Turbos, Wastegates, blow off valves, and intercooler info
Written by Dennis Grant (DSM GUY)
Part 1 How Turbos Work (or: The Closest Thing to a Free Lunch)
Before we start, we have to take a second to review a little grade 10 physics - The Ideal Gas Law. In short, gas temperature, pressure, and volume are all related. Compress a gas (reduce the volume) and pressure and temperature goes up. Let it expand, and temperature and pressure go down. Increase the temperature, and the pressure goes up (in an enclosed space) or the volume goes up (it expands). Finally, gases want to flow from a high pressure area to a low pressure area, and the greater the difference, the bigger the push. (Pop a baloon, little bang. Pop a welding O2 cylinder, big bang)
OK, a 4 stroke engine produces work by expanding a gas in a confined space where the high pressures created can push against a piston. Furthermore, that gas is heated by the process of creating it (unlike a steam engine) so you get even higher pressures - and more power. Unfortunately, most of that heat (which is the same as energy) is dumped overboard in the exhaust before we get any chance to use it. It's just not in the cylinder long enough to transfer all that heat into mechanical energy, and it's not practical to make cylinders "tall" enough to extract every last bit of work from that hot expanding gas.
So, what can we do about it? well, we can point the tailpipes out the back to try and get some thrust - except that aside from some very rare circumstances, the gas volume isn't high enough to get any worthwhile push. (A few older IndyCars actually created a couple of pounds of thrust from their exhausts, but that's not enough to be really useful)
OK, how about sticking some sort of auxillary engine in the exhaust flow? Steam engines did this for years...
Enter the turbocharger, a turbine fed by exhaust gasses, connected to a compressor via a shaft that compresses intake air into the engine. More air in the cylinder means more fuel can be burnt per power stroke, more burnt fuel means more hot gas, more hot gas means more power - and more boost too.
This is the closest thing to a free lunch you'll find in engineering, because you're taking heat (energy) that would otherwise be wasted and getting usable work out of it, with almost no tradeoffs. You gain a little complexity, and added manufacturing costs, but there is no real performance hit from adding a turbo.
"But doesn't the turbo increase exhaust backpressure?" Under boost conditions, no. Here's why: when the exhaust valve opens, the pressure inside the cylinder is much much higher than the pressure at the turbo inlet. That cylinder pressure "blows down" very quickly, but we're on the exhaust stroke - the cylinder volume is decreasing very rapidly, and from the Ideal Gas Law, that tends to keep the cylinder pressure higher than the turbo inlet pressure. Finally, when the exhaust stroke is nearly done, and the pressures are nearly equal, the intake valve opens, the intake pressure (we're under boost here!) "blows down" into the cylinder, and presto! we have a higher cylinder pressure again.
Part 2 Turbochargers continued, focus on the exhaust side
Allright, yesterday we determined that a turbo was a device that could be used to get useful work out of otherwise wasted energy, to day we'll discuss how that happens in more detail. It is a common misconception that the exhaust turbine half of a turbo is driven purely by the kinetic energy of the exhaust smacking into it (like holding a kid's tow pinwheel behind your tailpipe) While the kinetic energy of the exhaust flow does contribute to the work performed by the turbo, the vast majority of the energy transfered comes from a different source.
Keep in mind the relationship between heat, volume, and pressure when we talk about gasses. High heat, high pressure, and low volume are all high energy states, low heat, low pressure, and large volumes are low energy states.
So our exhaust pulse exits the cylinder at high temperature and high pressure. It gets merged with other exhaust pulses, and enters the turbine inlet - a very small space. At this point, we have very high pressure and very high heat, so our gas has a very high energy level.
As it passes through the diffuser and into the turbine housing, it moves from a small space into a large one. Accordingly, it expands, cools, slows down, and dumps all that energy - into the turbine that we've so cleverly positioned in tho housing so that as the gas expands, it pushes against the turbine blades, causing it to rotate. Presto! We've just recovered some energy from the heat of the exhaust, that otherwise would have been lost.
This is a measureable effect: Stick an EGT upstream and downstream of the turbo, and you see a tremendous difference in temperature.
So, in real world terms, what does this tell us?
All else being equal, _The amount of work that can be done across an exhaust turbine is determined by the pressure differential at the inlet and outlet_ (in english, raise the turbo inlet pressure, lower the outlet pressure, or both, and you make more power) Pressure is heat, heat is pressure.
