Optimizing Volumetric Efficiency and Engine Performance

Volumetric Efficiency and Engine Performance

Modern engines are manufactured to meet specific standards: high specific power, sufficient elasticity, low fuel consumption, and reduced exhaust contaminants. Current systems used to improve cylinder charging in aspirated engines include multivalve distribution and variable intake.

Volumetric Efficiency

The efficiency of filling cylinders is calculated by comparing the mass of gas introduced into the cylinder against the maximum quantity that could enter. In atmospheric engines, the result is always below 1.

Multivalve Distribution

Multivalve distribution improves cylinder charging by increasing the gas passage section, thereby reducing resistance and head losses. Increasing the valve diameter would increase mass, leading to inertia and heat evacuation problems, while being limited by the cylinder diameter. The most favorable solution is using a higher number of valves, which reduces the diameter of each and optimizes space.

Advantages of Multivalve Distribution

• The input section increases by about 30% due to better utilization of the cylinder diameter and increased valve area.
• Helps improve the volume and shape of the combustion chamber.
• Valves weigh less, reducing inertia effects and allowing for higher RPMs.
• Springs can be softer, avoiding knocking and rebound effects.
• The impact against the seats is smaller, resulting in less noise.
• Smaller valve size enhances cooling.

Multivalve System Dynamics

Gas velocity entering the cylinder is a function of RPM. At high regimes, it is necessary to introduce a large volume of gas in a short time. High speeds cause mass loss due to friction against intake duct walls; the multivalve system reduces these losses and increases the gaseous mass intake.

Variable Intake

Variable intake modifies the intake manifold to adapt to different speeds, improving cylinder filling at both low and high RPMs. This results in higher torque. Two techniques are used:

1. Gas Inertia: Manifold dimensions (diameter and length) determine the regime at which better filling is achieved. As RPM increases, the length decreases and the diameter increases to maintain gas momentum.
2. Resonance: Based on the oscillations of the gaseous mass occurring inside the intake manifold due to the opening and closing of cylinder valves.

ACAV System

The ACAV system is based on the technique of acoustic resonance. The intake manifold splits into two lines of different lengths and sections, which are used independently or simultaneously depending on the RPM via a butterfly valve.