The Science Of Boost

it's a simple concept that drives the design of any reciprocating internal combustion engine chasing after more power combine as much oxygen as possible with an appropriately matched quantity of fuel compress it to its limits and ignite of the two components of combustion fuel is far easier to add in quantity due to its density the supply of oxygen however is limited by the much lower density of air as well as an engine's ability to efficiently induct air through the inherent restrictions of the intake tract into the partial vacuum created within its cylinder this ratio of air fuel mass captured by the cylinder during induction to the air fuel mass displaced by the cylinders volume at ambient air density is known as volumetric efficiency on older or less refined engines this efficiency can get as low as 75 percent however in more advanced designs the resonance of the intake manifold and the mass of the air at a specific rpm range is used to achieve pressures greater than the atmosphere at the intake valve pushing volumetric efficiencies up to 110 percent in some race engines even the inertia of exhaust gases exiting a cylinder can be tuned to aid in increasing volumetric efficiency by design reciprocating engines are air pumps they compress the aspirated air fuel charge ignite it convert this expansion of hot gases into mechanical energy and then expel the cooler lower pressure gases the amount of energy converted is determined by the pressure exerted on its pistons by combustion and the length of its expansion cycle by increasing how aggressively a given mass of air fuel charge is compressed higher combustion pressures are achieved allowing more energy to be extracted and thus creating more mechanical power output the measure of how much an engine compresses its intake volume is known as its compression ratio and it's a key parameter of its thermal efficiency as the compression ratio rises more power can be extracted from combustion however how high this ratio can be taken is limited by the properties of the fuel used as air is compressed rapidly within a cylinder its temperature rises for certain fuels like gasoline this hotter air fuel charge may cause pockets of air fuel mixture to ignite outside the envelope of normal combustion this is known as detonation and it dramatically decreases thermal efficiency and can cause engine damage because of this limitation most modern gasoline engines hover around compression ratios of 10 to 1 to 12 to 1. engines on the higher side of the spectrum tend to require higher octane rated gasoline which lowers the tendency to detonate other fuels that are more resistant to detonation allow for more efficient higher compression ratios in their respective engines ethanol and methanol engines for example can safely operate at 14 to 1 to 16 to 1 ratios while diesel engines which rely on the temperatures created by compression for ignition can run at ratios as high as 23 to 1. limited by the properties of fuel atmospheric pressure and intake restrictions making more power would come down to either making an engine larger or operate at a higher rpm though these options are easily hampered by a combination of cost reliability and practicality however early on in the history of the internal combustion engine a novel idea started to emerge by adding an external air pump to feed an engine the properties of an intake charge could be modified before it even enters a cylinder this concept of forced induction allowed for cooler air that is pressurized higher than the atmosphere to enter the cylinder eliminating the limitations of natural aspiration in 1859 two brothers philander higley roots and francis marion roots founded the roots blower company in connersville indiana one year later they would be granted a patent for their air pump design known as the roots blower while the roots blower was initially designed for pumping air into blast furnaces used to melt iron and mine ventilation in 1900 gottlieb daimler of daimler benz was the first to fit a roots blower known as a supercharger onto a four-stroke engine while a handful of race cars utilizing route superchargers would be built over the next two decades it would not be until september 23rd 1921 at the berlin motor show that the mercedes 620 hp and 1035 hp the world's first series built supercharged passenger cars were announced these vehicles were designated compressor models a mercedes-benz badging signature that continues even today root superchargers operate by pumping air with a pair of meshing lobes resembling a set of stretched gears the incoming air is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust side of the blower they're typically driven from the engine's crankshaft by a toothed or v-belt roller chain or gear drive roots blowers are a part of category of devices known as positive displacement pumps these deliver a nearly fixed volume of air per revolution at all speeds making the volume of air pumping into the engine predictably linear with the rpm of the engine while they are ideal at moving large volumes of air they do not directly compress it as the blower transfers air at ambient pressure into the cylinder the intake charge begins to pressurize within the cylinder itself this characteristic is known as external compression because rotary lobe pumps need to maintain a critical clearance between the lobes root superchargers can only pump air across a limited pressure differential due to the expansion of the lobes from the heat of compressing air they also pump air in discrete pulses potentially causing turbulence downstream while variants of the design that change the quantity shape and twist