DN Staff

October 1, 2001

8 Min Read
Lean burn lives!

The efficiency gains provided by 42V-de-pendent technologies such as integrated starter generators, electric water pumps, and by-wire brakes and steering have been well publicized. However, a little-known consequence of the 42V migration could be the realization of "lean burn," which involves diluting the fuel entering an engine's combustion chamber with air to improve gas mileage and reduce emissions. But it's a Catch-22: the leaner the fuel mixture, the harder it is to ignite. Underestimating the significance of the ignition spark in combusting lean fuel mixtures, many researchers gave up years ago, or focused on a lesser form of lean burn called stratified charge.

CEI claims its high-energy 42V ignition alone can improve fuel efficiency by 5-10%, but when used in conjunction with the high flows that naturally occur inside engines, the technology can extend an engine's lean misfire limit to 30:1 AFR.

In contrast, Michael Ward, president of Combustion Electromagnetics Inc. (Arlington, MA) focused his research on ignition design, and never lost sight of his mission to improve fuel efficiency and emissions using homogenous charge lean burn. The work he has done in the last decade has paid off in the form of two major breakthroughs: First he developed and patented a 42V-ignition system that produces a flow-resistant spark. Then he coupled that high-power spark to the engine's flow field. Ward claims that flow-coupled spark discharge is the key to combusting extremely lean fuel mixtures. But, he says, it requires an ignition system that produces spark with an energy of at least 100mJ.

Design News covered CEI's 42V coil-per-plug ignition system, which Ward now calls ECO-FIRE(TM), in 1997 (See 11/3/97). The article included results from 1995 tests at Chrysler with ECO-FIRE in-stalled as a simple replacement to a standard ignition on a 4-cylinder engine, with no other engine modifications. Test results showed an increase in fuel economy of up to 20%, and reduction of NOx emissions by as much as 90% over standard ignitions. In terms of engine stability, the results were the best ever seen, according to Chrysler's head of advanced ignition system development Anson Lee.

Since ECO-FIRE produces a spark with five times the flow-coupling capability of standard ignitions, the Design News article highlighted the ignition-flow-coupling theory, even though it had yet to be tested. Ignition flow coupling involves placing the spark directly in the fuel mixture flow field of the engine's combustion chamber, in contrast to the more typical approach of sheltering the spark from the flow region by recessing it. CEI's high-energy spark withstands flows up to 20m/sec, says Ward, "so flow becomes an asset when igniting lean fuel mixtures be-cause it stretches the spark kernel, distributing its energy over a larger volume."

By combining a 42V ignition with a combustion chamber that enhances flow coupling, CEI claims the ECO-FIRE test engine runs leaner and achieves a faster burn with greater efficiency than any known carbureted or port fuel-injected gasoline engine.

In recent years, Ward has been busy putting this theory to the test, presenting his latest results in a technical paper at the SAE 2001 World Congress in Detroit in March. The paper documents what may be the best overall fuel economy ever achieved in a gasoline engine.

SAE paper (2001-01-0548) titled "High-Energy Spark-Flow Coupling in an IC Engine for Ultra-Lean and High EGR Mixtures" is based on ECO-FIRE tests in a single-cylinder, two-valve, homogenous charge gasoline engine. Designed with high "squish" flow and dual high-energy spark plugs located in the combustion chamber's flow field, the engine achieves lean misfire limits beyond air-fuel-ratios (AFR) of 30:1. According to the paper, the result is a 10-20% fuel economy increase at light engine loads, and reduced emissions throughout the entire engine operating range.

Ricardo engineers designed one of the most advanced homogeneous charge 2-valve lean burn engines with a compression ratio of 13-15:1, and teh combusion chamber entirely below the exhaust valve. In contrast, ECO-FIRE operates at a more practical 11:1 compression ratio. Its combustion chamber is in the cylinder head close to the exhaust valve, partially under the intake valve.
In the AFR range of 23-26:1 with BSFC of 0.50 to 0.52 lb/hp-hr, an optimal trade-off between fuel efficiency and emissions occurs. With lower emissions, BSFC is 20% less thant the Ford engine and 10% less than the Ricardo engine.
Since squish decreases at low engine speeds and loads, supplementing it with swirl or tumble flow in the 25-40 degree timing advance range may extend BSFC and reduce HC emissions further.

Ignition efficiency. Even though CEI's ignition provides three times the spark energy and six times faster rise time for the same size coils in a standard ignition, Ward says, it doesn't draw any more current from the battery because it's 2-3 times as efficient. This means that the fuel consumption to produce the highpower spark is insignificant. "A very small amount of ignition energy goes a long way," Ward explains. In fact, his test engine, operating at light load and 3,000 rpm, produces approximately 4 hp (3,000W). Firing once every 40 msec, the ignition draws just 7.5W from the battery, or approximately one-quarter of one percent of the engine brake power produced.

"Our patented 42V dc-dc converter is 90% efficient," Ward says. "It supplies 40-60W for the typical 4-cylinder engine, and up to 120W or higher for an 8-cylinder racing engine. It operates over a wide range of battery voltages from 6-25V, which is important because battery voltage decreases in very cold weather." The design uses a low-cost, low RDS FET switch, a ferrite-based transformer, in-put and output capacitors, an output diode, and a unique comparator-based controller.

"Our ignition is suitable not only as part of a complete lean burn/high EGR engine system," Ward says, "but as a replacement for standard inductive ignition systems. It has the capability to meet the entire internal combustion engine industry's needs for superior coil-per-plug ignition and could help meet ever tightening emission and fuel economy standards in passenger car, motorcycle, small engine, and marine applications.

Ben Sheaffer, retired senior staff scientist for Mercury Marine (Fond du Lac, WI) sees promise in Ward's work. "It's very advanced," Sheaffer says, "and certainly could become of major importance." With more than 40 years experience with IC engine design, Sheaffer's experience in lean burn stems from advanced research at Mercury that included the design and development of direct injection 2-stroke engines. "Ward has an excellent design for a 42V ignition system that has the capability to improve fuel efficiency, reduce emissions, and take lean burn even further," Sheaffer explains.

Moderately lean mixtures between 15 to 23 AFR are forbidden since conventional 3-way catalysts can't reduce NOx in an oxygen abundant atmosphere. However, NOx drops to acceptable levels under the ultra-lean conditions ranging from 24 to 26 AFR.

Ward is currently upgrading laboratory prototypes into pre-production systems that will be available to powertrain engineers for testing and evaluation by the end of the year. Following the industry trend, he will be designing these systems comprised of an ignition ECU, one or more ignitors de-signed for four IGBT switches (two ignitors for an 8-cylinder engine), and ignition coils, wires, and spark plugs. "The ECU will integrate the power converter to make the 42V step up transparent to the user, and will include a feature that makes the system user friendly and simple to implement," Ward explains. "The unique patent pending feature is based on a micro-controller which will allow the user to operate with only an ignition firing signal, instead of the usual cam or crank reference."

Due to the high cost of implementation, most analysts predict it will be at least a decade before entry-level vehicles come equipped with standard 42V electrical systems. But with or without 42V, Ward contends that "right now, with minimal complexity, lean burn is the only affordable technology that can simultaneously solve the problems of fuel conservation and air pollution." Ward estimates his system will add $10 or less to the cost of a 4-cylinder vehicle with a 12V electrical system. That's the estimated cost of CEI's 42V dc-dc ignition converter. "On vehicles already equipped with a 42V system, ECO-FIRE will cost the same or less than standard ignition systems."

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