Overview: supercharger vs. turbocharger

[texan]

The intention with this thread is to shed some light on the theories behind supercharging the modern engine, and the wonderful advantages it gives us in both speed and basic engine design issues. This post is also to try and give people a better understanding of supercharger systems and how they work to make such a simple thing as an engine even simpler (speaking in terms of tuning). I personally am tired of seeing people bicker over the differences between the two basic designs, and would like everyone to note the vast similarities among them instead. So without further delay... I will start with this simple statement:

"It makes no sense to build a naturally aspirated engine if what you are looking for is good power output."

To understand why this is so obviously true, we need to break the concept of an engine down into its simplest parts. There are 4 main parts to an engine that concern airflow:
-the induction system
-the cylinder head(s)
-the exhaust system
-the camshaft(s)

Now if you think of the engine in terms of airflow and forget about fuel for a minute, it becomes a very simple matter really. What we want to do is best flow air through the cylinder head, from the induction system to the exhaust system and then out into the world again. This is best & most naturally accomplished by pressure variation, because as almost anyone with a high school education knows, air naturally flows from areas of higher concentration (pressure) to lower concentration (pressure). Now let's assume for a minute that we are talking about an engine at or near sea level, well we can just forgo the exact physics of things and say that at both the induction system's inlet & the exhaust's outlet we have equal pressure (just under 15psi absolute pressure). So in order to flow air into this system we must always be working a balancing act between the three fundamental sections of the engine, which are exposed to each other only through the camshaft's orchestration of the valves. So forget everything else you know about engines and start thinking of what's under your hood in this way for the rest of this post .

NA ENGINES (naturally aspirated)
These must work within a maximum pressure variation of 0psi (which is really hard to create without massive pumping losses) and 14.7psi (maximum atmospheric pressure @ sea level). To add to the basic problem of how to flow air into and out-of this system, both ends of the system start out at the same pressure, meaning air doesn't naturally want to go IN or OUT. This can be accurately termed as a “pain in the ass”. Now engineers and enthusiasts alike have long been fascinated with how to make power from this setup, but I am talking specifically about supercharged engines here, and as I already stated "It makes no sense to build a naturally aspirated engine if what you are looking for is good power output." So forget about how you can best accomplish this through piston movement and it's effects on cylinder pressures, and understand that it's just a whole lot easier to get an engine to work if it's supercharged.

FI ENGINES (forced induction)
From a pure engine design standpoint, it makes MUCH more sense to pressurize the intake system than to run NA. When only the intake system is running under pressure well above atmospheric, it becomes perfectly obvious that air is going to want to flow through the engine exactly the way we want it to, and both cam timing & exhaust sizing becomes much less important to getting the system to work right (as it was before in NA setup). The air will naturally want to flow into the cylinder head, and then after the very strong power stroke (thanks to all that air) it will naturally want to flow out into the lower pressure exhaust system afterwards. Everything in the engine will be working at pressure above atmospheric and the pressure differences will be greatest in the induction system, so all air will want to exit out the tail pipe quickly and efficiently. One other thing should be said here: turbos technically ARE superchargers. A supercharger is ANY device that pressurizes the intake to above atmospheric pressure, and turbos do this exactly like superchargers do. The only difference is in how a turbo gets the energy necessary to perform it's job, and also that the turbo contributes to supercharging the exhaust system (or more accurately a portion of it, the exhaust manifold).

THE CASE FOR SUPERCHARGING
Since a crank driven s/c (s/c = supercharger) is what people are normally talking about when they use the term supercharger, I will no longer say "crank driven" to make the distinction between it and a turbo. Now using a supercharger makes a ton of sense simply because it only has a direct effect in pressurizing the engine on the side we want it to, the induction side. Since pressures will always be higher here than in any other part of the system (except of course during the engine's power stroke, but that's always sealed off from the rest of the system so we can forget about that complexity), it's very easy to make this combination a powerful one. NA engines often use large amounts of valve overlap to get the whole system to work properly at higher RPM, which has obvious drawbacks in that it's possible for the intake system and exhaust systems to interact in a negative way (since they operate at similar pressures). It's sometimes just as easy to get air flowing backwards through the system as it is to go forwards in an NA setup, which is one reason camshaft choice is so important to where in the RPM band best power will be produced. And here is where the beauty of supercharging is; neither valve overlap amounts nor perfect exhaust system designs are completely essential to keep everything flowing in the right direction. No matter how long the exhaust is exposed to the intake system through valve overlap, air should NEVER pass backwards through the system unless the supercharger stops working.