Raising the inlet pressure is possible, but tough. Lowering the outlet pressure is easy - just bolt on a bigger, free flowing exhaust. I've seen a couple of posts from people who added aftermarket exhausts, who report "my turbo spools up faster now" Well, that's because by lowering the outlet pressure, you increased the pressure differential, and now the exhaust gas can expand more, and do more work. That increased work pushes harder on your turbo, and it spools up faster. You should also see less boost drop at redline, because if an exhaust system is flow-limited, once you pass the flow limit of the system, any additional gasses you try and force through it only raise the outlet pressure. Higher outlet pressure, lower pressure differential, less work, less boost.
Part 3 Turbochargers continued - the compressor side
Having covered what a turbo is, and how the exhaust turbine works, we now turn our attention to the compressor side of the turbo. (If you thought yesterday's post was a little verbose, just wait 'till you see this one
If you can extract work from an expanding gas via a turbine, then it stands to reason that you can compress a gas by driving the turbine shaft with a power source. In other words, the compressor side is just the turbine side driven backwards. The exact same physical lays apply, just now in reverse: we take a low pressure, low temperature gas, do work on it with the compressor vanes, and get a high pressure, high temperature gas at the outlet.
That temperature increase is unfortunate, and will cause us problems later on - and we''l come back to it in a bit.
While the turbine and compressor sides of the turbo are essentially the same, they are _not_ mirror images of each other, and the reason why is due to the chemistry of combustion. A given volume of air will burn an exact amount of fuel, in a ratio of air:fuel about 14:1. The volume of exhaust produced is much greater than the volume of the air used to create it, and the resulting exhaust pressure is much higher than the boost pressure will ever be, so the wheel and housing designs are completely different.
Which leads us to turbine/compressor design.
Turbines are wonderful devices. They are light, and _very_ efficient, but they also tend to suffer from a limited RPM range. That is, a turbine/compressor is very efficient at a certain RPM/flow capacity, but if you vary the shaft RPM very much, the efficiency drops. Run too fast, and the turbine blades cavitate and (aerodynamically) stall, and flow drops. Run too slow, and the blades aren't getting enough "bite", and flow drops.
Here's an example. The M1A1 Abrams tank weighs about 55 tons, most of it in armour. (Steel and depleted uranium) It has a gas turbine engine that produces 1800HP at the wheels... er, tracks, which is enough power to move that beast at about 70 MPH. The turbine is amazingly small, and while I don't remember exactly how much it weighs, it seems to me that it's on the order of 300-500lbs. Compared to the weight of the rest of the tank, the engine might as well not be there!
However, the design of the turbine was optimised for WOT operation. At WOT, the turbine gets better gas milage than an equivelent diesel at the same power point, but at idle, the turbine efficiancy drops, to the point where gas milage (per minute of operation) is **lower** at idle than it is at WOT!
Turbines are fantasic powerplants for vehicles that can run at a constant RPM all day - like tanks, boats, airplanes, IndyCars, etc. For vehicles that need to be run at different engine speeds, they don't work so well. (although if somebody invents a good infinately-variable-ratio transmission, look out!)
So, getting back to turbochargers, what does this mean?
Well, a turbo is really a single speed device. We're only producing enough exhaust to generate boost at WOT, and we have boost-limiting devices to keep the turbo running at a constant speed (once it gets there) so, if we know how much boost we want to produce at WOT, and we know how much air we are consuming at WOT and full boost, then we can select a turbo (really, we're selecting a compressor wheel and housing combo) to maximise the turbine efficiency at that flow point.
Well what does _that_ get us?
A smaller turbo.
That is better, because the smaller the turbo, the less rotational inertia you have to overcome, and the faster the turbo accelerates to it's WOT speed (and the associated boost level) The time delay between opening the throttle and the production of full boost is commonly referred to as "turbo lag" and is the single most hated "feature" of turbos. Ever wonder why the turbo on the 2G is so small? It's been exactly matched to the air consumption of the engine for the driving style of Joe Public - who rarely, if ever, exceeds 4500RPM.
Reducing lag has another important side effect though. If you have a datalogger, and plot the boost curve of your vehicle, the area under that curve determines your transitional power band. Do a litle calculus, and you find that increasing that area - even without increasing the peak boost point - increases the torque available to accelerate the car by a large amount. One of these days, one of our tuner guys is going to get a flow bench, and a dyno, and work out the air consumption of his motor at a certain boost point, and select a compressor wheel and housing combo that maximises efficiency at that point (describing how is beyond the scope of this post - in a nutshell, you compare pressure maps) and go really, really fast.
Part 4 Wastegates, blow off valves and intercooler
The story so far: We have determined what a turbo is, how the exhaust turbine functions (and what affects its performance) what the inlet compressor is (and what affects its performance) and hinted a little at what selecting a turbo requires.
