What makes a turbo work (for those who are interested)

[zguy36]

For those who want to know what actually makes a turbocharger work and the theromdynamics behind its operation, please read. The other posts that concerned this topic turned into more of an arguement with people that only half understand the topic at hand. If you do not think that I am qualified to write this post, then I will let you know that I am taking this information from someone who is much more qualified than I am. I will be referencing the following book:

 

Thermodyamics

An Engineering Approach

by Yunus A. Cengel and Michael A Boles

 

This is a great book on thermodynamics that covers the cycles of an internal combustion engine and also the operation of a turbine. If you want to read up on it, by this book.

 

I'll start out with the cycles of an engine. I don't want to insult anyone's intelligence, but I think that there are a few key points that might be misunderstood. The following diagram shows the cylces of the engine and what is happening.

 

You can see the difference between the actual engine and the ideal cycle that is shown. The upper half of the actual engine cycle does closely resemble the idea cycle, just with the corners rounded off. This is due to losses of heat into the cooling system and also due to internal friction. Once again, these losses don't matter for understanding the topic at hand. Just realize that the equations that follow don't take into account these losses and can't be used directly without taking a measured efficiency into account.

 

For the purposes of how a turbocharger works, I am going to focus on the part of the cycle that goes from points 4-1, or the exhaust cycle. A more illustrative figure on the energies involved in the cycles of an engine is displayed in the following figure:

 

In this diagram, T is temperature and s is entropy per unit mass (kJ/kg*K). The definition of entropy would take a lot more explanation for what is necessary here. In short, to understand, entropy is a measurement of heat transfer divided by the temperature the transfer takes place at. Other terms of interest are u and q. u is internal energy with units of kJ/kg. This is the energy stored in a certain mass. q is heat energy with units of kJ/kg. These two terms are very similar.

 

To find the work output of an engine, we need to know the internal energies of the air inside the cylinder at each point in the cycle. A point that seemed a bit shaky is from points 2-3 of the cycle, or the heat addition. When fuel is burned, it releases its stored chemical energy as heat. This is why heat is not a inconsequential byproduct of combustion, but in fact the only part of combustion that means much at all. When fuel is burned, the air in the cylinder is superheated which causes it to expand which builds pressure. Without this heat, the air would not expand and cause the pressure to push the piston in the cylinder. The equations for work output of an engine are:

 

q_out=u_1-u_4

q_in=u_3-u_2

work=q_in-q_out

efficiency=work/q_in

 

q_in is the heat energy added by combusting the fuel. q_out is the heat energy released from the exhaust to the atmosphere. In order to maximize efficiency and work of an engine, we want to maximize q_in and minimize q_out. The more heat we add with fuel and the less we lose by dumping it out of the exhaust, the more power we can make. The problem with an internal combustion engine is how thermally inefficient it is. According to the text, "thermal efficiencies of actual spark-ignition engines range from 25-30 percent." If you think about it, you can easily see why this is true. Think of all the heat lost to the atmosphere out of the cooling system, exhaust, and also heat radiating from the engine itself. This is all heat energy being dumped that isn't being converted to kinetic energy. By using a turbocharger, some of the heat energy lost through the exhaust can be recovered. This is the part that everyone is interested in. Next I will explain the turbine section of the turbo.

 

Terms for this section are:

m = mass into the system (kg)

h = enthalpy (kJ/kg)

 

Enthalpy is very similar to u (internal energy) and I will not cover the differences here. It gives the energy values for the state that the air is in. Enthaply for a gas depends on its temperature and pressure. Values for air can be found in tables in the back of this book. Higher pressures and temperatures give higher values of energy. The work transfered from the compressor to the turbine is found by:

 

work=m*(h_in-h_out)

 

Higher temperature and pressure going in and lower temperatures and pressures going out maximize the work in the turbo. The fact that the exhaust expands as it goes through the turbo causes it to cool and the pressure to drop. The more pressure drop, the more temperature drop and also more energy recovered. This is where things have been controversial regarding heat. Looking at the tables for enthalpy, it can be seen that there is a larger amount of energy transfered into the turbo with higher temperatures going into it. The function that defines the values of enthalpy for air has exponential decay as temperatures get closer and closer to ambient temperature. With the sts turbo systems, they rely on pressure alone and waste the heat energy that is available. I am starting to lose focus on what people are confused about, so please ask me some more questions so I can clarify.

[(goldfish)]

I would like to clarify that the heat is not doing the work of spinnig the turbo. It's the pressure difference. With tempature and pressure directly linked, it makes for an indicator?. I think that's a good word, but the pressure change, at least for a system like an automoblie, must be accompined by a tempature drop.

