I. Working Principle of the Exhaust Gas Turbocharger

The turbocharger comprises three parts: the turbine end (exhaust side), the compressor end (intake side) and the centre section (bearings, oil cooling and oil passages), which are rigidly connected on the same axis.

1. Exhaust-driven turbine

High-temperature, high-pressure exhaust gases from the engine’s combustion process surge from the exhaust manifold into the turbine housing, striking the turbine blades at high speed and causing the turbine shaft to rotate rapidly (normal speed: 80,000–200,000 revolutions per minute). The energy in these exhaust gases would otherwise be wasted by being discharged directly through the exhaust pipe; the turbine recovers this kinetic energy.

2. The coaxial drive compresses air in the compressor

The other end of the turbine shaft is connected to the compressor impeller, which rotates at high speed in synchronisation with the turbine, drawing in fresh air from the air filter and forcibly compressing it.

3. Intercooling reduces temperature and increases air mass density

Compressed air heats up and its density decreases; the high-pressure air produced by compression first passes through an intercooler (air-cooled or water-cooled) to be cooled, and the low-temperature, high-density air is then fed into the cylinders.

4. Loop Logic

More air = more fuel injection = greater torque and higher power; a small-displacement engine delivers the power of a large-displacement engine, whilst improving fuel economy and reducing exhaust emissions.

5. Lubrication and cooling system

Engine oil is continuously fed into the intermediate floating bearings to lubricate the high-speed rotating shaft and dissipate frictional heat; some models feature a separate water-cooling circuit, which provides delayed cooling after shutdown to prevent high-temperature carbon deposits from causing bearing burnout.

II. At what engine speed does the turbocharger begin to engage and build boost pressure?

It is divided into three stages: the intervention speed (turbo rotation), the boost-in speed (effective boost) and the maximum torque speed (full boost):

1. The turbo begins to spin (no boost)

At idle speeds below 1,000 rpm, exhaust gas flow is low and the turbocharger spins slowly, generating virtually no boost; at around 1,200 rpm, the turbocharger spins noticeably, but there is only slight pre-compression, and no perceptible increase in power is felt.

2. Boost-on speed (noticeable surge of power, effective boost)

Small-displacement 1.2T/1.5T petrol engines for passenger cars: between 1,500 and 1,700 rpm, boost pressure of around 0.4 bar begins to build up, resulting in a noticeable increase in power;

Mainstream 2.0T petrol cars: boost pressure builds from 1,400–1,600 rpm;

Diesel lorry turbocharger: delivers strong torque at low revs, building pressure as early as 1,200–1,400 rpm.

3. Maximum boost pressure range

Petrol engine: Maintains rated boost pressure between 1,800 and 4,000 rpm; above 4,000 rpm, the exhaust gas flow becomes excessive and the bypass valve relieves the pressure.

Note: Modern low-inertia, small-blade turbochargers (lightweight turbochargers) build boost earlier, delivering effective boost from as low as 1,300 rpm; older, larger turbochargers suffer from significant lag and only start to deliver power from 2,000 rpm onwards.

III. Structure, function and activation conditions of the bypass valve (pressure relief valve)

1. Two types of bypass valves

(1) Exhaust bypass valve (mechanical pressure relief valve, the most common type): Fitted on the exhaust side of the turbine, this metal valve blocks the turbine exhaust passage.

(2) Electronic pressure relief valve (electronically controlled actuator, new models): motor-driven, with precise control of the opening.

2. Key role

(1) Limits the maximum boost pressure to protect the engine and turbocharger; the bypass valve diverts excess exhaust gas to control the maximum boost pressure (rated boost pressure for petrol engines: 0.8–1.2 bar).

(2) Reducing turbo lag
At low revs, the valve closes completely, allowing 100 per cent of the exhaust gas to flow through the turbo for rapid pressure build-up; at high revs, the exhaust gas is diverted to stabilise the pressure.
(3) Shut-off and pressure relief to protect the compressor
When the throttle is released, the throttle valve closes instantly; compressed air cannot enter the cylinder, and the high pressure in the piping pushes back against the impeller, which may damage it; the bypass valve works in conjunction with the pressure relief to prevent surging.
(4) Optimising exhaust flow velocity during cold start and idling conditions
Open the throttle slightly whilst the engine is cold and idling to accelerate the warm-up of the three-way catalytic converter and reduce cold-start emissions.

3. Activation timing (mechanical bypass valve logic)

The mechanical version is driven by a pressurised diaphragm:

Low speed (1200–1800 rpm): The intake boost pressure is low, the diaphragm chamber exerts no thrust, the bypass valve is fully closed, and the turbine is driven entirely by the exhaust gases.

When the engine speed rises to the full-boost range (above 1,800 rpm): the boost pressure in the intake manifold reaches the set threshold (approximately 0.9 bar), and high-pressure air is fed into the actuator diaphragm chamber, causing the connecting rod to push open the bypass valve.

The higher the rotational speed and the greater the load: the wider the valve opens, the more exhaust gas is diverted, and the boost pressure remains constant, ceasing to rise further.

Rapid acceleration whilst climbing a steep gradient under heavy load: briefly close the throttle completely to provide a short burst of boost; open the blow-off valve fully the moment the throttle is released.

Electronic bypass valve logic: The ECU adjusts the valve opening in real time based on signals relating to engine speed, throttle position, intake pressure and coolant temperature, ensuring more precise control and eliminating the pressure fluctuations associated with mechanical valves.

