PROBLEMS in combustion at low engine speeds have been created by the high levels of compression ratio in current U. S. passenger cars. These are: Hot-starting noise, hot-cranking difficulty, and after-running. These combustion irregularities are not major field problems at present, but their severity and frequency of occurrence will probably increase as compression ratio continues to rise.
An investigation of these low-speed combustion irregularities has been conducted in the laboratory and on the road. It has been found that hot-starting noise usually occurs during the first revolution the engine makes when being cranked. Although starting noise may result from compression ignition, it usually follows spark ignition. It can be influenced by engine operating conditions, compression ratio level, engine water jacket temperature, cranking speed, and fuel type. The starting noise behavior of a fuel has been related to its octane quality, volatility, and aromaticity.
Hot-cranking difficulty, the slowing down or stalling of the starting motor, is due to early compression ignition of the fuel, and appears related to cetane quality. After-running at low engine speeds, attributable to compression ignition following ignition cutoff, also appears related to fuel cetane quality. The three low-speed combustion difficulties were found to be worse when operating on a paraffinic fuel than with other fuel types of the same research octane number.