gas Interference: The overlapping exhaust pulses from different cylinders can interfere with each other, reducing the effectiveness of the turbine.
- Complex Exhaust Manifold Design: Pulse systems require intricate exhaust manifold designs to optimize the timing and strength of the exhaust pulses, increasing manufacturing costs and complexity.
- Backpressure: Pulse systems can generate higher backpressure compared to constant pressure systems, which can negatively impact engine performance.
- Noise: The pulsating nature of the exhaust gases can lead to increased noise levels.
Due to these challenges, pulse turbocharging systems are often used in combination with other turbocharging technologies or replaced by constant pressure systems in certain engine applications.