Ken's corner #5: Turbocharging

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Ken's corner #5: Turbocharging

Postby ken.lagarec on Mon May 12, 2003 3:20 am

Well, I promised a friend I'd write something this week-end and I figured some of you might have a few questions about turbocharging that I can answer. So the topic for today is turbocharging. I'll present the basics of what it is and maybe you can ask questions that will lead to more interesting and more advanced topics related to turbocharging. I'll be writing this as I go, so sue me if you don't think it's coherent...

1. Rewind back to the naturally aspirated engine.

Of course, before understanding turbocharging and how it can help get more power out of an engine, it's necessary to understand how a gasoline engine works (diesel is similar).

A gasoline engine works by using the energy produced by the combustion of gasoline. Combustion is the chemical reaction between a combustible (gasoline) and oxygen (which is found in air). Basically, an engine works by filling a cylinder with gas and air (a 14.7:1 weight ratio of air and fuel will burn all the gas and leave no excess oxygen) and igniting it with a spark. The resulting hot and high density gas then expands by pushing the piston and transferring some energy to the piston, which causes the crankshaft to turn. The rest of the energy produced is in the form of heat which is either transferred to the engine block or still present in the evacuated exhaust gases. In fact, only ~30% of the energy from the combustion results in mechanical work. Over 70% is wasted through the exhaust and the radiator (as the coolant transfers the heat from the block). Enter turbocharging, a method to recover some of the heat from the exhaust gases and put it to good use.

As the engine is turning, it acts as a pump, sucking air into the cylinders. The amount of air it can ingest is usually the limiting factor in producing power. Because gasoline is liquid, it can easily be squirted into the cylinder (in a 240sx, one cylinder is 600cc. The amount of fuel required to get a 14.7:1 A/F ratio if the cylinder is full of air is 0.6g, not much). But 600cc of air is the MAXIMUM that can be drawn in there (volumetric efficiency of 100%). A turbocharger will use some of the energy in the exhaust gases to PUMP more air into the cylinders, by doing so at a higher pressure (atmospheric pressure is 1 atmosphere = 1 bar = 14.7 psi).

2. How does the turbocharge do it?

The turbocharger is composed of two components:
. the turbine, which is made to convert the energy from the exhaust gases into mechanical energy: the exhaust gases spin the turbine, which is connected to the compressor by a shaft.
. the compressor, which is a standard pump-type compressor that draws air from it's inlet and expells it through the outlet. If it's spinning fast enough, the air coming out of the outlet will be pressurized.

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Because the air from the compressor is pressurized, it will cram more oxygen molecules in the cylinder than normally. When somebody says he's running 10 psi, the total pressure of the air filling the cylinders is 10 + 14.7 = 25 psi, which represents a 67% increase (25/15-1) compared to an NA engine. Given additional fuel, this can thus produce 67% more power than the NA engine. Of course, the volumetric efficiency of the turbocharged engine might not be the same as the NA engine (it can be higher or lower), so you won't always get 67% more power when you're running 10 psi.

3. So what do you need?

Instead of dumping the exhaust gases into the exhaust , through the cat and muffler as in an NA car, you need to insert the turbine in there, so you need a new manifold.

You also need to direct the air from the compressor outlet to the throttle body. A thermodynamic side effect of compressing the air is that it will also heat up. Because hot air is not desired (it increases the risk of detonation), it can be worthwhile (read imperative if running medium to high boost) to have an intercooler cool the air before it gets to the TB.

To supply the extra fuel, you can a) get bigger injectors if the stock ones aren't sufficient or b) raise the fuel pressure so that the stock ones flow more for the same duty cycle. a) typically requires a new ECU and b) can be done with a rising rate fuel pressure regulator for low ot medium boost (<8 psi).

Well, just as the theory of an NA engine is simple, its implementation is nothing but simple, and the same goes for turbocharging: it sounds simple enough, but implementing it is much more complicated and costly.

Does it work? Formula 1 cars were running around 60 psi in the late eighties, producing up to 1500 hp from 1.5l engines (in qualifying form)! That's a higher hp/liter than ANY type of racing has ever acheived, including top fuel dragster.

Please, ask questions (or ask for details)!

Ken
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Postby JJM_240sx on Tue May 27, 2003 1:42 am

Hey I have read all the ken's corners and they’re great. You were also the one that helped me out with my FMU. I was just wondering if you could explain the components of the actual turbo charger its self. The main thing I wanted to know is if the a/r on the intake side. Is the a/r determine by the wheel or the housing. As same goes for the exhaust A/R side. I have a turbo with a .60intake and .63exhaust. Now if I got a .42intake housing would it work with the .60a/r turbine wheel? Or if I get a .42wheel would it work with a .60compressor. As same goes for the exhaust side. I hope I didn’t confuse you, but I am so confused on this.
Thanks!
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Postby ken.lagarec on Tue May 27, 2003 3:12 pm

Thanks for appreciating the "column"! Questions such as these are what I welcome most!

