Two Guys Rally

Turbochargers! - Part 3 [ June 20th, 2008 ] By:Mark Ozimek

Welcome back to the Turbochargers! series, where I get to have fun rambling on about one of the most effective ways to create a lot more power from an internal combustion engine. If you haven’t done so already, I recommend reading part one and part two and grabbing a snack before continuing on with this one.

There are several things I left open ended in part two that I would like to cover this time around. First is compressor efficiency, and other things, like how having a turbocharger affects the engine itself, will follow.

Simply put, the efficiency of a turbocharger is how much work is put into compressing the air compared to how much work would have been done in an ideal world. What is different about the ideal world? Well, things like turbulence, heat transfer between the blades and the air, the effects of sound, air’s high and low pressure points within the compressor wheel, and so forth. Nothing can ever be 100% efficient, so we just try to get as close as possible. Newer turbos are generally more efficient than older ones, thanks to improvements in modeling technology, more experience in design, improved bearing tech, and stronger materials to name a few.

The efficiency of the turbo really affects two things:

1. Exhaust pressure right before the turbine inlet
2. Intake air temperature after the compressor wheel

Both of these are very important things to keep as low as possible. I’ll touch on exhaust pressure (commonly referred to as back pressure) more later, since it ties in with a few other important things regarding turbo selection and engine design. The intake air temperature is pretty obvious, the lower the temperature at a given pressure, the more dense the air is, which means more air can get into the cylinders per stroke, mass-wise. This ultimately means more power, if it is not immediately obvious why, I have gone over the effects of temperature on engine performance in more detail before.

I won’t get into detail on the theory behind the calculations involved with efficiency and intake air temperature, but if you really need to know this stuff for some bizarre reason, go do some research on adiabatic compression. For the calculation, you need to know 4 different things to find the compressor outlet temperature, which I will designate as To for temperature outlet:

1. Ti: Compressor inlet absolute temperature (ie: Kelvin or Renkin, add 293.15 or 457.69 to Celcius and Farenheit, respectively)
2. Pi: Compressor inlet absolute pressure (ie: psia)
3. Po: Compressor outlet gauge pressure (absolute works too, but you will have to modify the equation)
4. n: Compressor efficiency (ranges from 1 to 0, typically around 0.7 to 0.6)

This may seem a little messy, but it is straightforward. Plug the values into the upcoming equation and you have the outlet temperature.

We can find the pressure ratio to help us simplify the final equation, and help us relate to the compressor maps, since they are given in terms of airflow and pressure ratio between the inlet and outlet pressures:

Pressure ratio (Pr) = (Po + Pi) / Pi

The equation used for finding the compressor outlet temperature:

To = ((Ti*Pr)^0.283)-Ti)/n+Ti

So if we have a car that is running with an 80ºF inlet temperature, 14.2 psia inlet pressure, 10.0 psig outlet pressure and 70% efficiency…

Pr = (10 + 14.2) / 14.2 = 1.704

To = (((80ºF + 459.67) * 1.704^0.283) - (80ºF + 459.67)) / 0.7 + (80ºF + 459.67)

Do the math and you get To to be 665.2ºR. The units are significant here, since we did all the temperatures in absolute value due to the ratios involved, the result is an absolute value. To convert, just simply subtract the number needed to convert it back to relative, 459.67 for imperial units (Fahrenheit and Rankine) and 273.15 for metric (Celsius and Kelvin)

So the outlet temperature is 206ºF, I usually round to the nearest integer, since these calculations are hardly accurate due to the complexities involved. Either way, that is pretty warm, eh? It gets much hotter with more boost and less efficient compressors. This is what we use intercoolers for.

Many intercoolers are rated up to a certain horsepower, but I find this a rather silly notion. The calculations involved with the temperature drop across the intercooler are quite complex due to the nature of the geometry of the intercooler, and I will omit them simply because we usually don’t know things like the fin height, depth, thermal resistance between the plate and fin, and so forth. It is possible to calculate the outlet temperature based on an airflow speed through the intercooler, speed of the intercooler through the air, and a lot of geometry, but it’s still an estimation at best.