Today: Intercoolers and Wastegates and BOV's - Oh My! So, yesterday we left off with high pressure air leaving the compressor outlet. Unfortunately, physics has worked against us this time, and the act of doing work to our inlet air to compress it has raised its temperature. This is bad. Not only are we reducing density, we're increasing the possibility of the great bugaboo - detonation. Remember, the onset of detonation is usually the limiting factor on the amount of power a given engine can produce, and that increased intake temperature (as measured at the intake valve) increases the chance of detonation. So we have to cool the air back down again, without losing any pressure. That's the job of the intercooler, basically a "air radiator" placed in the flow stream between the turbo compressor outlet and the intake manifold. There's really not much else to say about them, except:
1) The more you can cool the air flow, the better. This _normally_ means the bigger the intercooler, the better. (There are some smaller coolers that are better designed than the lower-end "big" coolers though, so size does not necessesarily indicate effectiveness.
2) The cooler must be placed in a location where ambient air can flow through it. This means that your cooler must have an intake path and an _exhaust_ path. Mounting a cooler flush against a plate does no good!
3) There's always a pressure drop across a cooler. How much depends on the cooler design.
Wastegates
A turbo is a positive-feedback device. The more boost you make, the more exhaust you make, which makes more exhaust, which makes more boost... in a vicious circle. So we have to have some way of limiting boost. What we _really_ want is a way of keeping the turbine operating at a constant speed (see yesterday's post) so that we can maximise the compressor efficiancy - remember that turbines like to run at a single speed. However, as measuring turbo RPM is not practical, and as boost level is directly related to turbo speed, keeping the boost constant is the wastegate's job. The wastegate is just a valve that opens when we have exceeded our desired boost level, and allows exhaust to flow around the turbine, instead of through it. This lowers the pressure differential across the turbine, less work is done, and the turbo slows down. The only "gotcha" with the wastegate is that it must be able to flow enough gas to let the turbo slow down. If it can't, then you get "boost creep" where boost levels slowly grow as the car remains under boost. Bad.
BOV
Everybody likes BOV's because of the nifty sneeze sound they make. However, a BOV is an evil device. It's taking your precious boost and venting it to someplace else. Bad! Unfortunatly, it's a necessary evil, and we have to live with it. Here's why: You're under boost, the turbo is fully spooled, and life is good - then you shift. That means your foot comes off the gas - and the throttle plate slams shut. Suddenly, instead of flowing in a continuous stream through the engine, the intake air smacks into the closed throttle plate. The turbo, which is still spinning and producing boost because if it's rotational inertia keeps producing pressure, and the intake stream, caught in between a rock and a hard place, jumps in pressure. In fact, you get a high-pressure shockwave that travels from the throttle plate back to the compressor vanes, that once it gets there, is a little like poking a stick into the spokes of a bike wheel. The repeated shock is hard on the compressor vanes and the shaft bearings, and in any case acts like a brake, slowing the turbo, and requiring it to be spooled up again. The BOV sits in between the turbo and the throttle plate, and if it detects the shockwave created by a shift, vents it elsewhere - either to atmosphere, or back to the inlet side of the turbo. So, we lost boost pressure, but we kept the turbo spooled... tough to say without a dyno if that was a fair trade on a race vehicle. On a street vehicle, it was definately a good idea, becuase we spared our expensive turbo a mechanical shock.
Part 5 Turbochargers summary Summary:
1) Turbos reclaim energy that would otherwise been dumped overboard in the form of heat, by using exhaust gasses to spin a turbine, which in turn spins a compressor, which compresses the intake air.
2) Compressed intake air makes more power, because it allows you to burn more fuel per power stroke of the engine, and because it helps scavenge the engine. (The new, compressed, intake charge "blows out" the remaining exhaust gasses)
3) The amount of work done by a turbo's turbine depends on the amount of gas flow through it, and the pressure differential across it.
4) You can improve the turbine pressure differential by installing an exhaust system with a higher flow capacity than stock. It's impossible to have "too much" exhaust flow downstram of the turbo.
5) The intake compressor works best when it has been specifically sized for an engine's flow requirements and boost levels.
6) The best way to choose a compressor wheel and housing is to call the manufacturer of the turbo, and answer all their questions.
7) Intercoolers are our friends. They redunce the temperature of the compressed intake charge after the compressor has heated it.
8) An intercooler is only as good as the air flow into it ***and out of it***
9) Wastgates limit boost levels by acting as a "rev limiter" for the turbo.
10) "Boost creep" indicates a wastegate that is too small.