of the lobes attempt to address these issues they generally begin to drop off civilian efficiency at around one bar of pressure above the ambient ear or one bar of boost root superchargers typically operate most efficiently at moderate speeds and low boost in 1935 swedish engineer alf lifesome patented a new air pump design as well as a method for its manufacturer that improved upon the limitations of the roots blower based on an old german patent from the 1870s lysome had replaced the lobes with screws creating the rotary screw compressor in a rotary screw compressor based supercharger or twin screw supercharger two very closely meshing asymmetric helical screws known as rotors are used to compress air as the meshing rotors force air from the intake end through the screws the interlobe volume between the male and female rotors decreases along the length of the rotor until the exhaust port this change in volume compresses the air because twin screw superchargers compress air without the housing and do not rely upon downstream resistance to increase pressure they are characterized as internal compression devices furthermore the air compression process occurs in a continuous sweeping motion creating very little pulsation or surging by design they also have far lower leakage levels and parasitic losses allowing them to operate more efficiently across a broader range of speeds and at higher boost levels than route superchargers despite these advantages only a handful of production vehicles have ever been fitted with a twin screw supercharger their complex design makes them far too expensive for mass production when compared to alternative forms of available forced induction during world war ii a completely new type of supercharger had evolved from an 1899 pump design known as a centrifugal compressor initially used on piston aircraft to increase power at high altitudes where the air is thinner they have successfully migrated to the automotive world centrifugal superchargers are a type of dynamic compressor they work by spinning an impeller at a high speed to draw air into a compressor housing called a volute when air leaves the impeller it travels at high speed and low pressure through a diffuser and out the housing this rapid slowdown of the airflow as it moves towards the outlet of the supercharger causes it to pressurize centrifugal superchargers typically contain a transmission that provides a step up ratio from the engine driven input shaft to the impeller because impeller speeds can exceed 100 000 rpm high speed load must be taken into consideration in their design and manufacture because centrifugal superchargers utilize centrifugal forces to compress the air they offer a higher efficiency over positive displacement superchargers both in terms of power consumption and heat production they're also typically mounted off to the side on the front of the engine reducing heat transfer from the engine to the supercharger during operation while other less popular designs have been attempted such as the power plus sliding vane superchargers of the 1930s and volkswagen's notoriously unreliable g ladder scroll type superchargers of the 1980s variants of the roots type and centrifugal superchargers dominate the mass production automotive market forcing more air into a cylinder with boost easily creates more power in an engine by increasing the air mass of the intake charge beyond what is possible with natural aspiration this also inherently pushes volumetric efficiency well beyond 100 however with forced induction the new problem emerges a pressurized charge within a cylinder combined with too high for compression ratio dramatically increases the likelihood of detonation this is further exacerbated if the incoming air is heated from being compressed for detonation sensitive fuels such as gasoline this can be addressed by a combination of reducing the compression ratio and the maximum amount of boost allowed most gasoline engines for example are designed with compression ratios between 8 to 1 and 9 to 1 when operating under forced induction while this reduces the overall thermal efficiency of the engine the power output is increased due to the pure increase in air mass during combustion because forced induction occurs outside of the engine the properties of the air mass can be further enhanced by cooling by passing the compressed air through a heat exchange device known as an intercooler by lowering the intake air charge temperature under constant pressure it becomes more dense aside from the increased volumetric efficiency of denser air the lower temperature intake charge also significantly reduces the danger of detonation this now permits operation under higher levels of boost and greater power output in addition charge cooling by using wasteful fuel enrichment under certain engine conditions can also be reduced intercoolers come in various packages and configurations and can vary dramatically in size shape and design depending on performance and space requirements while most operate by air cooling some use water as an intermediate heat exchange fluid coupled with a remote radiator this is usually employed to accommodate space restraints roots and twin screw type superchargers for example often rely on water cooled into coolers due to their close proximity to the engine's intake track other systems such as centrifugal superchargers and turbochargers allow for more flexibility due to their mounting distance from the engine this allows larger more efficient air cooled intercoolers to be placed easily within an airstream permitting higher boost operation when the efficiency of forced induction devices are compared two contributing