JoneZZZ

iTrader: (1)

After all of the discussions about knock and boost spikes this winter I decided to educate myself on "stuff" that affect turbo charged motors. I pulled some of this together...hope others find it usefull too:

-"The primary input in determining which turbocharger is appropriate is to have a target horsepower in mind. This should be as realistic as possible for the application. Remember that engine power is generally proportional to air and fuel flow. Thus, once you have a target power level identified, you begin to hone in on the turbocharger size, which is highly dependent on airflow requirements.

Other important factors include the type of application. An autocross car, for example, requires rapid boost response. A smaller turbocharger or smaller turbine housing would be most suitable for this application. While this will trade off ultimate power due to increased exhaust backpressure at higher engine speeds, boost response of the small turbo will be excellent.

Alternatively, on a car dedicated to track days, peak horsepower is a higher priority than low-end torque. Plus, engine speeds tend to be consistently higher. Here, a larger turbocharger or turbine housing will provide reduced backpressure but less-immediate low-end response. This is a welcome tradeoff given the intended operating conditions.

Selecting the turbocharger for your application goes beyond “how much boost” you want to run. Defining your target power level and the primary use for the application are the first steps...." Garrett

-Wheel trim topic coverage

Trim is a common term used when talking about or describing turbochargers. For example, you may hear someone say "I have a GT2871R ' 56 Trim ' turbocharger. What is 'Trim?' Trim is a term to express the relationship between the inducer* and exducer* of both turbine and compressor wheels. More accurately, it is an area ratio.

* The inducer diameter is defined as the diameter where the air enters the wheel, whereas the exducer diameter is defined as the diameter where the air exits the wheel.

Based on aerodynamics and air entry paths, the inducer for a compressor wheel is the smaller diameter. For turbine wheels, the inducer it is the larger diameter...." Garrett

-"The A/R parameter has different effects on the compressor and turbine performance, as outlined below.

Compressor A/R - Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications.

Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise..... This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.

Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise.

-" Compression ratio with boost

Before discussing compression ratio and boost, it is important to understand engine knock, also known as detonation. Knock is a dangerous condition caused by uncontrolled combustion of the air/fuel mixture. This abnormal combustion causes rapid spikes in cylinder pressure which can result in engine damage.

Three primary factors that influence engine knock are:

Knock resistance characteristics (knock limit) of the engine: Since every engine is vastly different when it comes to knock resistance, there is no single answer to "how much." Design features such as combustion chamber geometry, spark plug location, bore size and compression ratio all affect the knock characteristics of an engine.
Ambient air conditions: For the turbocharger application, both ambient air conditions and engine inlet conditions affect maximum boost. Hot air and high cylinder pressure increases the tendency of an engine to knock. When an engine is boosted, the intake air temperature increases, thus increasing the tendency to knock. Charge air cooling (e.g. an intercooler) addresses this concern by cooling the compressed air produced by the turbocharger
Octane rating of the fuel being used: octane is a measure of a fuel's ability to resist knock. The octane rating for pump gas ranges from 85 to 94, while racing fuel would be well above 100. The higher the octane rating of the fuel, the more resistant to knock. Since knock can be damaging to an engine, it is important to use fuel of sufficient octane for the application. Generally speaking, the more boost run, the higher the octane requirement.
This cannot be overstated: engine calibration of fuel and spark plays an enormous role in dictating knock behavior of an engine......." Garrett

-"Air/Fuel Ratio tuning: Rich v. Lean, why lean makes more power but is more dangerous

When discussing engine tuning the 'Air/Fuel Ratio' (AFR) is one of the main topics. Proper AFR calibration is critical to performance and durability of the engine and it's components. The AFR defines the ratio of the amount of air consumed by the engine compared to the amount of fuel.

A 'Stoichiometric' AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric , A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel. The stoichiometric AFR depends on fuel type-- for alcohol it is 6.4:1 and 14.5:1 for diesel.

So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture.

