1 Parts of the Compressor Map:
The compressor map is a graph that describes a particular compressor’s performance characteristics, including efficiency, mass flow range, boost pressure capability, and turbo speed. Shown below is a figure that identifies aspects of a typical compressor map:
Pressure Ratio
Pressure Ratio (
) is defined as the Absolute outlet pressure divided by the Absolute inlet pressure.
Where:
o
= Pressure Ratio
oP2c = Compressor
oDischarge Pressure
oP1c = 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. This is important when determining P2c. For example, a reading of 12 psig on a boost gauge means that the air pressure in the manifold is 12 psi above atmospheric pressure. For a day at standard atmospheric conditions,
12 psig + 14.7 psia = 26.7 psi absolute pressure in the manifold
The pressure ratio at this condition can now be calculated:
26.7 psia / 14.7 psia = 1.82
However, this assumes there is no adverse impact of the air filter assembly at the compressor inlet.
In determining pressure ratio, the absolute pressure at the compressor inlet (P2c) is often LESS than the ambient pressure, especially at high load. Why is this? Any restriction (caused by the air filter or restrictive ducting) will result in a “depression,” or pressure loss, upstream of the compressor that needs to be accounted for when determining pressure ratio. This depression can be 1 psig or more on some intake systems. In this case P1c on a standard day is:
14.7psia – 1 psig = 13.7 psia at compressor inlet
Taking into account the 1 psig intake depression, the pressure ratio is now:
(12 psig + 14.7 psia) / 13.7 psia = 1.95.
That’s great, but what if you’re not at sea level? In this case, simply substitute the actual atmospheric pressure in place of the 14.7 psi in the equations above to give a more accurate calculation. At higher elevations, this can have a significant effect on pressure ratio.
For example, at Denver’s 5000 feet elevation, the atmospheric pressure is typically around 12.4 psia. In this case, the pressure ratio calculation, taking into account the intake depression, is:
(12 psig + 12.4 psia) / (12.4 psia – 1 psig) = 2.14
Compared to the 1.82 pressure ratio calculated originally, this is a big difference.
As you can see in the above examples, pressure ratio depends on a lot more than just boost.
◊ 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.
◊ 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.
◊ The Choke Line is the right hand boundary of the compressor map. For Garrett maps, the choke line is typically defined by the point where the efficiency drops below 58%. In addition to the rapid drop of compressor efficiency past this point, the turbo speed will also be approaching or exceeding the allowable limit. If your actual or predicted operation is beyond this limit, a larger compressor is necessary.
◊ Turbo Speed Lines are lines of constant turbo speed. Turbo speed for points between these lines can be estimated by interpolation. As turbo speed increases, the pressure ratio increases and/or mass flow increases. As indicated above in the choke line description, the turbo speed lines are very close together at the far right edge of the map. once a compressor is operating past the choke limit, turbo speed increases very quickly and a turbo over-speed condition is very likely.
◊ Efficiency Islands are concentric regions on the maps that represent the compressor efficiency at any point on the map. The smallest island near the center of the map is the highest or peak efficiency island. As the rings move out from there, the efficiency drops by the indicated amount until the surge and choke limits are reached.
Now that you know how to read compressor maps, it’s time to move onto the next step of the process, learning to use them to find out which turbo is right for you:
Quote: As found on Garrett.com’s Turbo Tech103 Article 2. Plotting Your Data on the Compressor Map
In this section, methods to calculate mass flow rate and boost pressure required to meet a horsepower target are presented. This data will then be used to choose the appropriate compressor and turbocharger. Having a horsepower target in mind is a vital part of the process. In addition to being necessary for calculating mass flow and boost pressure, a horsepower target is required for choosing the right fuel injectors, fuel pump and regulator, and other engine components.
◊ 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 lb/(Hp*hr) 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.
For the equations below, we will divide BSFC by 60 to convert from hours to minutes.
To plot the compressor operating point, first calculate airflow:
Wa = HP * A/F * BSFC/60
Where:
• Wa = Airflowactual (lb/min)
• HP = Horsepower Target (flywheel)
• A/F= Air/Fuel Ratio
• BSFC/60= Brake Specific Fuel Consumption ÷ 60 (to convert from hours to minutes)
Now it is time to put that information to use:
Quote: From Garrett’s Turbo Tech103 EXAMPLE:
I have an engine that I would like to use to make 400Hp, I want to choose an air/fuel ratio of 12 and use a BSFC of 0.55. Plugging these numbers into the formula from above:
Wa = 400 * 12 * .55 / 60 = 44.0 lb/min of air.
Thus, a compressor map that has the capability of at least 44 pounds per minute of airflow capacity is a good starting point.
Note that nowhere in this calculation did we enter any engine displacement or RPM numbers. This means that for any engine, in order to make 400 Hp, it needs to flow about 44 lb/min (this assumes that BSFC remains constant across all engine types).
Naturally, a smaller displacement engine will require more boost or higher engine speed to meet this target than a larger engine will. So how much boost pressure would be required?
◊ Calculate required manifold pressure required to meet the horsepower, or flow target:
Where:
• MAPreq = Manifold Absolute Pressure (psia) required to meet the horsepower target
• Wa = Airflowactual(lb/min)
• R = Gas Constant = 639.6
• Tm = Intake Manifold Temperature (degrees F)
• VE = Volumetric Efficiency
• N = Engine speed (RPM)
• Vd = engine displacement (Cubic Inches, convert from liters to CI by multiplying by 61.02, ex. 2.0 liters * 61.02 = 122 CI)
EXAMPLE:
To continue the example above, let’s consider a 2.0 liter engine with the following description:
• Wa = 44 lb/min as previously calculated
• Tm = 130 degrees F
• VE = 92% at peak power
• N = 7200 RPM
• Vd = 2.0 liters * 61.02 = 122 CI
= 41.1 psia (remember, this is absolute pressure. Subtract atmospheric pressure to get gauge pressure (aka boost):
41.1 psia – 14.7 psia (at sea level) = 26.4 psig boost