I rode in a friend’s Ferrari (1978 308) recently and while I love how it sounds… I often can not get enough of the turbocharged sound. So if you love the sound of turbochargers doing work here you go:
Link for you RSS peeps.
It may be an older video of ours, but I love it and cannot get enough.
Update: Apparently I decided to post this exactly two years after uploading it to YouTube. Odd.
Last time, I left off talking about how intake and exhaust restrictions should be minimized with a turbocharged setup to increase efficiency and overall power at the same boost level. I meant to cover how the engine must be changed to accommodate a turbocharger, but discussing efficiency ended up to be more involved than I thought it would be.
At any rate, there are quite a few things that must be considered, mostly to prevent the engine from going KABOOM! The things I will cover this time around are:
Now, when I said KABOOM, I meant it quite literally. The primary concern with increasing the boost is knocking or detonation, which is when the air/fuel mixture explodes, instead of burning outward from the spark plug like the engine is designed for. The explosion flame front travels a lot faster than combustion, over 300 m/s compared to around 30 m/s for combustion. This causes a rapid spike in pressure before top dead center (TDC), which can be pretty damaging to the engine. Here is a handy diagram from Volvo that shows what happens when knocking occurs:
As you can see, the expanding circles represent areas that are burning. When knocking occurs, there is detonation instead of, or in addition to the normal combustion.
First, we must understand why knocking occurs. It really boils down to one thing: Excessive temperature. This high temperature can be caused by a few things, like intake temperature being too high, compression ratio too high, excessively hot cylinders and pistons. Something as unavoidable as the increase in temperature and pressure from the normal combustion can cause detonation in another part of the cylinder. Lower octane fuel also burns more readily, contributing to knocking. As quick side note, this is why most turbocharged cars recommend premium fuel.
Increasing the boost pressure increases the final pressure and temperature within the cylinder significantly, which as we now know, greatly increases the chances of knocking. In order to increase the boost a lot to make more power, we must try to prevent knock.
The first way to prevent knocking is to retard the ignition timing. The ECU usually does this on the fly based on signals from the knock sensors mounted to the cylinder block. Retarding the timing may seem counterintuitive, since waiting longer to ignite the mixture means the temperature and pressure is higher, because the mixture is still being compressed. Ignition is almost always before TDC, and ignition timing is measured in degrees before TDC, a negative value indicating that the timing has been retarded to after TDC. Once again, a nice little diagram from Volvo for visual reference:
The curve indicates pressure within the cylinder, a spike occuring after ignition, and a sudden drop-off when the exhaust valve opens at the very end of the cycle.
However, later ignition means that the hot gas from combustion stays in the cylinder for less time, reducing temperature, preventing knock. It also means that if knocking persists, the pressure build-up occurs later, while the piston is traveling downward, decreasing the intensity of the pressure spike. However, the negative side to retarding timing is reduced power output, so we want to run it as close to the optimal timing for the RPM as possible without causing knock. This will typically range from 40º to 30º before TDC, depending on the geometry of the cylinder head and piston.
Another easy way to prevent knocking is to richen the Air/Fuel Ratio (AFR). When there is more fuel present, the final exhaust temperature is ultimately higher, but the extra fuel acts as a thermal damper of sorts, since it takes more energy to heat up more fuel during compression. The temperature during compression is what is ultimately what determines if there will be knock. Making the AFR too rich will also reduce power, and cause the engine to consume a lot more fuel, a doubly bad thing to do. Despite this, many ‘high performance’ tunes will richen the mixture significantly to allow much higher boost levels to be run without knocking.
Getting into the engine itself, there is a very straightforward way to allow for a lot more boost without causing knock. Just reduce the compression ratio, it reduces the final pressure within the cylinder, preventing knock. This is why most turbocharged engines have pretty low compression ratios compared to their normally aspirated (NA) counterparts. For example, my S70 has a compression ratio of 8.5:1, while the NA version of my engine has a compression ratio of 10.3:1. Charles’ WRX has a compression ratio similar to mine, at 8.2:1, and the NA Impreza of the same vintage has a compression ratio of 10:1.
The reduced compression ratio compensates for how there is almost twice as much air in the cylinder as there would be at WOT without the turbocharger. The nice thing is that the final pressure within the cylinder is higher with a turbocharger than is possible with an NA engine, because of how the air is cooled down in the intercooler between the two compression stages. Without getting into the specifics of the math, a higher pressure usually yields a higher efficiency, meaning the engine extracts more power out of a certain amount of fuel. In theory, a turbocharger can be used to increase the fuel economy when trying to reach a specific horsepower target. In reality, turbocharged cars often get worse gas mileage due to the lower compression ratio, and tuning of the ECU for extra power over efficiency.