For Example:
15.0:1 = Lean
14.7:1 = Stoichiometric
13.0:1 = Rich

Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.

The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.

There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power........" Garrett

-"Pressure Ratio

Pressure Ratio ( ) is defined as the Absolute outlet pressure divided by the Absolute inlet pressure.

Where:
= Pressure Ratio
P2c = Compressor
Discharge Pressure
P1c = Compressor Inlet Pressure
It is important to use units of Absolute Pressure for both P1c and P2c. Remember that Absolute Pressure at sea level is 14.7 psia (in units of psia, the a refers to “absolute”). This is referred to as standard atmospheric pressure at standard conditions.
Gauge Pressure (in units of psig, the g refers to “gauge”) measures the pressure above atmospheric, so a gauge pressure reading at atmospheric conditions will read zero. Boost gauges measure the manifold pressure relative to atmospheric pressure, and thus are measuring Gauge Pressure.....For a day at standard atmospheric conditions....." Garrett

- "Mass Flow Rate
Mass Flow Rate is the mass of air flowing through a compressor (and engine!) over a given period of time and is commonly expressed as lb/min (pounds per minute). Mass flow can be physically measured, but in many cases it is sufficient to estimate the mass flow for choosing the proper turbo.
Many people use Volumetric Flow Rate (expressed in cubic feet per minute, CFM or ft3/min) instead of mass flow rate. Volumetric flow rate can be converted to mass flow by multiplying by the air density. Air density at sea level is 0.076lb/ft3
What is my mass flow rate? As a very general rule, turbocharged gasoline engines will generate 9.5-10.5 horsepower (as measured at the flywheel) for each lb/min of airflow. So, an engine with a target peak horsepower of 400 Hp will require 36-44 lb/min of airflow to achieve that target. This is just a rough first approximation to help narrow the turbo selection options.
....." Garrett

-"During cold weather, remember to watch the boost. The Chips are programmed to increase boost based upon temperature, so if it's cold enough your boost could be higher than recommend. This is one of the reasons for a boost gauge! ....." xcelleration.com

-"Boost Creep (Boost Spikes) The waste gate on the turbo is controlled by the ECU and is looking for a certain amount of pressure on both sides of the waste gate. When you change the pressure by using an exhaust system that is too free flowing, then the waste gate doesn't have the proper pressure and doesn't open at the correct time. This causes the boost to 'spike' ....."xcelleration.com

-"Boost Creep
Boost creep occurs when the wastegate/turbocharger combination is improperly sized and cannot dump enough exhaust gasses......."TurboGlossary

JoneZZZ

iTrader: (1)

-"Estimating Required Air Mass Flow and Boost Pressures to reach a Horsepower target.

· Things you need to know:
· Horsepower Target
· Engine displacement
· Maximum RPM
· Ambient conditions (temperature and barometric pressure. Barometric pressure is usually given as inches of mercury and can be converted to psi by dividing by 2)

· Things you need to estimate: · Engine Volumetric Efficiency. Typical numbers for peak Volumetric Efficiency (VE) range in the 95%-99% for modern 4-valve heads, to 88% - 95% for 2-valve designs. If you have a torque curve for your engine, you can use this to estimate VE at various engine speeds. On a well-tuned engine, the VE will peak at the torque peak, and this number can be used to scale the VE at other engine speeds. A 4-valve engine will typically have higher VE over more of its rev range than a two-valve engine.

· Intake Manifold Temperature. Compressors with higher efficiency give lower manifold temperatures. Manifold temperatures of intercooled setups are typically 100 - 130 degrees F, while non-intercooled values can reach from 175-300 degrees F.

· Brake Specific Fuel Consumption (BSFC). BSFC describes the fuel flow rate required to generate each horsepower. General values of BSFC for turbocharged gasoline engines range from 0.50 to 0.60 and higher. The units of BSFC are
Lower BSFC means that the engine requires less fuel to generate a given horsepower. Race fuels and aggressive tuning are required to reach the low end of the BSFC range described above....." Garrett

cessna

Great writeup!!! Thanks

JoneZZZ

iTrader: (1)

Intercoolers:

An intercooler is a heat exchanger.