THE EVIL OF SUPERCHARGING
The evil of supercharging is that some of the power we finally get from combusting the air/fuel mixture must go back into powering the supercharger. So here we have designed this whole system that works so well, yet we have to power it with some of our hard earned torque. This is not a good thing, but then again nothing so simple is ever going to come for free. Do superchargers work? Of course they do, which is why many racing engine uses the technology unless the rules prohibit it. The net result is more total power from the system, but a portion of this power must be sapped from our output to make it all work.

THE CASE FOR TURBOCHARGING
This section is easy to write, because it's exactly the same thing as the supercharger portion. We have all of the same advantages, except for one major benefit. That benefit is that turbocharging runs off what is largely wasted energy, so that damn drawback of needing to power the system with some of our hard earned torque is removed. In this way, a turbocharger addresses the one main drawback to using a supercharger, but as you will see in a second the supercharger addresses the one main drawback of turbocharging.

THE EVIL OF TURBOCHARGING
Hopefully you now understand why it makes so much sense to forgo designing engines for NA use and just supercharge the sucker instead, at least when we are talking about how to best make power. And if you have been following what I have said, you will also understand the bad effect turbos have on our little perfect world of pressure variation.
A turbo is an ingenious little design that harnesses the wasted kinetic energy we dump out through the exhaust system to actually force more air into the engine. This is good for the same reasons that supercharging is good, but it has one major drawback: it of course increases the pressure within a portion of the exhaust system. While turbocharging a motor increases the amount of air that can be flowed into it, it has a negative effect on how easily we can flow it back out again. This weakens our positive pressure difference between these two fundamental sides of the engine, and causes both cam timing & exhaust system design to again become extremely important to making good power. This is most certainly not a good thing, but can a turbo overcome this drawback with the other inherent good it possesses? It certainly seems so, because in most current forms of racing where the rules don’t probihit the use of tubos or slap restrictions on their use, the turbo reigns supreme in terms of engine power output.



Now I didn't post this to make a statement about which system will work better for your intended use, because the answer is (as usual) "it depends". Sorry, but if there was such a clear cut answer do you really think people would still be debating this topic? A long time ago someone would have proven everyone else wrong, and either turbos or superchargers would no longer exist. Remember, these systems were designed and in use on production vehicles long before most of us were born, so it's not like this is a new debate. The purpose here is to educate people on exactly why we would want to supercharge or turbocharge an engine in the first place. Also I wanted people to see, from a basic and theoretical perspective, how each system is different in its function and it's relative pros & cons. Hopefully this discussion of basic theory helped some of you come to a better understanding of FI engines, and that my leaving out any real world examples actually made it easier to understand. I have written an article concerning the technical differences of the common systems, giving examples of different supercharger designs and their advantages/disadvantages. You can read it HERE. And please, any questions or comments, post ‘em here.

Thanks for reading, peace!


© 2001 AutomotiveForums.com. All Rights Reserved.

[enzo@af]

More here

[autofan]

Oh man!!!!
This is so awesome!!
Thanks texan.
Now i know the diff.-
supercharges use torque, and turbos use the wasted kinetic enegry
did i get i all right?
But I see this as turbo being on top of the supercharger.
Can you explain what Superchargers have over Turbo?
I did not get the explanation you gave in the article.
GREAT JOB texan!!
Now, im off to reading your other article. very kewl

[Racer 20]

I didn't read it all but you did an awesome job .

[igor@af]

racer 20,
you should really read the whole thing...
it has some good info in it.

[texan]

autofan- The biggest advantage all supercharger systems have over the turbo concept is simplicity. The drive system for the compressor is just much more complex on turbos than a simple crank driven pulley, which is why turbo systems have more parts than s/c systems. The other big advantage comes in compressor design choices, because you simply don't have any with a turbo. Turbos have to use centrifugal compressors (although suitable copmpressor sections now exist for just about any application), which isn't necessarily a bad thing, but you can learn more about that in the "Superchargers: compressor design and choices" thread.

