The turbocharger is a centrifugal air pump driven by the engine exhaust gas. It forces additional amount of air or air-fuel mixture into the engine. This increases combustion pressure and engine power.
In most automotive engines, atmospheric pressure is the only force that pushes air into the intake mainfold. These engines are naturally aspirated (N.A.). The air they 'breathe' is at normal atmospheric pressure. The amount of fuel that can be burned in the cylinders is limited by the amount of air that the atmospheric pressure pushes in.
An engine can produce more power at the same speed (rpm) if more air-fuel mixture is forced into the cylinders. More air-fuel mixture means higher presssures during the power strokes and higher power output. Using a pump to force additional air-fuel mixture into the cylinders is called forced induction. It is one way to improve volumetric efficiency (V.E.). An engine with forced induction may produce 35 to 60 percent more power than a similar naturally aspirated engine.
This concise introduction to the Turbo-Charger is quoted from the textbook "Automotive Engines (8th edition)" by Crouse & Anglin, published by McGraw-Hill International. In three short, precise paragraph, it perfectly serves to introduce the Turbo-Charger in this article.
As explained above, an engine will produce more power if more air-fuel mixture is forced into the combustion chambers. In NA engines, the maximum possible amount of air that can be fed into the combustion chamber is the displacement of that cylinder. For a 4-cylinder 1.6l engine, this will be approximately 400 c.c. of air per cycle. Flowing 400 c.c. of air into a cylinder of the 1.6l means it is working at 100% volumetric efficiency (V.E.). Engines with variable valve timing and other racing derived technologies (e.g. DOHC-VTECs) do acheive V.E. of above 100% but only a little bit more as they are still ultimately limited by the amount of air normal atmospheric pressure can push in. But maximum power is derived by igniting an optimal air-to-fuel mixture. So if we want to get more power, we will need to burn more fuel. And to burn more fuel, we need more oxygen and that means more air. But there is no way to feed a lot more than 400 c.c. of air into each cylinder in N.A. form. However, using a turbocharger, although each cylinder might still be only 400 c.c. volume, the air is now compressed and there is more oxygen per volume of air than that at normal atmospheric pressure. Thus in forced induction, we can feed in more fuel because we have the necessary amount of oxygen in the air to sustain optimal power production. This then is the basic operating principle of the Turbocharger.
Turbo-charging is one of the most effective ways to significantly boost up the power of any engine at all rpms. While a turbo-kit would initially seems to involve complicated pumbling and fueling work, in actual fact once the design principles are understood, it becomes clear and straight-forward. In this article, I will attempt to introduce and explain the basic design principles of the turbo-charger application. While I will never claim to be an expert in this area, what I hope to acheive is to clarify sufficiently so readers will be able to understand the important components of the turbo-charger. I will also attempt to look at and explore, very briefly, several important areas of turbo-charging that are often misunderstood. As usual, should any reader who are well versed in turbo-charging see any errors in this article, please feel free to contact me with your correction(s).
The material presented in this article is theoretically based on the "Automotive Engines" textbook quoted above. My understanding of the theories in that textbook has been supplemented by my own observations of several examples of real-life turbo applications in Honda cars.
The most important component of the turbo-charger package is the turbocharger itself. The photo above is a turbocharger that has been cut-open to illustrate its construction. The diagram on the right clearly shows the air and oil flow through the turbocharger.
As explained, the turbocharger is basically an air-compressor - it sucks in air and compresses it before pushing it out into the engine. An important part of the turbo-charger is therefore the compressor itself, identified by the number '1' in the picture. This is a specially designed rotary blade that when spun will suck air through the opening in the middle and delivers compressed air out of the surronding pipe.
In operation, the compressor spins at an extremely high speed, upwards of 100,000 rpm. It is driven by the turbine, identified by the number '2' in the picture. The compressor and the turbine are connected to each other via a specially lubricated shaft (number '3'). When the turbine is turned, it will turn the compressor via the shaft. The turbine itself is spun by blowing exhaust gas against it via the inlet. Spent exhaust gas will be exhaled through the center outlet (and into the exhaust system).
