Turbo Basics and Boost Control

[Moai]

TURBO BASICS AND BOOST CONTROL
Basic synopsis courtesy of user billwot (Bill Wotschak ) from www.mr2oc.com
Additions and some translation into teh proper english by Me.


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 as it is regularly fed clean, high quality oil from the engine lube system, and allowed to cool down before engine shutdown, will last nearly as long as the engine itself.


THE WASTEGATE
There are two 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, the air becomes pressurized. 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. Pressure and flow are directly related.
That means you can not get "more flow at the same pressure..."
The engine itself will only flow so much volume into the combustion chambers per cycle. [Volumetric efficiency]. The DENSITY of that volume determines how much fuel is injected and how many cfm's of exhaust gas EXIT the engine and go into the turbine housing. You can get more POWER out of the same boost pressure by increasing the density of the incoming air, which larger turbos, intercooling and nitrous all do, but that will be covered later.


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 usually be spotted near the turbo's compressor housing as some sort of cannister with hoses leading to it, mounted to the turbo. This is an internal style wastegate actuator, meaning that on the back of the cannister is a pushrod, connected to a flap door inside the exhast housing of the turbocharger assembly. The cannister contains a spring and a diaphragm. This is the mechanism that keeps boost levels steady. As pressure builds in the turbo, air begins to fill the actuator and pushes against the diaphragm. When the pressure exceeds the spring value, the actuator moves out, pushing a rod, and opening the wastegate, venting exhaust gases before they reach the spinning turbine. The factory spring value is whatever boost level your car runs on cold start. [NEVER boost a cold engine though!] It is intended primarily as a "safe" level when other boost control devices fail.
An external wastegate is very similar, except instead of the actuator and pushrod being mounted on and through the turbo, the entire assembly is moved outside and mounted on a pipe which is in turn mounted in the exhaust stream before the turbine inlet. The pushrod is deleted.
In this design, the turbocharger no longer has an operating flap-door wastegate inside its turbine housing. The wastegate retains the simple spring and diaphragm construction of an internal wastegate, with a hose on top of the assembly leading to a boost-pressure source, same as it was with the internal wastegate. The external wastegate dumps regulated gases out a tube connected to its 'dump' flange. This wastegate dump tube can be left open to the atmosphere or integrated back into the exhaust stream after the turbine.
Often when faced with the option of choosing the spring value for a wastegate, it is best to choose a lower value which will serve as the 'safety boost' in case any aftermarket boost control device malfunctions and the wastegate itself must limit the boost ingested by the engine.


BOOST PRESSURE SENSOR AND FUEL-CUT
The boost pressure sensor is simply a pressure transducer mounted somewhere in the engine bay that monitors manifold pressure and reports it to the ECU as a voltage value. Some cars calculate boost pressure through airflow and rpm calculation and some measure it directly with a pressure sensor. Regardless of the method, if the voltage from the sensor or airflow and rpm exceed a certain value, the ECU initiates the fuel-cut sequence. It de-energizes the factory boost control solenoid to lower boost, retards ignition timing, and restricts fuel delivery by limiting the injector cycle. The "check-engine" light may come on , and in some applications a code will be stored in the diagnostic memory. Depending on the operation of the fuel cut circuitry, the ECU may place the car in "limp mode" leaving you unable to boost again until you shut off the engine and restart it.


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 at a certain psi, you must somehow eliminate it or alter it to boost above that level. Fuel-cut is initiated when there is a volt signal or rpm/airflow calculation sent to the ECU, so ALL fuel cut defeat (FCD) systems either prevent that signal from happening, or delay it . Some methods simply eliminate FC completely by preventing any thing over a certain voltage from ever reaching the ECU. This includes the Greddy BCC, the Zener diode, disconnecting the hose to the pressure sensor [only works on some applications], etc. The HKS FCD is an adjustable FCD that has one setting that raises fuel cut to a certain psi without eliminating it. The danger of raising or eliminating fuel cut is inherent in the elimination of any safety device. Damage that you are trying to avoid to your engine may occur due to the removal or alteration of the factory fuel cut feature. When removing or altering this system, it is best to ensure that your control over boost pressure is rock solid, and to have a properly functioning boost gauge installed in your vehicle before taking this step. Additionally, boost controllers such as the Blitz DSBC have a warning/limiter feature which limits boost to a pre-set level and sounds a warning tone in case conditions allow the boost to rise to a level above that desired by the user.


TEMPERATURE AND CHARGE DENSITY
A lower ambient temp means there is an increase in the density of the outside air. Pressurizing (compressing) the charge also increases the density, but lowering the temp does not increase the pressure. Anything that increases the density increases the charge mass. Increasing the mass means that the amount of oxygen, nitrogen and other gases contained per certain amount of volume will be higher. A higher concentration of these gases in an area will require a proportionately higher amount of fuel to maintain a proper air/fuel ratio. Since air + fuel = power and More air + more fuel = more power, more air and fuel introduced into the cylinder will result in higher cylinder pressure, and therefore an increased propensity for knock. This increased knock potential is regulated by the ECU's control over ignition timing. When knock occurs, the ECU retards the timing until it no longer detects knock though the knock sensor, at which point it builds the timing back in stages until it detects it again and retards timing again. This cycle repeats many tens and even hundreds of times per second.


