3 ms·

Internal combustion engines do in fact attempt to burn fuel and oxidizer at maximum stoichiometric efficiency, and they also are trying to burn it as quickly as

Internal combustion engines do in fact attempt to burn fuel and oxidizer at maximum stoichiometric efficiency, and they also are trying to burn it as quickly as possible to capture all of the heat rather than exhausting a partially burnt mixture and wasting fuel. Effective explosives have the exact same chemical goals and nearly the same thermodynamic goals. If that isn’t a carefully controlled explosion I don’t know what is. I mean, come on, a piston moving up and down thousands of times per minute isn’t explosive enough for ya?

jonsen 6y ago
> ...trying to burn it as quickly as possible... No, the speed of burn in an internal combustion engine is not explosive: “Knocking (also knock, detonation, spark knock, pinging or pinking) in spark ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.” - WP on engine knocking, which is undesirable and may damage the engine.
- RealityVoid 6y ago
  Sounds to me that the problem is not the speed of the explosion that happens when engine knocking, but the timing of it.
Obi_Juan_Kenobi 6y ago
Engines combustion is designed to occur as a propagating flame front in the cylinder initiated by the spark plug. Great efforts are made to ensure that this is precisely what happens. The alternative is a detonation, i.e. engine knocking. This causes a spike in cylinder pressure and will quickly destroy an engine. It is for this reason that engines are absolutely not trying to burn fuel as quickly as possible. This is a difference of detonation vs. deflagration. Whether you consider both of these to be forms of explosions is a semantic issue.
namibj 6y ago
There are HCCI engine prototypes, but they are finicky, because the mixture (mostly regarding initial temperature and pressure) has to be in a narrow flammability band, so that it ignites precisely at Top Dead Centre from adiabatic compression heat. If it ignites earlier, the pressures are far higher than normal. If it doesn't trigger, it won't for the complete cycle, as the piston expands and the temperature drops. Free-piston engines have an advantage there, as they don't force the compression ratio with a crankshaft. They would decelerate the piston somewhat earlier and harder than necessary, but there are no bearings that have to handle this load. Their issue has more to do with extracting energy from the oscillation, and keeping a stable operating point that's neither prone to stalling nor prone to runaway.

jonsen 6y ago

> ...trying to burn it as quickly as possible... No, the speed of burn in an internal combustion engine is not explosive: “Knocking (also knock, detonation, spark knock, pinging or pinking) in spark ignition internal combustion engines occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.” - WP on engine knocking, which is undesirable and may damage the engine.

RealityVoid 6y ago
Sounds to me that the problem is not the speed of the explosion that happens when engine knocking, but the timing of it.

Obi_Juan_Kenobi 6y ago

Engines combustion is designed to occur as a propagating flame front in the cylinder initiated by the spark plug. Great efforts are made to ensure that this is precisely what happens. The alternative is a detonation, i.e. engine knocking. This causes a spike in cylinder pressure and will quickly destroy an engine. It is for this reason that engines are absolutely not trying to burn fuel as quickly as possible. This is a difference of detonation vs. deflagration. Whether you consider both of these to be forms of explosions is a semantic issue.

namibj 6y ago

There are HCCI engine prototypes, but they are finicky, because the mixture (mostly regarding initial temperature and pressure) has to be in a narrow flammability band, so that it ignites precisely at Top Dead Centre from adiabatic compression heat. If it ignites earlier, the pressures are far higher than normal. If it doesn't trigger, it won't for the complete cycle, as the piston expands and the temperature drops. Free-piston engines have an advantage there, as they don't force the compression ratio with a crankshaft. They would decelerate the piston somewhat earlier and harder than necessary, but there are no bearings that have to handle this load. Their issue has more to do with extracting energy from the oscillation, and keeping a stable operating point that's neither prone to stalling nor prone to runaway.