Once the darling of automakers looking for a quick and dirty hit of horsepower, turbocharging may be on the cusp of a renaissance in North America. A renaissance that, its supporters say, will couple ideas like air bearings, variable nozzle technology, and electric drive with up-to-date engines and electronic controls to eradicate that old saw, "There's no replacement for displacement."
"We have shown automakers that it's possible to replace a 4.0-liter V8 with a 2.5-liter four cylinder in an SUV, while getting better fuel economy–about 30% better–and the same performance," says Joe Panella, Garrett Engine Boosting Systems' director of Global Commercial Vehicle Product Marketing. Most of the time, he says, the average engine is operating under part- or closed-throttle conditions, and carrying the pumping losses and weight associated with multiple cylinders when not under load. Replacing the power produced by these "extra" cylinders with energy produced by channeling waste gasses through a turbocharger can–with the right choice of technologies–decrease fuel consumption by as much as 30% while producing the same or better horsepower and torque. Or so the claim goes. So far, however, he says that the response from the OEMs has been lukewarm.
One reason for this, Panella suggests, is the need to compete on a cylinder-for-cylinder basis in the market. "Automakers think it's going to be hard to sell customers on the concept of a four that can keep up with an eight," he says. "They're worried that the smaller engine will appear to be highly stressed and too complex, which may raise reliability and durability questions in the consumers' mind." That and the fact that in the light truck market, the number of cylinders is inextricably linked to manhood and off-road ability. "Yeah," Panella sighs, "that probably has something to do with it too."
Freed of these restrictions, however, an installation of the type Panella describes would liberate underhood space, a precious commodity, and help lower overall vehicle weight. "Imagine an SUV that was both space- and fuel-efficient," he says. "Now try imagining one that met performance and cost targets too."
Varying the charge
There are a number of ways Garrett claims to be able to meet the OEM's cost and performance needs. At its most basic, the process would start with the addition of a variable nozzle turbocharger (VNT) to the downsized powerplant. By varying inlet geometry, VNT turbos can increase boost at low engine speeds, and match it to demand without overspeeding the turbine at high exhaust flow levels.
Currently, Garrett has two styles of VNT turbos in production. The first uses multiple airfoil-shaped vanes arrayed around the inner circumference of the exhaust side of the turbo housing. Each pivots on its axis in response to either vacuum actuation or from commands sent by the engine management system to a rotary electric actuator. The latter gives the engine computer precise control over vane position, boost pressure, and turbine speed. Currently, BMW's 740D uses this system.
The company's second VNT design casts the vanes as part of the housing, and uses a piston to vary the width of the chamber. Like the pivoting VNT design, the idea is to provide optimal flow to the turbocharger regardless of engine speed. Restricting the passage at low speeds increases exhaust velocity through the scroll, augmenting boost pressure. At higher engine speeds, the piston increases the nozzle width to balance exhaust flow and boost.
Further enhancements in performance and efficiency can be found through the use of a twin-scroll exhaust nozzle. In a single-scroll design, exhaust gasses are combined and fed into the turbo. As engine speed rises, back pressure increases, and exhaust gasses are drawn into the combustion chamber where they mix with the intake charge. In a twin-scroll design, however, cylinders are paired, and their exhaust released into a dedicated entry. "This way," says Paolo Carmassi, product line director for Garrett Engine Boosting Systems, "you don't have a build up in back pressure affecting the intake charge for the other set of cylinders."
Air not oil
And while a twin-scroll design can help simplify an OEM's emission strategy, the oil used to lubricate the turbo's bearings causes other concerns. "In the 1980s, coking [turning the thin oil film on the shaft bearings into an abrasive solid through hard use and inadequate cooling] was a problem in certain, poorly designed applications," says Panella. "Properly integrating the turbocharger into the overall system takes care of that. Plus, the turbo unit can be water cooled, if necessary, to keep temperatures below the critical level." That's only half the problem. It is the migration of minute amounts of oil from the spindle bearings into the intake charge, especially on low-emission vehicles, that is of greatest concern to some potential turbocharger customers.
"That's where air bearings come in," smiles Robert Gillette, president of Garrett Engine Boosting Systems. "Preventing oil from entering the intake charge is important in any gasoline or diesel engine, but vital in low emission vehicles. Any amount of oil in the system potentially could foul the system."
Air, the lubricating medium, is forced into a reservoir surrounding the bearing. Here it passes through precisely metered holes into a narrow gap between the bearing bore and shaft, and this air curtain separates and cools the two surfaces. With an air bearing, frictional drag is lessened, bearing life increased, and higher shaft speeds are possible. "Balance is critical to an air bearing in order to prevent metal-to-metal contact," says Gillette, "but turbochargers already require exceptional balance due to the high speeds they run. The other advantage we have is that we can draw on [parent company] Honeywell's experience with the air bearings it uses in aircraft environmental units."