 

Well, my understanding.

[tannji]

That was a wonderful depiction and description. It wont solve anything, because it is over some peoples heads, and won't get the attention of some. It also doesnt address those who want to prove that STS remote mount turbos are craptacular, or those who think for certain people in certain situations, the STS system does it's job. The major dispute is over some people trying to hammer home the point that you cant have a turbo system at maximum efficiency (or real world practical approximation of it, at any rate) if the system is remote mount. Some of these people are offended by STS in its entirety, and are under the impression that anyone who uses it or brings it up is advocating it as the answer to every turbo application, as well as being the best possible implementation of cutting edge turbo tech. Some of us just think remote mount is interesting, all the more so because it not only does things so well that it shouldnt, but also because it is fun watching people run through the cyclic litany of whats wrong, and what will go wrong, and why it is inferior. They start out with it's location and the danger of a hot turbo near the gas tank, being exposed to water puddles, taking impacts from speed barriers, and as the intake is attached to the turbo, the danger of particulate and aqueous contamination of the intake. To anyone that has read much of the user feedback on the STS systems, these points turned out to be non-issues for the most part, or had solutions available from STS. So, for those who insist on perpetuating some kind of rivalry between the two methods, the only real option remaining was to take the tack that if STS is selling these systems, they must be held to the same measuringstick as a front mount. Furthermore, many insist on attacking STS for marketing these systems as somehow being more efficient, or at least an appropriate option for the hard-core racing enthusiast. The ONLY statements I have seen that could support this view are some made by people that took their STS-equipped cars to the track and humbled a few non-believers. STS never made any claims that I ever came across.

That is the basic dispute, in a nutshell. Being a sucker for both underdogs and opportunities to be a Devil's Advocate, I get involved when I see certain claims or accusations made.

To my knowledge, NO ONE who is technically familiar with turbocharging in general and STS in particular is under the impression that the remote mount will ever be more efficient. I would love to see any such claims copied and posted here, as long as they were not obvious ignorant fan-boy rants. Unfortunately, this falls into the same category as flames on FWD VS RWD. Those who hate FWD (to the point of obsession) will never completely appreciate a fast Civic or Sentra. An 8-second Civic is still FWD garbage.... = ) and a 9 or 10-second STS vehicle is still a remote mount that doesnt bow to the Gods: Entropy and Enthalpy. Personally, I respect them both... but there is a mere human-like hero on the scene now called displacement....(Heracles?) and that apparently drives a turbo decently as well. Efficiently... no. But hey... a pressure drop is a pressure drop, and if there is no major negative to using the system, I have no beef with it, and enjoy the consternation it continues to cause.

[thehelix112]

tannji,

 

Being a devils advocate simply means asking questions, you've asked them. This is the answer.

 

zguy36 is not saying STS systems don't work. He is simply stating quite correctly that heat increases pressure differential across a turbine, and there is less heat at the back of the exhaust (both of which didn't I see STS say themselves as quoted by someone in a previous thread?).

 

Dave

[tannji]

Your quite right, but I was responding to his final statement, more so than the post in particular, and I didnt take what he said as an attack in the slightest, and didnt intend my post to be antagonistic.

He said: "I am starting to lose focus on what people are confused about, so please ask me some more questions so I can clarify."

I didnt have any questions, but clarified the situation, as I dont think most people who simply like STS are really questioning anything he addressed, so much as others who dont like STS presume. In all reality, most of the people I have talked to or observed in other forums are not confused in the slightest. Many of them have owned, designed, or built nice conventional examples of turboed and SCed cars prior to trying STS... and had some legitimate reasons for liking what STS achieved, with regards to their unique needs or requirements. In all actuality, I think that perhaps the worst press STS will ever get will be from people with no prior experience having STS installed for them, and in their ignorance and delight, embarking on a career of suggesting STS for every situation, which would of course be unfortunate and improper.

Perhaps you misunderstood my meaning because I used the term "Devil's Advocate" (either arguing for the sake of arguing, or arguing with the intent of clarifying and purifying a point or position) and thought I was trying to be one in response to Zguy36... I actually meant what I said to him, it was a quality post, and appreciated, but didnt address what I take to be the underlying reason for the the acrimony over STS.

[thehelix112]

All good.

 

Dave

[cygnusx1]

Now that we all have enough information here at HybridZ to understand how a turbo works, I want to give a brief synopsis of traditional vs sts arrangements.

 

Both systems are better than not having anything.

 

-Traditional systems have the 'potential' for more power because of the energy available near the point of combustion.

-STS runs in a less aggressive environment so it may offer greater durability as a trade off for less potential power production.

