The internal combustion engine's last hurrah: Homogeneous-charge compression-ignition may be the answer

Automotive Industries, Dec, 2004 by Don Sherman

It sounds too good to be true: significantly higher fuel economy, greatly reduced N[O.sub.x] and particulate emissions, and the ability to run on today's fuels. Homogeneous-charge, compression-ignition (HCCI) engines combine the very best gasoline and diesel engine properties. This alternative combustion process, which uses lean mixtures ignited without a spark or flame front, has been under study for more than 25 years. The late Smokey Yunick touted his version of this technology in the pages of Popular Science magazine more than 20 years ago. Momentum behind HCCI research is now building so rapidly that every major engine manufacturer and a consortium of five engineering schools backed by Department of Energy funding are pursuing this path to low emissions and high fuel economy. Powertrain experts believe HCCI could be the internal combustion engine's last gasp before fuel cells take over.

More than a dozen different names have been used to describe what is now called HCCI. In 1979, engineers at the Nippon Clean Engine Research Institute in Japan perfected a cleaner, more efficient two-stroke using what they labeled Active Thermo-Atmosphere Combustion for use in a 14-horsepower portable generator. Concurrently, Toyota engineers determined through spectroscopic analysis that HCCI chemistry and ignition were fundamentally different from the normal spark-ignition combustion process. Both were more reminiscent of what would be seen in conventional engine experiencing auto-ignition (detonation). Smokey Yunick called his approach hot vapor injection.

Gasoline HCCI

Like the name says, fuel and air are well mixed throughout every region of the cylinder. Time-lapse photos taken through a window in the combustion chamber reveal no localized high-temperature flame front; instead, there are hundreds of evenly distributed specks of light with HCCI, indicating spontaneous combustion of the entire charge. High exhaust-gas dilution and lean mixtures greatly reduce peak temperatures and heat-transfer losses. The extra EGR also facilitates running the engine virtually unthrottled, minimizing pumping losses. Thermal efficiency gains come from squeezing peak combustion temperatures into a narrow window timed for maximum energy release during the expansion stroke. The potential is diesel-engine fuel efficiency--15-20 percent gains over today's gasoline engines--with N[O.sub.x] emissions so low that reduction-catalyst after-treatment is unnecessary.

Unfortunately, HCCI works better in theory than in practice. Achieving sufficient thermal energy late in the compression stroke to trigger auto-ignition consistently is the most serious challenge. The diesel approach of using a high compression ratio was an obvious avenue for investigation but researchers quickly discovered that ratios beyond 12:1 cause severe knock during full-load operation. Other concerns are excessive rates of pressure rise and the combustion-generated noise associated with too-high compression. Today, there's agreement that elevated compression ratios are not well suited to smooth HCCI operation.

A more practical avenue is using high levels of EGR to supply the desired thermal energy. A strategy called recompression traps exhaust in the cylinder by closing the exhaust valve early. Another approach called rebreathing captures exhaust after it has left the cylinder. The intake valve can be opened briefly during the exhaust stroke, the exhaust valve can be opened slightly during the Intake stroke, or exhaust can be recycled by simply postponing the exhaust valve closing. Researchers have investigated all of the above possibilities, including mixes of recompression and rebreathing; many have been proved effective.

Achieving the necessary homogeneous fuel-air charge is relatively easy using early injection of the fuel into the intake port to maximize the time available for mixing prior to combustion. But researchers also discovered that late injection directly into the chamber is the best means of triggering combustion without a spark. Direct injection avoids premature ignition, thereby facilitating a higher compression ratio. Varying the injection timing during the compression stroke is also an effective means of balancing the amount of N[O.sub.x] versus HC and CO emissions produced.

Several fuel-injection schedules have been investigated. Injecting fuel into retained and compressed exhaust gasses prior to the intake stroke commences the chemical activity that results in auto-ignition at the end of the compression stroke. A secondary fuel injection event has also been used to trigger combustion, though emission levels suffered with this method. Trying every possible avenue, experimenters have even used a spark plug to initiate a flame that burns a portion of the charge which in turn drives the remainder into auto-ignition. Results were mixed with this strategy. Mitsubishi engineers experimented with a 6-stroke gasoline engine using two combustion processes: a stratified ultra-lean combustion triggered a second HCCI combustion phase. N[O.sub.x] reductions and fuel efficiency were good but the operating range was limited to light-load conditions.