Comment Re:Cannot explode but can be used in cars? (Score 5, Informative) 603
Um, not really.
A combustion event, aka 'explosion' occurs at the beginning of every power stroke in a reciprocating internal combustion engine. When an engine 'knocks' there is a combustion event as well. What makes it a 'knock' instead of a normal part of the power cycle is that it occurs at the wrong time. Knocking indicates perhaps a spark timing issue or the use of a fuel with an improper octane rating (which indicates resistance to knock). Octane ratings describe the resistance of the fuel to spontaneous ignition relative to a mixture of iso-octane (by definition Octane rating of 100) and n-heptane (by definition an octane rating of 0). Extrapolation is what allows for an octane rating of greater than 100. Diesel fuel has a similar concept, a Cetane number which indicates susceptibility to "spontaneous" combustion, since diesels use compression to ignite combustion events rather than an electrical spark.
Modern cars do depend on a much higher octane rating than historical vehicles. This allows for running on a much higher compression ratio and/or the use of turbo-chargers which allow for an engine that is thermodynamically more efficient (as compression ratio approaches infinity, thermodynamic efficiency approaches unity). This is one reason why diesels (compression ratios in the 20's rather than 5-10 for gasoline vehicles) get better mileage for a comparable vehicle/power output.
You are, however, entirely correct about the relative difficulty of causing a gasoline burn or explode. Only the vapor state is flammable and only at a narrow range of particle size.
A combustion event, aka 'explosion' occurs at the beginning of every power stroke in a reciprocating internal combustion engine. When an engine 'knocks' there is a combustion event as well. What makes it a 'knock' instead of a normal part of the power cycle is that it occurs at the wrong time. Knocking indicates perhaps a spark timing issue or the use of a fuel with an improper octane rating (which indicates resistance to knock). Octane ratings describe the resistance of the fuel to spontaneous ignition relative to a mixture of iso-octane (by definition Octane rating of 100) and n-heptane (by definition an octane rating of 0). Extrapolation is what allows for an octane rating of greater than 100. Diesel fuel has a similar concept, a Cetane number which indicates susceptibility to "spontaneous" combustion, since diesels use compression to ignite combustion events rather than an electrical spark.
Modern cars do depend on a much higher octane rating than historical vehicles. This allows for running on a much higher compression ratio and/or the use of turbo-chargers which allow for an engine that is thermodynamically more efficient (as compression ratio approaches infinity, thermodynamic efficiency approaches unity). This is one reason why diesels (compression ratios in the 20's rather than 5-10 for gasoline vehicles) get better mileage for a comparable vehicle/power output.
You are, however, entirely correct about the relative difficulty of causing a gasoline burn or explode. Only the vapor state is flammable and only at a narrow range of particle size.
