Spark ignition internal combustion engine efficiency improvement - a variable compression ratio option

Abstract

Pressure to reduce energy consumption is increasing. The problem of vehicle fuel consumption and emissions is approached by exploring various vehicle propulsion options, assessing their net effectiveness on a energy conversion basis and on a usability (consumer appeal) basis. Life Cycle Assessment (LCA) of various options indicates that internal combustion engine powered vehicles compare favourably because of low production cost in spite of only achieving modest energy conversion efficiency in operation. Spark ignition (SI) homogeneous charge engines have dominated as passenger vehicle power plants, and are likely to maintain their prevalence for passenger vehicle propulsion into the future, but efficiency improvements are required and achievable. Throttling losses are a significant contributor to reduced efficiency at low load for SI engines which is the load range most employed in standard driving behaviour. An Induction Air Motor (IAM) was conceived, designed, simulated and prototyped to evaluate the potential to recover some of the work the engine does to reduce its intake air pressure for low load operation. The prototyped IAM produced work which potentially could contribute to the engine output while reducing the intake pressure resulting in improved efficiency. However, further effort is required to reduce the friction in the IAM and optimise the work produced by the IAM. An alternative strategy for efficiency improvement involves high Compression Ratio (CR) in conjunction with a reduced compression stroke volume achieved by Late Valve Closing (LVC). Such an arrangement of the Atkinson cycle is shown by simulation to produce improved brake efficiency in SI engines. In this cofinguration, the maximum power produced by the engine is considerably lower than the maximum power that is achieved by the same displacement for a full compression stroke. To achieve both the improved efficiency at low load using the Atkinson configuration and the power achievable from a full induction stroke, the engine requires Variable Compression Ratio (VCR). Assessment of VCR concepts from literature and patents identified that the complexity of continuously variable compression ratio designs prevented their development to production-ready configurations. A simulation of fuel consumption over a standard driving cycle showed that a two-position VCR arrangement produces the same benet as a continuously variable CR for physically achievable piston-rod-crank cofingurations. Experiments with supporting simulations were performed for a previously patented two-position VCR device, an eccentric link in the big-end of the connecting rod. This work concludes that the eccentric link is not a viable VCR mechanism. An alternative VCR device involving a hydraulic connecting rod was prompted by further experiments and simulations which identified the behaviour of oil when compressed at high rates in a hydraulic cylinder impacted by a falling mass. The oil impact work suggested that oil chambers of cross-sectional area that could be arranged in a conventional connecting rod could readily support the loads experienced by the rod in a conventionally configured engine, so the design and prototyping of a hydraulic connecting rod proceeded. Experiments and simulation confirmed that a relatively easily manufactured hydraulic connection rod can be successfully operated in an engine, achieving controllable two-position VCR. Further development of the hydraulic connecting rod control device and improved production techniques are recommended for this new two-position hydraulic VCR device.