The engine generates power by burning fuel within the cylinders to perform work. As the amount of fuel input is constrained by the volume of air drawn into the cylinders, the engine's power output is consequently limited. Should the engine's operating performance already be at its optimum state, increasing output power can only be achieved by compressing more air into the cylinders to increase the fuel quantity, thereby enhancing the engine's capacity to perform work.
The turbocharger system is one of the most common forced induction systems in supercharged engines.
If, within the same unit of time, a greater volume of air-fuel mixture can be forcibly compressed within the cylinder (combustion chamber) for combustion (a small-displacement engine can “draw in” the same air volume as a large-displacement engine, thereby increasing volumetric efficiency), it can generate greater power output than a naturally aspirated engine at the same rotational speed. The principle is akin to directing an electric fan into the cylinder, forcibly pumping air into it to increase the volume and thereby achieve greater horsepower. The difference lies in the power source: instead of an electric motor, the fan is driven by the exhaust gases expelled from the engine.
Typically, engines incorporating such forced induction achieve at least a 30-40% power increase. This remarkable effect is precisely why turbochargers are so highly prized. Moreover, delivering optimal combustion efficiency and substantial power gains represent the core value a turbocharged system offers to a vehicle.
So how exactly does a turbocharger function?
First, the exhaust gases expelled from the engine drive the turbine impeller at the exhaust end (right side in the diagram above), causing it to rotate. This rotation in turn drives the connected compressor impeller on the opposite side (left side). The compressor impeller then forcibly draws air through the inlet. This air is compressed by the rotating blades before entering progressively narrower compression passages for secondary compression. The temperature of this compressed air exceeds that of the intake air, necessitating cooling via an intercooler before injection into the combustion chambers. This cyclical process constitutes the operating principle of the turbocharger.
The function of the throttle valve
Within the engine's intake system, there are two principal components: firstly, the air filter, which is responsible for filtering impurities from the air; and secondly, the intake manifold, which directs the air into the cylinders. Within the intake manifold lies a crucial component known as the throttle valve.
The throttle valve's primary function is to regulate the volume of air-fuel mixture entering the cylinders. When driving, the depth to which we depress the accelerator pedal directly controls the throttle valve's opening. The deeper the pedal is pressed, the wider the throttle valve opens, allowing greater air-fuel mixture intake and consequently increasing the engine's rotational speed.
Traditional cable-operated throttles utilise a steel cable connected at one end to the accelerator pedal and at the other to the throttle valve, employing a 1:1 transmission ratio. This method yields less than ideal control precision. In contrast, modern electronic throttle systems employ position sensors to transmit data such as the force and amplitude of the accelerator pedal depression to the control unit for analysis. This process interprets the driver's intent, after which the ECU calculates the actual throttle opening required and issues commands to operate the throttle motor, thereby achieving precise throttle control.
Is the intake manifold length variable?
A control valve installed within the intake manifold can divide it into two sections by opening and closing, thereby altering its effective length. This adjustment of the intake manifold's length primarily serves to enhance the engine's intake efficiency across varying rotational speeds, thereby improving power delivery performance throughout the entire rev range.
Why does the exhaust manifold have such an odd shape?
The vehicle's exhaust system primarily comprises the exhaust manifold, three-way catalytic converter, silencer and exhaust pipes. Its principal function is to expel the combustion exhaust gases from the cylinders into the atmosphere.
The exhaust manifolds we commonly observe feature rather peculiar shapes. This design serves to minimise interference between exhaust gases from different cylinders or the occurrence of backflow, thereby preventing any detrimental impact on the engine's power output.
How does a turbocharger boost pressure?
Turbocharging is commonly abbreviated as Turbo or T. When we see markings such as 1.4T or 2.0T on a vehicle's rear, it indicates that the engine is turbocharged.
The turbocharger primarily consists of two components: the turbine and the compressor, connected by a drive shaft. The turbine's inlet is linked to the engine's exhaust manifold, while its outlet connects to the exhaust pipe. The compressor's inlet is connected to the intake manifold, and its outlet is connected to the intake manifold. The exhaust gases expelled from the engine strike the turbine, causing it to spin at high speed. This, in turn, drives the coaxial compressor to rotate rapidly, forcibly delivering pressurised air into the cylinders.
Turbocharging primarily harnesses the energy from engine exhaust gases to drive a compressor, thereby boosting the intake air pressure. This process consumes virtually no engine power, delivering excellent sustained acceleration. However, at low speeds, the turbocharger cannot engage promptly, resulting in a certain degree of lag.
What about mechanical supercharging?
Mechanical supercharging primarily involves using the crankshaft's power to drive a mechanical air compressor, thereby compressing air. However, this process incurs a certain degree of power loss from the engine during operation.
As the mechanical supercharger is directly driven by the crankshaft, it commences operation whenever the engine is running. Consequently, the engine delivers impressive torque output even at low revs. However, during high-speed operation, the mechanical supercharger imposes significant power losses on the engine, resulting in less pronounced power gains.
