Lesson Notes By Weeks and Term v5 - Grade 12

Lesson Video

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Performance objectives

• Explain the principles of Variable Valve Timing (VVT).
• Describe the operation of turbocharging systems.
• Describe the operation of supercharging systems.
• Analyse the pros and cons of each technology.
• Diagnose common faults in these systems.

Lesson summary

This week, we delve deeper into advanced engine technology, focusing specifically on Variable Valve Timing (VVT) and Forced Induction systems (Turbocharging and Supercharging). These technologies are crucial for improving engine efficiency, power output, and reducing emissions – aspects that are incredibly relevant in South Africa. Consider the fluctuating fuel prices in our country; efficient engines can significantly impact household budgets and the profitability of transport businesses. Similarly, the need to meet increasingly stringent environmental standards makes understanding these technologies vital for future mechanics and engineers.

Lesson notes

2.1 Variable Valve Timing (VVT) VVT is a technology that allows the timing and/or lift of the intake and/or exhaust valves to be varied while the engine is in operation. This provides significant advantages over engines with fixed valve timing. Why is VVT important? Fixed valve timing is a compromise. The optimal valve timing for low-speed torque is different from the optimal valve timing for high-speed power. VVT allows the engine to optimize valve timing for different engine speeds and loads, resulting in improved: Fuel Efficiency: By optimising valve timing, VVT can improve combustion efficiency, reducing fuel consumption.

Power Output: VVT can increase both low-end torque and high-end horsepower.

Reduced Emissions: Improved combustion efficiency reduces harmful emissions.

Types of VVT: Cam Phasers: These systems use a hydraulic or electric actuator to rotate the camshaft relative to the crankshaft. This changes the valve timing. They are the most common type of VV

T. Variable Valve Lift: These systems alter the amount the valves open (valve lift). This is typically achieved through complex rocker arm mechanisms or by using multiple cam lobes with different lift profiles.

Variable Valve Duration: This changes the amount of time the valve is open. Usually this is achieved by integrating it with variable valve lift systems.

How Cam Phasers Work (Example): A cam phaser typically consists of a housing bolted to the camshaft and a rotor inside the housing. The rotor is connected to the camshaft and can rotate relative to the housing. Oil pressure is used to control the position of the rotor. The engine control unit (ECU) controls the oil pressure to the phaser, based on engine speed, load, and other parameters.

Worked example

Imagine a Toyota bakkie used for both commuting and hauling loads. Without VVT, the engine would be optimized for either low-speed torque (good for hauling) or high-speed power (good for highway driving), but not both. With VVT, the ECU can adjust the valve timing to provide strong low-end torque when hauling a load and then optimize for higher horsepower at higher RPMs when driving on the highway.

2.2 Turbocharging

Turbocharging is a forced induction system that uses exhaust gases to spin a turbine, which in turn drives a compressor. The compressor forces more air into the engine, allowing more fuel to be burned, resulting in increased power.

Components of a Turbocharging System:

Turbine: Converts the energy of exhaust gases into rotational energy.

Compressor: Compresses the intake air.

Intercooler: Cools the compressed air, increasing its density and further boosting power. Colder air is denser and contains more oxygen per unit volume.

Wastegate: A valve that bypasses exhaust gases around the turbine to limit boost pressure. Prevents overboost and engine damage.

Blow-Off Valve/Diverter Valve: Releases excess pressure in the intake system when the throttle is closed, preventing compressor surge and damage to the turbocharger.

How a Turbocharger Works:

Exhaust gases from the engine are directed to the turbine housing.

The exhaust gases spin the turbine wheel.

The turbine wheel is connected to the compressor wheel via a shaft.

As the turbine spins, it also spins the compressor wheel.

The compressor wheel draws in air and compresses it.

The compressed air is then cooled by the intercooler.

The cooled, compressed air is then directed into the engine's intake manifold.