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How Turbocharged Piston Engines Work
II. Moving Air...
III. Forced Induction
IX. Multiple Turbos
Turbocharging is perhaps the best way to increase the performance of a reciprocating internal combustion engine, gasoline or Diesel. While the basic concept is simple, the subtleties of a turbocharger system are actually very complex. In this section I will do my best to describe how and why turbocharged engines work.
A piston engine is an air pump. It draws air into its cylinders by creating a lower than atmospheric pressure (vacuum) inside the cylinders, by motion of a reciprocating piston. It then traps that volume of air, compresses it with the piston, mixes it with a measured and proportiate amount of fuel, and burns the fuel and air mixture to release energy in the form of heat. Then, the same piston is used to convert the heat energy into mechanical energy by way of a connecting rod and crankshaft. Spent exhaust gases are then expelled by the piston to make room for a fresh charge of fuel and air. As anyone in the business of tuning engines knows, the amount of power an engine can make is relative to the amount of air it can flow into its cylinders, and so, the way to make more power is to flow more air into the cylinders. This can be done in essentially three ways:
The first way is to increase the physical volume of the cylinders, or the displacement. By having larger cylinders, more air will flow into the cylinders. This is how power is made in a large displacement engine such as the overhead valve V10 found in the Dodge Viper sports car. The ten massive cylinders in the Viper engine displace a total volume of 8.3 liters, allowing the big and heavy engine to produce 500 horsepower in factory trim, at a relatively slow engine speed.
2. High RPM
The second way is to increase the ability of an engine to rev higher. By spinning fast, the pistons cycle more often, and more air can flow into the cylinders per unit time. It is in this way that an 8 cylinder 2.4 liter Formula 1 engine, revving at close to 20,000 rpm, can produce around 750 horsepower. Though the F1 engine's displacement is around 3.5 times less than that of the Viper, by spinning around 3.5 times as fast, the Formula 1 engine can flow just as much air as the big Viper engine. The fact that it is a racing engine with better volumetric efficiency and higher compression allows it to make even more power with the same level of theoretical airflow.
3. Increased Volumetric Efficiency
The third way to increase power is to increase an engine's volumetric efficiency. VE is the measure of how much air actually makes it into the cylinder on an intake stroke compared to the actual volume of the cylinder. For example, a cylinder may displace 1 liter. That does not mean, however, that on the intake stroke, 1 liter of air actually makes it into the cylinder. Due to many factors, including friction, backpressure, scavenging, and engine speed, the cylinder does not fill up fully with charge. A typical engine may have a peak VE of around 70-75%. A higher performance tuned engine may have a peak VE of 80-85%. And by use of high performance cam shafts, combined with careful design of the intake and exhaust passages, a skilled race engine tuner can develop resonant effects at certain rpm ranges to increase the VE to 100% or even slightly higher in race engines, although this almost always comes at the expense of drivability, because the VE increase usually only occurs within a very narrow rpm range, and everywhere else in the rev range, VE is greatly compromised. Honda and other automobile manufacturers have developed variable valve timing and variable intake and exhaust path systems to help reduce this compromise.
There is one other way to increase volumetric efficiency, and to do so dramatically. This method is generally referred to as Forced Induction, and with it, VE's of 100% are academic, and in reality, VE's of 200% or greater are possible. Forced Induction is the act of using an external air pump, or compressor, to force more air into the engine's cylinders than it would otherwise be able to ingest atmospherically. This increase in intake manifold pressure above atmospheric pressure is often referred to as "boost". By boosting the engine's intake charge, or supercharging it, tremendous levels of power can be realized from a relatively small sized engine. This external air pump must be powered somehow, and it typically requires a great deal of power just to drive this pump.
There are two basic ways to supercharge an engine. The first way is referred to as mechanical supercharging, or just simply supercharging. A supercharger comes in two basic forms: the first is a positive displacement blower, often referred to as a Roots Style Blower. And the second form of mechanical supercharger is a dynamic compressor, a high speed centifugal compressor very similar to those seen on many smaller and medium sized gas turbine engines. In either case, the mechanical supercharger is driven directly off of the rotation of the engine, either by belts, pulleys, gears, or a combination thereof. Mechanical supercharging can be a very effective way to pressurize an engine, but it faces one major downside: In many cases, the mechanical supercharger consumes a large portion of the engine's total power output just to drive itself. Increase the boost by increasing blower speed, and the power requirement goes up. Coupled to the relative inefficiency of a Roots style blower, or the exponential boost rise of a centrifugal compressor which leads to a peaky powerband on smaller displacement engines, and the mechanical supercharger can fall short. A third type of mechanical supercharger, referred to as a Lysholm supercharger, combines the immediate boost response of a Roots blower with the high efficiency of a centrifugal compressor. However, it still robs a good amount of power from the engine crankshaft.
