Predicting automotive technology trends isn’t an easy job for suppliers, who also must focus on how the trends will impact their product lines. But the global trend to engine downsizing has given Honeywell Turbo Technologies a clear picture.
The company projects an upward curve for its signature product line, growing from almost one quarter of global vehicle production today to over 40% in four years. And by 2020, turbo penetration is expected to exceed 66% worldwide.
The increase is tied to regulatory demand for improved fuel economy but also to customer expectations of performance. In the U.S., where the turbo market share increased from 6% to 10% in the past year, it is expected to reach 23% by 2016. This compares with 56%—primarily diesels—in Europe. Few naturally aspirated diesels remain in production for mainstream automotive use, and indeed Honeywell's newest design is for an Audi A6/A7 diesel. So the greatest amount of future growth will be for gasoline engines.
The overall benefit of downsized, boosted engines is evident in this recent performance comparison: a 2.0-L turbocharged four-cylinder develops the same 200 hp (149 kW) rated output as a naturally aspirated 3.0-L V6, while delivering at least 20% better fuel economy. Complementary technologies such as direct injection (DI) and stop-start also benefit fuel efficiency, but the turbo certainly is the key addition for engine performance. Indeed, there are 2.0-L DI turbocharged gasoline engines producing up to 274 hp (204 kW).
The lighter weight of turbocharged four-cylinder engines vs. larger displacement V6s also enables a reduction in vehicle mass, further contributing to fuel economy.
According to a Honeywell study, the average U.S. passenger-vehicle engine in 2011 is a 3.3-L V6. In Europe the typical engine is a 1.8-L four (including diesels); in China it's a 1.6-L four. Turbo penetration in China is approximately the same as in the U.S., also due to that market's lower diesel penetration than in Europe.
Integration is critical
But for turbo suppliers (Honeywell is the world's largest), the issue goes deeper—namely what must be done to advance the turbo technologies in ways that will meet OE requirements and be cost-competitive in the marketplace. According to Honeywell's Mike Stone, Application Engineering Leader for gasoline turbos, there is no single approach to turbocharger design nor an area of the design that is escaping engineering attention.
Speaking at the annual Test Days program of the International Motor Press Association (IMPA), at the Monticello (N.Y.) Motor Club track facility, Stone noted that turbocharger systems are now part of the initial engine design, rather than being a performance add-on. He described some early industry approaches as "just make it [the turbo unit] physically small and let the wastegate get rid of any excess pressure."
Today's configurations range from the simple single turbo with a wastegate to relieve excess exhaust pressure (including single separate turbos for each bank of a V-type), to integrated parallel and series flow two-stage sequential flow designs feeding a single intake manifold.
There are also dual-scroll designs, including those joined to split-interior-path exhaust manifolds to smooth and speed exhaust flow. And since 1990, variable nozzle technology (variable vanes for the turbine) has provided even greater opportunity to tune turbos for specific engines.
Honeywell's newest design, the TwoStage Module for the Audi A6/A7 3.0 diesel V6, is also its most compact. It’s a series-flow type, in which a small turbo is joined to a larger turbo. A bypass (control) valve enables the two turbos to operate together or in single low-pressure stage mode. This provides good response throughout the rpm range as well as low emissions.
The TwoStage is sized and shaped so it packages neatly in the engine valley. The smaller (high-pressure) turbo has a variable nozzle and is mounted transversely. The larger (low-pressure) turbo is mounted longitudinally, so the TwoStage was a challenge for integration, castability, and overall manufacturability.
Thus equipped, the Audi engine produces a claimed 313 hp (233 kW), a 30% increase in power vs. the baseline 3.0-L diesel V6. That's about the same power as a 30% larger displacement engine but with 10% better fuel economy.
Ford's 6.7-L V8 diesel in the 2011 Super Duty truck is another unique design. Its single sequential turbocharger features a double-sided compressor wheel driven by a single variable nozzle turbine. A compact design that is mounted in the engine valley, the Super Duty turbo has the reduced inertia of a single turbine wheel turning a single compressor shaft, so it spools up quickly for fast response.
The primary compressor on the Ford diesel unit operates at low speed, and at a specific rpm or boost pressure, the bypass (control valve) opens and the secondary compressor also comes into play. And because both are on the same shaft, the transition is seamless.
Exhaust temperature challenge
At the IMPA Test Days program, Honeywell had five turbocharged cars for test driving. Two were the single wastegate turbo for gasoline engines, both installed on 1.4-L engines, in a Chevrolet Cruze and an Alfa Romeo Giulietta. There was also a European market VW Golf with a 1.6-L diesel, a Lincoln MKS with a single turbo for each bank of the 3.5-L V6, and a Mercedes-Benz E-350 3.0-L diesel V6 with a single turbo with variable nozzle technology.
Proving the point that no area is escaping attention, the Mercedes unit has a shaft ball bearing instead of a plain sleeve, both for durability and better performance (primarily in transient operation), Stone explained. And although the Cruze turbo is a single scroll, it’s integrated with the exhaust manifold to improve flow characteristics and packaging. The design also eliminates a bolt-together joint, which simplifies assembly.
The search for new turbine materials is ongoing, particularly for gasoline engines which run at higher exhaust temperatures than diesels (approximately 950ºC vs. 800ºC). Ceramics can take the temperatures but they’re brittle, posing a durability problem. And debris particles are more of a problem than with steel.
An “exotic” material, titanium aluminide, is used in some commercial aircraft turbines and some race car turbos, particularly those used by Porsche. The lightweight alloy enables the turbo to spool up faster and also is both heat and corrosion-resistant. However, it has never been used in mass production cars.