CHARGE DENSITY, POWER AND VOLUMETRIC EFFICIENCY
The increase in charge density is how larger turbos make more power at the same boost pressure than smaller turbos do. The reason for this phenomenon is due to the size and speed at which the compressor wheel spins. A smaller turbo's compressor wheel will need to spin fast in order to flow a certain amount of air being ingested by the engine. A larger turbo's compressor wheel can spin slower, because of the increase in area on the blades. It can simply move more air with each revolution than the smaller wheel can.

Now please note that the ENGINE determines how much volume is ingested per revolution, NOT the turbocharger, hence a larger turbo will not FLOW more cfm's THROUGH the engine at the same engine rpm. A larger turbo may move more air with each revolution, but it will NOT affect the volume of air being moved THROUGH the engine as a whole. Remember that the larger turbo will be spinning more slowly, in order to maintain the SAME AMOUNT of airflow. It is a common misconception that a larger turbo will make power by increasing total CFM flow THROUGH the engine. It will not. As soon as air passes the compressor blades, the turbocharger will only raise the DENSITY of the mixture, which in turn will ignite and create more cubic feet of gas on its way out of the motor. It is DENSITY, not VOLUME.

On a bench, disconnected from the engine, you may measure the volumetric flow of a turbocharger. As long as it is connected to the engine however, the engine acts as a container. [i.e. each cylinder]. A cylinder's volume can be measured. It is simply the cubic product of the cylinder's dimensions. NOTHING is going to change that unless you change those dimensions. Hence, while manufacturers may measure a turbocharger's flow capability through CFM's [or cubic feet per minute], as soon as its attached to an engine, that measurement becomes fixed as far as the engine is concerned. The turbocharger blades will still flow in terms of CFM, but the engine will only ingest as much CFMs as it has the room for. When the turbocharger throws more cubic feet at the engine than the engine has room to ingest, the air becomes pressurized.
Remember, turbine blades flow air. Air flow has its own units. Cylinders have volume. Volume has its own units. In our case, Cylinders are measured in Cubic Feet. Turbines are measured by Cubic feet PER MINUTE. Time is the differentiating factor between the two.
Flow and Volume can work together [and do in our case] but are still two different entities.

On a 2.0L 4 cylinder engine, each cylinder's volume will be 500 cc's or cubic centimeters. With each stroke, the MAXIMUM volume you can ingest per cylinder is 500cc's of air. That will not change regardless of what turbocharger you put on it. However, you can change the DENSITY of the air going into that volume. By increasing the density of the existing airflow, you create a potential for a power increase. This increase in density of incoming charge air in a turbocharged engine CAN BE CONTRASTED to the increase in cfm's by increasing an engine's displacement. One engine alters the properties of the volume's density to make power, the other engine simply gulps in more volume to make power.
Now let's say you have a cylinder gulping in a maximum of 500cc's per stroke. In reality, it will gulp in a little less - say 90-95 percent. This is your VOLUMETRIC EFFICIENCY, also called VE. VE matters a lot when it comes to turbocharged engines. With a 500cc displacement per cylinder, let's say the charge air coming in is twice as dense as normal air in the atmosphere. Ideally, two times the air means twice as much fuel must be injected. That means that ideally, you have the potential to make twice as much power as an engine gulping in air at the normal atmospheric density. [Though again, reality dictates that it will always be less than twice the amount of power produced]. Ideally, a naturally aspirated 500cc cylinder that has its VE improved by 5%, gains the equivalent of a 25cc increase in displacement. Ideally it can now gulp in 25cc's more air than it used to, thanks to improved pumping efficiency. A forced induction cylinder of the same displacementne will also [ideally] gain 25cc's, but because those 25cc's are pressurized, there are more air molecules per CC than the naturally aspirated engine, which require more fuel to burn at the appropriate air/fuel ratio. Realistically, the actual gains are nowhere near the ideal gains, but the main rule governing power still stands: more air + more fuel = more power.

All things being equal between the two, a forced induction engine has the potential to gain more power per percent increase in volumetric efficiency than a naturally aspirated engine does, of the same displacement.

Add-ons such as cams, intake manifolds and headwork all help to improve the VOLUMETRIC EFFICIENCY of an engine. It is the VOLUMETRIC EFFIENCY that will alter the volume of air flowed by an engine, by improving the engine's ability to pump air through the cylinders. [Also known as improving pumping efficiency or decreasing pumping losses].