Because the turbocharger spins at speeds of beyond 100,000rpm, two extremely critical parts of it are the lubrication and the cooling. Both of these are normally done by engine oil. When operating, engine oil is delivered to the shaft at high pressure. This serves to 'float' the shaft in a layer of lubricating engine oil and allows the turbocharger to spin at high speeds with little or no friction and wear. Specially designed oil passages also permeate the turbocharger casing and oil flow through these passages together with that floating the shaft serves to cool the turbocharger. Ensuring proper delivery of engine oil into the turbocharger is crucial to its operation and reliability. After extended operation, the turbocharger will glow red hot and we will need to maintain oil flow through it in order to conduct heat away. The cutaway turbocharger above has the casing sides painted in yellow and the oil passages can be clearly seen.
With this explanation, the construction of a turbo-charger system can be explained in a clear manner. The most important part is to properly place the turbine. The turbine is driven by exhaust gas so for optimum placement, it will be located just after the exhaust ports, where the headers originally are. This means we need a special manifold to feed exhaust gas from all the cylinders into the turbine. Then we need to connect the turbine into the exhaust system so the spent exhaust gas will be discharged.
We now basically need only two more pieces of piping. One is to fit the air-filter to the compressor part of the turbocharger. Any complicated piping here is simply to allow us to place the filter into an optimum position for air-flow. We then need to connect the compressor outlet to the throttle body so that compressed air will be fed into the engine. The image on the left shows conceptually the relative location of the turbo-charger in relation to the cylinder and its intake and exhaust valves as well as the piping that is needed to connect the turbo-charger to the cylinder head.
The other important connection to the turbocharger is the oil delivery line, normally a special steel-braided oil hose to tap engine oil from a suitable location. The oil outlet from the turbine then needs to be connected to the oil pan.
A turbocharger system often includes an intercooler. The Intercooler is a special cooling coil that is mounted in between the compressor outlet of the turbo-charger and the throttle body. When air is compressed, it gets heated up. Hot air contains less oxygen per volume than cooler air (all other parameters being equal). The intercooler 'intercepts' the compressed air from the turbo-charger and cools it before sending it out to the throttle body.
However an intercooler has the disadvantage that it introduces throttle response lag into the system. From light or closed throttle cruising condition, when the throttle is suddenly floored open, the turbo-charger now needs to first pressurize the air inside the intercooler before air can be fed into the engine. This causes a lag between the time when the throttle is floored open to the time the engine responds. However, the lower air temperature given by the intercooler allows more power to be generated, as well as allowing more stable engine operation and consistent power delivery.
To complete the turbo-charger system, we will need various ancilliary devices to help regulate and control its operation.
The turbocharger will normally include a waste-gate. There are many types but the most common would be the actuator waste-gate. This is an air-pressure driven device that opens a flap located on the turbine part of the housing. When the flap is open, it provides an alternate, low resistance escape route for the engine exhaust gas into the exhaust system. The flap is open via a lever that is connected to an air-pressure switch on the compressor. The amount the flap is open varies directly with air pressures from the compressor. A spring is used to hold the flap shut against the housing. By adjusting the spring, it is possible to control at what air pressure the flap will start to open. In operation, this serves as a method to approximately control the amount of boost the turbocharger will deliver to the engine. The photo on the right shows the flap that opens the by-pass 'escape route' from the turbine to the exhaust system. There are other variations of the waste-gate, but all of them are designed to control the boost to the engine.
In the piping from the compressor to the throttle body called the 'pressure pipe', there is also often a blow-off valve. The function of the blow-off valve is to relieve pressure off the pressure pipe when the throttle body butterfly is closed. When this happens, the turbocharger is still spinning (often at maximum speed) and pressure build-up inside the pipe will push back against the compressor blades. This has the effect of slowing the turbocharger down and will cause a delay in response should the throttle be open again immediately. The blow-off valve will relieve the pressure build-up by venting the air out of the pressure pipe. Most turbocharger systems will vent the pressure back into the air-filter connecting pipe, others will simply vent the air back out into the atmosphere (there are advantages to doing this). The mechanism of venting these excessive pressure are also varied, with the most famous and popular being the Sequential Blow-Off Valve invented by HKS.
In stock form, most Honda engines are naturally aspirated. Honda uses a pressure sensor, the MAP sensor to measure the amount of air flowing into the engine. MAP sensors measures the air pressure just after the butterfly valve in the throttle body. The higher the pressure (or more accurately, the less the vacumn), then more air will be flowing into the cylinders. In NA form, the maximum possible air-pressure at the throttle body is therefore normal atmospheric pressure or air at 1.0 bar. The stock Honda ECU therefore will have PGM-Fi programs that will only work until 1.0 bar MAP pressure value.