Simplicity, reliability, ease of installation and greater design flexibility are the general plus's to crank driven s/c systems. Ultimate power output, tunability and scalability are what the average turbo system has in it's favor. And never forget the bane of all turbo motors; turbo lag. It's not really the huge downfall people make it out to be, but it certainly isn't something the positive displacement supercharger crowd has to deal with. Again though, there's more on that in the other thread (and in threads I am about to write on the subject). Hope this helps explain things, peace.

[Adam]

Just thought I'd add to the topic....
SCI
Turbo and Blower Design

A turbo consisits of a centrifugal compressor unit [something like a multi-bladed fan or tiny jet engine] coupled to a separate turbine unit [another ducted fan] sharing the same shaft and driven to speeds of up to 180,000 rpm by hot exhaust gases traveling through at speeds approaching 400 mph. The more boost put into an engine, the more exhaust is generated. In turn, the more boost that can then be generated and the more exhaust you get, and so on in a feedback loop of sorts. Centrifugal compressor airflow increases as the square of compressor rpm.
In order to keep a turbo engine from rapidly self-destructing due to overboost at wide open throttle, a wastegate --controlled, these days, by a computer-- is employed to bypass just enough exhaust around the turbo to maintain the desired level of boost in the engine's intake manifold. At low compressor speeds, centrufigal compressor airflow falls off dramatically, but when coupled with a wastegate, the exponential nature of centrigual compressor boost acutally becomes a great asset for getting low-rpm boost up quickly.
The main fallout of exhaust-driven centrufigal compressors is not a lack of low speed boost but the brief fractin of a second [sometimes infinitesimal] of "turbo lag" that occurs upon application of full throttle at idle or light cruise while the compressor spools up to make boost. In good turbo cars, turbo lag is virtualy non-existent.
Superchargers, on the other hand, are driven directly off the crankshaft by gears or belts. A Top Fuel Roots blower at full howl is consuming 1000 horses of the engine's power just to turn the blower [which is still a pretty good deal]. By contrast, turbos are largely driven by exhaust gases and heat that would othrwise be wasted. Some superchargers--like the Vortech and Paxton-- are essentially turbo compressors that are belt-driven off the crank, but there is a fundamental problem here: When you match the centrifugal blower's gearing to achieve the correct multiple if engine speed for correct boost at maximum power rpm, given the exponential nature of air flow changes with speed, your compressor speed will then typically be too slow to make any boost at low engine speed. [Fixed-displacement-- also called positive-displacement-- blowers like the Roots deliver nearly instantaneous low-end boost.]
Turbos have no such problem. Driven by some or all of the exhaust gases [depending on wastegate position], they are not limited to a particular multiple of engine speed. The wastegate concept enables the turbine and compressor configuuration to be optimized to be optimized to deliver plenty of bost air flow at low rpm and low exhaust flow, while diverting enough exhaust past the turbine to keep boost from getting out of hand at wide open throttle and higher rpm.
The Roots-type Eaton supercharger differs from turbos and centrifugal superchargers in that, internally, there are no fans at all, but internal moving rotors that gulp air, close the "gates" behind them, and then squish or move the air in the general direction of the engine intake. Positive-displacement blowers essentially pump a fixed amount of air per cycle, reguardless of the speed at which they are rotating [this breaks down at very low and high speed due to changes in volumetric efficiency].
The bottom line is, both turbos and blowers can pack extra air in an engine; an important difference between the two is the blowers are a slave to the speed of the enigne while turbos are not. While a supercharger cannot lag behind engine speed, it also can't surge ahead like a turbo to make more power while an engine lugs at low speed. Maximum boost, delivered mechanically on blowers according to the size of the installed blower-belt pulleys, cannot be dynamically increased to compensate for altitude as it can on a turbo, and it cannot be controlled by a computer to compensate for fuel quality or to provide short bursts of extremely high boost for passing. Without computer controls, in general, superchargers on street cars cannot operate in as high a state of tune as cumputer-controlled turbos.
A supercharger is like an engine that has to drive a car with only one gear, whereas the turbo is like an engine coupled to an infinitely-variable transmission. The need for flexible blower speed for a crititcal performance is illustrated by WW II fighter aircraft like the P-51 Mustang, which used a two-speed supercharger [essentially all modern piston-engine aircraft use turbos]. Pre-war European F1 racers--Alfas in particular-- have been known to use multi-stage supercharging.
From a technical point of view, the really interesting thing about turbos is that, within limits, the more power they make, the more power the CAN make. From a marketing pint of view, turbos and blowers both make more power.

