Abstract
Strategies to mitigate cold-start emissions are critical to meet stringent standards for particulate and regulated gaseous emissions from direct injection spark ignition (DISI) engines. The objective of the current work was to establish an experimental protocol for cold-start studies and to identify the in-cylinder phenomena important during engine warming in terms of engine power and stability and engine-out particulates, NOx, and unburned hydrocarbon emissions. Metal-engine experiments were conducted with ambient intake air temperature and coolant air temperature of 20 °C to quantify the sensitivity of the performance of a single-cylinder engine to timing of a strategy using two injection events per power cycle and late spark ignition. The results showed strong sensitivity to the timing of the second injection event and weak sensitivity to the timing of the first injection. Complementary high-speed imaging experiments were conducted with the same engine where the fuel spray and combustion characteristics were visualized during cold-start split-injection operation. The imaging data showed the sources of particulates were due to fuel impingement on combustion chamber surfaces and due to fuel rich regions, in particular regions where the fuel re-condensed due to the conditions associated with late ignition timing. The combination of engine-out particulate and imaging measurements indicated smaller particles (10-20 nm) were associated with fuel impingement on surfaces, and larger particles (20-200 nm) were associated with fuel rich “pockets”.
