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Old 04-29-2013, 02:40 PM   #6
NewellCrazy
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Originally Posted by TomGauger View Post
I drive up mountains with the turbo needle pegged depending on the waste gate to dump excessive pressure. Am I doing this wrong? The Allison temp never reaches the red line and the Cummins ISM500 is cool.The question is this: Is it possible to over-boost the engine?
Hi Tom,

Over-boosting an engine frequently can cause damage to the engine in a variety of ways including pre-ignition, overheating, and over-stressing the engine's internal hardware.
For example, to avoid engine knocking (aka detonation) and the related physical damage to the engine, the intake manifold pressure must not get too high, thus the pressure at the intake manifold of the engine must be controlled by some means. Opening the wastegate allows the excess energy destined for the turbine to bypass it and pass directly to the exhaust pipe, thus reducing boost pressure. The wastegate can be either controlled manually (frequently seen in aircraft) or by an actuator (in automotive applications, it is often controlled by the Engine Control Unit).


[h=3]Turbo lag[/h]Turbocharger applications can be categorized according to those which require changes in output power (such as automotive) and those which do not (such as marine, aircraft, commercial automotive, industrial, locomotives). While important to varying degrees, turbo lag is most problematic when rapid changes in power output are required.



Turbo lag is the time required to change power output in response to a throttle change, noticed as a hesitation or slowed throttle response when accelerating from idle as compared to a naturally aspirated engine. This is due to the time needed for the exhaust system and turbocharger to generate the required boost. Inertia, friction, and compressor load are the primary contributors to turbo lag. Superchargers do not suffer this problem, because the turbine is eliminated due to the compressor being directly powered by the engine.



Lag can be reduced in a number of ways:
  • lowering the rotational inertia of the turbocharger; for example by using lighter, lower radius parts to allow the spool-up to happen more quickly. Ceramic turbines are of benefit in this regard and or billet compressor wheel.
  • changing the aspect ratio of the turbine.
  • increasing the upper-deck air pressure (compressor discharge) and improving the wastegate response
  • reducing bearing frictional losses (such as by using a foil bearing rather than a conventional oil bearing)
  • using variable-nozzle or twin-scroll turbochargers (discussed below).
  • decreasing the volume of the upper-deck piping.
  • using multiple turbos sequentially or in parallel.
  • using an Antilag system.
[h=3] Boost threshold
[/h]Lag is not to be confused with the boost threshold. The boost threshold of a turbo system describes the lower bound of the region within which the compressor will operate. Below a certain rate of flow, a compressor will not produce significant boost. This has the effect of limiting boost at particular rpm regardless of exhaust gas pressure. Newer turbocharger and engine developments have caused boost thresholds to steadily decline.


Electrical boosting ("E-boosting") is a new technology under development; it uses an electric motor to bring the turbo up to operating speed quicker than is possible using available exhaust gases.[SUP] [/SUP]An alternative to e-boosting is to completely separate the turbine and compressor into a turbine-generator and electric-compressor as in the hybrid turbocharger. This allows the compressor speed to become independent to that of the turbine. A similar system utilising a hydraulic drive system and overspeed clutch arrangement was fitted in 1981 to accelerate the turbocharger of the MV Canadian Pioneer (Doxford 76J4CR engine).

Turbochargers start producing boost only when a certain amount of kinetic energy is present in the exhaust gasses. Without adequate exhaust gas flow to spin the turbine blades, the turbo cannot produce the necessary force needed to compress the air going into the engine. The boost threshold is determined by the engine displacement, engine rpm, throttle opening, and the size of the turbo. The operating speed (rpm) at which there is enough exhaust gas momentum to compress the air going into the engine is called the "boost threshold rpm". Reducing the "boost threshold rpm" can improve throttle response.
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