--Sports Car International--January 2001-- Page 57

[Psman32@af]

Ok there are a few thing that are not being said here that I feel should be.
There are three different kinds of superchargers and two different kinds of turbochargers.
Two of them have been covered. The roots type and the centrifugal. Roots do NOT compress air but they do give a constant pressure to the engine. A centrifugal basically is just like a belt driven turbocharger. There are two types of turbochargers. One is the average with a max fixed PSI and one is a variable pressure turbo. The variable pressure turbo is better the a regular turbo, but for instant horsepower nothing beats a roots type super charger or the third type that has not been mentioned yet. This is the twin-screw supercharger. A twin-screw supercharger improves on the roots supercharger by putting "fins" on top of the rotors. This will compress the air giving you boost. Twin-screw superchargers offer extremely high amounts of volumetric efficiency, as high as 95%. It also gives a very high amount of heat efficiency for a supercharger, as high as 80%.
If you add a supercharger or turbocharger to an engine, there will be a noticeable response after there is an additional 3 psi going into the engine. This will give u about 1/5 horsepower increase. For a turbocharger to do this it will take .4-.5 seconds. A supercharger will take .1-.2 seconds to reach 3-4 psi.
All in all, it depends if you have a long revving large displacement engine or a higher revving, usually smaller displacement, engine as to what will affect your horsepower and torque more to your taste. I am biased toward large V-8's. I drive a 1989 5.0 Lincoln Towncar. One other thing I think I should mention here is the fact there is an equation for how about much horsepower a person can get by adding a supercharger or turbocharger.
The equation is as follows
original horsepower times pressure ratio times corrected thermal efficiency times volumetric efficiency. From this subtract the power needed to drive the mechanism. to figure out the pressure ratio, take 14.7 and add the number of psi the turbo gives. Then divide this by 14.7. The ratio for the purpose here will be 1.05. A turbo, on a redline of 7000 rpm, usually takes 2 horsepower and can take as much as 10 horsepower, depending how efficient the exhaust is. At 7000 RPM the average supercharger takes 11 horsepower.
A roots corrected efficiency is 85%, so it would be .85 in the equation
A twin-screw has an efficiency is 88%, so it will be .88 in the equation.
A turbo has 89% efficiency, so it would be .89 in the equation.
I do not know what it is for a centrifugal supercharger.
I will give an example for all three types.
They will all have a 6 psi boost and 140 horsepower base
Roots:
(140)x(1.41)x(.85)x(1.05)-11=165
Twin-screw:
(140)x(1.41)x(.88)x(1.05)-11=172
Turbo:
(140)x(1.41)x(.89)x(1.05)-2=184
I must give credit to the magazine Hot Compacts and Imports for most of this information. Some of this information is from Whipple's web site. I have a few opinions which I expressed but nothing to affect the info. Talk to me later if you want more info than this because I probably can get it.

[texan]

Psman32- First off, check the other discussions in my forum, there's plenty of info on the different compressor designs commonly in use today.

Secondly, I should clarify the term "compressor" as I am using it; not as an all-in-one design which compresses air internally, but one that's capable of compressing air when on an engine. And all of these designs do just that; compress air in the intake system.

By your calculated efficiency rating, I assume you are speaking of adiabatic efficiency. But in every case, this number fluctuates heavily depending upon blower design and operating RPM, so a single catch-all number simply won't be accurate. Also, the stated power consumption of each design also fluctuates greatly, Magnuson has this information on the Eaton designs available on their home site. And how much power restriction a tubocharger in the exhaust path presents is highly dependent upon turbocharger selection, exhaust system design, valve timing (speaking largely of overlap amounts here) and boost level. Put simply, there is no equation to accurately estimate HP output from a supercharged engine. Usual power increases fall in the area of 35-50% more peak horsepower, and that's about as accurate as you can really get without taking several other factors into account. But I get what you are saying, you can form a vague idea of peak power levels through some basic math.