How does a twin-turbocharged engine work?
A twin-charged engine, as the name suggests, refers to an engine equipped with two superchargers. Should an engine utilise two turbochargers, it is termed a twin-turbocharged engine.
To address turbocharger lag in exhaust gas-driven systems, two identical turbines are connected in parallel on the exhaust manifold. At low engine speeds, the reduced exhaust flow can drive the turbines to high rotational speeds, generating sufficient boost pressure and thereby minimising turbo lag.
As previously noted, turbochargers exhibit lag at low engine speeds, yet deliver substantial boost at high speeds, markedly enhancing engine power without consuming significant engine power. In contrast, mechanical superchargers are directly driven by the engine's rotation, eliminating turbocharger lag but incurring some power loss and offering lower boost levels. Combining these two systems allows their respective advantages to complement each other.
As with the 1.4-litre TSI engine fitted to the Volkswagen Golf GT, the designers combined a turbocharger with a supercharger. The supercharger is mounted on the engine's intake system, while the turbocharger is positioned on the exhaust system. This arrangement ensures effective boost delivery across the entire engine speed range, from low to high revs.
Working Principle of a Turbocharger
The operating principle of a turbocharger is actually quite straightforward. Simply put, it harnesses the exhaust gases emitted by the engine to drive the turbine. This turbine, in turn, drives a fan connected to the same shaft, continuously forcing fresh air into the engine cylinders. Consequently, more air enters the cylinders per unit time, thereby enhancing the engine's efficiency.
Turbochargers are high-temperature components, as the gases driving their operation are drawn directly from exhaust gases expelled from the cylinders, reaching temperatures of 900°C to 1000°C. Under full load conditions, the turbine's rotational speed can attain 180,000 to 200,000 revolutions per minute. Operating at such extreme temperatures and rotational speeds necessitates exceptionally efficient and stable lubrication. Moreover, functioning within high-temperature and high-pressure conditions demands that all turbocharger components and lubricants exhibit excellent heat resistance and sealing properties. Consequently, maintenance should focus on the following aspects:
1.Selection of engine oil
Many motorists find themselves at a loss when selecting engine oil for replacement. Damage to turbochargers typically stems from compromised oil seals between the unit and intake manifold, resulting in significant oil consumption. Professional investigations reveal that a significant proportion of oil seal failures stem from owners failing to change their engine oil at the recommended intervals, or using substandard oil. This prevents the floating turbocharger main drive shaft from receiving adequate lubrication and heat dissipation, causing the oil seal to deteriorate under high temperatures and resulting in oil leakage.
It is therefore recommended that turbocharged engines should be fitted with high-quality engine oil that offers excellent high-temperature resistance and oxidation stability, whilst also ensuring that the oil change interval is appropriately shortened.
2.Ensure the turbine remains clean
The clearance between the turbocharger shaft and its bearing sleeve is extremely small. Consequently, if contaminated engine oil is used, or if impurities enter due to a dirty oil filter, this will cause excessive wear on the turbocharger. Furthermore, should the intake air contain significant impurities, these dust particles entering the high-speed turbine impeller will collide with it. This results in unstable turbine operation and accelerated wear of the bearing bushings and seals. Consequently, vehicles equipped with turbochargers must pay particular attention to promptly replacing both the oil filter and air filter to maintain the cleanliness of the turbocharger.
3.Cold starts should be gradual; allow the engine to idle for a moment after warming up before switching it off.
During the initial stages of a cold start, engine oil is typically at a low temperature and exhibits greater viscosity, resulting in suboptimal lubrication. Reaching normal operating temperature requires a gradual process and time. Forcing the turbocharger to operate at full load within these first few minutes will increase wear and tear, thereby shortening its lifespan.
The correct approach is as follows: during the first few minutes of driving, maintain a slow speed for several minutes to allow the oil to reach its optimal state before accelerating to high engine speeds. This is particularly crucial in northern winters, where it is essential to let the vehicle warm up after a cold start before proceeding. This benefits both the vehicle and the driver.
When parking, as the turbo operates at extremely high temperatures, it is advisable to idle the engine for a short while before switching it off. At this stage, the engine's oil and cooling systems remain active, allowing the turbo's temperature to gradually decrease. Note that after switching off the engine, the turbo will continue to spin due to inertia, and it still requires oil lubrication. If you abruptly switch off the engine, the entire engine system ceases operation, abruptly halting both the turbocharger's cooling system and oil supply. It would then rely solely on natural cooling, which can drastically reduce the turbocharger's lifespan.
4.Regular inspections are important
Much like regular health check-ups for the human body, cultivating this good habit helps us detect turbocharger faults promptly and prevent problems before they arise. For instance: inspect the turbocharger's exterior for damaged sealing rings; check oil inlet and return pipe connections for looseness or leakage; examine the exhaust outlet for residual oil; verify the compressor inlet duct walls are free of oil; and listen for unusual noises or abnormal vibrations. Should any of these irregularities be detected during routine operation, prompt professional servicing is essential to prevent more severe component damage.
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