The other method of supercharging an engine is referred to as a turbo-supercharger, also known as a turbocharger, or just a turbo for short. The turbo is an exhaust gas turbine driven centrifugal supercharger. In a piston engine, approximately 2/3rds of the heat generated in combustion is normally wasted. A turbocharger places a small radial inflow turbine in the exhaust gas stream to recapture a portion of that energy, and use it to supercharge the engine. The turbine is driven at extremely high speeds by the exhaust energy, and a shaft transmits this rotation to a centrifugal compressor, which is used to boost the engine. Because the compressor is not driven mechanically, but rather is driven by the otherwise wasted exhaust heat, the net power gain can be substantially higher in a properly designed turbo system.
The limiting factor for increased performance in a gasoline engine as brought about by a turbocharger is the level of boost that the engine can take before the increased heat and pressure causes pre-ignition, or detonation, which can destroy an engine in a matter of seconds. Of course, any properly designed turbo system will take a number of measures to minimize the possibility of detonation, at a given boost limit. These will be discussed later. However, all things being equal, there will always be a practical boost limit for any given application. Boost then becomes a function of compressor speed, which itself is a function of turbine speed. Turbine speed is a function of exhaust gas flow combined with exhaust gas velocity at the turbine wheel. Flow is essentially a function of engine power output, but velocity at the turbine wheel can be widely varied as a function of the relationship between the turbine wheel size and the size of the turbine housing. This relationship is known as the Area/Radius Ratio, or A/R ratio. The A/R ratio is the ratio between the area of any given cross section of the exhaust path area through the turbine scroll to the radius measurement from the center of the turbine wheel to the center of that cross section. Regardless of where in the turbine scroll the measurement is taken, the ratio will be the same for a given turbine and housing combination. The larger the A/R ratio, the slower the turbine speed for a given rate of flow. The upside to a larger A/R ratio is reduced engine backpressure, which can improve top end power dramatically.
The simplest turbo system is referred to as a free floating turbo system. In a free floating system, the relationship between the turbine wheel size and turbine housing size is such that compressor speed will reach the point of maximum boost at maximum engine revs and full throttle. At any other point in the rev range and throttle opening, turbine speed, and thus compressor speed and boost will be lower than the maximum. This type of system would theoretically produce the maximum peak horsepower, due to the minimal amount of backpressure, but engine response would be terrible and it would not be considered a drivable engine.
In practice, to make a turbocharged engine more responsive, boost controls must be used. The most common type of boost control is the wastegate. The wastegate is a device which can bypass some exhaust gas around the turbocharger when necessary to limit boost to a pre-set value at a lower engine speed. The key is that the wastegate must be used with a smaller A/R ratio than what would be found on a free floating system. The smaller A/R would mean higher exhaust velocity for a given flow, so the turbo would begin to spool at a much lower rpm. As power and engine speed increases, the flow will increase to the point where maximum allowable boost is reached. However, the engine will still have room to accelerate, which would lead to an overboost condition. At this point, the wastegate will begin to bypass exhaust flow around the turbine, maintaining turbine speed and boost at the pre-set level, all the way up to engine redline. In this way, boost can be achieved at a much lower rpm, which means increased engine responsiveness and torque, combined with high levels of peak horsepower. The downside to the smaller A/R ratio is increased backpressure, which will reduce peak horsepower, and will also increase the heat in the system, which can lower the threshold of detonation. A properly engineered turbo system will find a good compromise between turbine backpressure and turbocharger responsiveness.
A wastegate can be integral to the turbo, or can be a standalone device which must be plumbed into the exhaust system. The wastegate usually operates pneumatically. A pressure line from the compressor discharge is fed to the diaphragm on the wastegate, and a spring tension is set so a certain amount of compressor discharge pressure will causes the wastegate to open, thus limiting the boost to that pressure.
