Increasingly restrictive limits on Oxides of Nitrogen - NOx levels and desire for low methane emissions from gas engines are driving the change from lean-burn to stoichiometric combustion strategies on heavy-duty on-highway natural gas engines in order to take advantage of inexpensive and effective three-way catalyst technology. The change to stoichiometric combustion has led to increased tendency for engine knock due to higher in-cylinder temperatures. To suppress engine knock, Exhaust Gas Recirculation (EGR) rates from 10 to 30% are used. While high EGR rates nominally improve Brake Thermal Efficiency (BTE) and reduce exhaust gas temperatures, they also slow down combustion. However, by deploying a controlled spark triggered homogeneous charge volumetric ignition, very short burn durations can be achieved without the destructive effects of engine knocking towards high efficiency gas engines.
In the interest of achieving 45% BTE in spark ignited an on-highway class 8 truck engines fueled on natural gas and to meet EURO 6 and future California emissions standards of 0.02 gm/kw-hr NOx, Controlled Auto-Ignition (CAI) is herein demonstrated on a 15 liter truck engine. CAI is enabled by (a) having a combustion device capable of exceptionally good combustion stability in the presence of high EGR rates (COV of IMEP < 0.75 %), (b) cylinder pressure based combustion feedback, and (c) fast closed loop combustion control (using a Woodward RT-CDC control system).
This system enables significant reduction in burn duration by controlling a two phase combustion event. The first phase is normal spark ignited propagating flame, which then triggers the second phase which is volumetric auto-ignition. The location and percentage of fuel that burns in the volumetric auto-ignition event is controlled relative to that which occurs via the conventional spark ignited flame propagation process by use of high speed combustion in the loop feedback control. Auto-ignition mass fraction burned (MFB) ratios of 25–50% have been achieved yielding higher heat release rates at the end of combustion than at the center of combustion with the result being a shortening of the combustion burn duration from a nominal 20–30 degrees to a near optimal 10–15 degrees even with EGR rates as high as 25%. A novel and patent pending burn duration control strategy is employed to stably maintain this knock-free combustion strategy even with compression ratio as high as 14:1. The benefits are significant increase in Brake Thermal Efficiency and substantial reduction in engine out methane emissions without sacrifice of transient responsiveness.