So when picking an intercooler, my advice is to use as big of an intercooler as will fit in the area you’re working with, since bigger intercoolers can remove more heat and usually have a smaller pressure drop across them, which means your turbo can do less work to get the same pressure at the intake manifold. Just remember that the more volume it has, the more air must be put into it when the boost pressure increases (read: throttle response time increases)

In a similar vein, be careful of how much tubing is used to install the intercooler. The bigger the diameter, the less restriction, which is always good, but there is more volume. To avoid excess restriction, try to use as few bends in the intake path as possible, and when you need them, use a bend with as large of a radius as will fit, since that will give the least restriction to the airflow. The whole idea with the intake is to allow it to flow as freely as possible without increasing the volume, thus lag, too much. This is something that you will have to figure out on your own, or talk to other people who have done similar modifications on the same car as yours to find their opinion on how to set things up.

The same thing applies to the exhaust side of the engine too. There are two evils with exhaust restrictions, reduced power and increased exhaust gas temperatures.

I see people say things like “This engine needs a little bit of back pressure to perform properly”, and then I end up laughing a lot. The camshaft profile was designed to create optimal torque with some specified amount of back pressure. Reducing the pressure may reduce torque, but only because that is how the cam profile is set up. Change the profile some and you will end up with more power with less pressure. I’m not going to get into cam profiles yet, since it is an area that is beyond my understanding for now. With turbocharged engines, this is not a concern at all, since the turbine creates an enormous amount of back pressure.

This pressure is created by the work needed to spin the compressor wheel, and the geometry of the turbine wheel and housing. The smaller the overall turbine assembly is, the more pressure it generates at a given airflow. This is why larger turbos tend to generate more power at the same boost level as a smaller turbo. However, as we went over in part two, a larger turbo almost always spools later in the RPM band. This means that while the peak power will be higher, the total amount of energy the engine is capable of putting down to the road is lower.

Getting back to what I was talking about before with back pressure, with a turbocharged car, it is best to keep the back pressure as low as possible, since the turbine generates a substantial amount of pressure for the engine to deal with. This pressure is not constant either. Increasing the boost increases the back pressure even more, since neither the turbine wheel or the compressor wheel are 100% efficient.

In addition to this, the turbine creates energy through the difference in pressure between the inlet and outlet of the turbine wheel. Once again, due to the nature and inefficiencies of the turbine, every small increase in pressure after the turbine wheel creates a larger increase in pressure before the turbine wheel.

Why is this so bad? Well, as I pointed out before, you can make more power with less backpressure. You may have to modify the cam profile to make full use of it, but the net result is more power, which is our goal. The other is equally important. Higher exhaust pressures increase the exhaust gas temperature (EGT) with everything else being held constant. When pushing an engine close to it’s limit, a close eye needs to be kepts on the EGTs to make sure that ridiculous things like melting a piston or warping the manifold don’t happen. Plus, lower EGT’s typically mean a longer engine lifespan, since there is less thermal stress on the parts.

So how to reduce exhaust pressure? Quite simple really, use the biggest diameter exhaust pipe you can fit into the car, straight-through mufflers are a huge plus, use as few bends as possible, and possibly most importantly, the part known as the downpipe must be capable of supporting the airflow.

The downpipe is often the most restrictive part in the exhaust after the turbo (known as the turbo-back, all the parts after the turbine housing) aside from the mufflers, because the exhaust is the hottest in that part. Hot air means low density, which means a high volume for the same mass. This low density creates a high airflow velocity, and drag increases exponentially with velocity. Just like the rest of the air stream, try to ensure that the downpipe has a large diameter, smooth bends, a smooth interior surface (roughness causes more turbulence, which almost always increases the resistance to flow), and the turbo will thank you.

Well, I think that’s enough for this time around. I didn’t cover quite as much as I wanted, but the topics I did cover were gone into a lot of detail, which is good. On the plus side, I already have a few ideas for part four. I always welcome comments, questions or suggestions, so feel free to ask and I’ll do my best to help you out.

So what is everyone’s personal motto for the next month? Less restriction is better!