Boosting Potential

Hot Rod May 2001

by Terry McGean

An informative look at “unraveling the mystery of turbocharging.”

About a half-hour after the invention of the internal combustion engine, somebody figured out that it would make more power if the intake air could be forced into the engine rather than sucked in by the down stroke of the piston. Since then, hot rodders have been trying to devise new and better ways to stuff cylinders full of mixture for maximum output. Many of these efforts involved crank-driven devices to compress the air, but one of these gearheads, a Swiss gent named Dr. Alfred J. Buchi, came up with the idea of using exhaust gases to drive the compressor.

It was a brilliantly simple concept: Put the energy contained in the exhaust to work rather than letting it go to waste. That was back in 1915, and the engineering sketch looked remarkably similar to what we now know as a turbocharger. Most of its early applications were on wartime aircraft, serving as a means to overcome the problem of air density at altitude, but naturally, it didn’t take long to migrate to the automobile.

Internal Combustion Basics

Before we delve into the mechanics of turbocharging, let’s take a brief look at the basics of internal combustion. All internal combustion engines, piston- or rotor-driven, whether running on gasoline, diesel, or other fuels, require atmosphere to produce power. The air is ingested and combined with fuel before being compressed in the combustion chamber and then burned to generate power. Of the two ingredients needed to produce this power, air is generally the more difficult to deliver to the combustion chamber. This is primarily due to the fact that a naturally aspirated engine must “inhale” the air using the vacuum generated when a piston traveling downward on its intake stroke creates a low-pressure environment in the intake tract. How effectively this process is able to fill the cylinders is a major factor in determining how much power the engine is capable of developing, and is measured in terms of volumetric efficiency (VE). An engine capable of completely filling the volume of its cylinders by the end of the intake stroke would have 100 percent VE; however, there are a number of factors that hinder this efficiency. These include the shape, size, and length of the intake tract, the quality of the piston seal, the timing of the valve events, and the efficiency of the exhaust evacuation during the preceding stroke. The deficit in cylinder fill imposed by these factors (and others) is referred to in terms of pumping losses. A typical naturally aspirated engine operates at around 80 to 85 percent VE.

Of course, the essence of hot rodding has been to overcome these obstacles, or at least diminish them. But while racing engines often exceed 85 percent VE substantially, doing so requires extensive engineering based on the specific application. Even then, the VE is generally only at its highest value for a narrow rpm range. That’s where the forced induction alternative comes in-increasing cylinder fill using external means to push air into the engine, rather than relying on the draw of the descending piston. With these methods, VE can easily exceed 100 percent.

Supercharging is the process of force-feeding air into a combustion engine; in fact, early turbochargers were referred to as exhaust-driven superchargers. However, modern use of the term supercharger usually involves a compressor that is driven off of the crankshaft, either using pulleys and a belt or a direct shaft. These designs draw power from the crankshaft, often in the range of 30 percent of the engine’s output at maximum boost. A turbocharger has no direct connection to the engine, but rather uses the force contained in the exiting exhaust to drive the charge air compressor, usually drawing only 5 to 10 percent of engine output.

Turbo Basics

A turbochrager is actually an assembly, consisting of several components. The two primary components are the turbine, which is driven by the exhaust, and the compressor, which pressurizes the intake. A common shaft, supported by a center-mounted bearing assembly, joins the two. The turbine and the compressor are similar in configuration, though one uses airflow to generate rotary motion while the other uses that rotary motion to generate airflow.

Looking at the photos, it’s easy to see why turbochargers are often called “snails” or “hair dryers.” The housings for both the compressor and the turbine are similar, shaped somewhat like a donut with an offshoot stemming from its outer diameter. The turbine housing is usually constructed of cast or ductile iron, allowing it to cope with the intense heat and pressure developed as the exhaust is driven in from the engine. Inside is a small wheel with fins or blades that fit very close to the inside of the turbine housing without actually touching it. The incoming exhaust blows into the “offshoot” portion of the housing and is routed against the fins, driving the wheel and then exiting through the center outlet of the housing. Because the air enters at the outer diameter of the housing and flows over the spinning turbine wheel toward the center exit, it is classified as a centripetal, radial-inflow device.