IV. Common faults, symptoms and causes relating to turbochargers.

(1) Oil leaks (the most common fault)

1. Oil leakage from the compressor-end intake duct

Symptoms: Oil on the air filter, heavy sludge build-up on the throttle body/intake manifold, blue smoke during cold starts, and excessive engine oil consumption.

Possible causes:

① The PCV valve in the crankcase ventilation system is blocked, causing excessive pressure in the crankcase, which forces engine oil into the turbocharger’s return oil passage.

② The turbine oil return pipe is crushed or blocked, causing poor oil return from the intermediate housing, which results in oil pressure forcing the oil seal out of position.

③ Excessive wear clearance in the plain bearings, resulting in oil seal failure.

④ Prolonged short-distance journeys with a cold engine cause engine oil and carbon deposits to block the return oil passages.

2. Oil is leaking from the exhaust end of the turbocharger, and blue smoke is coming from the exhaust pipe.

Ageing of the turbo oil seal and bearing wear can cause engine oil to enter the exhaust system, leading to blue smoke from high-temperature combustion and a tendency for the three-way catalytic converter to become blocked and fail.

(2) Insufficient boost, lack of power, and sluggish acceleration

Symptoms: Poor acceleration, lack of power when climbing hills, warning light illuminated, and intake pressure readings significantly below the standard value.

Possible causes:

1. The bypass valve is stuck or remains open, causing a large volume of exhaust gas to be diverted directly, preventing the turbocharger from building up pressure (due to spring fatigue in the mechanical valve or carbon build-up causing the valve to seize).

2. Cracks in the intake piping, intercooler or vacuum hoses, resulting in compressed air leaks.

3. The turbocharger impeller is coated with carbon deposits and oil, resulting in high rotational resistance.
4. The electronic wastegate actuator has failed, causing the ECU to continuously command the valve to remain open.
5. The three-way catalytic converter is blocked, resulting in excessive exhaust back pressure; the exhaust gases are unable to drive the turbocharger.

(3) Excessive boost pressure, engine knocking, fault warnings

Symptoms: Jerky acceleration, engine fault light illuminated, warning of excessive boost pressure; in severe cases, cylinder head gasket failure.

Possible causes:

1. The bypass valve is seized due to carbon build-up and is completely closed; there is no exhaust gas diversion, and the boost pressure exceeds the limit.
2. The vacuum line for the relief valve has come loose or ruptured; the diaphragm assembly lacks thrust and is unable to open the valve.
3. The electronically controlled relief motor is seized and cannot execute the valve-opening command.

(4) Abnormal noises from the turbine

1. Whistling (continuous, high-pitched screech)
Most noticeable at high speeds; caused by air leaks in the intake or intercooler pipework, slight deformation of the impeller, or air leaks through gaps in the housing.
2. Metallic scraping or clicking noises
Worn bearings; impeller scraping against the housing (lack of oil, prolonged dry running at high temperatures, or foreign objects entering the turbocharger).
3. Hissing sound from pressure relief
Prominent when lifting off the throttle; this is normal pressure relief. A continuous hissing sound at idle indicates that the bypass valve is not closing properly.

(5) Severe turbo lag; lack of power at low revs

Symptom: Power only kicks in after pressing the accelerator for more than one second; the car lacks power when overtaking at low speeds.

Possible causes: 
1. A high-inertia turbocharger paired with a small-displacement engine.
2. Carbon build-up blocking the exhaust manifold and turbocharger housing, resulting in insufficient exhaust gas flow.
3. The bypass valve is slightly open, diverting exhaust gas at low revs.
4. A leak in the exhaust pipe, causing a loss of exhaust gas energy.

(6) Impeller damage, turbine seizure (serious fault)

Symptoms: No boost pressure whatsoever, lack of acceleration, unusual noise on start-up, and the turbocharger shaft fails to rotate.

Possible causes: 
1. Insufficient engine oil supply or complete loss of oil, resulting in the high-speed bearings overheating, seizing up and becoming locked.
2. Foreign matter (carbon deposits or metal debris from the exhaust) enters the turbocharger and strikes the blades, causing the impeller to fracture.
3. Prolonged exposure to high temperatures causes the intermediate housing to deform, resulting in the shaft becoming seized.
4. Switching off the engine immediately after driving at high speed causes the oil circulation to cease, leading to the bearings and oil seals being scorched by high temperatures.

(7) Overheating; turbocharger turning red.
It is normal for the turbocharger housing to turn red after prolonged climbing under full load; however, if it turns red even at idle, this indicates a blocked exhaust, excessive fuel injection causing delayed combustion, or a stuck bypass valve, resulting in the continuous build-up of high-temperature exhaust gases.

V. Key Points for Supplementary Maintenance (to Reduce Breakdowns)
1. When starting the engine cold, allow it to idle for 30 seconds to ensure the turbocharger is fully lubricated before revving it up.
2. After long-distance, high-load driving, allow the engine to idle for 1–2 minutes before switching it off to prevent overheating due to a lack of oil for cooling.
3. Use the manufacturer’s specified fully synthetic engine oil throughout, and change the oil filter regularly.
4. Clean the PCV (Positive Crankcase Ventilation) valve regularly to prevent oil leakage caused by excessive pressure in the crankcase.
5. Avoid frequent short journeys, as this can cause the engine oil to emulsify and lead to carbon deposits blocking the return oil passages.