The other important factor that has not been discussed in detail yet is controlling the amount of pressure that the compressor makes. As I explained in part 1, the compressor and turbine wheel are attached by a shaft. To control the amount of boost the compressor makes, the speed at which it is spinning must be controlled somehow. This is usually done by letting exhaust around the turbine wheel, through something called the wastegate, instead of forcing it through the wheel. The external view of a wastegate that is integrated into the turbine housing looks like this:
(click for larger picture)
The wastegate itself is a vent hole right before the turbine wheel that allows exhaust flow into the exhaust pipe with a valve that is pulled closed by an actuator. In the above picture, you can see a rod come out of the right side of the picture, and end at a small arm. That rod and arm are connected to the wastegate and the actuator. The actuator is vacuum driven in this case, boost pressure is supplied to a solenoid that is controlled by the ECU, with two output ports, one to the unpressurized portion of the intake, and the other to the actuator. The solenoid bleeds off pressure as needed so that the actuator can be controlled by the ECU as the boost level changes.
How the wastegate is controlled changes some important factors, such as how quickly the boost pressure ramps up, if there is “overshoot”, where the turbo temporarily exceeds the target boost level, and so forth. In most stock turbo setups, the wastegate starts opening at a pressure significantly below the target boost level, causing a slower increase up to the maximum boost. This gives the least amount of overshoot, which is good for safety reasons, but bad for performance. One option is to increase the pressure required to start opening the wastegate, which will decrease spool time, but potentially creating overboost situations that may damage the engine if you’re running close to the limit of it’s capabilities.
Another choice that must be made is the wastegate type. There are some turbos that do not have an internal wastegate, and require an external one, either from something like the 5 bolt Garrett flange that has a port for an external wastegate, or by using an exhaust manifold with a tube coming off to go to a wastegate. Typically, the more air the wastegate can flow, the better control over boost pressure there is, to a point. Once there is too much flow, it is hard to have fine control over the boost levels. If there is not enough flow, boost will creep above the target level, which is not good.
So not much math this time around, although if anyone wants me to, I’d be happy to review my thermodynamics notes and explain the Otto cycle and why higher combustion temperature and pressure is better. There is always more stuff to cover on turbochargers, so stay tuned for part 5, coming soon!
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:
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.
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.
This is actually a very very complex topic with a lot of things to consider, so I will be splitting it up into several parts. The first one will cover the basics: What a turbocharger does, and how. If you’re already familiar with turbochargers… well, this might be a bit boring for you, but keep an eye out for the later articles where I get into more detail.
To put it very simply, a turbo increases the amount of air flowing into an engine to create more power. If you read the article on temperature and engine power, you’d see that compressing air (by increasing the ‘boost’) increases the density. This means a higher mass in the cylinders, which will be compensated for by more gasoline. The end result of all this is more power.
How does it do that? Well, pretty simply actually. A turbocharger is made up of two centrifugal turbine wheels. One is exposed to the exhaust gas, referred to as the turbine wheel, and the other is exposed to the intake air, and is referred to as the compressor. The two are connected by a shaft, and the whole assembly is free to rotate inside the housing. Typical rotational speeds range from 100,000 rpm to 200,000 rpm at normal boost and airflow rates at full throttle.
What happens is the exhaust gas flows from the turbine housing, into the turbine wheel, and then out the center of the turbine wheel towards the exhaust pipe. The force and velocity of the air from the engine creates enough power to spin the turbine. The compressor wheel works in the opposite fashion. Spinning at high speeds, it sucks in air through the center and expels it out the circumference of the wheel at a high pressure. The pressure of the air is commonly referred to as boost, and is measured in relative terms, meaning 0psig (g for gauge) is actually atmospheric pressure, usually 14.7psia (a for absolute). Then at say, 10psig of boost, the real pressure of the air in the intake stream is 24.7psia, assuming that the car is being driven at sea level. The difference between gauge and absolute pressure is trivial until we start trying to select a turbo for a specific application, so don’t worry about the absolute pressure until then.
For the visual learners among us, like myself, here’s a handy diagram from Garrett that makes the turbochargine process pretty simple:
The following steps are labeled on the diagram:
While turbochargers offer the ability to significantly increase power output with very little weight addition, there are a few downsides.
One often cited one is “turbo-lag”, which can refer to two different things. At lower engine speed, there is not enough airflow through the engine for the turbo to create a significant amount of boost. As a result, turbocharged engines typically do not have very much torque below 2500-3000rpm, unless the turbo is sized very small for the displacement. The other thing is actually lag, when the gas pedal is depressed, it takes some time for the turbine wheel to “spool up”, and spin faster to make more boost to meet the engine demands. Bigger turbochargers typically spool slower and need more engine speed to reach the target boost level, but is less restrictive on the exhaust and can ultimately yield more power in the high end of the rpm band. The opposite is true for smaller turbos.
Another is increased complexity. The ECU has to monitor the pressure going into the engine, and add more fuel, cut back on timing, and restrict the boost level as needed to prevent the engine from detonating or knocking with lots of boost. Left uncontrolled, the turbocharger will create upwards of 30-40psig, which most engines cannot handle.
So why use it? As I said before, it is an excellent way of increasing power output without adding very much weight. For rally racing, this is ideal, since we want to keep the cars as light as possible for good handling. Getting the power that the engines make now without a turbocharger will mean making engines at least 50% bigger, which is a lot more weight.
Powered by WordPress | Blue Weed by Blog Oh! Blog | Entries (RSS) and Comments (RSS). | 