The turbine wheel mounts on one end of the common shaft and drives it. The shaft passes through the bearing assembly, which is lubricated with pressurized oil from the engine, and continues to the compressor impeller, which is surrounded closely by the compressor housing, usually made of aluminum. Since the compressor wheel is driven from an outside source (the turbine), it generates airflow rather than reacting to it. Air is drawn in through the center inlet of the housing and forced out through the offshoot, which is plumbed to the engine’s intake tract. The path of the air through the compressor classifies it as a centrifugal, radial-outflow device.

As the throttle is opened, the amount of air ingested by the engine increases. This, in turn, increases the flow and temperature of exhaust out of the engine, which acts to increase the speed of the turbine, simultaneously increasing the speed of the compressor. With increased compressor speed comes increased airflow into the engine, and the process continues until engine rpm stabilizes to match throttle opening, as during cruising, or until the turbo’s output is curtailed, which can be controlled through varying means that we will discuss in the following section. The turbine wheel and compressor impeller are sometimes referred to as the rotating team, and this assembly can accelerate quite rapidly. Where a positive-displacement supercharger accelerates in direct proportion to the rpm of the engine it’s mounted to, a turbo is more load-sensitive, and can accelerate much faster than the engine powering it, often reaching speeds in excess of 100,000 rpm.

The output of the compressor is generally referred to in terms of boost or positive manifold pressure-the increase in pressure created in the intake tract, usually measured in pounds per square inch (psi). European-designed turbo systems may express boost as units of “BAR,” each one equaling one unit of barometric pressure, or 14.7 psi. The amount of boost generated depends on a number of factors, including the size of the turbine housing, the size of the compressor housing, and the shape, angle, and arrangement of the fins on the turbine wheel and on the compressor impeller. Most turbochargers are designed as modular assemblies, so that compressors and turbines can be combined in widely varying pairs to suit the applications they’re coupled with. Altering the specifications of the turbine and/or the compressor alters the performance characteristics of the turbocharger assembly, in turn altering the performance of the engine to which it is attached.

Other terms used in describing turbo performance include turbo lag-the time lapse between opening the throttle and realizing boost. It is during this period that the turbo is said to “spool up,” a term used to describe the acceleration of the compressor to a point where boost is realized.

Boost Control

Since the output of a turbocharger climbs at a rate that is greater than, and not proportional to, engine rpm, boost levels in many applications can quickly exceed the limits of the engine. There are some systems that produce just enough airflow to suit the engine-a relationship known as a “floating match”-but in most automotive applications, some form of boost control must be employed. Many early automotive turbo systems used variable restrictions in the inlet path to diminish boost after reaching the desired level, but most modern turbochargers accomplish this using a wastegate, a device that controls the speed of the turbine. The wastegate uses a valve to bleed exhaust pressure from the turbine housing and divert it into the exhaust system after the turbine. It can be a separate unit, mounted just ahead of the turbine inlet, or it can be an integral part of the turbine housing. The point at which the wastegate opens is most commonly controlled by a diaphragm assembly, which references intake pressure. When the pressure reaches a preset amount, the diaphragm overcomes the spring tension holding the valve shut, thus opening the wastegate and dumping some of the exhaust pressure from the turbine. This slows the turbine, thereby limiting the speed of the compressor and its output.

On factory-built turbocharged vehicles, the manufacturer determines the amount of maximum boost, and the wastegate diaphragm is preset accordingly. Aftermarket turbocharger assemblies often use adjustable wastegates, controlled manually via a set-screw or electronically, with a cockpit-mounted remote. Some factory-built computer-managed vehicles control the wastegate electronically as well.

