In cylinder visualization of stratified combustion of E85 and main sources of soot formation

Abstract

The combustion process and soot formation in spark ignited spray guided stratified combustion of E85 was investigated in a single cylinder optical engine with direct injection of fuel using an outward opening piezo actuated injector. The effect of engine rotation frequency, fuel quantity, injection sequence and ignition timing was studied. Combustion, soot formation and soot oxidation was analysed using cylinder pressure measurements, images recorded using high speed video cameras, the flame emission spectrum and OH * chemiluminescence and soot incandescence imaging. A maximum injection duration was found to exist for direct ignition of the fuel spray. Engine rotation frequency had little effect on the initial and maximum rate of combustion. The maximum rate of combustion decreased with increasing cycle fuel mass when a single injection was used. The rate of combustion and indicated mean effective pressure increased and the combustion variability decreased when the single injection was split into multiple injections in close succession to deliver the same total fuel mass and the last fuel spray was ignited. Ignition of the first fuel spray resulted in a more pronounced change. The absence of soot incandescence during the initial flame propagation suggested flame propagation in a partially mixed fuel and air mixture with stoichiometric to fuel lean regions. A single fuel injection resulted in piston pool fires due to fuel spray impingement on the piston and was the primary source of soot formation. The pool fires persisted until after conditions favourable to oxidation of the soot had ended. Soot formation in the gas phase occurred while favourable soot oxidation conditions existed and was efficiently oxidized. The magnitude of the piston pool fires was reduced using multiple injections. The reduction is attributed to a reduction of the fuel spray penetration length and a smaller effective injection orifice area, resulting in a shorter total duration of fuel spray impingement on the piston crown. Soot formation occurred primarily in the gas phase when the first of two fuel sprays was ignited and persisted due to the second fuel spray entering an existing flame leading to fuel rich combustion.

Combustion and Sooting Behavior of Spark-Ignited Ethanol–Isooctane Sprays under Stratified Charge Conditions

“…Especially for ethanol, a discrete strong emission line can be detected at around 600 nm. This line is attributed to atomic luminescence of sodium, which may be contained in the fuel in small quantities because of the refining process . This atomic emission could explain also the red/orange luminosity (“bright intensity spots”, “red spots”) which were noticed in other studies. , …”

“…This line is attributed to atomic luminescence of sodium, which may be contained in the fuel in small quantities because of the refining process. 29 This atomic emission could explain also the red/orange luminosity ("bright intensity spots", "red spots") which were noticed in other studies. 6,9 Furthermore, it needs to be analyzed whether chemiluminescence influences the signal at the soot radiation observation wavelength of 568 nm.…”

Low-pressure gasoline direct injection system development and emissions analysis for a reciprocating two-stroke engine

“…One of the reasons is that in the engine cylinder the piston already provides pressure (compression phase), and in order to deliver the fuel a higher pressure is necessary. The other reason is that the fuel must be introduced in the shortest period as possible, and it must be injected in an atomized form to facilitate a good mixture with air, mainly when the engine works with a homogeneous mixture for combustion [21]. That pressure is provided by the fuel pump, which, in this case, is a rotary pump from Volkswagen group, and the released fuel has a pressure greater than 10 bar (1 MPa), which is why the pressure must subsequently be regulated.…”

Optical Investigation of a Partial Fuel Stratification Strategy to Stabilize Overall Lean Operation of a DISI Engine Fueled with Gasoline and E30

“…For both fuels and each ϕ, spectra were featured by OH* indicating an occurring oxidation. In reaction mechanism models of gasoline and ethanol the OH radical is involved in the initial decomposition of the fuel molecule, it is produced during oxidation as a product of chain branching and propagating reactions and is a reactant in exothermic chain propagating reactions [28].…”