Turbo responsiveness is usually measured in two ways. The first is boost threshold. Boost threshold is the engine rpm point where the turbo begins producing effective boost pressure, at full throttle. Below the boost threshold point, the engine will usually feel a little mushy and unresponsive. Once the boost threshold is reached, the boost will rise toward peak boost and the engine will begin producing significantly more torque. A well designed wastegated street turbo system designed for high power output may have a boost threshold of around 2,500 to 3,000 rpm. Peak boost will then be produced by around 3,500 to 4,000 rpm, and should hold all the way to redline.
Turbo lag is sometimes confused with boost threshold. While turbo lag is also rpm dependent, it is more of a direct measurement of turbocharger response, as opposed to engine response. Turbo lag is the time it takes for the turbo to spool up from the moment the throttle is pressed to the moment maximum torque at that particular rpm is developed. Obviously, turbo lag widely varies based on engine rpm. For example, if the boost threshold is 3,000 rpm, and you floor the throttle at 1,500 rpm in sixth gear, then turbo lag could be 20 seconds or more, as it will take a long time for the engine to accelerate, off boost, from 1,500 rpm to 3,000 rpm in sixth gear. Conversely, if the engine is at 5,000 rpm, and you floor the accelerator, it may only take .25 seconds for the turbo to spool up again. Perhaps the best way to quantify turbo lag is the slight delay in maximum power delivery that occurs in between full throttle shifts, or when you floor the accelerator to pass someone on the highway. A properly designed turbo system can be both responsive and produce high levels of power, though it is not easy to acheive the right balance.
While the wastegate has been the primary method to control boost in turbo systems for many years, a new technology is coming to the forefront which will revolutionize turbocharging systems, particularly on automobile engines. This is referred to generally as variable turbine geometry. VTG has been used on Diesel engines for quite a while, but the higher exhaust gas temperatures in gasoline engines have precluded their use in that application. For some time, a company called Aerodyne produced a small variable geometry turbine for aftermarket gasoline applications. However, a lack of reliability of the system led to the demise of the concept in gasoline engines until very recently, when Porsche developed the first VTG system in a production gasoline powered automobile, the 911 Turbo (997 model).
VTG works by using movable vanes in the turbine housing, whose pitch can be changed by an actuator. The vanes can allow for a very small nozzle area, (simulating a small A/R ratio) a very large nozzle area, (simulating a large A/R ratio) or anything in between. When cruising at low power and low rpm, the vanes form a large nozzle area, minimizing any restriction in the exhaust stream while the turbine freewheels. At full throttle and low rpm, the vanes snap shut, creating a very small flow path, which increases the exhaust velocity even at low flows, kicking the turbine up to speed at very low rpm, lowering boost threshold and minimizing turbo lag. As engine speed increases, the nozzle vanes open up with engine rpm to maintain a pre-set boost limit. At full throttle/maximum rpm, the vanes may be wide open to control boost. Because of the high A/R effect, the turbo can run with minimal backpressure, which opens up the door to very high levels of power with minimal heat, which also improves reliability and fuel consumption. The VTG turbo is the best of both worlds; ultra fast turbo response and engine response, without sacrificing top end power. The VTG turbo will revolutionize turbocharging on gasoline engines.
The turbocharger's purpose is to compress air and stuff it into the engine. Compressing air always has the unwanted side effect of increasing the temperature of the air. Increasing heat is bad for a few reasons. The first is that it makes the air less dense. The whole point of turbocharging is to increase the amount of air that gets into the cylinders. Compression allows this to occur, by packing a mass of air into a smaller space. However, when the air is compressed, it is also heated, and heating air makes it take up more space again. Fortunately there is a net gain in density; however, it is most desirable to cool the air after compression using an intercooler (sometimes referred to as an aftercooler on a Diesel engine) An intercooler is basically like a radiator, which removes some of the heat of compression of the charge air before the air enters the cylinders. By removing heat, the density can increase even further, and the air density is the critical factor in making more engine power. The other downside of increasing the heat of the air is that it makes the engine more succeptible to detonation, which is the major enemy of any gasoline turbocharged engine.
There are two basic types of intercooling: Air to air, and air to water. Air to air is the simplest setup, and is basically like a radiator, except that instead of coolant flowing through the core, compressor discharge air flows through. Water to air intercoolers have advantages and disadvantages. Since water has a much higher thermal conductivity than air, the water intercooler can be much smaller in size, and will still have a higher efficiency. Because of this compact nature, the water intercooler can be neatly integrated into the intake manifold of the engine without an increase in engine volume. However, once the water is used to cool the intake charge, it too must be cooled. This is accomplished by using an electric pump to circulate the dedicated water circuit to a radiator, where ambient air is used to cool the water down. Because air is used to cool the water, the overall efficiency of the system is about the same as an air to air intercooler.