It's all in the package
Another important benefit of air bearings is packaging, especially the ability to mount the turbo at any angle. "With oil-fed bearings," says Carmassi, "the turbo housing must be mounted so that the spindle joining the impellers is horizontal. Air, on the other hand, flows in any direction without starving the bearing surface of lubrication, which makes packaging much, much easier." This means the turbo can be mounted to the exhaust manifold, close to the engine.
"A turbo soaks up a lot of heat," says Gillette. "Therefore the catalyst can be moved closer to the manifold without having to worry about it exceeding its optimal operating temperature.
That makes for a major improvement in both underhood packaging and catalyst durability." This ability to soak up heat may become even more important as automakers struggle to reduce hydrocarbon emission limits. Under full-load conditions, engines soon will be run at fuel ratios that remove the cooling effect provided by excess fuel, driving exhaust temperatures up and close-coupled catalyst life expectancy down.
As if this wasn't enough, Garrett has one final trick up its sleeve: electrically assisted turbochargers. An ultra-high speed electric motor/generator drives the turbocharger to provide boost at engine speeds where adequate exhaust flow is unavailable or unequal to the demand. "This means an electrically driven unit can provide boost from a cold start, just off idle, or in transient response conditions, making boost totally independent of engine speed," says Gillette. "Plus you can use the electric motor to eliminate the need for a separate wastegate, eliminate throttle lag, and take advantage of an air bearing's lower friction by using a smaller motor/generator unit."
Says Panella, "Reduced emissions and improved fuel economy are important benefits, but the ability to provide boost just off idle greatly increases torque, and torque is what Americans drive on. Unlike Europeans, American drivers place greater emphasis on the ability to surge away from a stop than on top speed, and this design gives them that response."
Another benefit of electric drive, Garrett claims, is that excess energy can be redirected to the generator side of the system to provide electrical power. "We can use this to provide a source of extremely high voltage for hybrid electric vehicles," says Panella, "or use it as a supplemental 42-volt power source. That's the beauty of ‘air on demand'."
The one question remaining is whether or not customers and OEMs will accept the cost and complexity necessary to do more with less displacement. "Over the past decade, we've seen an 18% reduction in displacement go hand-in-hand with an approximate 18% increase in power output," says Gillette, "and engines have gotten more costly and complex. So the argument can be made that customers and OEMs already have come to grips with this." Perhaps. But it remains to be seen if they will pay the projected $100 to $200 premium for electric drive with all the goodies.
Belting Out the Boost
Eaton's largest supercharger customer today is Mercedes Benz. The company offers supercharged engines in its SLK, CLK, C-Class, and E-Class lines. GM is another large customer with supercharged variants of the Buick Regal and Park Avenue, and the Pontiac Grand Prix and Bonneville. Meanwhile, Ford offers the SVT Lightning—a pumped-up F-150—and Jaguar the XKR sedan and XJR coupe. Even BMW's Mini division will join the fray with a supercharged Mini Cooper S. "We're working on approximately 20 new programs, including a number of diesel applications," says Streeter. But perhaps the greatest growth, at least in North America, will come in the light truck market.
Nissan's supercharged Frontier and Xterra are the first light truck applications (other than the limited edition Ford Lightning) to offer a supercharged engine as an option, but they doubtless will not be the last. "In the past, supercharging has been a pure power play," says Streeter, "but we see a definite move toward keeping a competitive power level while offering better fuel economy. Americans love torque, and smaller engines don't provide the performance buyers expect. Plus, a supercharger lets you get the torque up while running a (numerically) lower gear ratio for fuel economy." As for how quickly this shift will happen, Streeter says that depends on changes in CAFE legislation.
Like a turbocharger, a supercharger is under boost only 5% to 10% of the time. At full boost, parasitic losses can reach 40 kW (depending upon the size of the unit, of course), dropping to less than 0.5 kW in non-boost conditions. But a supercharger has a few advantages over a turbo, says Streeter. "First, it doesn't soak up exhaust heat and delay catalyst light-off. Second, you can use it as an air pump on cold start. Third, you can choose one unit for a family of engines, and use different drive pulleys to alter the boost level—something I think we will see much more of. Fourth, it is emissions neutral, though you also can use it to put EGR back into the intake charge and homogenize the mixture. If there is a drawback, it's in having to pick up a [drive] belt line, but if you plan for a supercharger from the start, it's a clean installation." Packaging room, he claims, can be saved by integrating the unit into the intake manifold, like Cadillac did on its Evoq concept car.