At wide open throttle and full boost the hot compressed air coming from a turbocharger is probably between 250 and 350 deg F depending on the particular turbo, boost pressure, outside air temperature, etc.. We want to cool it down, which reduces its volume so we can pack more air molecules into the cylinders and reduce the engine's likelihood of detonation.

How does an intercooler work? Hot air from the turbo flows through tubes inside the intercooler. The turbo air transfers heat to the tubes, warming the tubes and cooling the turbo air. Outside air (or water) passes over the tubes and between fins that are attached to the tubes. Heat is transferred from the hot tubes and fins to the cool outside air. This heats the outside air while cooling the tubes. This is how the turbo air is cooled down. Heat goes from the turbo air to the tubes to the outside air.

There are some useful equations which will help us understand the factors involved in transfering heat. These equations are good for any heat transfer problem, such as radiators and a/c condensers, not just intercoolers. After we look at these equations and see what's important and what's not, we can talk about what all this means.

Equation 1
The first equation describes the overall heat transfer that occurs.
Q = U x A x DTlm


Q is the amount of energy that is transferred.
U is called the heat transfer coefficient. It is a measure of how well the exchanger transfers heat. The bigger the number, the better the transfer.
A is the heat transfer area, or the surface area of the intercooler tubes and fins that is exposed to the outside air.
DTlm is called the log mean temperature difference. It is an indication of the "driving force", or the overall average difference in temperature between the hot and cold fluids. The equation for this is:

DTlm = (DT1-DT2) * F
ln(DT1/DT2)


where DT1 = turbo air temperature in - outside air temperature out
DT2 = turbo air temperature out - outside air temperature in
F = a correction factor, see below

Note:

The outside air that passes through the fins on the passenger side of the intercooler comes out hotter than the air passing through the fins on the drivers side of the intercooler. If you captured the air passing through all the fins and mixed it up, the temperature of this mix is the "outside air temperature out".


F is a correction factor that accounts for the fact that the cooling air coming out of the back of the intercooler is cooler on one side than the other.

To calculate this correction factor, calculate "P" and "R":


P = turbo air temp out - turbo air temp in
outside air temp in - turbo air temp in

R = outside air temp in - outside air temp out
turbo air temp out - turbo air temp in


This overall heat transfer equation shows us how to get better intercooler performance. To get colder air out of the intercooler we need to transfer more heat, or make Q bigger in other words. To make Q bigger we have to make U, A, or DTlm bigger, so that when you multiply them all together you get a bigger number. More on that later.


Equation 2
We also have an equation for checking the amount of heat lost or gained by the fluid on one side of the heat exchanger (ie, just the turbo air or just the outside air):
Q = m x Cp x DT

Q is the energy transferred. It will have the exact same value as the Q in the first equation. If 5000 BTU are transferred from turbo air to outside air, then Q = 5000 for this equation AND the first equation.
m is the mass flowrate (lbs/minute) of fluid, in this case either turbo air or outside air depending on which side you're looking at.
Cp is the heat capacity of the air. This is a measure of the amount of energy that the fluid will absorb for every degree of temperature that it goes up. It is about 0.25 for air and 1.0 for water. Air doesn't do a great job of absorbing heat. If you put 10 BTU into a pound of air the temperature of it goes up about 40 degrees. If you put 10 BTU into a pound of water, the temperature only goes up about 10 degrees! Water is a great energy absorber. That's why we use water for radiators instead of some other fluid.
DT is the difference in temperature between the inlet and outlet. If the air is 200 deg going in and 125 deg coming out, then DT = 200 - 125 = 75. Again, on the cooling air side the outlet temperature is the average "mix" temperature.