In turbo-charging, we now have the unusual situation where the MAP sensor will see air pressure of beyond the 1.0 bar normal atmospheric pressure. When the MAP sensor feeds such a condition to the ECU, under stock conditions, this constitutes an error. In stock form, PGM-Fi sets an error code and goes into back-up mode. PGM-Fi sets the error code not really just to notify that the MAP sensor signal is out of range, more importantly, the stock PGM-Fi fuel and ignition maps do not have data for MAP values beyond 1.0 bar. So, to avoid engine damage, PGM-Fi quickly goes into its back-up mode. Therefore in turbocharged Hondas, we will need to work around this design logic of PGM-Fi.
There are a few ways to take care of this. To avoid having PGM-Fi go into back-up mode, the traditional method is to add special check valves into the vacumn hose that connects the MAP sensor to the throttle body. Later Hondas have their MAP sensors mounted directly to the throttle body so the MAP sensor will then have to be relocated to a remote location. These check valves prevents air pressure to the MAP sensor from building up beyond normal atmospheric pressure. The more popular way nowadays is to modify the electronics such that although the MAP sensor do see boost (anything above normal atmospheric pressure), the actual signal to the ECU itself is modified so that it will never indicate a boost condition.
Taking care of the error-code condition of the ECU is just one small part of the modification needed. Because PGM-Fi does not have fuel and ignition map values that cater for boost, we will need to ensure that at boost conditions, the injectors continue to feed sufficient fuel into the engine. Again, there are several different ways to do this.
The most simple way is the use of an Additional Injector Controller (AIC). This is an electronic device that works when boost is detected by the MAP sensor. One or more injectors are mounted such that they feed additional fuel into the engine. The most popular location is along the pressure pipe (though some mounts it at the intake manifold plenum and others at the runners). When boost is detected, the AIC unit injects fuel into the air stream. Thus the air arriving into the cylinder head is already pre-mixed with a set amount of fuel so when the original injectors opens, they're effectively adding fuel into the mixture. By controlling how much fuel is fed by the additional injectors, the required air-fuel mixture can be quite closely approximated using this method.
A method popular in the earlier days and still popular now is to replace the stock ECU with an aftermarket one. The most famous amongst such devices are HALTEC and MOTEC aftermarket computers, with the APEXi PowerFC being a more recent introduction. They basically have fuel and gnition maps that will need to be calibrated not only for the original NA operating range of MAP sensor values but also for the new boost range of MAP signals. However, because they replace the original ECU, they will also alter the operating characteristics of the engine.
Japan's HKS tuning company introduced the concept of the 'piggy-back' computer with their FCON series of computers. An FCON computer intercepts the wiring harness coming into the ECU. The stock ECU is connected to the FCON computer instead of directly to the wiring harness. By this method, FCON is able to manipulate both the input and output signals of the stock ECU. This innovative device introduces the idea of new fuel and ignition maps that works in conjunction with the original PGM-Fi maps. The map values in a HKS FCON computer are used to modify the original NA injector and ignition timing values of the stock PGM-Fi maps while the boost range of values are controlled directly by FCON.
Specifically for Hondas only however is the introduction of the new HONDATA customisable PGM-Fi codes. HONDATA's Boost Option actually changed the original PGM-Fi code to work with MAP sensor signal values of beyond 1.0 bar normal atmospheric pressure as well as expanded the stock fuel and ignition maps to cater for boost conditions. Together with HKS' FCON computers, it is the most advanced method to take care of fuel and ignition management for turbo-charged Hondas.
Common optional components of the turbo-charger system are typically various meters. They are used to monitor various operating parameters of the engine, e.g. the operating boost of the engine, and other operating conditions like exhaust gas temperature.
An often indispensible component of the turbo-charger system is the oil-cooler. When a certain amount of fuel is burned, only a portion of it gets converted into useful work (driving the pistons and generating power). Most of it actually gets converted into heat ! Therefore it is unavoidable that more powerful engines will produce more heat, irregardless of whether it is NA or turbo-charged. Installing a turbo-charger on a Honda engine will quickly increase its power by a large amount and consequently the amount of heat it produces. An oil cooler will quickly and efficiently disperse the heat, keeping the engine running at optimal temperature.