There is also the concept of using refrigerant as the intercooling media, or using a refrigerant system to cool the water in a water to air intercooling system. The basic concept is to use an air conditioning system to cool the water to much lower than ambient temperatures to cool the intercooler. In theory the concept should work, although one problem is that running an air conditioning compressor can sometimes rob more horsepower from the engine than it can gain from the charge cooling process. With some effort the system should be made to work. The downside is increased complexity.
There are many applications where a single turbocharger is sufficient to do the job. However, many engine designers have come up with multiple turbo applications. The most common is the twin turbocharger. In a typical twin turbo system, a vee or horizontally opposed engine configuration is used, and one cylinder bank feeds one turbo, while the opposite bank feeds the other turbo. The main advantage in this case is that two smaller turbos can be used for a given amount of airflow, and smaller turbos have lower inertia, and can spool up more quickly. The twin turbo arrangement also eases packaging on these engine configurations.
Taking this concept one step further, some engines exist with a quad turbo system, where four turbos are used. The most notable application of this is the Bugatti Veyron, whose 16 cylinder engine uses four turbos to generate 1,000 horsepower. In reality, the quad turbo arrangement is more of a gimmick than anything else, because those who have driven the Veyron have noted considerable turbo lag, clearly a case of diminishing returns.
Some high performance vehicles have utilized a system called sequential turbocharging, namely the Porsche 959 and the Toyota Supra Turbo. A sequential system typically uses one small turbo and one large turbo. At low engine speed, all flow passes through the small turbo, to spool up quickly and produce boost at low rpm. As the exhaust flow increases, the second, large turbo, is phased in to produce high boost at high levels of exhaust flow. In theory this concept works well, however, the amount of plumbing involved, along with all of the electronically controlled valves to route the exhaust and compressor flow makes this system very complex, and in practice, boost response is no better than a well engineered wastegate system.
Very high performance diesel engines can sometimes use staged turbos to generate very high boost pressures, when a single compressor stage cannot acheive the desired pressure efficiently. In this case, multiple turbos are used in series to do the job. In a simple two stage turbo system, the first, larger turbo (low pressure turbo) will feed an intercooler, which then flows into a smaller high pressure turbo. The high pressure discharge will then feed an aftercooler before feeding the Diesel engine.
The most extreme example is a three stage turbo diesel tractor pulling engine. The first stage actually uses twin parallel turbos to provide the required airflow. Twin parallel turbos don't provide more pressure, just more airflow. The outlet from the twin parallel turbos (low pressure turbos) feeds an intercooler and is then sent to a single intermediate pressure turbo. The intermediate pressure turbo increases the pressure further, and sends the flow to another intercooler, which then sends the flow to a high pressure turbocharger, which will be smaller in size. The high pressure turbocharger feeds an aftercooler, which then feeds the engine with up to 250 psi of boost pressure. No gasoline engine could withstand this level of boost. With this much boost, a Diesel tractor pulling engine can produce something like 5,000 horsepower from a basically stock tractor block.
Detonation, sometimes referred to as engine knock, is a condition where the fuel air mixture in the cylinder explodes instead of burning in a controlled fashion. In normal gasoline combustion, the spark plug ignites a corner of the fuel air mixture, and then the resulting flame propagates through the combustion chamber, burning at a controlled rate. Detonation, on the other hand is a situation that can occur where the mixture either starts burning at the spark plug but then suddenly all burns at once, or can also occur as preignition, where a hot spot or other condition in the combustion chamber causes the mixture to burn or explode even before the spark plug fires. The result of the explosion is a dramatic spike in combustion pressures that can burn a hole in the piston, break a connecting rod, blow a head gasket, or even launch a cylinder head off the block.
Detonation is bad for any engine, but is a particular problem in high performance turbo engines. The possibility of detonation must be minimized in the following ways:
High Compressor Efficiency- A highly efficient compressor reduces heat from compression. A cooler charge is less likely to detonate.
Intercooling- An intercooler is essential to reduce charge temperature on any high performance turbo system
Reduced Static Compression- Due to the compression of the turbo, engine compression ratio must be reduced to bring the overall compression ratio to a safe range, which will reduce charge temperatures in the cylinder. A typical compression ratio in a high performance turbo street engine is around 9:1. Non turbo engines can go as high as 11.5:1 on the street.