If you know 3 of the 4 main variables on one side of the exchanger (the amount of heat transferred, the inlet and outlet temperatures, and the fluidís flow rate) then this equation is used to figure out the 4th


Caveat:
These equations are all for steady state heat transfer....Cruising on the highway you would definitely see steady state. Perhaps at the big end of the track you may see it too, I don't know. As various people on the mailing list have pointed out in the past, the material of the intercooler itself will rise in temperature when you hit full throttle, absorbing more heat than what these equations would lead you to believe. For example, at steady state idle the intercooler body may be at 100 deg F. At steady state full throttle it may be 175 deg F. The energy it takes to heat it up to that temperature comes from the turbo outlet air, and so the cooling of that air is what is removed by both the flowing outside air and the absorption of the intercooler body. How long does it take to get to the new steady state? Beats me, but the graphs I've seen of intercooler outlet temperatures over the course of a quarter mile run lead me to believe that it is approached before you get to the end of the quarter mile, since the intercooler outlet temperatures reached a steady level.

So, now that we've got these equations, what do they REALLY tell us?

Heat transfer goes really well when there is a large temperature difference, or driving force, between the two fluids. This is shown in equation 1 as a large DTlm. It doesn't go as well when there is a small temperature difference between the two fluids (small DTlm). The closer you get the intercooler outlet temperature to the outside air temperature the smaller DTlm gets, which makes the heat transfer tougher.


The difference between the intercooler outlet temperature and the outside air temperature is called the approach. If it is 100 degrees outside and your intercooler cools the air going into the intake manifold down to 140 degrees, then you have an approach of 40 degrees (140 - 100 = 40). To get a better (smaller) approach you have to have more area or a better U, but there is a problem with diminshing returns. Lets rearrange the first equation to Q/DTlm = U x A. Every time DTlm goes down (get a better temperature approach) then Q goes up (transfer more heat, get a colder outlet temperature), and dividing Q by DTlm gets bigger a lot faster than U x A does. The upshot of that is we have a situation of diminishing returns; for every degree of a better approach you need more and more U x A to get there. Start with a 30 deg approach and go to 20 and you have to improve U x A by some amount, to go from 20 to 10 you need to increase U x A by an even bigger amount.


I would consider an approach of 20 degrees to be pretty good. In industrial heat exchangers it starts to get uneconomical to do better somewhere around there, the exchanger starts to get too big to justify the added expense. The one time I checked my car (stock turbo, stock IC, ported heads, bigger cam) I had an approach of about 60 deg. The only practical way of making the DTlm bigger on an existing intercooler is to only drive on cold days; if you buy a better intercooler you naturally get a better DTlm.


You can transfer more heat (and have cooler outlet temps) with more heat transfer area. That means buying a new intercooler with more tubes, more fins, longer tubes, or all three. This is what most aftermarket intercoolers strive for. Big front mounts, intercooler and a half, etc... are all increasing the area.

A practical consideration is the fin count. The area of the fins is included in the heat transfer area; more fins means more area. If you try to pack too many fins into the intercooler the heat transfer area does go up, which is good, but the cooling air flow over the fins goes down, which is bad..." turbocalculator.com

JoneZZZ

iTrader: (1)

"The Turbo Exhaust
Response, Power & Reliability
By MJ Ferrara

Aftermarket exhaust systems are usually at the top of the list in any performance buildup. When your buildup involves a turbocharged vehicle, the benefits of a well-engineered cat-back exhaust system are immense. Significant increases in turbo response, power output and reliability will be the result of just bolting on the exhaust system on nearly all turbocharged vehicles. However, some vehicles will not show significant benefits from the addition of the exhaust system by itself. On these applications, the factory engine control unit (ECU) may reduce boost pressures, change timing curves or change fuel delivery after the exhaust system is added. As a result of these adaptive ECU's and their uncooperative nature to see more power generated, little or no power is seen at the wheels on some vehicles. On these vehicles, the OEMs are making it harder to increase horsepower. Fortunately, the speed bumps to performance can be crossed as long as our intelligence is not lower than the bump itself. Understanding what to look for in an aftermarket performance turbo exhaust system and knowing how to beat the roadblocks that your factory ECU may throw at you will allow you to realize the maximum performance from your vehicle.

