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DPNA295.qxp 2/23/07 2:38 PM Page 1
ENGINE TECHNOLOGY
A cutaway of the supercharger used on the Kawasaki250X personal watercraft, along with a schematic of theengine system. Two air bypass valves balance air flowupstream, downstream or wastegate air under variousconditions of load and speed.
SUPERCHARGERS COMINGBACK INTO FOCUS?With engine efficiency demands growing and new design concepts, superchargers are getting another look
BY ROB WILSON
When the subject is superchargers, the kneejerk response by motorheads far
and wide is “parasitic losses.” Thatsimple phrase often puts consideration of superchargers to rest in thecurrent era where emissions, efficiency and fuel economy are thedrivers for engine manufacturers.But new technology and more efficient designs seem to be promptinganother look.
The Roots blower design on whichmany superchargers are based, actually predates the internal combustionengine, and these devices were beingused to optimize blast furnace outputin the 1870s. Uses have aboundedover time.
Twocycle diesels, of course, usethem to scavenge exhaust gas. Foraggressive air handling in tight spaces,the reputation of Roots blowers wasnever in dispute.
The same can be said for high performance and for power density. In
many ways, superchargers were justthat — super.
So perhaps it comes as no surprisethat for the 2007 model year, Kawasakiis launching a highperformance personal water craft (PWC) called the Ultra250X that derives some of its “hair onfire” capability through a supercharger.The Rootsblown, 1.5 L, inline fourcylinder, intercooled gasoline engineproduces 250 hp remarkable for anengine that size. That’s 166 hp per L,which is nothing short of astonishing.
Kawasaki claimed the 250X is themost powerful PWC in the world. Thesupercharger produces large boosteven at low rpm, the company said,and acceleration from idle is not hampered by turbo lag.
The blower for the 250X forcefeeds2 L of air for every revolution, yieldinghighpressure intake air at all enginespeeds and a flat, beefy torque curveover the operating range. Air pressureis managed, balanced upstream ordownstream or dumped under various
conditions of engine speed and loadby two air bypass valves.
The Roots blower used in thisapplication is produced by Ogura
nSomerset, N.J.
“The Kawasaki application is certainly high profile and Ogura is excitedto be a part of it,” said Frank Flemming,president of Ogura Clutch Co. “Withthe 250X, the use of supercharging isstrictly performance driven and theresults speak for themselves.
“High performance has also characterized most applications for supercharging but there is an evolution happening where other factors are nowcoming into play. And the supercharger we are bringing to the market todayhas far more flexibility and capability.”
Jim Ebisawa, engineering managerat Ogura Clutch Co., pointed out thatthe Ogura TX series has adoptedWankel rotary geometry design,which increases the working or
XX DIESEL PROGRESS NORTH AMERICAN EDITION March 2007 March 2007 DIESEL PROGRESS NORTH AMERICAN EDITION XX
Clutch Co. in Japan, with U.S. headquarters of Ogura Industrial Corp. i
DPNA295.qxp 2/23/07 2:38 PM Page 2
ENGINE TECHNOLOGY
Ogura has adopted Wankeltype geometry to improve volumetric efficiency. Thetwin counterrotating rotors have acrosssection shaped like a figure eight.The rotors are hollow from end to end forlow inertia.
sealed displacement of the pumpingarea and also improves volumetricefficiency of the blower over the operating range.
Ebisawa said the TX volumetric efficiency advantage over other Rootsblower designs can be more than20% at low speed and 10% on the topend. The design has fewer parts andis lightweight throughout its design.
Twin counterrotating rotors blowing the air within the superchargerhousing are hollow from end to end,resulting in a very lightweight design,reducing their moment of inertia andthus the energy required to makethem rotate.
The TX series superchargers aredesignated by their theoretical discharge per revolution. Thus the smallest, the TX04, has a discharge volume of 0.4 L/rev, while the largest, theTX28, has a discharge volume of 2.8L/rev. The smallest weighs 11.6 lb.and the largest 31.3 lb.
Maximum continuous speeds forthe TX series range from 9000 to12,000 rpm and the maximum pressure ratio is given at 1.8:1.
It happens that Ogura Industrial isactually better known for its capabilities in clutches, but it has combinedthis capability with the Wankel supercharger design, and this is whatresults in a completely new tool for engine aspiration.
“On the Kawasaki 250X jet ski application, the only interest is high performance and so that unit does notincorporate a clutch,” said Flemming.
“There, the supercharger is driven allthe time, but that need not be the case.
“For most of the pilot programs andinquiries we are currently pursuing, aclutch is certainly a big motivator ofthe interest level. Almost all of oursuperchargers have electric clutchesmounted on the front end and theengine management system turns thepower on and off as needed.”
This capability is probably the realgame changer. When torque is not required, the supercharger is switchedoff in milliseconds and only a belt andidler pulley remain as parasitic losses,very small ones at that. At any rate, thedrive belt is there driving other accessories. When not required, the supercharger is completely out of the circuit.
To give an idea of response time,engagement time is about 50 ms onthe smallest superchargers, on thelargest about 50 ms. Disengagementis 30 to 50 ms for all models.
When torque is required, the engine management system cues theelectromagnetic clutch to pick up thelow inertia supercharger, producing aburst of power on demand.
Where Flemming and Ebisawa seethe best fits are:
• vehicles requiring high torque atlow speed, with stopandgo driving, hilly and poor road conditions;
• wherever large engine performance or burst power is requiredfrom a small engine and whereweight limitations might call for asmall engine;
• in combination with turbocharg
ing on larger engines to optimizelowspeed and highspeed performance;
• as an additional, fast acting, airhandling device to help emissioncompliance, improve fuel economyor both.
Ogura has been working on development programs for diesel and gasoline engines, mainly with engine displacements smaller than 5 L, but a fewupwards of 10 L.
One program involves the KAD42Volvo Penta marine diesel. Poweredby a turbocharged 3.6 L engine, theboat simply couldn’t get up on planedue to low torque at low speed. Theaddition of the Ogura supercharger tothe 3.9 L diesel raised the torque at100 rpm by a factor of three and theboat came up on plane. At 2500 rpm,the turbocharger takes over and theboat remained on plane.
Europe is described as somewhatahead in active development programsfor supercharger application. There isalso greater recognition in Europe thatsupercharging has great potential forcoming emissions hurdles. Better combustion on diesel engines at lowerspeed is of particular interest.
And perhaps it can now be calledadaptive supercharging. Why not?The modus operandi is completelychanged. Parasitic losses are nowfiltered out when the superchargeris disengaged. Now the supercharger can bring a lot more air into playon demand without the traditionalturbo lag.
Or maybe the requirement is formore cool air to keep combustion temperature in the correct zone whereNOx generation remains under control.
For many compelling reasons, itseems the time for reexamination ofsupercharger technology is at hand.The potential is much changed. dp
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XX DIESEL PROGRESS NORTH AMERICAN EDITION March 2007 March 2007 DIESEL PROGRESS NORTH AMERICAN EDITION XX