The promise of an internal combustion engine offering near-diesel levels of fuel efficiency, with emissions that would make the cleanest gasoline engines proud, has moved another step closer to production. Engineers in GM’s Advanced Powertrain group have further broadened the operating range of their 2.2-L Ecotec homogeneous-charge compression-ignition (HCCI) engine, currently under test in Chevrolet Malibu mules.
According to HCCI Staff Engineer Vijay Ramappan, the engine’s speed/load map in HCCI mode now extends down to idle (800 rpm). That is 200 rpm lower than in January when AEI last sampled an HCCI-equipped GM vehicle. (See feature, March 2009 AEI, p. 23.) The upper limit “at the moment” is restricted to 3000 rpm, Ramappan noted, due to excessive combustion noise.
Operating in HCCI mode at 3000 rpm translates into approximately 60 mph (100 km/h) in steady-state driving conditions, as AEI’s summer test drive showed. On public roads surrounding GM’s Warren, MI, tech center, the car operated in fuel-saving HCCI mode for slightly more than 75% of the test drive, according to onboard data acquisition equipment. During this time the engine’s leanest air/fuel operation was 28:1.
Compared with just six months earlier, transition from HCCI to spark-ignition modes is more transparent, both audibly and by seat-of-the-pants feel. There is still some minor off-idle “dieseling” noise, but the mules are devoid of any NVH attenuation. “We still have work to do in this area,” Ramappan acknowledged.
Because HCCI’s sensitivity to ambient temperatures, pressure, humidity, and fuel blend presents control issues—as do combustion chamber deposits—GM’s first-generation HCCI production units will likely have multimode capability. They will start on stoichiometric mixtures and spark ignition at very light loads and also operate in this mode at 40-100% load. In between they’ll transition to HCCI mode, where in typical freeway running, GM engineers claim, the engines will return up to 15% greater fuel efficiency than comparable conventional gasoline engines.
HCCI also produces inherently low peak temperatures, so engine-out NOx is significantly lower compared with conventional gasoline ICEs. In GM’s vehicle steady-state NOx emissions testing, measured at 60 mph in sixth gear, HCCI engines have 0.186 g/km engine-out NOx emissions while operating in HCCI mode, compared with 1.938 g/km NOx in spark-ignition mode. Additionally, unthrottled operation minimizes pumping losses while improving low-load efficiency.
Success in HCCI development is measured in very small incremental steps. GM’s program is among the auto industry’s most advanced. Production HCCI engines are expected to debut within three model years, according to supplier sources.
In related developments, GM engineers are finding that “HCCI plays well with other technologies,” quipped Paul Najt, Powertrain Lab Group Manager.
GM is finding ethanol to be “a very attractive fuel for auto ignition—it offers high octane and no propensity for soot or smoke,” said Najt, who has been working on HCCI R&D at GM since the early 1980s. Alcohol fuels also present easier combustion control and appear to minimize carbon accumulation. He noted that ethanol in combination with HCCI combustion provides aggressive catalyst light-off characteristics.
Najt explained that GM also has investigated boosting strategies and use of hybridized drivelines and CVT transmissions to serve as HCCI enablers, helping to minimize the disruptive combustion events. He said HCCI would make sense in an extended-range electric vehicle such as the upcoming Chevrolet Volt during part-load operation when the ICE is charging the battery.
“And it’s scalable—100-mm cylinder bores don’t scare HCCI,” he said.
The HCCI program has remained a priority at GM despite the automaker’s financial woes and recent reorganization. “Our mission for this technology is unchanged; nothing that’s been in planning since 2001 has changed,” asserted Uwe Grebe, Executive Director, Global Advanced Powertrain. “I’m very proud of the team’s speed in getting this technology out of the lab and heading toward production.”
He explained that additional resources have been applied in algorithm development—HCCI electronic controls are widely considered the most sophisticated of any ICE control system—in order to achieve transparent operations, plus have full onboard diagnostics capability and assured 150,000-mile emissions compliance.
Grebe said HCCI control systems are being developed to allow the engine controller to “learn and compensate” for combustion-chamber carbon buildup. The MathWorks Simulink model-based design, simulation, and analysis software has been a key to GM’s progress in HCCI developments, engineers said.
Besides the considerable technical hurdles that remain, Grebe noted another challenge that stands between HCCI and the showroom—a catchy name. “This is cool technology, so the name for it also needs to be cool,” he said. “I’m working with our marketing guys. I want the name for HCCI to boil down to what’s relevant to the customer because we intend to use this across our brands.”