General Motors is rolling out two test vehicles powered by a completely new form of internal combustion engine that GM said would use less fuel and produce less pollution than current technology powering cars and light trucks.
GM’s new Homogeneous Charge Compression Ignition (HCCI) engine platform, when combined with direct injection, electric cam phasing, variable valve lift and cylinder pressure sensing, provides up to a 15% fuel savings while meeting current emissions standards, said Tom Stephens, group vp-GM powertrain and quality.
“In an integrated engine concept, HCCI, along with other technologies, approaches the engine efficiency benefit of a diesel but without the need for expensive lean NOx after-treatment systems,” he said. “Its efficiency comes from burning fuel at lower temperatures and reducing the heat energy lost during the combustion process. Consequently, less carbon dioxide is released because the vehicle’s operation in HCCI mode is more efficient.”
Unlike a spark-ignition gas or diesel engine, Stephens noted that HCCI produces a low-temperature, flameless release of energy throughout the entire combustion chamber so all of the fuel in the chamber is burned simultaneously. “This produces power similar to today’s conventional gas engines, but uses less fuel to do it,” he explained. “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, meaning there is a larger percentage of air in the mixture. This helps the engine approach the efficiency of a diesel while requiring only a conventional automotive exhaust aftertreatment compared to what’s needed for diesel engines today, Stephens pointed out. It can also operate on either gasoline or E85, he added.
Stephens said GM’s HCCI-powered concept vehicles are a production-based Saturn Aura and Opel Vectra, both with a modified 2.2L Ecotec four-cylinder engine. He said they drive like conventionally powered vehicles but offer up to 15% improved fuel efficiency relative to a comparable port fuel-injected engine. However, he stressed that this fuel efficiency improvement varies depending on vehicle application and how it’s driven.
GM said the test vehicles currently only operate on HCCI up to approximately 55 mph, as they need to switch over to traditional spark ignition to handle higher vehicle speeds and during heavy engine load. “Perhaps the biggest challenge of HCCI is controlling the combustion process,” said Prof. Dr. Uwe Grebe, executive director for GM Powertrain Advanced Engineering. “With spark ignition, you can adjust the timing and intensity of the spark, but with HCCI’s flameless combustion, you need to change the mixture composition and temperature in a complex and timely manner to achieve comparable performance.”
