Is Less More? - four-cylinder turbocharging engines

Automotive Manufacturing & Production, May, 2001 by Christopher A. Sawyer

The Folks From Garrett think it is, but automakers and their customers may need more coaxing to accept Four cylinder engines that are more efficient than, and outperform, V8s.

Once the darling of autornakers 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 over-speeding 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.