[Mikelly]

Although I must admit I've learned a bit from this discussion, as Im insterested personally in the aspects of the STS kits, I think it should be made clear that each of these kits, used in the right kit is worth installing.

 

I have no doubt that the STS kit isn't nearly as efficient as a traditional exhuast manifold mounted setup. However, after pricing TT kits for C5 Corvettes and seeing the installed prices for well over $10K (Closer to $12K is realistic), There simply can't be any arguement against the practical self installation of the STS setup, Especially on something like a Camaro, or in my case a Corvette, where the gas tank issue is a real NON-issue!

 

Will it perform as well as the traditional nightmare install on a C5? No. But I bet I'm closer to getting the wife to buy off on that install at less than $5K as opposed to more than DOUBLE that. Is it a compromise? Sure. Is it worth the additional weight per HP gain? From what I've read, yea 50-70HP and substantial Torque increase is well worth it.

 

To me, this is just one more "option" to look at from a practicle standpoint.

 

Mike

[mobythevan]

When fuel is burned, the air in the cylinder is superheated which causes it to expand which builds pressure.

 

This brings up another good point, that more "material" in the cylinder to be heated can allow more energy to be transferred to physical force. There was a good write up in one of the mags a while back talking about the benefits of NOS becasue it put more "material" in the cylinder allowing higher cylinder pressures not just because of the extra chemical reaction but because there was more "material"/molecules to transfer the heat energy to physical energy.

 

Seems like water injection would help in this aspect.

[tannji]

Funny you should mention the Vette, here is an impressive STS Vette:

 

http://www.ls1tech.com/forums/showthread.php?t=415194

 

Here is a 1/4 track vid:

 

http://www.learntodragrace.com/mike933.wmv

 

Not exactly what you are talking about budget-wise, but pretty damn impressive. I particularly like the Wheelie = )

[zguy36]

Tannji,

Well done in your support for the STS systems, I enjoyed reading your post. My beef on here isn't with the STS system, just with the people who don't understand how the conventional turbo works concerning heat issues. I actually find the STS system quite intruiging. It just seems like a lot more work to me to plumb all that piping in, but in a car with a tight engine bay, that is pretty much the only option. You did hit the nail right on the head by not saying that the STS system is more efficient. I have done some more reading and found some interesting stuff in Advanced Engine Technology by Heinz Heisler

 

"A measure of the energy transformed from the exhaust gas to the compressor turbine-wheel is the amount the exhaust gas temperature drop on its way through the turbine blades. In its simplest form, the thermal energy absorbed by the turbine wheel can be given by the fomula

 

Q=mC(T2-T1)

Q= rate of energy transfer (kJ/min)

m=gas flow rate (kg/min)

C=specific heat capacity

T1=temp at turbine inlet ©

T2=temp at turbine outlet©"

 

I have been trying to think of how this equation would apply to the STS system, since the inlet temperatures are so much lower. The answer is that the outlet temperatures are a bunch lower too. It can be equated to releasing compressed air from a cylinder and the cylinder getting cold. In a conventional turbo system, the temperature changes go from really hot to hot... but the sts systems temperature drop goes from not so hot to cold. Just some food for thought....

 

Moby,

Interesting thought on water injection. Adding more material to the combustion chamber doesn't always "allow more energy to be transferred to physical force. " This is only the case when you add more combustibles. Like you said about nitrous, adding more mass to the chamber that will either burn (fuel) or aid combustion (nitrous) will add more energy. Water injection inhibits combustion. In the combustion chamber, water is inert (combustion wise) and really only gets in the way. It doesn't help at all if you only add water to your setup. The only reason that it helps is because it cools down the incoming charge and internal surfaces of your engine that you can turn up the boost without detonation issues. The extra power comes from the extra boost that was previously detonation limited. On my current (or at least before I tore it out yesterday) of stock ECU and a supplemental fuel injector, I could run 15psi on 85 octane with water injection. I could only do this on days where it was 10-20F outside, but without the water I had detonation issues at 11 psi. Anyway, this wasn't on topic so much for this post. At least I hijacked my own post this time!

-jeremy-

[tannji]

Zguy36.... you reiterated the point I have been making for a long time. I referred to it again by saying a pressure drop is a pressure drop, regardless of the temp. Obviously, there is more energy to be scavenged if you drop pressure at a higher starting temp, but the engine is always going to insure that there is enough pressure to justify the remote turbo. As you point out (and to be fair, STS has been pointing out all along) there is still heat, just not as much as a conventional mount. I believe STS says that the temps average out in the 1200 degree range, compared to a conventional 1700 or so degrees, and that is still plenty hot enough to have an appreciable affect on what is going on. I am familar with why the heat is important in the front mount, but I would love to see a comparitive analysis done based on the fact that both systems on the same power plant will have exactly the same number of molecules hitting the turbine blades. I am wondering what the comparison would show as far as final performance numbers, understanding that it can never be really be an apples to apples setup. I personally am inclined to believe that the frontmount will have a measurable but small advantage in both lag and final HP.... but I really think that it would be smaller than conventional thought would suppose. The only thing resembling empirical data I have is the first impressions of knowledgeable people after driving an STS equipped car for the first time.... they are always impressed, and one of the first things they comment on is the lack of lag, at least as compared to what they were expecting.

