Advanced Engine Valve & Timing Technologies
Multivalve Engine Design Principles
Multivalve distribution involves doubling the number of valves to reduce the diameter of each valve and effectively utilize available space in the combustion chamber. Valves are typically angled between 20° and 50°, allowing for a compact combustion chamber design with a very favorable surface-to-volume ratio. This design can easily induce significant turbulence, leading to better utilization of heat energy and a positive impact on fuel consumption.
Advantages of Multivalve Engines
- The intake cross-section increases by approximately 30%, optimizing cylinder filling and valve flow.
- Improves combustion chamber volume and shape.
- Reduces valve inertia for faster opening.
- Allows for softer valve springs, minimizing rebound effects.
- Reduces noise.
- Facilitates easier cooling of valves.
Addressing Low RPM Performance in Multivalves
While the larger valve cross-section reduces losses and allows more gas into the cylinder, improving high-speed performance, a challenge arises at low RPMs. The wide intake section causes gas velocity to decrease, leading to poor cylinder filling and insufficient turbulence, resulting in reduced low-end torque. To mitigate power loss and enhance intake, manifolds are designed with specific dimensions to maintain proper gas velocity, or variable intake systems are employed.
Variable Intake Manifold Technology
A variable intake manifold modifies its geometry to suit different engine speeds, improving cylinder filling at both low and high RPMs, thereby achieving greater torque across the range.
Optimizing Intake Runner Length & Diameter
As engine revolutions increase, the length of the intake runner typically decreases, and its diameter increases to prevent charge losses. Conversely, for low speeds, longer and narrower runners are preferred to maintain gas velocity.
Harnessing Acoustic Resonance for Performance
Pressure waves (both positive and negative) travel through the intake ducts at a constant speed. However, their frequency varies with RPM. At certain engine speeds, these oscillations create a resonance effect that supercharges the intake, improving low-speed torque and increasing high-RPM power.
Variable Valve Timing Systems Explained
These systems allow for two or more distinct valve timing diagrams. One diagram optimizes cylinder filling for strong low-RPM torque, while another provides high performance at high speeds. This improves cylinder charging across the entire RPM range and can reduce emissions.
VarioCam: Porsche’s Variable Timing
The VarioCam system varies the valve timing diagram by using a hydraulic tensioner that transmits rotation between two camshafts, primarily affecting intake valve timing.
Honda VTEC: Intelligent Valve Control
- At Low Revolutions: Independent rocker arms are operated separately by cams with a slight gap, optimizing cylinder filling.
- At High Revolutions: A valve is actuated, allowing oil pressure to displace pistons that synchronize the rocker arms, causing them to move as a single unit. They are then powered by a central, larger-diameter cam, resulting in greater valve lift and improved cylinder filling. This system varies valve lift and opening duration based on engine RPM.
VTEC-E: Economy-Focused Valve Timing
In the VTEC-E system, exhaust valves operate with constant lift. The system primarily affects intake valves: at low RPMs, only one intake valve opens, while at high RPMs, both intake valves open.
VTEC-E Operational Details
- At Low RPM: The rocker arms are disengaged and operate independently with different lifts. This gap prevents gas accumulation in the second intake runner. The gas flow creates strong turbulence inside the cylinder, promoting good mixture combustion even with a lean fuel mixture.
- Above 2500 RPM: The engine control unit (ECU) opens a valve, allowing hydraulic pressure to connect the rocker arms. The primary lever then moves both valves with the same increased lift, boosting power as RPMs rise.