Automotive HCCI Engine
The homogeneous-charge compression-ignition (HCCI) strategy has caught the attention of automotive and diesel engine manufacturers worldwide because of its potential to rival the high efficiency of diesel engines while keeping NOx and particulate emissions extremely low. However, researchers must overcome several technical barriers, such as controlling ignition timing, reducing unburned hydrocarbon and carbon monoxide (CO) emissions, extending operation to high and low loads, and maintaining combustion stability through rapid transients.
HCCI engines can operate using a variety of fuels. In the near term, the application of HCCI in prototype automotive engines typically adopts mixed-mode combustion in which HCCI is used at low-to-moderate loads and standard spark-ignition (SI) combustion is used at higher loads. This type of operation using standard gasoline-type fuels requires a moderate compression ratio of 10:1 to 14:1 for SI operation and variable valve timing to achieve HCCI operation.
The CRF’s automotive HCCI engine project comprises three parallel endeavors performed in collaboration with partners in industry, academia, and national labs:
- experimentally characterize in-cylinder processes including fuel-air mixing, ignition, combustion, and emissions to build our understanding of automotive HCCI combustion and facilitate its implementation;
- develop laser-based diagnostics capable of delivering the in-cylinder measurements required to characterize HCCI combustion; and
- develop, validate, and apply computational tools for simulating automotive HCCI combustion strategies including detailed fluid dynamics and chemical kinetics models.
The automotive HCCI engine lab houses a versatile light-duty engine designed to enable investigation of in-cylinder processes during HCCI operation. The automotive-sized engine (0.63 liters/cylinder) has a 3-valve pent-roof head and is equipped with extensive optical access for the application of advanced laser-based diagnostics, including a full height quartz cylinder and an optical piston. The air system provides intake pressures up to 2 bar and heating to 250 °C. These high intake temperatures allow investigations of HCCI operation with lower compression ratios (10:1 to 12:1). Alternatively, valve timings that retain large fractions of hot residual gases can be used to induce HCCI combustion. The engine is equipped with a centrally mounted gasoline-type direct injector, a port fuel injection capability, and a fully premixed fueling system, allowing investigations of both well-mixed and stratified HCCI operation.
As an example of current efforts, researchers have developed a new tunable diode laser diagnostic designed to capture time-resolved, spatially averaged measurements of CO in the engine. Such measurements are needed for the investigation of recompression strategies in which exhaust valves are closed early to trap and recompress residuals in the cylinder. Partial fuel injection during recompression is advantageous for rapidly controlling HCCI combustion phasing, and quantifying the extent of reaction of this fuel is a prime objective of the laser-absorption diagnostic. In other recent work, researchers have applied two-wavelength laser-induced fluorescence in the engine to simultaneously map both composition and temperature during recompression operation. Details of in-cylinder temperature distribution are important for understanding HCCI ignition and combustion performance.