Design and analysis of a variable-compression, ratio internal-combustion engine : the Alvar engine concept

The internal-combustion engine operates over a wide range of conditions. These conditions include boosted intake pressures and increased fuel rate to deliver maximum power output and, most of the time, part loads with emphasis on fuel efficiency. Inherent in the internal-combustion engine design is the compression ratio. The compression ratio affects engine efficiency and the charge temperature and pressure prior to ignition, and thus the engine's tendency to knock. Maximum power output of an engine is therefore influenced by the compression ratio. The Alvar engine represents an attempt to capitalize on the benefits available from varying the compression ratio of the internal-combustion engine. One main advantage of the Alvar concept stems from the ability to operate at higher power output at a given engine size (power density) via a combination of higher intake pressure (boost) and lower compression ratio at the maximum load condition to avoid knock. Another advantage of the Alvar approach is the thermal efficiency gain by operating at higher compression ratios at part loads. This work includes the design and analysis of the Alvar engine. Through a performance analysis using a quasi-dimensional four-stroke cycle engine simulation, the Alvar concept has been found to effectively extract the benefits associated with the compression-ratio flexibility. The analysis included in this thesis reveals that close to six percent improvement of engine efficiency at very light load and 3.8 percent improvement in energy-weightedaverage urban-driving-cycle efficiency are attainable by the Alvar engine. Furthermore, this work reveals a significant advantage in the reduction of knock likelihood in the Alvar engine during turbocharged operation, allowing an increase in power density. This thesis recommends an optimal and practical design of the Alvar combustion chamber to be sized at 34.0 mm secondary bore and 39.9 mm secondary stroke, secondary connecting-rod length at 101.1 mm, and a clearance volume of 42.3 cc to fit onto an existing engine block of a Volvo 850 engine. Physical testing of the proposed design will further determine the success of the Alvar engine concept. Thesis Supervisor: Dr. Victor W. Wong Title: Principal Research Scientist Sloan Automotive Laboratory Lecturer Department of Mechanical Engineering Table of