In part one, I went over what a turbocharger does in relatively basic terms. This article will expand on the concepts I explained there, so if something does not make sense, read of it again! There is a lot to understand when it comes to designing a turbocharged engine, or selecting a specific turbo for an engine that you already have. The latter is undoubtedly more important to us, since we don’t have the resources to design an engine from the ground up.
So what things do we need to keep track of? Well, there are a few important things that take precedence over all others, and can depend on the goals of your design:
- Air flow supported by turbocharger at different boost levels
- Compressor trim
- Turbine A/R
I’ll go over trim and A/R before airflow, since the pieces will fit together in a more sensible manner that way.
To understand trim, we need to know what the inducer and exducer are on the compressor wheel. Both are pretty straight forward. The inducer size is the diameter of the compressor blades at the inlet. The exducer is, as you guessed, the diameter of the compressor blades at the outlet. The opposite is true for the turbine wheel, since the exhaust gas flows in at the edge and out through the center.
We can calculate trim with a simple equation: (Inducer)² / (Exducer)² * 100
So for an Inducer of 61mm and an Exducer of 82mm, the trim is 55. As a general rule of thumb, the larger the trim, the more air a turbo can flow, while smaller builds boost earlier, assuming that all other things are held equal. That said, it’s easy to change things to make a smaller trim turbo flow more than a larger trim, since the trim is just a ratio, not an actual size descriptor.
The Turbine A/R is equally straight forward, it’s the ratio between the Area of the turbine housing inlet to the Radius of the turbine housing.
Pretty simple, just divide the area by the radius to get A/R. A turbocharger with a smaller A/R will generally spool sooner and faster than one with a larger A/R, with everything else held constant. Again, like with the trim, it’s a ratio, so a larger turbo with a small A/R may spool slower than a small turbo with a larger A/R.
Now that we have those two out of the way, the air flow is pretty straight forward. Most turbocharger manufacturers have “flow map” that show how much airflow the turbo can support at various boost levels, along with the approximate wheel rpm and overall efficiency. For example, the relatively small Garrett GT2860R’s flow map looks like this:
It may be a lot to look at for the first time, but it’s actually pretty straight forward. Along the x-axis is the mass air flow, and going up in the y-axis is the amount of boost that the turbo can make at that airflow. The lines that start out horizontal and curve down to the right is the rpm the turbine and compressor wheels would be spinning at to make that amount of boost at that air flow rate.
Go past the right edge of the map and you get into a pretty inefficient region of the turbo, meaning that the compressor wheel will be heating up the air a lot when compressing it. The left edge is called the “surge line”, and you want to avoid running the turbo in this region at all costs, as it is very bad for the turbo and will lead to a premature failure. More on that later.
The y-axis’s label is exactly what it means. The pressure ratio is how much higher the output air pressure is than the inlet pressure. If the inlet of the compressor sees 14psia, at a pressure ratio of 2, the outlet will be 28psia, which is 13.3psig of boost at sea level. This is where it is important to keep the difference between relative and absolute pressure in mind. Relative pressure is the pressure above the ambient pressure, which is usually around 14.7psia (sea level). Absolute pressure is exactly that, the actual pressure created. Absolute pressure cannot be below 0, ever.
The airflow can be calculated pretty easily, though there are a few assumptions we’ll make to simplify the process.
- “Standard” atmospheric conditions, meaning air density is 0.0749lb/ft3, or 0.002645lb/L
- Engine volumetric efficiency is constant at 95%, meaning the throttle valve is fully open.
- Pressure losses in intercooler, air hoses and air filter are negligible. (They aren’t in reality)
Now all we have to do is multiply a couple things:
Mass Air Flow = Air Density * Engine Displacement * (RPM/2) * Volumetric Efficiency * Pressure Ratio
For example, a 2.3L engine at 6000RPM with 10psig of boost:
Pressure ratio = (10psig + 14.7psi) / 14.7psia = 1.68
0.002645lb/L * 2.3L * (6000RPM / 2) * 0.95 * 1.68 = 29.12lb/min
So how does this help us interpret the flow map? Well, here is where 29.12lb/min and 1.68 pressure ratio lies on the map:
As you can see, it’s outside of the efficiency range of the turbo. While it does appear that it can flow that much air at that pressure, it will be heating it up excessively in the process, putting a large demand on the intercooler, and increasing the backpressure in the engine to excessive levels. A bigger turbo would be preferred, but we have to be careful how big we go. Two obvious things, and one not so obvious. First, the bigger the turbo, the more air it can flow. Second, bigger turbos take more time and airflow to spool up. Third, and possibly the most important, but often overlooked, is that we need to make sure we don’t go past the surge line of the turbo. We can prevent this when the throttle is suddenly closed by using a compressor bypass valve or blowoff valve, but we must make sure that the turbo isn’t so large that the engine causes the compressor wheel to hit the surge line under moderate load (low air flow) with high boost pressure.
We know now that the GT2860R is too small for our example engine, so let’s move a step up in the Garrett turbo family to the GT3071:
This might actually be a little bit too much turbo, but leaves the option for us to increase the boost a whole bunch without running out of breath. We’ll just have to be careful not to hit the surge limit at lower rpm if we do so. Careful boost control with the wastegate and electronic boost controller will work well.
Speaking of wastegates, there are two very important things used to control the pressure ratio and airflow of a turbo to prevent overboosting and crossing over into the surge area. The wastegate is one of them, the compressor bypass valve (CBV) and blowoff valve (BOV) is the other.
What the wastegate does is allows exhaust gas to flow around the turbine wheel, instead of through it. This allows the engine to flow a lot of air without forcing the turbo wheels to spin at full speed all the time. Typically what happens is the wastegate is closed when below a certain boost level, and slowly opens past that level to control the amount of pressure the compressor wheel generates. This also reduces backpressure on the engine a bit, which is always a good thing.
The CBV and BOV are on the intake side of the engine, and are meant to prevent the compressor from surging. They do the exact same thing, with one minor difference. The BOV vents air to the atmosphere, while the CBV puts the air back into the intake, after the mass air flow sensor and before the compressor. What happens is when the pressure gets above a preset value, the valve opens to allow the compressor to maintain the air flow it was doing before. Typically what happens is the throttle valve will close suddenly while turbo is making a lot of boost with lots of airflow (ie: upshifting during hard acceleration). The closure of the throttle basically drops the air flow to 0lb/min. Look at the compressor map, and note how this is left of the surge line, definitely a bad thing. The CBV and BOV allows air to keep flowing during this time to prevent surge. Then the boost will decrease or engine airflow will increase again, at which point the valve will shut. There is a very important thing I would like to note about CBVs and BOVs. Never use a BOV on an engine that uses a mass air flow measurement (MAF) system. When you vent air to the atmosphere at the BOV, it has already gone past the MAF sensor, and the ECU thinks that this air will be going into the engine. As a result, there will be fuel injected for that amount of air, when it is really less. This leads to an excessively rich condition in the engine. This definitely not an optimal solution, for a whole bunch of reasons, from how excess fuel decreases performance to the damaging effects on the catalytic converter when the engine runs too rich. Since the engine only runs rich while the BOV is venting, it’s not the end of the world.
Now you know a little more about turbochargers, and what you need to do in order to select the right turbo for your applications. There is still more to cover though, such as more in depth calculations, what to do with the AFR and ignition timing when boost increases, why compression ratio must be decreased to allow for more boost, and so forth, so stick around for part 3.
May 6th, 2008 |
