squeaking noise from WRC cars

[urabus-denoS2000]

I think that the sound of the Escort WRC has it's own charm...

[zerodegreec]

ALS explained..

Firstly, boost is controlled very acurately by using 2 fuel injectors as air valves, these feed the boost/atmospheric pressure to the waste gate actuator, allowing minute and very fast acting boost control.
Secondly, the normal method of controlling engine idle via an idle control valve, that bypasses the throtle butterfly is dispensed with, the static idle is set in the old fashioned way by a locked off bypass screw.
Thirdly, where the idle control valve used to be, a new valve is fitted that has a solenoid controlled valve (looks like an engine inlet valve internally) that can open/close the throttle butterfly bypass orifice very qickly.
Forthly, a miriam turbo shaft is used to withstand the massive increase in turbo temperature generated by ALS.
Fifth, there is no dump valve used on an ALS equipped engine, the whole point of ALS is to keep boosting the engine, even on closed throttle.
The way it works is that when you come off the throttle, for example braking hard into a corner as you describe.. the throttle butterfly closes as per normal, once the boost drops to a certain level, the ECU starts to pulse the Throttle butterfly bypass valve, this has the same effect as you constantly stabbing the throttle on and off very quickly, this means that the engine is being driven by the ECU as though it wants to accelerate again, however this occilation of the throttle bypass valve is happening so fast, it has the effect of allowing the turbo to stay spinning yet produces no additional torque to drive the car forward. The ignition timing is also controlled during this off throttle period to again reduce torque yet maintain boost presure.
The effect of all this is that massive heat is generated in the turbo exhaust side, which ignites all the excess fuel, there is no deliberate overfuelling going on to explode the turbo impellar area.
The most modern of ALS systems on the current world rally cars are far more efficient due to them being so finely controlled unlike 10 years ago, which is why the ALS banging is less apparent on the modern cars.

thanks for the post. You mentioned that they are not intentionally running rich and burning it in the exaust. (I thought that is what they were doing...) How then do they keep the turbo RPM up without the mass flow from the engine? if they pulse the butterfly valve and cut fuel they are not going to get the needed flow to keep it up to full or almost full boost... I must be missing something, How can they keep the boost from going to high without a blow off? You would think that the back pressure would grenade the impellers.

And back to the original question, where does the squeak come from?

[anstis]

And back to the original question, where does the squeak come from?

Don't quote me on this but I think I read somewhere a long time ago, from memory it might have been David Lapworth explaining, that when the throttle is closed the compressed air, having gone through the turbo, almost stops as there is less being fed into the engine. The compressed air then momentarily expands forcing some backwards through the compressor wheel as it's still spinning, creating a chirp.

I could be mistaken though...

[Gard]

Don't quote me on this but I think I read somewhere a long time ago, from memory it might have been David Lapworth explaining, that when the throttle is closed the compressed air, having gone through the turbo, almost stops as there is less being fed into the engine. The compressed air then momentarily expands forcing some backwards through the compressor wheel as it's still spinning, creating a chirp.

I could be mistaken though...

[Nenukknak]

Don't quote me on this but I think I read somewhere a long time ago, from memory it might have been David Lapworth explaining, that when the throttle is closed the compressed air, having gone through the turbo, almost stops as there is less being fed into the engine. The compressed air then momentarily expands forcing some backwards through the compressor wheel as it's still spinning, creating a chirp.

I could be mistaken though...

I really can't imagine that's correct.

[Nenukknak]

How can they keep the boost from going to high without a blow off? You would think that the back pressure would grenade the impellers.

The ignition is being delayed, so that ignition happens at the time when exhaust valves are (somewhat) open, thus creating exhaust pressure to spin the turbo, while not giving the car forward momentum. At least that's the way I always understood it.

[Daniel]

The ignition is being delayed, so that ignition happens at the time when exhaust valves are (somewhat) open, thus creating exhaust pressure to spin the turbo, while not giving the car forward momentum. At least that's the way I always understood it.

Yes the compressor would stall and that is exactly what you don't want.

[OldF]

Here are some more readings.

http://www.turbobygarrett.com/turbob...o_tech103.html

“Surge Line

* Surge is the left hand boundary of the compressor map. Operation to the left of this line represents a region of flow instability. This region is characterized by mild flutter to wildly fluctuating boost and “barking” from the compressor. Continued operation within this region can lead to premature turbo failure due to heavy thrust loading.
* Surge is most commonly experienced when one of two situations exist. The first and most damaging is surge under load. It can be an indication that your compressor is too large. Surge is also commonly experienced when the throttle is quickly closed after boosting. This occurs because mass flow is drastically reduced as the throttle is closed, but the turbo is still spinning and generating boost. This immediately drives the operating point to the far left of the compressor map, right into surge.

Surge will decay once the turbo speed finally slows enough to reduce the boost and move the operating point back into the stable region. This situation is commonly addressed by using a Blow-Off Valves (BOV) or bypass valve. A BOV functions to vent intake pressure to atmosphere so that the mass flow ramps down smoothly, keeping the compressor out of surge. In the case of a recirculating bypass valve, the airflow is recirculated back to the compressor inlet.”

http://www.kosunenracing.com/eals.html

“As you might be aware of, turbochargers display what is known as lag time which is the time needed for the turbine to reach its full throttle from an intermediate rotational speed state. During lag time the engine is much less responsive and its output well below nominal. Essentially, exhaust gas is used to spin a turbine which compresses intake air. But what happens when you shift gears? You have to lift your foot off the throttle. Exhaust gas velocity drops, and the turbine blades slow down. Intake boost pressure drops. For the next gear, you have to wait until the turbine spins up again for the boost to kick in.

Example Motec Mode2: When the throttle is permanently open and a switch is used to indicate to the ECU that Overrun Boost (ALS) should be activated. When the driver lifts his foot from the throttle the ECU (motec, ecutek, gems, etc.) alters ignition timing is altered of delay (retard, 10-40 degrees) and the intake air and fuel supply mixture is made richer. The inlet butterfly is kept slightly open or an air injector is used to maintain air supply to the engine. This results in air/fuel mixture that keeps getting in the combustion chambers when the driver no longer accelerates. The ignition being delayed, the air/fuel mixture reaches the exhaust tubes mostly unburned. RPM. Additional fuel during and-lag is often required to help control EGT.

A value between 10 - 30% extra is usually beneficial. When the spark plug fires, the exhaust valve is starting to open due to the ignition delay mentioned above. The turbo sits right there and the explosion keeps it turning. Racing ALS version can maintain a pressure of up to 1 - 1.5 bar in the inlet manifold with the throttle closed. In decent rally/race cars anti-lag action is terminated by excessive engine temperature to safe engine! NOTE! ALS can add over 200°C to the temperature of the turbo forcing it up over 1000°C.

Rally cars use a different way of introducing petrol into the exhaust manifold. There are separate injectors that introduce fuel directly into the exhaust manifold. Some of this mixture will continue burning in the turbine and down the exhaust system, resulting in the flames and explosions you hear coming out of the exhaust muffler."

[zerodegreec]

Very Nice OldF thanks for that.

Now I understand that whats in my head as to how the ALS works is bang on what you posted, but still that original question...

what causes that squeak???

[Zico]

what causes that squeak???

I know Wiki isnt usuallly the most accurate of sorces but see the 'Wastegate chatter myth' section on here.. http://en.wikipedia.org/wiki/Wastegate