Auto Electronics

Variable Valve Train Technology Helps Enhance the Gasoline Engine

Valve train improvements are an important part of the continued efficiency advancements of internal combustion engines. Today's electronics allow more precise control and different approaches to valve train design.

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While the motoring public waits breathlessly for the promise of a revolution in powertrain technology to materialize, the venerable gasoline-powered internal combustion (IC) engine has quietly been evolving into an increasingly cleaner, more fuel-efficient power source.

In fact, today's cars and trucks deliver significantly higher-performance benefits in smaller, lighter, more efficient packages that use conventional 87 octane unleaded gasoline. And they do so without the need for massive investments in the fueling infrastructure, exotic technologies or sacrifices in engine performance.

The reason, of course, has been an evolution in engine technology made possible by an array of innovations in electronics beginning in the early 1980s with electronic fuel injection and “variable displacement” cylinder deactivation. It wasn't until about 10 years ago that electronics really began moving the needle in engine technology with the high-volume application of sophisticated electronically controlled variable valve train technologies. These developments are enabling the gasoline-powered engine to cost effectively realize its full potential in fuel economy, emissions and performance.

The most significant advances have occurred in three areas:

  • continuously variable cam phasing;
  • cam phasing combined with two-step valve lift; and
  • cylinder deactivation.


While the future of automotive powertrains appears to be hybrids in the near and mid-term and fuel cells long-term, advanced electronic systems like these valve train control systems provide practical, cost-effective answers today.

Like racing — “How fast can you afford to go?” — there is a cost associated with fuel economy, emission and performance improvements, and these valve train enhancements provide cost-effective gains that are competitive with more glamorous, revolutionary approaches.

For example, vehicle manufacturers can get a 2% improvement in these qualities with a modest investment of say about $50 in valve train enhancements vs. about a 15% gain with a hybrid that requires an investment of several thousand dollars. So the dollar per percent improvement in valve train technology is extremely affordable.

Today's IC engines perform very well in the real world in terms of fuel economy and are relatively robust to environmental and operational variation. Hybrids, which deliver good fuel economy in controlled test environments, may not perform as well in real-world driving conditions. People don't drive in everyday life the way test procedures dictate.

The evolution toward sophisticated valve train control began some 60 years ago when Delphi developed the first hydraulic valve lifter and then incorporated the first roller hydraulic valve lifter into production vehicles in the late 1970s. The hydraulic system replaced a mechanical, fixed lash system (no automatic compensation for component wear over time) with one in which hydraulically actuated adjusters allowed the valve train system to take up lash, significantly increasing valve-train life and eliminating service requirements. Today, approximately 70% of all engines produced have hydraulic lash adjustment.

Electronic control of the valve train, a relatively new approach that has gained popularity over the past five years, has raised this automatic adjustment capability to an entirely new level with the high-volume application of continuously variable cam phasing.

CONTINUOUSLY VARIABLE CAM PHASING

Continuously variable cam phasing changes the timing of the valve train as the engine operating conditions change — idling in traffic, launching and accelerating, towing a trailer or cruising at highway speeds — improving fuel economy and reducing hydrocarbon and NOx emissions. This is accomplished by shifting the intake and/or exhaust cam as required on dual overhead cam engines to broaden the torque curve, increase peak power at high rpms and improve idle quality.

In the Delphi continuously variable vane-type cam phaser system, the variable cam phaser (VCP) replaces the standard pulley or sprocket. This enables it to optimize their angular relationship while the engine is running based upon the operating parameters of the engine.

Various strategies can be used in applying this technology: one on the intake cam, one on the exhaust or both — a dual independent cam phasing strategy. Fuel economy gains can range from 1% to 4%, depending on the fuel economy vs. performance trade-off strategy used.

For example, if the value to the vehicle builder is $20 per percentage point per vehicle, some very large paybacks can be realized from investments in this technology alone. Today, Delphi estimates that at least 50% of the market has engines using cam phasing and likely this technology will be applied to virtually all production engines within the next six years or so. The value equation makes sense. The system cost is minimal and the engine builders can usually eliminate their EGR system while getting better fuel economy, more horsepower and torque and improved emissions — so they get about three times as much value from the investment.

HOW VCP WORKS

The angular position (or phase relationship) is controlled by the VCP's internal rotor/stator pack. Commands from the engine powertrain control module control the oil control solenoid valve mounted on the cylinder head to regulate engine oil flow to either side of the rotor “vanes,” pressurizing that side, which advances or retards the camshaft, in turn adjusting the camshaft timing accordingly. Benefits are the result of increased volumetric efficiency, reduced pumping losses and internal charge dilution control that come from varying the cam timing.

TWO-STEP VALVE LIFT CAM PHASING

Delphi has developed a new technology that promises to raise engine efficiency and performance to an even higher level. This system combines two-step valve lift with variable cam phasing to provide improved fuel economy, lower emissions and increased performance. The system offers most of the benefits of more complex continuously variable valve lift systems at approximately half the cost with relative ease of packaging on new and existing engines.

Typically, an engine will open an intake valve about 10 mm and that remains constant throughout its operating range. With the Delphi two-step system, the intake valve lift is reduced to 3 mm to 7 mm for part-load fuel economy and then seamlessly switched to high lift for torque and power. Fuel economy improvements up to 6% to 7% can be achieved on the EPA federal test procedure. High lift operation combined with cam phasing delivers a balanced torque curve over the full speed range. The system can be configured to provide increased power at high engine speeds without compromising idle stability or fuel economy. Other benefits include up to 50% reduction of hydrocarbon emissions during cold starts by improving mixture preparation.