The blow-off valve is often confused with the wastegate, although its function is quite different. A blow-off valve is actually a particular type of pressure-relief valve. When a turbocharged engine is accelerated, the exhaust pressure accelerates the turbo, driving up the level of boost. However, when the throttle is released suddenly-a situation known as throttle-drop-the turbo continues to spin, since it isn’t directly connected to the engine. This means that boost is still being developed, even though the engine isn’t being called on to use it. Under these circumstances, the boost charge slams into the closed throttle blades, reverting back towards the compressor. This back-pressure stalls the compressor, so when the driver gets back in the throttle, there is no boost, and the turbo has to spool up to boost rpm all over again.

A pressure-relief valve alleviates this situation by releasing the residual pressure developed by the free-wheeling compressor during throttle-drop. Usually, intake manifold pressure is sourced to control another diaphragm-activated valve. When the throttle is released, manifold pressure goes from positive (boost) to negative (vacuum), and the valve opens, dumping the excess boost pressure. You’ve likely heard this occur at a racetrack or on the street; the sound is a sporadic chirping or a sharp, hissing burst.

Charge Air Coolers

One of the negative side-effects of turbocharging and supercharging is the heat it generates, much of which winds up in the intake charge. The friction created between the intake air and the compressor impeller is partially responsible, while the rest is simply the nature of compressed air-it gets hot. This heat counters the desired effects of forced induction, as it compromises the density of the air fed to the engine. Since the purpose of forcing the intake charge is to get more oxygen into the combustion chambers, the quality of that air will also have a profound effect on the engine’s output.

Encompassing all of the more familiar terms used in this area, like intercooler, aftercooler, and so on, charge-air coolers were developed to combat the heat by-product of forced induction. The basic function of the CAC is simply to remove as much heat from the intake charge as possible without posing a significant restriction to airflow from the compressor to the intake manifold. Still, even the most efficient CACs restrict airflow slightly, costing a couple pounds of boost. However, the power increase resulting from the cooler charge air should more than make up for it on systems producing at least 10-psi max boost.

There are two main types of charge-air coolers: air-to-air and air-to-liquid. Both designs appear similar to conventional coolant radiators: a core made up of many small passages covered with cooling fins and bookended by a pair of thanks. An air-to-air design passes ambient air over the core to pull heat from the compressor charge passing through it. An air-to-liquid design uses a small coolant flow system, often incorporating an electric pump to circulate the coolant through the charge-air cooler core to a separate heat exchanger, usually mounted in the front of the vehicle so that it can receive ambient airflow.

Each has its advantages. An air-to-liquid unit might seem more effective, but its pump imposes some parasitic draw (significant even if it’s electric), and its efficiency is limited by its coolant capacity and dissipation rate of its heat exchanger. The coolant can become saturated with heat if the heat exchanger can’t remove it fast enough, at which point the charge-air cooler is rendered ineffective.

Air-to-air units don’t have to rely on coolant, but usually need to be physically larger than a comparable air-to-liquid cooler to achieve the same effects, which isn’t always feasible, depending on the packaging of the host vehicle. Where packaging permits, air-to-air designs are often favored, as the single heat exchange of this type makes for a simpler system.

In either case, the benefits of cooling the compressor charge are significant-for every 100 degrees the temperature of the air is reduced, its density rises by 12 to 13 percent; a more relevant relationship may be that for every 10 degrees of temperature reduction, horsepower will increase by approximately 1 percent. As a bonus, cooling the intake charge often allows for more boost to be used before encountering detonation.

Turbo Selection/Tuning

Understanding how a turbocharger functions is but one step toward engineering an effective engine package. We consulted with the turbocharging experts at Gale Banks Engineering for more insight on the subject. Banks has been developing and applying turbocharger technology to gasoline and diesel engines in street, strip, salt and marine applications for decades.

First of all, the commonly held belief that turbochargers provide “free” horsepower is false. Banks points out that a typical engine operates using four strokes, and only one of them actually produces power. Although the turbine is driven off of exhaust flow using energy that would otherwise be wasted, it still imposes a drag on the host engine in the form of an exhaust restriction. This restriction makes it harder for the pistons to evacuate the cylinders during the exhaust stroke, as the exhaust flow is working against the turbine. At the same time, the increased cylinder filling makes it harder for the pistons to squeeze the mixture on the compression stroke.

