The Ward's 10 Best Engines competition celebrates 13 years of recognizing outstanding powertrain development. In this sixth of a series, Ward's explores Toyota's 3.5L V-6, which packs a punch with performance and an innovative fuel injection strategy. Watch for features on the other 2007 winners throughout the year.

Toyota Motor Corp. engineers admit the company raised a few eyebrows when rumors began buzzing about the No.1 Japanese auto maker's newest 3.5L DOHC V-6, the 2GR-FSE, a variant of Toyota's widely deployed GR-series engine family.

The clamor focused on the new engine's industry-first combination of direct-injection gasoline (DIG) fueling with conventional port fuel injection.

The application of twin injection systems generated a lot of “What are those guys thinking?” sort of speculation, says Daniel Yerace, senior principal engineer-engineering design, powertrain, at Toyota Technical Center U.S.A. Inc.

But what might appear to be an extravagant redundancy is the result of careful research focused on the design goals for the engine, launched in late 2005 for the '06 Lexus IS 350 and the winner of two consecutive Ward's 10 Best Engines honors.

Toyota engineers in Japan say they were searching for the best methods to increase performance while simultaneously reducing emissions. Much of the industry is migrating to DIG fueling as a way to improve performance and fuel consumption.

Although DIG, by itself, has been effective in achieving those ends, Toyota says conventional direct-injection fueling also forces certain compromises the auto maker couldn't accept.

According to a Toyota engineering spokesman in Japan, “Most traditional (gasoline direct-injection) systems use high amounts of charge motion to mix the air and fuel to a homogeneous mixture.

“The charge-motion generation devices required for this mixing (charge-motion flaps or a swirl port or tumble intake port) tend to limit volumetric efficiency and wide open throttle engine performance. Additionally, on start-up, (DIG) engines typically produce a spike in emissions due to the use of a (required) high-pressure fuel pump.”

Unhappy with those concessions, research showed combining DIG fueling with familiar indirect port fuel injection (PFI) could help maximize the benefits of DIG without curbing volumetric efficiency or causing high levels of cold-start emissions.

The twin-injection design is dubbed D-4S, a variation of Toyota's base D-4 engine architecture in use since 1996 for 4-cyl. engines and later migrated to inline 6-cyl. and V-6 engine families. The new “S” nomenclature denotes “superior” for the twin-injection D-4S system.

“The D-4S system addresses these issues (reduced volumetric efficiency and excess cold-start emissions) by delivering fuel in a more evenly distributed spray pattern in the cylinder precluding the need for high levels of charge motion,” the Toyota spokesman says.

“The cylinder-head designer could then focus on optimizing the intake port for maximum flow to achieve high engine performance. (And) by using the port fuel injection during the engine-cranking portion of engine startup, the emissions spike noted with other (DIG) engines can be reduced significantly.”

Twelve fuel injectors — a DI injector and a port injector for each of the engine's six cylinders — is the stuff of an injector-supplier's dreams, but the data indicate there's a meaningful gain from the novel fueling design.

Engineers indicate brake-specific fuel consumption improves about 4% compared with a comparable DI-only engine, and start-up emissions are reduced by about 20%.

Yerace says the PFI system typically is used by itself only during the actual cranking part of the engine startup phase. He adds that this is when a substantial portion of the “raw” emissions of a cold start are produced, which for DI engines typically is exacerbated by the emissions “spike” during cranking that comes from immediate delivery of highly pressurized fuel.

At most other times, sophisticated engine management mapping controls the intricate interplay of the DI and PFI systems to achieve the best results depending on a multitude of variables.

But during high-load operation, only the DI injectors are used, maximizing DI's ability to deliver a cooler, denser air/fuel mix that promotes maximum power and torque.

At 306 hp, the 2GR-FSE is one of the highest specific-output normally aspirated V-6s available in the U.S., while delivering (in the IS 350) city-cycle fuel economy of 21 mpg (11.2 L/100 km) and a combined-cycle rating of 24 mpg (9.8 L/100 km).

The city figure ties Audi's A6 (DIG 3.2L DOHC V-6) for class leadership, and the combined fuel-economy number ties BMW's 330i (3L DOHC I-6) — yet the 2GR-FSE handily outpowers and out-torques both competitors.

Toyota engineers credit suppliers, particularly in the cylinder head and fueling areas. Making contributions were Yamaha Motor Co. Ltd., Aisin Industry Co., Nippon Soken Inc. and Denso Corp.

For now, the D-4S and its novel fueling arrangement will remain the province of premium-brand vehicles, Toyota sources say. But will the auto maker eventually move all its engines at least to DIG fueling?

“Currently, DI fuel systems have a much higher cost than conventional port fuel-injected systems,” the Toyota spokesman says.

“As with most technologies, greater adoption is usually accompanied by a reduction in cost based on economies of scale. As the cost of DI comes down, it would be reasonable to assume that availability of DI systems would be more widespread. Future emissions and fuel economy requirements could speed up the implementation of DI — but again, the customer needs to see a benefit and be willing to pay for this benefit.”

Meanwhile, although in other markets Toyota offers DIG engines that operate on higher-efficiency stratified-charge combustion, engineers also say all Toyota engines in North America that currently use DIG technology operate on homogenous air/fuel mixtures, largely because of past concerns regarding gasoline quality.

“With the proposed changes to corporate average fuel economy standards, these concerns may need to be revisited” to leverage the enhanced fuel economy that stratified-charge operation can generate, the spokesman says. Typically, however, engines that operate on air/fuel mixtures “leaner” than stoichiometric produce excess oxides of nitrogen emissions — a concern for the U.S., which has the world's strictest limits for NOx.

Toyota engineers say they decided to use the novel twin-injection design about 24 months before production of the 3.5L 2GR-FSE began in August 2005, and corporate approval for the engine came about 18 months prior to production.

For the foreseeable future, Toyota sources say high cost makes it unlikely the twin-injection D-4S system will be employed for engines in vehicles wearing anything other than the Lexus badge.

But today's 2-time Ward's 10 Best Engines honoree doesn't necessarily represent the end of the 2GR-FSE's development: The twin-injection technology is compatible with other fashionable engine enhancements, say Toyota engineers, including turbocharging.

So there may be intriguing companions for the economy and power-enhancing D-4S system yet to come.

ENGINE SPECS

Toyota Motor Corp.

3.5L DOHC V-6

Displacement (cc): 3,456

Block/head material: aluminum/aluminum

Bore × stroke (mm): 94 × 83

Horsepower (SAE net): 306 @ 6,400 rpm

Torque: 277 lb.-ft. (376 Nm) @ 4,800 rpm

Specific output: 87 hp/L

Compression ratio: 11.8:1

Assembly site: Kamiga, Japan

Application tested: Lexus IS 350

EPA fuel economy, city/highway (mpg): 21/28