Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain the operation of Variable Valve Timing (VVT) systems.
• Compare and contrast turbocharging and supercharging.
• Calculate boost pressure and its effect on efficiency.
• Diagnose common faults in VVT and forced induction systems.
• Describe the environmental impact of these technologies.

Lesson summary

This week, we delve into the fascinating world of advanced engine technology and its impact on performance. Specifically, we will explore variable valve timing (VVT) and turbocharging/supercharging systems. Understanding these technologies is crucial because they represent significant advancements in improving fuel efficiency, reducing emissions, and enhancing the power output of internal combustion engines – all factors relevant to the South African automotive market and our environment. With increasing petrol prices and growing awareness of environmental concerns, a solid understanding of these systems is vital for future mechanics, automotive engineers, and informed consumers.

Lesson notes

2.1 Variable Valve Timing (VVT)

Definition: Variable Valve Timing (VVT) is a technology that allows the timing and lift of intake and exhaust valves to be altered while the engine is in operation. This contrasts with traditional engines where valve timing is fixed.

Why VVT Matters: Fixed valve timing is a compromise. An engine optimized for low-speed torque will perform poorly at high speeds, and vice versa. VVT allows the engine to optimize valve timing for different engine speeds and loads, resulting in improved fuel efficiency, reduced emissions, and increased power across the RPM range. Imagine a bakkie used both for delivering goods in the city (low speed) and long-distance hauling (high speed); VVT enables optimal performance in both scenarios.

How VVT Works: There are several types of VVT systems, but the most common are: Cam Phasers: These systems alter the phase relationship between the camshaft and the crankshaft. This is typically achieved using hydraulic actuators controlled by the engine control unit (ECU). The ECU monitors engine speed, load, and other parameters to determine the optimal valve timing. Oil pressure then actuates a mechanism that rotates the camshaft relative to the crankshaft.

Consider a Toyota Hilux: these commonly employ VVT, where the intake camshaft timing is adjusted to optimize airflow into the engine at various speeds.

Variable Valve Lift: These systems change the amount the valve opens (the lift). This is more complex than cam phasing and often uses multiple cam lobes or complex rocker arm mechanisms. BMW's Valvetronic is a prominent example.

Effects on Engine Performance: Improved Volumetric Efficiency: By optimizing valve timing, VVT can ensure that the cylinders are filled more completely with air/fuel mixture, increasing volumetric efficiency.

Reduced Pumping Losses: At low engine speeds, VVT can reduce the amount of energy required to push air in and out of the cylinders, improving fuel efficiency.

Increased Power and Torque: By optimizing valve timing for different engine speeds, VVT can increase both power and torque across the RPM range. 2.2 Turbocharging and Supercharging Definition: Turbocharging and supercharging are forced induction systems that compress the air entering the engine, allowing more air and fuel to be burned and thereby increasing power output.

Why Forced Induction Matters: Naturally aspirated engines are limited by the amount of air they can draw in based on atmospheric pressure. Forced induction overcomes this limitation, allowing for smaller, more efficient engines to produce power comparable to larger, naturally aspirated engines. This is especially important in South Africa, where consumers often seek a balance between power and fuel economy. Consider the VW Polo TSI. It uses a small turbocharged engine to deliver good fuel economy with surprisingly good power for its size.

Turbocharging: How it Works: A turbocharger uses exhaust gases to spin a turbine. This turbine is connected to a compressor, which draws in and compresses air before it enters the engine. The hot exhaust gasses exiting the engine drive the turbine.

Advantages: Higher efficiency than superchargers (uses "free" energy from exhaust). Potential for greater power gains.

Disadvantages: Turbo lag (delay between throttle input and boost). More complex and expensive. Higher operating temperatures. Sensitive to incorrect oil viscosity and quality (important for longevity).

Supercharging: How it Works: A supercharger is mechanically driven by the engine's crankshaft, typically via a belt. It draws in and compresses air before it enters the engine.

Advantages: Instant boost (no lag). Simpler and less expensive than turbochargers.

Disadvantages: Lower efficiency (draws power directly from the engine). Typically lower power gains than turbochargers.

Comparing Turbocharging and Supercharging: | Feature | Turbocharging | Supercharging | |----------------|---------------------------------------------|-------------------------------------------| | Drive Source | Exhaust Gases | Engine Crankshaft | | Efficiency | Higher | Lower | | Lag | Present (Turbo Lag) | Absent | | Complexity | Higher | Lower | | Power Output | Generally Higher | Generally Lower | 2.3 Boost Pressure Ratio Definition: The boost pressure ratio is the ratio of the absolute pressure in the intake manifold with boost to the ambient atmospheric pressure.

Formula: Boost Pressure Ratio = (Gauge Boost Pressure + Atmospheric Pressure) / Atmospheric Pressure Where: Gauge Boost Pressure: Measured boost pressure in PSI or kPa.

Atmospheric Pressure: Approximately 14.7 PSI or 101.3 kPa at sea level. Varies slightly with altitude.

Example Calculation: A turbocharged engine is running with a gauge boost pressure of 10 PSI at sea level. Calculate the boost pressure ratio.