Seven Points of Exhaust Flow
The primary function of a vehicle's exhaust system is to muffle the sound coming out from the exhaust port of the cylinder head and to direct the exhaust gas out the back of the vehicle. If we follow the flow of the gases from the cylinder head to the tailpipe on a turbocharged vehicle, we find seven points of interest along the way. First, the exhaust port of the cylinder heads feeds its flow into the exhaust manifold. Unless this part has been upgraded, the chances are that the factory piece is a cast-iron manifold that supports and locates the turbocharger while directing the exhaust flow into the turbo. Second stop is at the turbine housing of the turbocharger. At this junction, the exhaust flow is directed through a nozzle (turbine housing) that increases the flow velocity to power the turbine wheel. While providing the power to turn the compressor, the turbine section also has a side benefit of dramatically reducing the exhaust noise. Essentially, the turbine section acts as a very effective muffler. Once the flow makes its way into and out of the turbine housing it finds its third stop at the downpipe. The downpipe provided the exhaust flow connection and direction into the catalytic converter. The catalytic converter provides our fourth point of interest. The converter aids in the conversion of exhaust gases into less polluting products. After the converter, one finds all of the "cat-back" elements. The cat-back components include the fifth, sixth and final stops (B-pipe, muffler, tail-pipe or tip). From the converter, the exhaust hits the Before-muffler exhaust pipe or B-pipe. Some B-pipes may use a resonator or pre-muffler, but it is pretty rare on turbo exhaust systems. The B-pipe feeds the sixth stop, the primary muffler. The mufflers job is to "muffle" or reduce the exhaust noise further. Finally, the exhaust meets the atmosphere through an exhaust muffler tip or actual tail pipes on some applications.

Objectives of the Turbo Cat-Back
The objective of a properly-engineered, aftermarket cat-back exhaust system is to provide additional performance while still delivering adequate sound control. Unlike an all-motor exhaust system, a turbo exhaust system suffers no ill effects from going as big as possible. Bigger is better in this case. The bigger or larger diameter exhaust pipes allow the back pressure to be significantly less than the factory exhausts system. As a result, the difference in exhaust pressure before and after the turbocharger is increased. The increase in the magnitude of the pressure difference allows the turbocharger to reach higher shaft speeds at lower engine operating rpms. As a result, boost response increases and boost pressures increase. More boost pressure at the intake manifold results more power at the wheels....."

www.tprmag.com

geoffc

Are you sure?

I think you'll find that 'useful' only has one L.

JoneZZZ

iTrader: (1)

oops....thanks for the correction(from across the pond at that !!)

dirtroad

iTrader: (1)

Great work - always wondered why my car was so fast!


Wow. What a great first post.

shezzzhot

iTrader: (5)

Awesome...will save this info! Thanks Morris!

failsafe306

iTrader: (38)

D350Z10

iTrader: (20)

Wow ... excellent write up!

BLN350Z

Nice Effort!

JoneZZZ

iTrader: (1)

I found the following to be a good read.........

"Controlling Turbocharger Boost Pressure and the Effect of Exhaust Back Pressure

Turbochargers operate on a very straightforward principle whereby exhaust gasses spin a turbine wheel. This turbine wheel is attached to a shaft and air compressor wheel so that as the turbine spins, it spins the compressor wheel, thus compressing air for the engine......the technology employed in modern turbocharger design is very advanced in order to deliver bulletproof durability, crisp response and high outright power.

Turbocharger Boost Pressure Control

A critical aspect of turbocharger operation is the management of boost pressure because this has a direct impact upon the total mass flow rate of air delivered to the engine in addition to controlling where the turbocharger operates in its compressor map....

Physically, turbocharger boost pressure is controlled by a waste gate (either internal or remote) and is designed to divert exhaust gasses from the passing through the turbine wheel and turbo housing assembly (turbine). By diverting exhaust gasses away from the turbine, the amount of exhaust gas energy used to spin the turbine is reduced which in turn affects the boost pressure generated by the turbocharger compressor.....