The two-step system replaces the conventional roller finger follower and camshaft with a device consisting of low mode and high mode cam followers; a tri-lobe camshaft, a regulating oil control valve and minor changes to the existing engine lube oil system. Roller bearings are used for the low mode followers to minimize friction. Sliding element is used for high lift, when oil film thickness improves sliding friction to acceptable levels. The engine management system selects the desired valve lift by controlling pressure in the lash adjuster oil gallery via the oil control valve; no additional oil galleries are required.

CYLINDER DEACTIVATION

One of the first working cylinder deactivation systems was on a six-cylinder engine in a prototype van developed by Ford using vacuum power and mechanical linkages in 1977. The system never went into production because of various problems and minimal fuel economy gains, according to reports at the time.

The first production system appeared in a 1981 Cadillac on the “V-8-6-4” engine — it could switch from four-cylinder to six-cylinder to eight-cylinder operation to meet varying operating conditions. The system was electronically controlled but the level of electronic sophistication at the time was limited and production was suspended when refinement issues couldn't be satisfactorily resolved.

Today's cylinder deactivation system is a much more advanced engine management system that selectively turns off an engine's cylinders under certain driving conditions. The system functions so smoothly and so unobtrusively that the driver is essentially unaware that the change has occurred. This is due in part to the incredible advances in computational power of automotive controllers over the past 20 years, and equally due in part to advances in vehicle integration of the cylinder deactivation technology.

More than just a valve train hardware supplier, Delphi is a full-line vehicle integrator, providing:

  • air induction system (1/4-wave tube, Helmholz resonator, bidirectional air meter);
  • exhaust system (catalytic converter, exhaust back pressure valve, specially-tuned mufflers) to optimize exhaust note;
  • hydraulic, multistate, and magneto rheological engine mounts to isolate the second-order shaking forces created when half the cylinders of a 90o V8 are deactivated;
  • high flow EGR and/or cam phaser as desired to achieve high dilution;
  • torque-based engine (air, fuel, spark) control algorithms for optimum mode scheduling, seamless mode transitions;
  • OBD II diagnostics strategies and algorithms (deactivation H/W and misfire) for the deactivation components and system functions; and
  • high-energy ignition solutions to permit increased EGR dilution in deactivated mode.


All of these innovations help to enable cylinder deactivation. They either improve the performance, pleasability or the amount of time the vehicle can operate in deactivated mode.

For motorists who primarily do a lot of highway driving, the system can deliver as much as 8% to 10% improvement in fuel economy.

These systems are in production and to date account for about 10% of the market and are expected to grow to approximately 25% by 2010.

Among vehicle producers, GM launched the Vortec 5300 V-8-powered 2005 Chevy TrailBlazer EXT and GMC Envoy XL with displacement on demand, Delphi's cylinder deactivation system, and all of Chrysler's new HEMI engines are available with a type of cylinder deactivation system known as multiple displacement system (MDS).

Other vehicles equipped with this technology include:

  • Honda Odyssey, and
  • Mercedes Benz S-Class V-8 and V-12 for the European market.


Cylinder deactivation is yet another immediate cost-effective solution to the growing concern of fuel consumption worldwide.

Cylinder deactivation seamlessly “turns off” the engine's cylinders under certain driving conditions. For example, with an eight-cylinder gasoline engine, four of the cylinders cease to operate when there is a light load on the engine. The engine continues to be even firing on the remaining cylinders. The system precisely closes the exhaust and intake on targeted cylinders using electronically controlled hydraulic actuation, while disabling fuel to those cylinders. Fuel economy is improved because the remaining cylinders operate at higher load, reducing the pumping losses of the overall engine.

In addition to fuel savings, the customer-specific design of cylinder deactivation provides optimized functionality and calibration flexibility. The system provides reliable, seamless switching between modes within one engine cycle that is undetectable by the driver. Low reciprocating mass in deactivated mode minimizes lost motion spring load and associated friction. The “drop-in” design of the deactivation lifter for most pushrod applications allows for ease of assembly, reducing time-to-market and cost for the vehicle manufacturer.

ELECTRONIC CONTROL

Advanced variable valve train and cylinder deactivation systems have been made possible by the incredible advances in computational power (speed, accuracy and reliability) of electronic control systems, as well as advances in the field of mechatronics, which has enabled the development of actuators that are optimized for their precise operating mission.

In the case of the valve train, the computer-controlled electric-solenoid oil-control-valves are an example. The ability to precisely control the amount of oil flow and instantly direct it to control engine hydraulic systems is a key factor in controlling the dynamics of the engine and optimizing its efficiency for greater fuel economy, more power and reduced emissions.

FUTURE DEVELOPMENTS

In order to improve the phase rate response time of cam phasers and to reduce noise and further reduce emissions, Delphi developed a robust engineering design of experiment focused on the high temperature, low-speed areas of the system's performance where phase rates tend to be slow and oscillation high.

The project included studying 16 variables, including phaser, oil control valve and various engine parameters at varying speeds to determine the best composition and combination of these variables to achieve the optimum system for any given application.

Today, the company develops valve train systems and components by focusing on optimizing the overall efficiency of the engine. By leveraging the company's comprehensive understanding of the engine management system, it can work with the customer to define exact needs from an engine output perspective.




ABOUT THE AUTHOR

Jean Botti is business line executive powertrain and executive director fuel cell, Delphi Corporation.

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