metrics are considered the first being the mechanical efficiency or how much engine power is consumed for the net power gained and the second being the device's adiabatic efficiency or the measure of its ability to compress air without adding excess heat to it in the world of superchargers centrifugal compressors dominate in overall efficiency but still an efficiency bottleneck amongst all supercharger designs remain present because superchargers are mechanically driven by the engine the load they present greatly reduces their efficiency in some extreme cases it can take as much as one third of the base engine's power to drive the supercharger in order to produce a net gain in power however by combining the adiabatic efficiency of a centrifugal compressor with an indirect drive mechanism that taps some of the wasted thermal energy of an engine a massive boost in efficiency as much as 35 percent can be achieved with the turbocharger the first turbocharger design was patented in 1905 by swiss engineer alfred buhe he had conceptualized the compound radial engine with an exhaust-driven axial flow turbine and compressor mounted on a common shaft initially called a turbo supercharger due to the use of a turbine the goal of these early designs was to help overcome the power loss experienced by aircraft engines due to the decreased density of air at high altitudes though his initial prototype proved to be unreliable attempts by others such as renault demonstrated their effectiveness for aircraft use by 1925 buhi began demonstrating commercial success with turbocharging on large diesel engines particularly those in marine use by world war ii turbochargers were routinely added to piston aircraft engines to boost their high altitude performance however it would not be until the 1960s for the technology to make its way into the automotive world early attempts at their use in the 1950s proved to be problematic due to drivability issues and packaging finally in 1962 the first turbocharged production car engine the osmobile turbo jetfire was released in the osmobile jetfire and later in the chevrolet corvair monza spider though it produced significantly more power over the natural aspirated engine reliability issues led to a cease and production of the engine just one year later by the late 1970s following dramatic changes in legislation and design goals due to the 1973 oil crisis and the 1977 clean air act manufacturers started to further explore turbocharging as a method to increase power and reduce fuel consumption and exhaust emissions turbochargers work by converting the heat and kinetic energy contained within engine exhaust gases as they leave a cylinder this is done by directing the hot gas flow through a radial inflow turbine spinning it anywhere from 20 000 rpm to almost 300 000 rpm in bursts radial inflow turbines work on a perpendicular gas flow stream similar to a water wheel and are generally simpler to manufacture and are more efficient at the power levels seen in most engines for engines that operate in the realm of thousands of horsepower axial turbines tend to be used over radial turbines due to their diminishing efficiency the turbine is directly connected via a shaft to a centrifugal compressor that operates similar to a centrifugal supercharger this shaft is housed within the center section of a turbocharger known as the sensor hub rotating assembly the center section of a turbocharger is by far the most challenging element of its design not only must it contain a bearing system to suspend the shaft spinning at hundreds of thousands of rpms but it must also contend with the high temperatures created by exhaust gases in automotive applications the bearing system found in most turbochargers are typically journal bearings or ball bearings of the two journal bearings are more common due to its lower cost and effectiveness it consists of two types of plane bearings cylindrical bearings to contain radial loads and a flat thrust bearing to manage thrust loads these bearings utilize hydrodynamic lubrication from oil pressure between the moving surfaces to prevent metal to metal contact this is similar to the bearings used within the engine itself on higher end turbochargers ball bearings generally contained within a cartridge unit are used these bearing assemblies typically consist of two rows of angular contact ball bearings to handle both radial and thrust loads because ball bearing rotating assemblies use less oil less energy is lost from oil shearing making them superior in efficiency and rotational acceleration these losses can reach as high as 3 horsepower in rpm ranges beyond 200 000 rpm in both bearing configurations it's common to use engine coolant to maintain a stable thermal environment in the sensor hub rotating assembly for proper bearing operation the volume of air compressed in a turbocharger is directly related to its rpm and because its energy comes from gas flow and not mechanical drive like a supercharger there is a delay between the engine's increase in rpm and the turbine's increase in rpm this is known as turbo lag furthermore the efficiency of a turbocharger is not constant and can vary considerably depending on operating conditions such as speed and load due to these characteristics the performance response and efficiency of a turbocharger is tailored to the application through a careful balancing of its size shape turbine and compressor a larger turbocharger for example typically provides top-end power and higher airflow potential while a relatively smaller turbo exhibits quick spooling characteristics at the expense of lower overall airflow at higher engine speeds two key properties