The Ward’s 10 Best Engines competition celebrates 15 years of recognizing outstanding powertrain development. In this latest installment of the 2009 series, Ward’s looks at the design philosophy behind Toyota’s 3.5L DOHC V-6.

What can you say about an engine that’s been on the Ward’s 10 Best Engines list four straight years?

“With a delectable balance of power, grace and technology, it is a favorite of several judges and impresses all with its class-leading horsepower, torque and fuel economy,” Ward’s editors exclaimed after testing the Lexus IS 350.

The engine’s most interesting feature is a unique and happy marriage of two complimentary fuel-injection systems. A conventional 45-psi (3-bar) port-injection system dominates at start-up because it’s more effective for that condition, while a separate high-pressure direct injection system takes over at full load for maximum performance.

“We’re always using the direct injection,” says Paul Williamsen, national manager of Toyota Motor Sales U.S.A. Inc.’s Lexus College, the brand’s in-house technical-training arm. “But there are a few conditions where we need to supplement that with the port injection to get ideal combustion.”

There are two different U.S.-market versions of Toyota’s 3.5L V-6. A port-injected version powers a wide range of front-wheel-drive Toyota and some Lexus models, while this dual-injection version is exclusive to the more expensive rear-wheel-drive Lexus IS and GS 350.

Williamsen says both versions are members of a diverse family of V-6s that range in displacement from 2.5L to 4.0L. A 4.0L version is used in RWD Toyota trucks, a 3.0L version in the GS 450 hybrid-electric vehicle and a DI 2.5L version in the U.S.-market IS 250. The 4.0L arrived first in Tacoma and 4Runner trucks, and the others soon followed.

“One top priority for the direct-injected Lexus versions was specific output,” Williamsen says. “At 3.5L, it’s putting out 306 hp, or 87.4 hp/L. That’s a pretty high number for a non-forced-induction engine.” The 204-hp DI 2.5L approaches that at 81.6 hp/L.

Another important priority, not surprisingly, was fuel efficiency.

When the engine was introduced, “Lexus didn’t have the benefit of 4-cyl. hybrids to bring up its corporate average fuel economy number, so these engines had to be run as lean as we could get them and still meet those performance targets,” Williamsen says.

That is one reason why there are two different injection systems. The 2.5L is able to meet all its needs through pure direct injection; the 3.5L – because of its larger port diameters and some other issues of cylinder filling – had to use port injection plus direct injection.

Two other priorities were low mass and minimal friction. “Mass was especially critical because we knew from the outset the 3.0L version would be in the GS 450h, where every component had to be light because we were adding a lot of components to make it a hybrid,” Williamsen says. “All the engines in this family benefit from very low-mass construction.”

The family also was optimized for minimal friction, and it’s the first time Toyota has begun to switch over to zero-weight (0W-20) motor oils. “We did not have zero-weight oil when we introduced these engines,” Williamsen says.

“We don’t require it now, but we started using it in the 3.5L RX last model year, and I expect we’ll change over the rest of the applications in coming years. We are gradually changing over to that as the factory fill.”

A major design challenge – not unusual with a V-engine designed to sit both east/west in FWD applications and north/south with RWD – was packaging. European pedestrian-impact-protection requirements made the job even tougher.

“Just fitting an engine between the fenders and under the hood doesn’t meet those (pedestrian-impact) requirements,” Williamsen says. “You now need crush space under the hood and behind the radiator support.”

In the U.S., as well as in Europe, “We pursue an internal strategy of GOA, for global outstanding assessment. That means for every Lexus model, our goal is to meet the strictest possible protection standards for every market in which we’ll sell the car.

“Quite a lot goes into meeting those pedestrian standards, so we had some issues with packaging the engine under the hood. Also, with ever-tightening emissions standards, we keep moving our catalytic converters closer and closer to the exhaust ports to keep them hot.”

Another key advantage of this engine, Williamsen adds, is very low emissions.

“It has a very fast warm-up cycle, which is important to get it off cold-start enrichment and into closed-loop operation as quickly as possible. A lot of the benefit of using direct injection is that it gives us a shorter loop for that closed loop between what the oxygen sensor senses and when we can change the mixture in the combustion chamber.”

Another interesting feature: The Denso Corp. starter and alternator use unique square-cross-section wiring, which increases the electrical field intensity while reducing mass.

“That allows us to use smaller bearings and lighter gears, gives us higher alternator output from less crankshaft energy and has the side benefit of reducing our use of lead for soldering,” Williamsen says.

“When you open the hood of this car, if you can find the alternator and starter, you’ll see that they are surprisingly small.”

Williamsen, a performance enthusiast, can see this V-6 replacing V-8s in some future applications.

“If there is a continuing demand for performance engines,” he says, “this one is well-suited. It has the potential to put a lot of good wake-up-level performance into quite a number of packages without a fuel economy penalty.”

Nevertheless, it’s unlikely Toyota will turn to forced induction to increase output in the future. Toyota is one of only two auto makers capable of building its own turbochargers and has a lot of experience with the technology, Williamsen says. Even so, Toyota believes hybridization is the better way to go.

“To give a big performance boost without a big fuel-economy hit, it’s better to slap a hybrid system on it than two turbochargers. I think it’s a great candidate for a really high-performance hybrid. The cost is still higher, but not as much as it was 10 years ago.”

But isn’t the cost of an entire hybrid-electric system, including the battery pack and sophisticated controls, still much higher than twin-turbos with DI?

“There’s a marginal cost-up when you go to direct injection, another when you go to forced induction, another when you go from gasoline to diesel and a big cost up when you go to hybrid,” Williamsen says.

“So it’s a sliding scale. I understand what Ford (Motor Co.) is doing with EcoBoost, and that makes sense. But as a corporation, we think there are better opportunities in hybridization. Yes, the cost is still a big challenge, but we’re getting better all the time at bringing it down.”

Can this engine be made still more fuel efficient to meet future CAFE requirements? “That’s a good question,” he responds. “It doesn’t have many weak spots. When I look at how much it is improved over its predecessor, it doesn’t seem that anything was overlooked.”