First of all, the components of a turbocharger. Essentially, the turbo is composed of 5 parts (see the pictures at http://pages.infinit.net/klagarec/240sx/turbo/turbo_pictures.html):
    . an exhaust housing, which attaches to the manifold and the downpipe.
    . an exhaust wheel that gets spun by the exhaust gases that enter the housing. This wheel has a shaft that spins the compressor wheel.
    . a compressor housing, which received air from the air filter and sends compressed air to the intercooler/throttle body.
    . a compressor wheel that connects to the turbine wheel shaft
    . a center section, which is lubricated and possibly watercooled which supports the rotating shaft of the turbin wheel. This part includes seals and stuff but is basically there to support the shaft and prevent heat from going from the turbine to the compressor.


The housings are characterized by their size (eg. T3, T4 and the various trims) and A/R ratio. The A/R ratio is the ratio between the cross-sectional area of the opening through which the gases (exhaust or air) flow and the radius between the opening and the center of the housing. A higher A/R ratio can be obtained by increasing the area (having a wider opening) or reducing the radius (a smaller housing diameter). For most turbine families, only the area is varied, as the external shape of the turbine housing is fixed. A larger area will reduce back-pressure, ersulting in better top end performance, but it will also reduce gas velocity, resulting in more sluggish low end and longer times to build boost. A smaller area will dramatically increase back-pressure in the top end, effectively choking the engine by killing its volumetric efficiency. However, this will increase spool-up speed and give quick building boost. Most manufacturers opt for small A/R to give good drivability, at the expense of high rpm power.

The wheel "size" and geometry will also determine how fast the gas will spin it and how much restriction (back-pressure) it will cause. A turbine wheel is very different from a compressor wheel. In its simplest form, it can be seen as vertical pales that the gases spinning around in the turbine housing will push, making it spin. If it is curved in slightly, it better "captures" the gas, thus spinning faster. But its also necessary to evacuate the gas or else you will choke the engine. So it's sligthly bend out towards the opening, enabling the gas to escape after having spent most of its energy to spin the wheel. Typically, a wheel for a given housing will have the same major diameter (the part that's pushed on by the gases in the housing, but the exducer diameter is varied for various trim levels. This will determine primarily how much flow (or how little back-pressure) it produces. For examples, check out the Turbonetics web site for their turbocharger specs (http://www.turboneticsinc.com doesn't have the wheel specs, but http://www.turbonetics.com does have them).

So to get back to your specific question... The A/R is not related to the wheel, so it is incorrect to talk about a specific A/R wheel, but I'll try to make the most of your question anyway. The T3/T4 I have also has a .60 A/R on the compressor side and a .63 A/R on the exhaust side and this works very well. Typically, when you choose a turbo, you can specify the housing A/R and the wheel diameters and trim separately, so they are interchangeable. There might, however, be combinations that don't work (the largest wheel might not fit all housings). The best thing for you to do is measure the wheel diameters and determine what spec it is. Then you'll have a better idea of what you're working with.

As a rule of thumb, on a KA you want at least .63 A/R housing on a T3 turbine. A .82 A/R might be better for top end. I have a stage III wheel and I build boost very early (around 2500-3000, I build full boost - 7 psi in 5th). With a T4 turbine, you should go with as low an A/R you can get, typically 0.54. If you can fit a divided housing, you will still get very good boost response.

As for wheels, the idea is to never have a large mismatch between the compressor and turbine major diameters: the small turbine wheel will never be able to spin the large wheel effectively. A rule of thumb is that the turbine wheel should be no less than .75 times the compressor wheel. For example, a T3 stage III wheel has a major diameter of 2.559" and a TO4E wheel is 2.950", giving a ratio of 0.87. A "massive" T-76 compressor has a wheel diameter of 4.03", which would give a ratio of 0.63 when matched to a T3 turbine. That won't work... Of course, most things turbo are trial and error, some combinations that should work according to rules of thumb aren't very good and some odd combinations can work well. Unfortunately, unless you're made of money, that's hard to test...

Maybe I should get back to work now...

Ken
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Postby JJM_240sx on Tue May 27, 2003 6:36 pm

hehe wow so detailed! That helped out a lot! one more small question. do you think its worth geting a turbo with both oil and water cooling abilitys? and if you have the water cooling option on the turbo you dont have to hook it up right?
thanks!
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Postby ken.lagarec on Tue May 27, 2003 8:19 pm

JJM_240sx wrote:hehe wow so detailed! That helped out a lot! one more small question. do you think its worth geting a turbo with both oil and water cooling abilitys? and if you have the water cooling option on the turbo you dont have to hook it up right?
thanks!


Water cooling is better. SAAB originally used oil cooled turbos but quickly switched to water cooled for longevity purposes. Basically, with a water cooled, the temperature is rarely enough to burn the oil (which is still needed for lubrication). With oil based cooling and lubrication, the oil temperature can be high enough to burn, particularly when the engine is turned off when the turbo is still hot (hence the use of a turbo timer). If you have a water cooled turbo, you have to hook it up, because the small amount of oil it uses is not enough to also provide cooling (the water passages in the turbo would be empty and not dissipate heat). It makes things more complicated, but better for the long term.

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Postby JJM_240sx on Tue May 27, 2003 9:51 pm

ok thanks! now time to find out how to hook up the water cooling pipes hehe.
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