Turbo Actuator

[Francois]

Hi All

When and where should the turbo actuator actually kick in? I understand the principle behind it that it prevents the turbo from running too high rpm... I think, but i have never seen my actuator actually move? I have revved the engine quite a bit and no movement on it at all. Does it only work when the engine us under load? It is not stuck although it is very stiff. WHat should i see when i connect a manometer to the line and what would a boost gauge actaully tell me?

Thanks

diylander

[Bush65]

Hi All

When and where should the turbo actuator actually kick in? ...

From memory a stock 200Tdi turbo is set for about 12 psi boost pressure. The waste gate will start to open at a little lower pressure and be fully open at the set pressure.

I understand the principle behind it that it prevents the turbo from running too high rpm... I think, ...

There is a flap valve, known as the waste gate, which when it opens, allows some exhaust gas to flow directly from the manifold to the exhaust pipe, by-passing the turbine. The spring inside the actuator holds the waste gate closed against the pressure in the exhaust manifold.

The length of the rod from the actuator to the lever on the waste gate determines the pre-load of the spring. Boost pressure (from the compressor) is applied at a diaphragm in the actuator so the waste gate opens when developed force overcomes the spring pre-load.

As you said, the waste gate actuator is adjusted (spring pre-load) to prevent the turbo from over speeding. It is also to reduce the back pressure in the exhaust manifold.

... but i have never seen my actuator actually move? I have revved the engine quite a bit and no movement on it at all. Does it only work when the engine us under load? ...

You are not seeing the rod from the boost actuator move when you rev the engine because the engine is not under enough load for the boost pressure to increase to overcome the spring pre-load.

When the load increases on the engine (hills, etc.), the governor of the fuel injection pump increases the fuel delivery rate to try and maintain the engine speed requested by the driver via the accelerator. Increasing intake air flow (engine speed and boost pressure) and fuel flow (injection pump) produces more heat energy and exhaust gas flow that can spin the turbo faster and thus increase the boost pressure.

... It is not stuck although it is very stiff. ...

I assume the stiffness referred to is with the actuator connected to the waste gate lever. This stiffness is the spring pre-load - you don't want the gate to open at low boost pressure (that would waste energy that is best used to drive the compressor instead of going out to atmosphere).

However if the waste gate is stiff when the actuator is disconnected, you have a problem with the waste gate that will require the turbo to be removed to find and rectify - best hope it is just carbon/coke accumulation.

WHat should i see when i connect a manometer to the line and what would a boost gauge actaully tell me?

If you connect a boost pressure gauge (or manometer) to the line between the compressor and boost pressure actuator, or to some other position where it will sense inlet manifold pressure, the gauge will measure the pressure at that point - note friction losses result in a small drop from compressor outlet to inlet manifold. Then if you go for a drive the boost pressure reading will:

• be negligible when the engine is idling
• increase a little when engine speed increases with low loads (e.g. low acceleration on level road)
• be low to medium at cruise speed on level or low road gradient
• increase quickly as you accelerate hard and engine rpm's increase
• reach maximum setting up a steep hill with your foot flat to the floor with high engine rpm's

Thanks

diylander

Edit:

If you also install a pressure gauge to measure the pressure in the exhaust manifold - known as drive pressure - you will see the drive pressure increase as boost pressure increases.

When drive pressure should be less than boost pressure at low boost pressure. The drive pressure is greater than boost pressure at high boost pressure.

Ideally the drive pressure should not be much more than about 1.5 time boost pressure at maximum boost pressure - if too high neither the turbo or engine are operating efficiently and the egt will be high - the turbine is too small or the compressor is operating well outside its efficient range and is putting too much heat into the charge air.

[Aragorn]

If you want to see it move you can stick a footpump or similar onto the small pipe and slowly build up pressure in the capsule. It should open somewhere just below 1 bar.

[Francois]

Thanks for the reply guys and bush65 for such a complete answer and explanation on the wastegate. Now i know what to look for and how to check. I always thought a turbo is a turbo and the faster it spins the better! Good thing i didnt start tinkering with it.....

[Bush65]

Thanks for the reply guys and bush65 for such a complete answer and explanation on the wastegate. Now i know what to look for and how to check. I always thought a turbo is a turbo and the faster it spins the better! Good thing i didnt start tinkering with it.....

The type of compressor used on turbos create pressure dynamically (not positive displacement). To achieve this they have to accelerate the intake air to high velocity as it leaves the tips of the compressor wheel.

Boost pressure is proportional to the tip velocity squared.

Air flow is proportional to the cross sectional area of the compressor inducer (diameter of wheel at inlet).

Compressor efficiency is related to compressor trim. Efficiency in non technical terms indicates to us how much heat the compressor adds to the charge air compared to ideal adiabatic compression. Compressor trim is the area ratio between inducer and exducer (diameter of wheel at outlet), calculated as [inducer dia squared / exducer dia squared] x 100.

The velocity of the air is limited to below the speed of sound and there are mechanical limits on the rotating assembly - shaft torque (provided by the turbine) has to increase to increase the speed of the compressor which increases the material stress.

The 300Tdi uses a compressor wheel that quickly becomes inefficient at boost pressure above 18 psi. It will increase the heat in the charge air without providing much more air flow - from the compressor wheel spinning in air that is not moving at a higher rate, not unlike a tyre spinning faster when it looses traction and burning rubber without much forward motion.