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15-Dec-2012, 08:58

Is the Gen V Engine’s Combustion System Really New?

by Mike Magda on December 10, 2012
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Given GM’s pre-introduction hype over the “all new combustion system” on the Gen V small-block, there is likely to be a measure of disappointment within certain progressive gearhead factions. The new Chevy LT1 still runs on the 136-year-old Otto cycle with four strokes of each piston, valves opening at conventional times with respect to crankshaft position and the all-familiar spark plugs igniting a stoichiometric air-fuel mixture. While there is the new-to-GM-V8 direct-injection fuel system, the LT1 lacks other high-profile features often associated with a high-tech engine: turbocharging, dual independent cam phasing, dual overhead camshafts and 4-valve combustion chambers. And the LT1′s scale of boast-worthy features hardly rivals the arousing passion of futuristic blue-sky breakthroughs like the Scuderi, IRIS or opposed-piston/opposed-cylinder concept engines.

In fact, GM didn’t even give its all-new system a clever moniker like SkyActive (Mazda), Multiair (Fiat) or OptiSpark (Oh, wait. GM tried that one with the previous LT1 engine). But still, not even a code name for identity and promotion? “All new combustion” in this marketing setting isn’t any more inspiring than an “all new flavor” for chewing gum.

An EngineLabs technical deep dive into the new LT1 engine, however, reveals just how delicately the the precise combustion dynamics are integrated into the overall powertrain strategy for this engine. Engineers racked up some 10 million hours of computational analysis in developing the LT1, with six million dedicated alone to the combustion.

Note the difference (left photo) in the piston top design from the LT1, left, and the LS engine. The LT1's eutectic aluminum alloy piston features a direct injection fuel bowl and deep enough valve reliefs to accommodate cam phasing. There's also a skirt notch for the oil jet. Some LS3 piston features that carry over include ring pack and oil drains that are both cast and drilled. The LT1's 6.125-inch connecting rod is a high-strength PM forging and features a tapered pin end. The 59cc combustion chamber (center photo) was redesigned to support direct injection and sized to achieve the 11.5:1 compression ratio. The spark plug was moved closer to center to help reduce knock. The hollow intake valves are 54mm (2.13 inch) while the hollow sodium-filled exhaust valves are 40.4 mm (1.59 inch). Engineers evaluated more than 75 iterations of the combustion system (right photo) that included spark plug placement, valve sizes, piston design and fuel delivery.
“The first two years of this program was based on nailing the combustion system,” stresses Dean Guard, executive director of GM’s North American engine programs. “I will put this combustion in any metric you want to talk about and against any overhead-cam engine in the world. I believe you can do a really good 2-valve combustion system and really bad 4-valve, and any area in between. We will not shy away from what this combustion system will do compared to anybody.”
While GM says the LT1 is a clean-sheet design, some traditional design elements found on every Chevy small-block since 1955 continued into the fifth generation, including a 90-degree V block with a single camshaft, two valves per cylinder and the trademark 4.400-inch bore-center spacing.

New to the Gen V

From there, very little carries over from the previous Gen IV LS architecture, either physically or in design. The starter bolts and valve keepers are the only common mechanical parts. Dimensionally, the deck height, overall length, rod length, piston compression height and crank-to-cam centerline carry over. The head-bolt centers are the same, although the LT engine has a larger head-bolt diameter.

Engineers used a sophisticated suite of CAD and simulation programs to first design the engine virtually. More than 75 combustion “iterations” alone were considered before lab testing combinations on a single-cylinder engine. Just what factors made up these iterations?

Here's a comparison (left photo) of the LT1 cylinder head, on the left, and the previous generation LS3. Note that the order of the valves has been reversed and the LT1 intake ports have been raised. The valves stand at 12.5 degrees intake/12 degrees exhaust and are splayed 2.5 degrees. The previous Gen IV heads had 15-degree valve angles. Also, all head bolts are now M12, compared to M11 in the Gen IV. The rocker arms are non-offset and sport a 1.8:1 ratio (right photo). The configuration of the exhaust port allows a small amount of exhaust gas to be drawn back into the cylinder for a more complete burn of the next combustion cycle and reduction of emissions.
“We looked at the momentum of the air, the direction of the air coming into the port,” says John Rydzewski, assistant chief engineer on the small-block program. “We moved the spark plug all over the place. Not only location but also intrusion into the cylinder.”

Power Density

First there was horsepower per cubic inch, then came area under the torque curve. Now the trending measurement is power density, which helps rate an engine’s efficiency and size. GM has made available numbers to compare the new LT1 with a BMW 4.4-liter twin-turbo found in the 5 Series. The BMW is rated at 400 horsepower with 450 lb-ft torque and has a weight of 503 pounds, or in simple terms, .795 horsepower per pound. The LT1 is likely to be rated at 450 horsepower and 450 lf-ft torque with a mass of 465 pounds, or .968 horsepower per pound. According to GM, the LT1 is also 4.3 inches shorter in overall height than the BMW.

With the introduction of direct injection, engineers couldn’t rely on design dynamics learned on GM’s other DI engines that are based on DOHC configuration. According to engineers, the flow field — that is the motion of the air-fuel mixture — is inherently more complex with an OHV configuration since DI requires more mixture swirling.

“DI has a lot of advantages,” explains Rydzewski. “It cools the piston, so we’re able to run a higher compression ratio. It allows you to inject into a piston bowl to help improve cold-start emissions. There’s also more flexibility in designing the intake port, since you don’t have to place an injector in the port.”

In addition to injector placement, engineers manipulated valve size and angles, the size and shape of the combustion chamber and the topology of the piston head — which is crucial to DI. The LT1′s dished pistons feature “risers” at the top to direct the fuel spray.

“We had to look at the direction and speed of the air going past the spark plug,” continues Rydzewski. “If too large or small, it’s not good for proper delivery of spark energy. We had to find that little window.