Tech Briefs

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April 2002
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Heavy-duty VVA from Jacobs

Jacobs concept to be used for engine testing and production viability analysis for heavy-duty VVA system.

As controller area networks (CAN) for vehicles become more commonplace, engine designers are pursuing advanced, next-generation variable-valve-actuation (VVA) technologies to achieve the elusive goal of direct, cycle-to-cycle, digitally controlled air management.

Jacobs Vehicle Systems believes that precision control of the last part of the ICE "trinity" (spark, fuel, and air) will bring even more astounding performance; it gave two presentations on the subject at the SAE Congress.

A partial list of engine benefits resulting from cycle-to-cycle ICE air management includes:

• Reduced fuel consumption
• Lower emissions
• Improved starting and idle
• Faster warm-up
• Increased low-speed torque
• Reduced transient response time (or, in "blown" engines, reduced turbo lag)
• Engine braking (compression release)
• Exhaust-gas-temperature control for aftertreatment systems
• Advanced combustion processes using multiple types of fuel.

The race toward throttleless gasoline engines and "variable displacement" involves a whirlwind of technologies. Tremendous promise is held in the application of increasingly flexible air and fuel systems, according to Jacobs. Most systems released and under study are for gasoline engines, where big benefits are easiest to reap. Some state that even diesel engines could benefit with the right system, especially in terms of emissions control/reduction and driveability. Which valvetrain architecture will be the basis of future engines remains a question.

Technologies can be divided into two groups: systems using camshafts and camless systems. Technologies may also be classified as primarily mechanical, electrical, or hydraulic (or various combinations of these).

The commercialization of high-volume, digitally controlled, lost motion, electrohydraulic VVA systems for diesel engines of all sizes is part of Jacobs' near-term corporate objective. Why diesel VVA? Because, Jacob says, the company has more than 40 years of expertise in precision valve actuation for compression-release engine braking in heavy-duty diesel engines. The low-lift secondary exhaust valve opening shortly before TDC compression (Jake Brake technology) turns power-producing diesels into power-dissipating vehicle retarders. Indeed, hydromechanically created compression-release valve events can be seen as an early form of VVA technology.

In pursuit of increased engine retarding power, engineers at Jacobs realized that variable timing of the compression-release event was needed for ultimate performance across the engine-speed range. It also led to the realization that application of the same hydraulic principles used for engine braking could significantly improve positive power engine performance and emissions control as well as enable further increases in retarding power at all engine speeds.

Jacobs is continuing to produce engine-retarding systems while concurrently developing VVA systems. With such inherent efficiencies of the diesel, worthwhile benefits remain a challenge. However, Jacobs has predicted benefits for a high-speed diesel engine that include:

• Reduced fuel consumption (4-5% average in various missions)
• Increased low-speed torque, 400 kPa (8333 psi)
• Improved startability
• Improved transient response (20% quicker vs. standard engine at low rpm)
• Braking power (80% of positive power).

Concepts have been initiated at Jacobs for gasoline engines to take advantage of known benefits in fuel economy and emissions reduction. Working within reliability paradigms of heavy-duty engines, the concepts add little in the way of incremental engine parts or expense and can even eliminate some existing engine hardware.

Relying on a conventional camshaft with special profiles enables engine benefits to be achieved reliably without concern for dreaded valve-to-piston contact. Cam-based lost motion systems are capable of the types of valve event flexibility that diesel or gasoline engine designers would like to have.

Capabilities of Jacobs VVA systems include control of valve lift within cam profile limits to enable early valve closing (Miller-cycle benefits); secondary valve events (e.g., compression-release braking, internal EGR); valve-lift cancellation (cylinder cut-out/variable displacement); and lift variation for reduced valvetrain power consumption, fuel economy, and breathing.

- Patrick Ponticel

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