||Last Updated: Aug 16th, 2006 - 11:01:00
The intention with this article is to shed some light on the theories behind turbocharging 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
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.
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