COMPRESSOR AND TURBINE THERMODYNAMICS
Back to the compressor wheel, the side effect of moving more air with fewer revolutions is that by spinning slower against the incoming air, it creates less friction and hence, less heat. By heating the incoming air less, you have a cooler intake temperature and since we already know that cooler air is denser air, the potential for making more power at the same boost pressure is greater.
Of course, nothing in life is free. With a larger turbo, the exhaust gases have more mass to spin on their way out. Additionally, most larger turbos have larger turbine housings in order to accomodate the increase in overall flow potential of the turbo. [if the turbo is properly sized].
A larger exhaust housing may flow more gas, but the speed at which the gas exits is decreased. To test this theory, all you have to do is whistle.
When your lips form a smaller exit for the air, it moves fast enough to create a sound as it pases. Now try whistling with your mouth open a little more. Doesn't work does it? A turbine operates in very much the same way. In order to get the turbine wheel spinning up to a workable rpm range, the exhaust gases must push against it on their way out. A larger housing simply means that in order for the turbine wheel to spin faster, more gas must flow through the housing to produce the same velocity with which they passed through the smaller turbine housing from the smaller turbo. It's very much the inverse of what happens at the compressor housing. Larger compressor wheel = denser air = more power. Larger turbine housing = slower gas velocity = more gas flow required to increase that velocity to get the turbine wheel spinning.
This is why larger turbos "lag" compared to smaller turbos in an otherwise identical setup. There are ways to get around the lag effect such as:
--installing a more free-flowing exhaust
--sizing the turbine housing differently
--using a variable exhaust housing


FREE FLOWING EXHAUSTS
The free flowing exhaust basically uses the pressure drop across the turbine to increase its response. I.E, if exhaust gas is pushing hard against one side of the turbine, the best way to get that turbine spinning faster, is to ensure that the exhaust gases can move from one side of those blades to the other as quickly as possible. A free flowing exhaust is what helps to usher these gases out of the way so that others can take their place. A turbine wheel spins quickest when the pressure drop across it is the greatest. A lot of pressure on one side and very minimal pressure on another allows the gas to use as much of its energy as possible to turn the wheel on its way out. Hence, you want as little backpressure in the exhaust stream as possible on a turbo car. Ideally, the best exhaust for any turbo application is "no exhaust", but if you want to get really technical, go look up the concept of Laminar Airflow to understand why we still use exhaust piping to usher the gases away from the turbine more efficiently.


DIFFERENT SIZE TURBINE HOUSINGS
Choosing which size turbine housing you wish to use allows you to tune the response of the turbo. Some turbos such as the T3/T4 have many options as to which size compressor and which size turbine housing you can choose. With these turbos, you aim for the most balanced solution that will minimize lag while maximizing how much area under the curve your dyno plot has. While a small exhaust housing will increase gas velocity and get the wheel spinning faster, it also acts as a bottleneck as engine rpm increases. Gas may flow faster through a smaller opening, but there is a limit to how much gas can fit through that opening. Hence, smaller turbos may hit fast and hard at lower rpm's, but as the engine rpm's rise and the amount of exhaust gas volume increases, the smaller turbo simply cannot flow enough gas out of the turbine housing to keep up. This is why many stock turbos tend to "wheeze" out at higher rpms. Additionally, since the gas cannot leave the turbine housing faster than it's coming in, a backpressure is created in the exhaust manifold before the turbine blades. The wastegate will try to vent as much of this pressure as possible, but sometimes the pressure is too great and can lead to the dangerous condition of overboosting or boost creep - where pressure in the exhaust manifold rises dangerously above the pressure in the intake tract. Pressure generates heat and heat can damage components. Also dont forget that while the turbine wheel is spinning away like mad to get those exhaust gases out, the compressor wheel is also spinning away like mad to bring more gases into the engine. We already know that the faster the compressor spins, the more it heats the incoming air. This is the point where a turbo enters the "overspeed" zone. It is operating at an rpm well above what it was designed for and is in danger of damaging the engine with hot air as well as simply failing and blowing itself to bits if there is the least defect in the turbine's structure.


PROPER SIZING
Ideally a properly sized turbo will have equal pressure in the intake and exahust manifolds at the desired boost pressure and will make that boost pressure in the middle of its efficiency range, which can be viewed on a compressor map as an "island". The middle island represents a zone where the compressor speed, pressure ratio [intake pressure + atmospheric pressure all divided by atmospheric pressure] and airflow through the engine, usually in terms of cubic feet or cubic meters per minute all contribute to the lowest intake charge temperature. The middle island is the happy island. You want to be on that middle island as long as possible. The middle island has the best compressor efficiency which means that in that particular zone's conditions [airflow, pressure ratio and compressor rpm], it is heating the air the LEAST. Outside of that zone, you begin to heat the air more and more as the efficiency drops and you move to other "islands".
Proper turbine sizing may be achieved by having a turbine housing that can change its size as the engine rpm varies. Aerodyne VATN turbos possess a series of "vanes" inside the turbine housing that are controlled by an actuator that's fed boost pressure. When pressure is lower, the vanes constrict to form a smaller path for the air passing through the turbine housing. This gives the effect of simply having a smaller turbine housing, which as we now know, increases turbo response. As the rpm's rise, the vanes open to give the effect of a larger turbine housing, which we now know increases total flow and keeps exhaust manifold pressures at a happy level. The VATN technology is still being refined, else we'd all own one. [or two!]

Okay folks, that's all.
Happy Boosting!