GM has begun public-road demonstrations of a Saturn Aura concept vehicle equipped with a modified 2.2L Ecotec four-cylinder gasoline engine that delivers mixed mode operation: homogeneous charge compression ignition (HCCI) at lower loads, reverting to spark-ignition at higher loads and speeds. HCCI can provide up to a 15% fuel savings, according to GM.
GM first unveiled driveable HCCI concept vehicles on its test track in August 2007 (earlier post); since then, the company has added the capability of operating in HCCI mode during idle in addition to highway driving. An extended range for HCCI operation is intended as further refinements to the control system and engine hardware are made.
An HCCI engine ignites a mixture of fuel and air by compressing it in the cylinder. Unlike a spark ignition gas engine or diesel engine, HCCI produces a low-temperature, flameless release of energy throughout the entire combustion chamber. All of the fuel in the chamber is burned simultaneously.
The basic idea is to employ a premixed air-fuel mixture that is sufficiently lean or dilute to keep flame temperatures below about 1900K to help keep NOx and particulate production low. Consequently, the HCCI engine with lean burn characteristics is a very good candidate for future clean and economical passenger vehicle applications.
In spite of these great benefits, it has been very difficult to apply HCCI technology to real production engines. There are major challenges that must be overcome to make the HCCI engine practical. First, the ignition timing and combustion phasing in the HCCI engine cannot be directly controlled because there is no direct trigger, such as spark timing in SI engines or injection timing in CI engines; second, it has low power density because of its lean combustion nature; and finally, the HCCI engine has limited operating range due to knock-like rapid combustion under some conditions and misfire under others.—Chang et. al. (2006)
To control the HCCI combustion process, the mixture composition and temperature must be changed in a complex and timely manner to achieve comparable performance of spark-ignition engines in the wide range of operating conditions. That includes extreme temperatures—both hot and cold—as well as the thin-air effect of high-altitude driving.
Heat is a necessary enabler for the HCCI process, so a traditional spark ignition is used when the engine is started cold to generate heat within the cylinders and quickly heat up the exhaust catalyst and enable HCCI operation. During HCCI mode, the mixture’s dilution is comparatively lean. This reduces the throttle losses of a conventional spark-ignited engine at low loads, helping a gasoline HCCI engine approach the efficiency of a conventional diesel, but requiring only a conventional automotive exhaust after-treatment.
Achieving HCCI operation at idle is a challenge because the relatively low-temperature and light engine load characteristics generally inhibit the proper thermodynamic conditions for successful, controllable auto-ignition. Also, heat is needed at start-up and idle to light-off the catalytic converter.
GM’s engineers overcame these challenges with advanced control of the direct injection system and an HCCI-specific cylinder pressure sensor system. After spark ignition is used to start the engine, the engine’s control system manipulates the combustion process via input from the cylinder pressure sensors so that auto-ignition can occur during idle.
Fuel consumption with a spark ignition engine is relatively high when idling, so this new development in our HCCI process helps to enhance fuel efficiency.—Dr. Matthias Alt, HCCI program manager, GM Powertrain
Gasoline HCCI, along with other enabling advanced technologies, approaches the engine efficiency benefit of a conventional diesel, but without the need for expensive lean NOx after-treatment systems. Its efficiency comes from reduced pumping losses, burning fuel faster at lower temperatures and reducing the heat energy lost during the combustion process.
HCCI, direct injection, variable valve timing and lift, and Active Fuel Management all help improve the fuel economy and performance of our internal combustion engines. I am confident that HCCI will have a place within our portfolio of future fuel-saving technologies.—Tom Stephens, executive vice president, GM Global Powertrain and Global Quality
The emerging HCCI technology offers several paths for implementation in a production vehicle. GM’s strategy combines the efficiency enhancements of HCCI and the power-on-demand attributes of spark ignition. This combination delivers enhanced fuel savings over a comparable, non-HCCI engine, but with the performance consumers have come to expect during higher engine load situations, such as passing or entering a freeway.
|The Saturn Aura HCCI concept vehicle.|
The Saturn Aura concept vehicle operates in HCCI mode up to about 55 mph (88 km/h) and switches to spark-ignition for higher-speed, higher-load conditions. It also engages spark ignition mode for passing at lower speeds and other higher-load demands.
The modified Ecotec engine produces 180 horsepower (134 kW) and 170 lb-ft of torque (230 Nm). It features a central direct-injection system, with variable valve lift on both the intake and exhaust sides, dual electric camshaft phasers and individual cylinder pressure transducers to control the combustion as well as deliver a smooth transition between combustion modes.
An advanced controller, using cylinder pressure sensors and GM-developed control algorithms, manages the HCCI combustion process, as well as the transition between HCCI combustion and conventional spark-ignition combustion.
Kyoungjoon Chang, Aristotelis Babajimopoulos, George A. Lavoie, Zoran S. Filipi and Dennis N. Assanis (2006) Analysis of Load and Speed Transitions in an HCCI Engine Using 1-D Cycle Simulation and Thermal Networks (SAE 2006-01-1087)