And one last thing; there is only one type of turbocharger. There are many different forms of boost management out there (as well as overall system design), but basic turbocharger design is static.

Thanks for the added information though, it's nice to see people engaging in the discussions around here. And I'm glad people see that I haven't discussed every concept surrounding forced induction here (not by a long shot), but please understand I tried to pick out the most important and necessary info. There have been many books written solely on the subject of forced induction; trying to compress that knowledge down to a few posts is not easy to do. So keep those posts coming, just understand the difficulties I have in trying to explain what's important in these threads. Hope this helps explain things, peace.

[Psman32@af]

Yeah thats what I meant. And I thought I said it was only an estimate, there is no equation for horsepower or torque taht will be perfect everytime. Those figures come from the Magazine Hot compacts and Imports. Turbo's can take up to 10 hp depending on the manifold and superchargers, from what they say at least, can range from 9-11 hp when the engine is at 7000 RPM. The numbers for effiency were put there by Corky Bell, who is an author of books on both Turbocharging and Supercharging.

[Dylan_Michael]

Since I am cannot stand here for 3 hours and read this epic, I'll just say something, and excuse me if it has already been said. An NA car I believe is more reliable than a turbo or supercharged car. Correct?

[Adam]

is a blower a supercharger?

[kepone]

i only have one thing to add here ( there is plenty that is unsaid, but i dotn feel like typing it all )

there are 2 different designs of turbos, not one.. there is your typical turbo, and the ball bearing turbo. and a true ball bearing turbo has superior response to any supercharger, the ability to reach maximum boost ( anything from 15-50 psi depending on the engine and the size of the ball bearing turbo ) before 1800 rpm..

an example of this is the incon systems turbo.

there is a lot that hasnt been said on boost vs compression and thermal/volumetric efficiency, and the use of intercoolers etc etc.. but ill put that up another day..

[texan]

Adam- Yep, "blower" is a common slang term for supercharger.


Dylan_Michael@af- It's true that reliability is easier to come by on simpler mechanical systems, and forced induction does add a measure of complexity to the engine. But reliability is much more a matter of proper design than pure simplicity; witness the incredible reliability of Supra Turbo motors (which are capable of handling over 700hp with stock internals) and Eclipse Turbos (capable of over 400hp without internal work). There are also plenty of examples of simple NA engines with terrible reliability. To me, an engine's reliability is all about working off a solid design and properly tuning the system, not about fundamental complexity.

Originally posted by kepone@af
i only have one thing to add here ( there is plenty that is unsaid, but i dotn feel like typing it all )

there are 2 different designs of turbos, not one.. there is your typical turbo, and the ball bearing turbo. and a true ball bearing turbo has superior response to any supercharger, the ability to reach maximum boost ( anything from 15-50 psi depending on the engine and the size of the ball bearing turbo ) before 1800 rpm..

an example of this is the incon systems turbo.

there is a lot that hasnt been said on boost vs compression and thermal/volumetric efficiency, and the use of intercoolers etc etc.. but ill put that up another day..

Please note there are currently three discussions in this forum about supercharger/turbocharger fundamentals, and what you are talking about is covered in greater depth in these.

Also, I would argue the point that any turbocharger has superior boost response to positive displacement superchargers, regardless of bearing design. In fact, one of the hardest parts about tuning a supercharged motor is in preventing throttle tip-in detonation, since boost literally builds faster than the ECU can detect and add fuel for (speaking of aftermarket kits here; the factories deal with this in OE applications). This is one of the reasons aftermarket rising rate fuel pressure regulators have gained in popularity over the last few years.

[TheMan5952]

Hey Texan. What about twincharging? Like the old MR2's. They were supercharged and Turbocharged. Can you explain that a 'lil, I am a little confused on how that works.

I don't know if you heard of the Twincharged MR2, but I have, I don't remember where though.