The Garret Aviation VNT-25 The idea of forced air induction by turbine, orturbo, is not new and has it’s mass production roots in WWII fighter planes.What is new, however, is its application to passenger automobiles. Unlike a nearconstant high RPM fighter engine, an automobile requires wide-open throttle (WOT)power availability throughout its entire operating range. Previous automotiveturbo applications acted like an on-off power switch with a five second delay,decreasing drivability, rather than providing the smooth linear powerband of anormally aspirated engine. Because the turbine is in a fixed position in theexhaust stream, it was plagued with sometimes uncontrolled production from thecompressor at high engine speeds, commonly referred to as boost creep, and asignificant decrease fuel economy versus a similar, but naturally aspiratedengine. The Garret Aviation produced VNT-25 solved all of these problems withits innovative Variable Nozzle Turbine. Hands down it is the most advanced turboever mass-produced and it was the first of its kind on production cars.
One ofthe most talked about problems with turbo charged engines is the lengthy time ittakes for the turbo itself to accelerate to operational speeds. This is commonlyreferred to as turbo lag or turbo spool up time. Under WOT, turbo lag results ina seemingly underpowered engine that suddenly comes to life as a delayed tiremelting rush of acceleration.
Previously, turbo lag was limited by decreasingthe size of the turbo itself. This resulted in lower rotating mass and moreimportantly, a smaller cross sectional area, which accelerated exhaust gasses atlower engine speeds. Although the turbo is able to spool quicker due to itssize, for the same reason its ability to move and compress large amounts of airefficiently is significantly reduced. Inherently a smaller turbo will produceless maximum horsepower than if it were replaced by larger turbo on the sameengine.
Previous turbochargers also used a fixed position turbine that poweredthe centrifugal compressor directly. Because the turbine is located directly inthe exhaust stream, the turbine is a huge exhaust restriction. This restrictioncreates a constant exhaust backpressure that decreases fuel economy even whenthe turbo is not in use. At high engine speeds, the restriction creates enoughpressure in front of the turbine (back pressure) that the wastegate can nolonger limit turbine power by bypassing the exhaust around the turbine. Theresult is that turbo compresses more air into the engine than is wanted.
Forexample, a turbo was set to produce a maximum 12psi boost pressure, but during aperiod of sustained wide open throttle high engine speeds the turbo is nowproducing 14.5psi of boost and still rising. This unwanted phenomenon is calledboost creep.
The VNT-25 solves all of these problems with an innovative turbinecalled a Variable Nozzle Turbine. Rather than a fixed turbine the VNT-25 uses aring of 12 moveable paddles aligned around a central, but very small turbinewheel. The entire exhaust charge is then directed to the small turbine by thepaddles. Moving the paddles varied the crossectional area that the exhaust mustpass through. When the paddles are nearly closed the exhaust is acceleratedtowards the turbine wheel to increase power. Decreasing the crossectional areaof flow accelerates normally slow, low engine speed, gasses and nearlyeliminates turbo lag while allowing a large and efficient compressor wheel forexcellent maximum engine power. Opening the paddles allowed the exhaust to flowslower and bypass the turbine to limit power.
This unique arrangementsignificantly reduced backpressure, greatly improved fuel economy, and allowsexcellent control turbine power at sustained high engine speed, without the useof a bulky external wastegate. The Garret VNT isn’t without its drawbacks. Inhigh performance applications it is a turbo that has little to be desired. Theengineers of this turbo, in their effort to reduce turbo lag as much aspossible, kept the compressor and turbine as small as possible. The smaller sizeof the turbine and the compressor decreases the size and therefore the weight ofthe turbo internals.
Keeping the weight as light as possible reduces rotationalinertia to an absolute minimum, which results in a much more responsive turbo.Because the exducer, that is the compressor, is of a compressor type,operational speeds are very high. It is not unlikely for a VNT to reach maximumoperational speeds of 173 thousand revolutions per minute even though resting or”cruise” speed of the turbine is only 2000-6000 RPMs. It is thislatency of the turbo to accelerate to operating speeds that is referred to asturbo lag. Although the small size of the turbine is ideal for a moderateperformance car, its size is a handicap in racing situations.
Inherent with asmall compressor is its ability to quickly reach operating boost pressure. Thisdoes not come with out a penalty. Effectively this small compressor tradesefficiency for speed. As any gas is compressed the temperature of it rises.Smaller compressors will tend to heat the compressed air more than would alarger turbo for a given pressure.
Bernoulli’s principal states that as a gas iscompressed the temperature increases as the volume decreases. The inefficiencyof the VNT at pressures over 15 pounds per square inch increases the temperatureof the gas more than it is possible for it to compress, or decrease the volume.The result is that the increase in boost pressure is inversely proportional tothe volume of air moved. As the compressor works to decrease the volume of air,the rise in temperature works to increase the volume. Eventually the volume ofair is expanded by heat more than it can be compressed. The point at which thishappens is referred to as the stall speed.
Because a larger turbo, although slowto respond, is much more efficient at higher pressures it will result in a muchcooler charge at a given pressure. A smaller compressor also cannot move largequantities of air at high pressures as would a larger turbo be able to. The sizeof the VNT, although ideal for 12psi as it was intended for, suffers greatly inhigh performance applications from stall speed of psi. The turbine also suffersfrom a small and compact A/R ratio. The A/R is the ratio at which the turbine orcompressor housing is cast. The A/R is the ratio at which the volume of thehousing as gasses enter the housing to the volume it exits. For instance, thesize of connection on the intake side of the compressor is two and one quarterinches inside diameter and has a volume of 323 cubic centimeters until itreaches the compressor.
The exit side is also two and one quarter inches insidediameter and contains a volume of 155cc’s. The volume of each path to thecompressor is misleading and cannot be determined from the diameter of the exitor intrance alone. The intake passage is a direct and simple path to thecompressor cartridge. The exit, however, is fluted from the from a very wide andnarrow, almost rectangular, passage at the side of the compressor to a standard2 inch inside diameter round pipe fitting. This fluted shape insures that thespeed of the compressed charge is kept relatively high.
The high speed maintainsthat the compressed charge is kept away from the compressor. If it were allowedto back up near the compressor, the compressor would have to work much harder tomove the already dense air. The result would be that the clready compressed airwould be further compressed and heated. Although the small inlet and outletsizes contribute to increased velocity With the introduction of the GarretVNT-25 it is now possible for a small displacement turbo charged engine tooperate and perform nearly identical to a much larger engine. The ON/OFF switchof turbo power is gone and is now replaced by the safer, smoother, and much morelinear acceleration comparable to naturally aspirated engines of much largerdisplacement. A VNT-25 equipped engine also has the potential to, and usuallydoes, produce much more power than engines twice its size. However, withdisciplined drivers, it does not loose the fuel economy characteristics inherentwith small, normally aspirated engines when the turbo is not in use.
The VNT-25combines the responsiveness of a small turbo with the efficiency and performanceof a much larger turbocharger. Simply stated, the VNT-25 is the idealturbocharger. It allows great power almost no turbo lag, great responsiveness,retains engine and compressor efficiency, and allows excellent turbine controlfrom boost creep. BibliographyRalph C. Bohn, Angus MacDonald. Energy Technology.
Fourth Edition, Peoria,IL: Macmillan/McGraw Publishing, 1992. Chrysler Passenger Cars Factory ServiceManual vol.1; Engine and Chassis 1990. www.alliedsignal.com/business/turbo/about_cas.htmlhttp://idt.net/~vnt4/vntrpt.html http://idt.net/~vnt4Physics