Optimizing Engine Performance: Multivalve, Variable Intake, and VVT Systems

Multivalve Engine Design Principles

Multivalve engine design improves cylinder charging by increasing the passage section for gases, thereby reducing resistance and decreasing losses. The optimal solution involves doubling the number of valves and reducing the diameter of each, though this takes more space in the combustion chamber. The primary aim is to enhance cylinder charge, giving preference to intake valves. This best use of thermal energy provides greater performance, lower fuel consumption, and achieves cleaner exhaust air.

Key Benefits of Multivalve Engines:

• Increased input section (+30%)
• Optimized volume and shape of combustion chambers
• Lighter, smaller valves with less inertia effect and improved cooling
• Softer springs, leading to less rebound and reduced noise

Variable Intake Systems Explained

Variable induction systems enhance cylinder charging by dynamically changing the characteristics of the intake manifold.

Inertia of Gases:

To improve cylinder filling and torque at low revolutions per minute (RPM), long and narrow collectors are preferred. However, at high RPM, wider and shorter collectors are more effective.

Acoustic Resonances:

These systems study variations in the mass of gas inside the intake manifold by opening or closing the valves of different cylinders. The dimensions of the manifold determine the noise characteristics.

ACAV System:

The ACAV System (Acoustic Control Induction System) splits the manifold into two tubes of different lengths and sections. It features a common entrance distributor where a gas butterfly valve is located. To feed the cylinder, the distributor has a long circuit for free passage and a shorter circuit controlled by four butterflies.

Variable Valve Timing Technologies

Variable valve timing (VVT) systems offer the possibility of having at least two distribution diagrams: one for low and mid-range RPMs, allowing better cylinder filling for good torque from low revs, and a second that provides high performance at high RPMs.

VarioCAM:

Porsche's VarioCAM system varies the distribution chart using a hydraulic tensioner on the chain that transmits rotation between two camshafts. It acts only on the intake camshaft and is controlled by the engine control unit (ECU). The valve overlap is reduced below 1500 RPM and above 5500 RPM. The fastest overlap is served from 1500 to 5500 RPM. The intake camshaft moves 7.5° relative to the exhaust camshaft (which remains fixed), equating to 15° at the crankshaft.

VTEC:

Honda's VTEC (Variable Valve Timing and Lift Electronic Control) system allows valves to open slightly at low RPM and open more at higher speeds. This system is applied to both intake and exhaust valves.

VTEC-E:

Similar to VTEC, but VTEC-E (Variable Valve Timing and Lift Electronic Control - Economy) primarily acts on the intake valves. At low RPM, only one intake valve opens, while at high RPM (typically above 2,500 RPM), both intake valves open. Below 2,500 RPM, primary and secondary levers act independently. Above 2,500 RPM, the rockers are connected, and the primary cam moves both intake valves.