Another critical component of the turbo-charger system is the turbo-timer. This is an electronic timer that is hooked into the ignition circuit. When the ignition is switched off, the turbo-timer keeps the engine running for an additional pre-set amount of time, after which the engine is automatically shut down. As explained earlier, the turbocharger can glow red-hot under heavy use. By keeping the engine running, the turbo-timer keeps oil flowing through the turbocharger, ensuring that it is properly cooled down before shutting down the engine.
It is now time to discuss various aspects of turbo-charging. One extremely key aspect is the target application the system is installed for. I have discussed about the two main approaches to modifying our cars in my Approaches to Modifying our Hondas editorial. These are what I arbitrarily named street-use and race-use approaches. Now, this applies to turbo-charging too, in fact even more so than other approaches.
Turbo-charger application for street use are often simple bolt-on mods. Street-use turbo-kits are designed for moderately big power gains with driveability and reliability. Driveability involves many factors like good throttle response, all round power (low-end, mid-range, high-rpm), and ease of driving. Reliability assures us the use of the car every-day without worry about the engine blowing up or having it spend 1 week in the workshop for every week that it is driven !
Turbo-Charging for race-use are in many ways directly in contrast to that for street use. For race-use, we want maximum possible power. Because racing allows us to keep engine revs more or less within a limited band, we can frequently tolerate very bad mid and low rpm power, as long as power within the important rev range is as high as possible. Good idling is certainly not a consideration at all while race engines typically only need to last for the duration of a single race - work will be continously done to improve the engine in between races anyway !
In practise, this has direct impact on the set-up of the turbo-charger system. The most important of this is the operating boost. In a turbo-charged engine, max power is more or less directly proportional to the boost we run it at. The higher our boost, the higher the maximum power. But I wrote maximum power for a very good reason. To sustain high boost, we need a larger turbocharger. The turbocharger must be big enough to create enough air-flow to sustain the desired boost to the engine, even more so when we have an intercooler added. When we have a large turbocharger however, we now have to contend with inertia. Inertia means the resistance of the turbocharger to spool up quickly.
One important characteristic of turbo-charging is turbo-lag. The text-book definition of turbo-lag is the delay one experiences between flooring the throttle to the time the engine responses. With a big turbocharger, while we will get huge power gains once the compressor spools up, we get the power only when the compressor spools up. Therefore one unavoidable characteristic of a high boost system is turbo-lag.
For this reason, a street-use turbo-charger system often use low to medium boost. For Hondas in basically stock form (no big engine works), boost are often within the 0.3 to 0.5 bar range. Honda engines, especially the super high specification VTEC versions uses very high compression ratios and this conflicts with higher boost levels. Detonation and engine knocking will become a big problem when boosts are pushed too high.
Running a very high boost set-up is often talked about as a matter of fact by enthusiasts but the implications are often also casually ignored. An extremely critical issue related to boost in a turbocharged engines is the excessive cylinder pressures in very high boost set-ups. When combustion occurs cylinder pressure is exerted in all directions. This not only pushes the pistons down but also attempts to push the cylinder head off the block. Very high boost levels generates very high power but also requires that the engine is strong enough to handle the stresses involved. This involves extended work like replacing the piston/conrod/crankshaft assembly, ensuring the block is strengtened, and many other things. The stock clutch system will not be able to handle excessive power increases. But clutches strong enough requires excessive foot pressure to operate while sports or racing clutch also have a very 'grabby' feel leading to jerky driving. Often overlooked is the transmission's ability to handle the extra power - the gearbox and even the drive shafts may break under excessive power. Also important is the rest of the car; brakes, suspension, indeed the whole chassis must be strong enough to handle the new power.
As can be seen, while turbo-charging will give big increases in engine power, crucially important is that proper work must be done to ensure the engine and indeed the whole car itself must be able to handle the new power. Equally important too is whether the power generated can be effectively utilized for the car's intended use, and how practical it is to use.
In this article I tried to explain the basic operating principle and structure of the turbo-charger system in as simple and clear manner as I can. I also looked at the approaches to turbo-charging as well as important characteristics of the two main approaches. I hope readers find this article informative and useful. Again, I have taken all possible means to ensure this article is accurate. Any error that creeps into this article will be totally unintentional.
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