Abstract
It has been respected to realize large-scale natural-gas engines. Low flame-propagation speed and poor spatial in-cylinder penetration of gaseous fuels impede the large-scale natural-gas practical. Premixed compression-ignition (homogeneous-charge compression-ignition, HCCI) engines have not such kind of problems, because natural-gas and air are supplied as mixtures, and burn out through speedy knocking-like combustion. However, a very narrow operating range could be established due to a lack of autoignition-timing control procedures. A formaldehyde-assisted premixed-compression-ignition natural-gas engine concept has been proposed previously by the authors to realize large scale natural-gas engines having ignition-timing control procedures. Generation/consumption histories of chemical species during the preflame induction period approaching to the final hot-flame ignition were investigated concerning the formaldehyde-assisted compression-ignition natural-gas engine. Small amount of formaldehyde is supplied as an additive into the premixed intake charge of natural gas and air. The formaldehyde addition has a strong promoting effect for lean mixture ignition of natural gas. When suitable amount of formaldehyde, even a hundreds ppm order of magnitude, is added into the intake fuel/air mixture the piston-compression ignition will occur adequately near the top dead center. This procedure enabled us to obtain ignition or hot-flame explosion even of the fuel/air mixtures nonflammable through a simple piston compression. The formaldehyde acts efficaciously as the ignition-promoting additive for the methane-based gaseous fuels which are weak in cool-flame generation during the preflame induction periods. Natural gas is a typical one of this kind. The in-cylinder gas composition histories were obtained by gas sampling / analyzing processes with a magnetic operating valve and gas chromatographs, concerning mainly to the methane, carbon monoxide, carbon dioxide and formaldehyde. Experiment was carried out using a single cylinder engine and a commercial natural gas 13A. The formaldehyde concentration shows a slight rise followed by prompt decrease at the final stage of the ignition delay period ; so-called blue-flame period, but seemingly stable during almost the whole induction period up to the ignition. A piston compression of a simple charge of natural gas and air with no intake formaldehyde addition showed a gradual formaldehyde generation during the preflame period and sometimes slight consumption at the final stage before the hot-flame occurrence. When the intake air is mixed with formaldehyde only, i. e., no fuel is included, the formaldehyde is consumed briskly near the top dead center, and shows a small but recognizable pressure rise due to a heat release. The effect of formaldehyde added into the fuel/air mixture leading to the ignition would not be an event antecedent to the natural gas preflame reaction but a promoting event of the preflame reaction of the main natural gas fuel. It is demonstrated that during the induction period preflame reaction of main natural gas fuel is under way the effect of formaldehyde addition become extremely efficacious at the final stage just before the heat release of hot flame occurrence.