that determine the turbocharger's performance are turbine aspect ratio this is the ratio of the area of the turbine inlet relative to the distance between the centroid of the inlet and the center of the turbine wheel and compressor trim this is the relationship between the compressor wheels inducer and exducer diameter in general increasing either metric will yield higher flow capacities but at the cost of slower response time and greater lag whereas reducing them will increase response time but at the cost of less efficiency and performance stalls at higher speeds particularly with lower aspect ratio turbine housings on both the compressor and turbine even curvature and the number of blades of the wheels can also affect the characteristics of turbocharger performance the performance characteristics of both supercharger and turbocharger compressors are represented by a compressor map compressor maps plot the flow rate against the ratio of air compression revealing both the limits of the compressor as well as its region of peak efficiency known as an efficiency island compressor maps aid in matching the properties of a particular forced induction device to the design goals of an application with turbochargers the goal is generally to size the constituent components in order to achieve the target boost pressure within the efficiency island with as minimal lag as possible because turbochargers are effectively free-floating they are not mechanically constrained to safe operating limits like a supercharger in order to prevent safe pressures and speeds from being exceeded a mechanism called a wastegate is employed wastegates work by opening a valve at a predetermined compressor pressure that diverts exhaust gases away from the turbine limiting its rpm in its most common form wastegates are integrated directly into the turbine housing employing a poppet type valve the valve is opened by boost pressure pushing a diaphragm against the spring of a predetermined force rating diverting exhaust gases away from the turbine because the pressure of exhaust gases may overcome the wastegate spring opening the wastegate too early a compromise between wastegate diaphragm size and spring force must be chosen in order to reduce premature opening yet still activate when intended this is particularly important for maintaining consistent boost during partial throttle operation on larger turbochargers that require more flow and a more assertive valve action larger wastegates tend to be fitted external to the turbocharger this is generally done to accommodate the packaging constraints of the larger spring and diaphragm used with the proliferation of computer engine management electronically controlled pneumatic valves that adjust the pressure feeding a wastegate's diaphragm would be added this is known as electronic boost control and it allows for engine management to modify the wastegate's action controlling boost electronically in some contemporary turbochargers fully electronic wastegate actuation have been employed for more direct and precise control while superchargers by design do not require a wastegate to operate a bypass mechanism that diverts air around the compressor section is sometimes employed for better fuel efficiency at partial throttle on engines with throttles such as gasoline engines a sudden closing of the throttle plate with the turbine spinning at high speed causes a rapid reduction in air flow beyond the surge line of the compressor on a compressor map above this line is a region of unstable flow where the compressor blades experience an aerodynamic stall causing a violent reversal of airflow as higher pressure intake air back flows into the compressor as the intake pressure drop and the forward flow is reestablished the cycle repeats itself this repeated high-speed cycling known as turbo flutter causes a cyclic torque on the compressor and may lead to damaging increased stresses on the bearings and compressor impeller to prevent this a device known as a blow-off valve or compressor bypass valve is fitted between the turbocharger compressor outlet and the throttle plate when the throttle is closed the pressure downstream of the throttle plate drops below atmospheric pressure and the resulting pressure differential is used to actuate the blow-off valve's piston the excess pressure from the compressor is then either vented into the atmosphere or recirculated into the intake upstream of the compressor inlet preventing compressor surge and damaging flutter early on in the use of forced induction especially in piston engine aircraft it became common to employ multiple force induction devices configured in a manner that would overcome each device's limitation two staged superchargers that had selectable boost for different altitude levels were found on some world war ii aircraft engines others combined a technique known as twin charging where a supercharger was used at lower altitudes and a larger turbocharger at higher altitudes twin charging started to appear in commercial automotive use during the 1980s with volkswagen being a major adopter of the technology in its most common configuration a supercharger would feed directly into a larger turbocharger the supercharger provides near instant boost eliminating turbo lag and once the turbocharger has reached operating speed the supercharger boost is either compounded or bypassed the concept of twin charging can be implemented with two turbochargers known as a twin turbo configuration in vehicles this is typically executed as a sequential twin turbo where a smaller turbocharger is compounded with a larger one to reduce overall lag yet still benefit from a larger turbocharger's