In almost all modern turbocharger applications, it is vital to allow for variation of turbocharger boost pressure relative to engine load/RPM. For example, on a particular engine it may be desirable to utilize say 10 psi at mid RPM (resulting in higher torque), but 7 psi at high engine RPM. This will also allow the turbocharger to operate within its high efficiency range envelope as specified in the compressor map ......


Modern engine management systems for turbocharged engines employ electronic forms of boost control that alter the boost pressure that is sensed by the waste gate actuator via a boost control solenoid. By altering the pressure on the waste gate actuator, one can now effectively control the turbocharger's boost pressure over a range of values and program the exact boost profile required by that engine.

A boost control solenoid valve is placed in line with the boost sensing hose to the waste gate actuator. The boost control solenoid valve pulses at a controlled frequency and effectively bleeds boost pressure from the actuator. The higher the pulsing frequency (or duty cycle), the greater the amount of boost pressure that is bled. In other words, the waste gate actuator senses a boost pressure that is lower than the actual boost pressure - hence allowing for greater boost pressure before the waste gate opens the swing valve......."

APS

JoneZZZ

iTrader: (1)

This too.....

"Exhaust Back Pressure

Since a turbocharger is effectively an exhaust gas driven compressor, it relies heavily upon the available exhaust gas energy to deliver charge air to the engine. Specifically, it relies upon the differential of exhaust gas energy across (between the turbine entry and exit) the turbocharger.

All of the above boost control discussions were based upon having sufficient energy differential to achieve the desired boost pressures. If however, a restrictive exhaust system is utilized, a great deal of backpressure is built up after the turbocharger (and to a lesser degree before the turbocharger) and there may not be sufficient differential across the turbine. In this case, regardless of the boost control mechanism employed, the turbocharger may not be able to achieve the target boost pressure. This is often seen with NA vehicles that are later turbocharged whilst using the stock exhaust system.

For the sake of simplicity, let us consider exhaust backpressure alone and ignore total exhaust gas energy as a measure of a turbocharger's ability to deliver charge air to the engine.......

As an example, let us consider a turbocharger system that experiences 30 psi backpressure before the turbocharger's turbine and 5 psi backpressure after the turbocharger's turbine. This effectively provides the turbocharger with 30 psi - 5 psi = 25 psi pressure differential across the turbine.

If a zero back pressure exhaust system is utilized, then the backpressure after the turbocharger is 0 psi. This results in 30 psi - 0 psi = 30 psi pressure differential - hence greater potential for the turbocharger to deliver higher boost pressure.

Conversely, if a very restrictive exhaust is utilized, the backpressure after the turbocharger may be as high as 10 psi. This results in 30 psi - 10 psi = 20 psi pressure differential across the turbocharger’s turbine - resulting in much lower potential for the turbocharger to deliver boost pressure to the engine.

The following table summarizes the above results:


Pre-turbo Exhaust Potential Boost
PSI Back PSI PSI Potential
Stock Exhaust System 30 psi 10 psi 20 psi Low
High Flow Exhaust System 30 psi 5 psi 25 psi Medium
Zero Back Pressure Exhaust System 30 psi 0 psi 30 psi High

The above discussion had assumed constant backpressure values. However, since backpressure varies according to engine load and engine RPM, a turbocharger unit's performance can change accordingly.

There are many cases where the exhaust system backpressure is the cause of lower boost pressure. As we will see this typically occurs at higher engine RPM and regardless of the waste gate control mechanism, the pressure differential across the turbocharger is not sufficient to achieve the desired boost pressure level.

The backpressure of an exhaust system is relative to the mass flow rate of exhaust gas passing through the system. The higher the engine's power output, the greater the mass flow rate of exhaust gas - hence the higher the back pressure from the exhaust system and indeed through all components after each exhaust port....."


APS

Zilvia

iTrader: (1)

awesome write up