Higher Octane Fuel- Octane is, by definition, a fuel's resistance to detonation. Higher octane means less likelihood of detonation.
Properly Programmed Fuel Injection with Adequate Fuel Delivery- Electronic fuel injection is, in general a much better choice for a turbocharged engine, because it is better able to handle the wide range of fuel flows required by a turbo engine. A turbo engine needs a commensurate rise in fuel flow to match the huge increases in airflow, and the injection system, from the fuel pump to the injectors, and everything in between, must be capable of providing the full range of fuel flows. Not only does the engine require fuel enrichment to maintain a stoichiometric flow of fuel, but it also should provide additional fuel enrichment at high levels of boost to remove heat. A turbo engine on full song should be running with a rich mixture of around 12:1. The excess fuel does not burn but rather is used to cool the charge to fight detonation.
Electronic Ignition with Proper Timing- Electronic ignition should go hand in hand with EFI, and should be used to increase power by maximizing the ignition timing on the threshold of detonation at all points in the rpm/load range. Like EFI, electronic ignition will be able to optimize engine control for all engine conditions, whether at idle, off boost cruising, or high load high rpm (boosted operation).
Knock Sensor- Most modern engines, turbocharged or not, have a device called a knock sensor, which literally listens to the engine for the sound of detonation, and when it detects the knocking sound, will retard ignition timing and possibly enrichen the fuel mixture to reduce knock. In a turbocharged engine, the knock sensor is even more important, and it call also be made to reduce boost pressure to avoid detonation.
Electronic boost controls- Electronic boost controls can allow boost limits to be varied slightly depending upon certain operating parameters. This means that the engine can be operated closer to its detonation threshold without fear that high ambient temperatures, or a lower octane fuel will cause the engine to melt down. The end result is better performance. Electronic boost control most often interacts with the wastegate actuator to control boost. Most commonly, a weak wastegate spring is used, and a solenoid cuts the pneumatic signal to the wastegate actuator until the boost limit is reached, and then the solenoid opens to allow the the pressure signal to open the wastegate. Some wastegate actuators use an electric step motor, and can be controlled directly electronically.
A turbocharger is probably the most effective way to increase the performance of a given displacement engine, and will yield the highest specific output and best power to weight ratio. While there are some compomises in utilizing a turbocharger to boost performance, a properly designed turbo system will maximize performance while minimizing the compromises. Indeed, turbocharging a gasoline engine is truly an art form, and is seldom done correctly. A properly designed high performance turbo system must almost definitely feature some form of boost control, either wastegate or VTG, an intercooler, a highly efficient compressor, and electronic fuel injection and ignition. These systems will allow for the least amount of compromise in drivability.
While turbocharging a gasoline engine is an art form, turbocharging a Diesel engine is actually easier and more commonplace. Diesels are much better suited to turbocharging for a number of reasons. First of all, exhaust temperatures on Diesels are much lower, so there is less wear on the turbine. Second of all, a Diesel is designed to withstand high compression, and because fuel is injected at top dead center and burns as it is injected, detonation is not a concern, so very high boost pressures can be used, while also maintaining the high static compression ratios of a Diesel engine. Unlike a gasoline engine, which is throttled and uses either a carburetor or electronics to maintain the air/fuel ratio within close parameters, Diesels do not have a throttle and can run at a wide variety of mixtures. A turbocharger allows a Diesel engine to operate at very lean mixtures throughout the power range, with plenty of excess air. The excess air keeps the exhaust cool and assures that all of the fuel is burned completely. A turbocharger improves a Diesel engine's performance in every way. It increases power output, decreases weight for a given power output, improves fuel economy, reduces smoking and other emissions, and reduces noise. There are very few modern Diesel engines that are not turbocharged.
Modern high performance turbo Diesel engines are beginning to make their way into high performance areas like automobile racing and general aviation which used to be the domain of the gasoline engine only. In fact, the Zoche Aero-Diesel engine, which was designed for general aviation applications, is quite an impressive adaptation of high pressure turbocharged Diesel engines to this engineering field. The Zoche Aero-Diesel ZO 02A model is designed to run on jet fuel, and the largest model produces 300 horsepower from 5.3 liters and weighs 271 lbs. This engine proves that Diesels can be lightweight and powerful, with the aid of turbocharging.
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