The key is to make up for these added pumping losses elsewhere by selecting the proper compressor/turbine combination. Accurate selection takes into account a dizzying array of factors too vast to describe in detail here. However, Banks was able to provide us with an overview.

To arrive at a proper turbine/compressor team for a given combination, he recommends selecting the compressor first based on the engine it will be applied to, the boost level desired, and the engine rpm at which this boost should be available. Turbo manufacturers can provide compressor maps-graphs charting the output of a particular unit-to assist in the selection process. Here’s a general guideline: A larger compressor takes longer to spool up, but moves more air once it does. A smaller unit reaches boost levels quickly, but also reaches its maximum output limit sooner.

Proper turbine selection is also important. As turbine wheel size grows, so does spool-up time, though back-pressure diminishes. A smaller turbine can reduce lag, but will have a lower choke point-the maximum level of exhaust flow it can handle before becoming a major restriction to exhaust flow, thus hindering evacuation of end gas from the cylinders. We briefly mentioned turbo lag previously, a term that is often misused, according to Banks. While it is true that the turbine and compressor must accelerate before boost is made, this generally happens very quickly, as throttle opening creates a sudden rush of exhaust flow. The “lag” felt is more often the effect of a poorly engineered system. A frequently overlooked facet is the proximity of the compressor outlet to the throttle. The closer they are, the quicker the boost will occur when the throttle is opened.

Engine Selection/Tuning

In the old days, planning a turbocharged engine focused largely on whether to use a blow-through system, where boost is fed into the carburetor, or a draw-through system, where the compressor inlet sucks air through the carburetor. Frankly, both have been made obsolete by today’s availability of electronically managed fuel-injected engines. In fact, many of the specialists that made those configurations work will agree with this viewpoint today. Fuel-injection alleviates most of the difficulties posed by combining carburetors with boost, and modern electronic controls offer the versatility and adaptability that eluded tuners in the past.

Virtually all turbocharged EFI systems are blow-through, and in fact, most lend themselves to this configuration, as the inlet to a typical EFI air intake almost seems made to accept a connection to the outlet of a compressor housing. In addition, the air metering systems incorporated in most EFI systems can usually “read” boost and adjust fuel delivery accordingly (to a certain point), even though the OE manufacturers of many of these engine packages probably never intended the use of positive manifold pressure.

Though this may seem like a match made in heaven, the engine specifications are still a major contributor to turbo effectiveness. As in the past, compression ratio is a primary consideration. The standard rule says to keep it between 8:1 and 9:1 for a street-going, pump-gas application, which would accept up to about 15 pounds of boost. More compression or more boost can induce detonation-the evil limiting factor in nearly all turbo engine packages. To avert detonation, timing advance should be less aggressive than a typical naturally aspirated curve; the general rule is to retard the timing 1 degree for every 2 psi of boost. Another feature of many electronically controlled engines is automatic spark retard based on knock sensor input. These sensors are mounted to the engine block, usually threading into the cooling jackets, and are calibrated to detect the vibration that results from detonation before it is audible to the driver. This can save an engine that isn’t tuned quite right, but Gale Banks points out that the amount of retard the system will dial in upon detecting knock is substantial, which can kill power. It’s better to run a little less initial timing and leave the knock sensor to deal with unusual circumstances.

Another major factor is camshaft profiling, with much of the attention focusing on the amount of overlap. A big, lumpy hot rod cam is likely to have lots of overlap, which can diminish the effects of boost. During overlap, both valves are open, allowing the incoming boost to go right out the exhaust, which diminishes potential cylinder filling. Some overlap is beneficial, as the incoming boost can help to evacuate the remaining end-gas left over from the previous exhaust stroke. Besides providing more space in the cylinder for the intake charge, eliminating end-gas also lowers the temperature in the cylinder, allowing more boost before reaching detonation.

The potential of turbo boost can be awe-inspiring, as we’ve seen recently in certain forms of drag racing, but the topics covered here merely scratch the surface of the subject. Don’t let this intimidate you; experts in the field have done the hard part, all you have to do is seek them out to boost your own potential.

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