Turbo Basics and Boost Control/ and How Engine Works...

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Turbo Basics and Boost Control/ and How Engine Works...

Postby drscam » Fri Feb 07, 2003 7:39 am

This is an Information post only (will remain locked and sticky) If you require further questions please refer to

[quote]Originally posted by billwot
THE TURBOCHARGER
The turbocharger is a centrifugal compressor driven by the otherwise-wasted energy in the exhaust stream. It is a 2 chambered housing with a shaft through the center extending into both chambers. A turbine wheel is mounted on one end of the shaft and is in the exhaust stream, and an impeller wheel is mounted on the other end. That is the compressor end, and it is connected to tubing that goes to the intercooler, and then to the throttle body. A turbocharger is really a very simple device, and as long is it regularly fed clean , high quality oil from the engine lube system, and allowed to cool down before engine shutdown, will last nearly as long the engine itself.


THE WASTEGATE
There are two alternate paths for the exhaust flow at the turbo. One is across the turbine, and the other is out the wastegate, allowing it to bypass the turbo. Since more energy at the turbo means more air to the engine, which means more energy to the turbo, which in turn means more air to the engine, which means...well, I think you get the point. The wastegate is necessary to limit the airflow output from the turbo.
The wastegate isn't simply open or closed; it modulates to maintain very precise control over the turbo's speed and output.

SOME BASIC PHYSICS
Compressors are pumps, and pumps create flow. When the turbo creates more airflow than the engine is consuming, then the air becomes pressurized. So boost pressure will rise and fall as the turbo output increases and decreases. Thats why the wastegte controls the speed and airflow of the turbo. And pressure and flow are directly related. That means you can not get "more flow at the same pressure..."

THE WASTEGATE ACUTATOR
The wastegate actuator is simply a can with a rubber diaphragm on one end, and 2 ports with hose fittings on the front end. Looking into the engine compartment form the driver's side, it can easily be seen just to the right of the turbo compressor inlet. The hose on the left is connected to the turbo, and the hose on the right is connected to the T/VSV. (more on that later) There is a spring that holds the diaphragm in place that has a yeild value of about 7 psi.. As pressure builds in the turbo, air begins to fill the actuator and pushes against the diaphragm. When the pressure exceeds the spring value, ~ 7 psi, the actuator moves out, pushing a rod, and opening the wastegate.


The T/VSV (Turbocharger Vacuum Switching Valve) is a "factory-installed boost controller". It is simply a solenoid-operated valve that, when energized (open) allows air to escape from the actuator, lowering the pressure in the actuator, and allowing the wastegate to modulate closed. This vented air actually flows back into the intake airstream between the airflow meter and the turbo intake so it does not distort the ECU's air/fuel ratio calculations.

The T/VSV is a normally closed-energized open valve. The ECU will energize (open) it whenever you boost IF 1) engine coolant temps are up to normal , and 2) ambient (intake) air temps are above ~ 32 deg F. (There may be some variation with that number). If the T/VSV is open (high boost mode) and the ECU gets a signal from the knock sensor, it will de-energize (close) the T/VSV. Likewise, if there is an over boost signal from the boost pressure sensor, it will de-energize the T/VSV as part of the "fuel-cut" sequence.

NOTE: Unless you have an aftermarket boost controller, disabling the T/VSV will lock you into low-boost mode!! It WILL NOT increase boost.

The normal range of max boost ranges from 7.1 to 11.8 psi, according to the Factory Repair Manual (BGB). Low boost problems are almost always a result of some problem in the boost control system and rarely with the turbo itself..

BOOST PRESSURE SENSOR AND FUEL-CUT
The boost pressure sensor is simply a pressure transducer that monitors manifold pressure and reports it to the ECU as a voltage value. If the voltage exceeds about 4.4 volts (~12 psi on '91, 92' and early 93's, ~16 psi on late 93's and up) the ECU initiates the fuel-cut sequence. It de-energizes the T/VSV to lower boost, retards ignition timing, and restricts fuel delivery by limiting the injector cycle. The "check-engine" light will come on for about 20 seconds, and a code 34 will be stored in the diagnostic memory. You will be in "limp mode" and unable to boost again until you shut off the engine and restart it. No other reset is required to return to normal operation. You do not have to pull any fuses or disconnect the battery (Internet legend!)

For a complete schematic diagram of the turbo and boost control system, go here:

http://member.newsguy.com/~gtfour/technicl.htm#Turbo
Click on "Boost Control"

BOOST CONTROLLERS
Since the rate and amount of airflow to the wastegate actuator control's its operation, we can increase the boost by either restricting the airflow into the actuator chamber, or by increasing the bleed rate of the air escaping the actuator. Boost controllers range from simple orfice plugs inserted into the hose connecting the turbo the the actuator, to complex multi-valve electronically operated devices, but again, the all do the same thing: they manipulate the amount of air (pressure) in the actuator.

FUEL-CUT DEFEAT SYSTEMS
Since the FC response is programmed in at ~ 12 psi*, you must somehow eliminate it or alter it to boost above ~12 psi. Fuel-cut is initiated when there is a 4.4 volt signal to the ECU from the sensor, so ALL fuel cut defeat (FCD) system either prevent that from happening, or delay it . Some methods simply eliminate FC completely by preventing any thing over 4.3 volts from ever reaching the ECU. This includes the Greddy BCC, the Zener diode, disconnecting the hose, etc. The HKS FCD is an adjustable FCD that has one setting that raises fuel cut to ~17 psi without eliminating it.

*(late 93's and later have FC set at ~ 16psi from the factory)

Here is some further information about the HKS FCD installed on the MR2:

http://www.mr2.com/ARTICLE/HKSFCDat.html
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drscam
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How Engines Generally Work (NEWBIE's)

Postby drscam » Fri Feb 28, 2003 9:26 am

MattmR2(IMOC) wrote:
I stumbled across this website:

http://www.autoshop101.com/autoshop15.html

There are dozens of PDF articles from Toyota describing how various engine control systems work, including details for troubleshooting. Topics include EGR, ISC, ECU, AFM, OBD-I, OBD-II, etc.


3mr2s wrote:
For beginners, learn the basics here first:

http://www.howstuffworks.com

And you can learn about transmissions, turbochargers and superchargers, and just about anything else!

Jim


Post in this IMOC thread if you have questions.
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