Fuel-flexible combustion control of modern compression-ignition and spark-ignition engines

Carrie M Hall, Purdue University

Abstract

Concern over the availability of fossil fuels as well as energy usage and harmful emissions output have produced an interest in alternative fuels, advanced combustion strategies, and increased usage of engine technologies such as variable valve timing (VVT), turbocharging, and exhaust gas recirculation. While alternatives such as biodiesel and ethanol are renewable and can reduce dependence on foreign sources of petroleum, challenges in using these fuels must be addressed in order for optimal performance to be achieved on fuel-flexible vehicles. On compression ignition (CI) engines, biodiesel combustion typically results in decreased particulate matter (PM), unburned hydrocarbon (uHC), and carbon moNOxide (CO) emissions. However, biodiesel usage also results in increased fuel consumption and nitrogen oxide (NOx) emissions relative to petroleum-based diesel. These challenges can be addressed through the use of a fuel-flexible combustion control strategy. The approach presented in this work consists of two parts: 1) estimation, whereby the engine control module (ECM) detects the biodiesel blend fraction being supplied, and 2) accommodation, whereby the ECM dynamically changes the control setpoints in order to improve engine performance. A closed loop control method, which accounts for the oxygen content and energy density differences between diesel and biodiesel, has been developed which can be added to the existing ECM control structure to create a fuel-flexible structure. Analysis of the control system indicates that the addition of this fuel-flexible control strategy will not detrimentally affect the overall engine control structure. Experimental validation of this control strategy on a 2007 6.7 liter Cummins ISB series engine at several very different operating modes shows that this fuel-flexible control method greatly reduced or completely eliminated increases in NOx emissions of up to 30% while largely maintaining the torque/power capacity of a modern diesel engine when operating with biodiesel. Since biodiesel can be created from a variety of different feedstock and can be further processed to alter certain fuel properties, its fatty acid structure can differ. The robustness of the proposed control technique to variations in fatty acid composition is also explored. Fuel-flexible spark ignition (SI) engines permit the increased use of ethanol-gasoline blends. Ethanol is a renewable fuel which has the added advantage of improving performance in operating regions which are typically knock limited due to the higher octane rating of the fuel. Furthermore, many modern SI engines are also being equipped with variable valve timing, a technology which can increase engine efficiency by reducing pumping losses through control of the in-cylinder burned gas fraction (BGF). However, the BGF and ethanol blend can have a significant impact on the combustion timing, such that capturing these effects is essential if the combustion phasing is to be properly controlled. Combustion efficiency is typically tied to an optimal CA50 (crankangle when 50% of fuel is burned) for an engine. A control-oriented model of combustion phasing has been developed and extensively validated across the operating range of a 4-cylinder Renault SI engine for 4 blends of gasoline and ethanol. Furthermore, the model was utilized to determine the impact of ethanol blend and BGF on combustion phasing and on the optimal spark timing. Leveraging the model in this manner provides direct evidence that accounting for the impact of these two inputs is critical for proper spark ignition timing control. The model created in this work has the potential to be used to improve air handling control to avoid the need for throttling and to improve spark timing control to ensure that CA50 occurs at its optimal timing despite changes in fuel and BGF. While fuel-flexible operation in conventional combustion modes presents significant challenges, an interest in abiding by stringent emissions and fuel economy regulations has motivated the investigation of advanced combustion strategies such as premixed charge compression ignition (PCCI) used in conjunction with alternative fuels. PCCI can achieve low emissions and high efficiencies in diesel engines; however, fuel property differences cause a more significant impact in such advanced combustion modes and can make control in these regions more challenging. When biodiesel is used at premixed operating conditions with the engine operating with the stock calibration for diesel fuel, the timing of start of combustion and peak heat release can shift by over 2°CA and 4°CA respectively, nitrogen oxide emissions can increase by over 100%, and torque output can drop by over 30% with respect to engine performance with diesel. A control framework for fuel-flexible PCCI has been developed and theoretical and experimental results demonstrate that: 1) combustion timing control and NOx control can also be achieved through control of in-cylinder oxygen fraction alone, 2) energy-based fueling provides consistent in-cylinder oxygen fractions between diesel and biodiesel while also matching torque outputs for the two fuels when considered at the same premixed operating point (speed, intake manifold temperature, charge flow, effective compression ratio, and combustion timing).

Degree

Ph.D.

Advisors

Shaver, Purdue University.

Subject Area

Mechanical engineering

COinS