11) High boost motors require good ignition systems. Most top end stumbles and misses are ignition problems.
12) There's no substitute for a day on the engine dyno.
There, that should about do it. Keep in mind that I've just summerised 80 years of turbocharger development and theory into a couple of pages over the last week. I haven't covered everything, and I've simplified a number of concepts where I could without losing the important stuff. (the physics of what goes on between the head and the turbine alone could make a book all on their own!) While my posts should help get you by, if you're really interested in this stuff, there's a lot of good books that cover this material better than I do. Finally, the response to this little series has been great. Thanks to everyone for their kind comments.
Part 6 Turbocharger sizing
Both ends of the turbocharger are turbines (well, strictly speaking the hot end is a turbine, the cold end is a radial compressor - but I'll use "turbine" here to mean "thingy with sorta-fan-shaped blades on it that rotates and does stuff with gases") - and one of the defining characteristics of turbines is that they are very efficient within a certain range of operation. That is, they like to be run at a constant speed, and at that constant speed, they will work _very_ well. Run them too fast, or too slow, and efficiency suffers very badly, as things like cavitation and surge add heat to the outflow. Consider this: The Ideal Gas Law tells us that when we compress air, the temperature of this air goes up. This is a law of nature, there is _nothing_ that can be done to prevent this from happening. However, it is certainly possible to impart even _more_ heat into the airflow by overspeeding/underspeeding the compressor. This is bad. To make the maximum amount of power possible, we need to keep every single excess calorie out of the output air we can. Yes, we will be running that air through an intercooler later, but there are practical limits to how much heat we can pull out of the airstream with our intercooler. The less heat coming into the intercooler in the first place, the lower we can make the outlet temperature. The lower the I/C outlet temperature, the more power we make. (Increased density, reduced chance of detonation) Heat MATTERS. As it turns out, the two variables that control the efficiency of a compressor are
1) The amount of airflow we need it to be able to push, and
2) how much we want this airflow compressed, expressed as a pressure ratio. Well whaddya know, we can compute this fairly easily!
First, we need the amount of airflow without any turbo, which is: (CID * MAX_RPM * VE) / (3456) = CFM Plugging in the values for the 2.0l Mitsu, we get: (121.9 * 7200 * .85) / (3456) = 216 CFM {Is my displacement value right? I'm working from memory}
Next, we need to decide how much boost we intend to run. That's right, decide how much boost you intend to run FIRST. Installing a turbo, then cranking up the boost to make it go faster is bass-ackwards. By cranking up the boost on a properly sized turbo, you are changing one of the variables that determine efficiency, and so you are imparting MORE heat into your intake charge. Yes, you make more power at higher boost levels, but you are making FAR LESS power than you would if the turbo had been sized for that boost level in the first place.
Heat MATTERS.
We'll use 14.7 PSI for this example. (you'll see why in a sec)
Pressure ratio is (BOOST + 14.7) / 14.7 (remember that 14.4 psi is atmospheric pressure, so our pressure ratio is (14.7 + 14.7) / 14.7 = 2.
Heh. So I cheat. Sue me.
Now we need to find out how much airflow we have at 14.7 psi of boost.
FULL_BOOST_CFM = PRESSURE_RATIO * NA_CFM = 2 * 216 = 416 CFM
Let's also work out the CFM we first want to see full boost at: (121.9 * 3000 * .85) / 3456 * 2 = 180 CFM
So, we now have an onset CFM value, a max CFM value and a pressure ratio value. We can now get our hands on a whole bunch of pressure maps from the turbo manufacturers, and pick the turbo that is the most efficient at these points.
Pressure map?
That's a graph that has CFM on the x-axis and pressure ratio on the y-axis. Lines that join points of equal efficiency are plotted in the middle, the same way contour lines join points of equal altitude on a topographical map. I can't do one in ASCII art, sorry. By plotting the intersection of onset CFM and pressure ratio, max CFM and pressure ratio, and joining the dots, we get a line that represents the operating range of the turbo. By comparing the maps of all the turbos we can find to this line, we can pick the turbo that gives us the best efficiency numbers across this line. ...or you can call the manufacturers, give them the numbers, and let THEM do the comparing for you.
Ta daa! We've sized our first compressor. (The hot side turbine we'll discuss later) This of course, begs the question: "How much power will this make?" And the answer is... a topic for later discussion.
PS: This post, and all proceeding it, have nothing to do with the Chrysler Corporation. Nothing I say or do in this forum should be perceived as the official Chrysler position on anything.
Turbocharger calculators: http://www.turbofast.com.au/