Everything I have seen or heard leads me to draw a comparison to computer performance. Like car buffs, cumputer buffs (gamers and overclockers) are looking for that last ounce of performance, and arguements rage over methods and efficiencies. When all is said and done, the difference between two similar machines tweaked according to competing "masters" will be something that an unknowing user will never see, feel, or appreciate. (and in reality, much like cars, a better gamer will still win on a "slower" machine) To the average user, fast is fast... and ignoring the smarty-pants that insist on proving STS at the track, STS is still marketed to the average user.

[zguy36]

Tannji,

Yes, it would be really interesting to compare both systems on a similar setup. The problem is that both systems don't use the same size turbos (STS uses smaller due to more density of incoming charge) so you really can't get a comparable result. You seem to be quite supportive of the STS systems, so maybe you can answer the most confusing of all questions. Why go to all the extra work to plumb the turbo in the back, if there don't seem to be any clear cut advantages? Taking your oil lines and pressure side of the intake clear to the back seems more difficult than just putting it all under the hood. It is all good stuff if you can make it work, more boost=more fun.

-jeremy-

[Mikelly]

I answered this in my post above.

 

The STS isn't a better solution than the more efficient manifold mounted turbo. However, it is an alternate that is space effective and cost effective.

 

Again, we're talking apples and oranges. Anyone who thinks the STS is the better solution would be incorrect. BUT, for those with space issues and on a budget, I think the STS should be a solution to consider, right along with NOS and superchargers.

 

Mike

[mobythevan]

This is only the case when you add more combustibles.

 

The article seemed to say that the inert material would help raise cylinder pressure. I don't know, and the only way I can describe it is like this: (not saying I am convinced one way or the other)

 

Heat is the energy and material (after combustion) is the lever to move the piston. The material is the lever because it expands from the heat. The more material to a certain point will give you a bigger lever to move the piston. If the ~75% of incoming air that is inert was removed from a combustion cycle, how much cylinder pressure would be lost because the heat has less molecules to expand (during/after combustion)? And the flip side, if more inert gas is added does cylinder pressure rise(with no more combustibles added)? Would be neat to have the equipment and time to do experiments like this. I'll have to dig the article back out of the stack.

[BrandonsZ]

On the turbine side:

 

What you want for the input side of your turbo is gas that is as hot and as high pressure as possible (most energy).

 

Where do you lose temperature? (bad)

 

1. In the turbo itself

2. In any length of runner before the turbo

 

where do you lose pressure? (bad)

 

1. as a consequence of losing temperature

2. as a consequence of runner losses due to friction

 

Obviously the longer the runner and the more the gas is allowed to cool the less energy you have to make power from the turbine (thereforte a smaller turbo to have any hope of spinning properly). Also insulation of the turbo and any runners to the turbo would be warranted as long as the increase of power offsets the added cost weight and complexity (reliability) concerns.

 

 

On the compressor side:

 

Because the specific volume of the gas increases with temperature, the efficiency of the turbo compressor decreases with increased temperature:

1. you want intake gas to be as cool as possible.

2. you want the compressor side of the turbo to be as cool as possible. It wouldn't hurt to add cooling mechanism to the compressor because efficiency decreases with increased temperature. However this should be tempered with the fact that cooling mechanisms (like fins, liquid cooling, etc.) add cost, weight and complexity, be sure you are getting enough power addition to warrant these factors but certainly don't insulate the compressor side of the turbo.

 

 

Finally the air exiting the turbo would want to be as cool as possible for two reasons,

 

1. because the cooler air is more dense for intake to the engine,

2. cooler air on the output side of the compressor side of the turbo would lose less pressure due to cooling (and resistance) in the intercoller and from historical evidence it is clear that thee loss of pressure due to cooling is offset by the increase in power due to density at the intake of the engine.

 

 

The final question:

 

You can't compare STS vs. Engine mount because the clear winner is engine mounted the true question is as pointed out by Mike and others:

 

Is the added weight, cost and complexity of a SMS turbo system warranted vs. N/A?

 

Let the Dyno fun-runs begin.