Francis Thomas Connolly

A simple model for cyclic variations in a spark-ignition engine

We propose a simple model that explains important characteristics of cyclic combustion variations in spark-ignited engines. A key model feature is the interaction between stochastic, small-scale fluctuations in engine parameters and nonlinear deterministic coupling between successive engine cycles. Prior-cycle effects are produced by residual cylinder gas which alters mean in-cylinder equivalence ratio and subsequent combustion efficiency. The model`s simplicity allows rapid simulation of thousands of engine cycles, permitting in-depth statistical studies. Additional mechanisms for stochastic and prior-cycle effects can be added to evaluate their impact on overall engine performance. We find good agreement with experimental data.

Controlling cyclic combustion variations in lean-fueled spark-ignition engines

This paper describes the reduction of cyclic combustion variations in spark-ignited engines, especially under idle conditions in which the air-fuel mixture is lean of stoichiometry. Under such conditions, the combination of residual cylinder gas and parametric variations (such as variations in fuel preparation) gives rise to significant combustion instabilities that may lead to customerperceived engine roughness and transient emissions spikes. Such combustion instabilities may preclude operation at air-fuel ratios that would otherwise be advantageous for fuel economy and emissions. This approach exploits the recognition that a component of the observed combustion instability results from a noisedriven, nonlinear deterministic mechanism that can be actively stabilized by small feedback control actions which result in little if any additional use of fuel. Application of this approach on a test vehicle using crankshaft acceleration as a measure of torque and fuel pulse width modification as a control shows as much as 30% reduction in rms variation near the lean limit.

Synchronization of Combustion Variations in a Multicylinder Spark Ignition Engine

Abstract : We report experimental observations of synchronization among combustion variations in different cylinders at fuel-lean conditions in an eight-cylinder spark ignition engine. Our results appear to confirm that synchronization readily occurs and that it becomes stronger as the overall equivalence ratio is reduced from stoichiometric. It also appears that the onset of synchronization is associated with bifurcation instabilities reported previously for combustion in single cylinders. We use both cross-correlation and symbolic time series analysis to quantify the apparent relationships between pairs of cylinders and multicylinder groups. Extension of a simple dynamic model for single-cylinder combustion variations to the multicylinder case appears to agree with the observations and provides a basis for further studies. The occurrence of significant cylinder-to-cylinder synchronization may have significant implications for engine diagnostics and control.

Controlling Cyclic Combustion Variations in Lean‐Fueled Spark‐Ignition Engines

Under conditions of lean fueling or high exhaust gas recirculation, interactions between residual cylinder gas and freshly injected fuel and air produce undesirable combustion instabilities in spark‐ignition engines. The resulting dynamics can be described as noisy bifurcations of a nonlinear map and are complicated by cylinder‐to‐cylinder coupling. We discuss the key dynamic features relevant to control and demonstrate simple feedback control of a multi‐cylinder test vehicle.