capacity staged twin turbo configurations where the smaller turbocharger outlet feeds the larger turbocharger are usually found on piston aircraft where lag is not an issue other common configurations of multiple turbochargers are bi-turbo systems where two equally sized turbochargers each feed a bank of cylinders or the entire engine in parallel this allows smaller turbochargers to be used reducing overall system lag on larger engine configurations such as v12 and w16 engines quad turbo parallel systems have also been implemented within the last 15 years the use of forced induction by automotive manufacturers has been growing rapidly this appeal to the technology is a direct result of advancements in engine management with technologies such as throttle by wire direct injection and variable cam timing smaller more fuel efficient engines would now be paired with forced induction with simple mechanical boost control replaced by sophisticated and highly integrated engine management systems manufacturers could now augment the power characteristics of a smaller engine with forced induction simulating the feel of a larger more powerful engine while still retaining the fuel efficiency and better emissions characteristics of a smaller engine the efficiency and versatility of turbocharging in particular has led to several notable developments that further enhance their all-around performance one notable example is the twin scroll turbocharger which operates on pulse turbocharging twin scroll turbochargers have two exhaust gas inlets that feed two gas nozzles one directs exhaust gases to the outer edge of the turbine blades helping the turbocharger to spin faster reducing lag while the other directs gases to the inner surfaces of the termite blades improving the response of the turbocharger during higher flow conditions such as under engine acceleration the two streams are separated by engine firing order allowing only matching exhaust gas pulses to share a stream eliminating pressure interference between cylinders this results in twin-scroll turbochargers having higher turbine inlet energy due to this exploitation of pressure waves making them more efficient and far more responsive at lower engine speeds however the inherent restrictiveness of the design does compromise high load performance making tight design integration critical to their use variable geometry turbochargers are another example of turbocharger development they generally work by allowing the effective aspect ratio of the turbocharger's turbine to be altered as conditions change because the optimum aspect ratio at low engine speeds is very different from that at high engine speeds the technique allows for increased boost pressure at low engine speeds while improving the response time of the turbocharger during transient engine operation phases they work by using movable components within the turbine housing to provide a variable cross-sectional area in order to modify the turbine's aspect ratio while variable geometry turbochargers started to appear on a handful of vehicles during the late 1980s they tend to be more common on diesel engines due to the lower exhaust temperatures and their ability to function as an exhaust break variable geometry turbochargers also do not require a wastegate as boost pressure can be regulated directly by turbine aspect ratio implementing a variable geometry turbocharger system is relatively complicated as coordination between the most optimal aspect ratio position and engine operating state must be tuned very precisely to benefit from their use this tends to limit their use to automotive manufacturers due to the development costs associated with them other noteworthy techniques used to enhance turbocharger characteristics especially in performance use are methanol or water injection where the air is cooled as the injected liquid evaporates cooling the combustion chamber and inhibiting detonation in a manner similar to a higher octane fuel allowing for both higher engine compression and more boost and anti-lag systems where the ignition timing is purposely delayed during momentary throttle closure allowing a small amount of fuel air charge to escape through the exhaust valves and combust within the hot exhaust manifold spooling the turbine reducing lag during throttle closure purely electric anti-lag systems that utilize an electric motor to maintain turbo shaft speed are also in development for production use in 2019 36 percent of the vehicles sold in the u.s used forced induction in europe the adoption rate was well over 70 percent and while more than 90 percent of these sales were turbocharged diesel engines a historical strong point of forced induction stricter government regulations for fuel economy and emissions targets as well as the general consumer demand for higher engine performance coupled with significant technical advances in drivability are expected to contribute to more widespread consumer adoption of turbochargers and superchargers especially within the emerging u.s market you

Channel: New Mind

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Keywords: turbocharger, turbocharger vs supercharger, roots blower, roots supercharger, roots supercharger install, twin screw supercharger, centrifugal supercharger, centrifugal supercharger vs turbo, turbo flutter, wastegate, wastegate vs bov, turbo aspect ratio, turbo aspect ratio explained, compressor trim, turbo trim and a/r explained, turbo trim explained, blow off valve, variable turbo geometry works, variable turbocharger, variable turbo geometry, twin scroll turbo, anti-lag

Id: gN10vCR-tPY

Length: 29min 14sec (1754 seconds)

Published: Tue Dec 01 2020