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Comment by kd5bjo | Hacker News Reader

From my understanding, it's more that the explosion phase of a detonation is more fuel-efficient than the burning phase, but can't last very long under normal circumstances because the flame front is moving so fast-- This is a trick to continue fueling the explosion to sustain it over an indefinite time period.

Think about it in terms of an old-fashioned gunpowder line fuse: If you lay it out in a ring and have some kind of mechanism to continuously place down new gunpowder on the ring in front of the flame, you can keep it going until you run out of fuel.

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What i've never understood is how detonation can be more efficient than deflagration. What does that mean? Both types of engine take a mix of fuel and oxidiser and turn it into hot combustion products. The hot combustion products then expand through a nozzle to produce thrust. How is that process different between the two? Does a deflagration engine leave some fuel unburnt, that a detonation engine burns? Does the combustion of the same fuel somehow produce more heat? Or less heat but more pressure? Is it something about the expansion?

To put it another way, if you set up a deflagration engine and a detonation engine next to each other, and fed them fuel and oxidiser at the same rate, how would the streams of exhaust gas coming out of them look different? What other external differences would you see?

This is mentioned in the introduction here [1]. The version I'm looking at is an image, so I cannot easily copy-paste the relevant passage, but it seems to say that the efficiency gain comes from detonation causing the combustion to occur at a higher pressure than in the case of deflagration. Generally speaking, higher peak pressures increase the efficiency of heat engines, as this allows for greater expansion of the working fluid, and thus more work being done for the same fuel consumption.

With regard to your comparison, I guess this means that the detonation engine can have a higher pressure in the combustion chamber, together with a larger bell, a faster-moving exhaust, or some combination.

[1] https://arc.aiaa.org/doi/10.2514/6.2013-3971

The energy produced by the combustion is the same but different fractions of it are converted into work. Just like efficiency differences in other types of heat engine.

η = 1 - ( T_c / T_h )

Carnot-Efficiency is the theoretical limit of the cycle’s ability to get work out of the thermal energy where T_c is the exhaust (coldest) temperature after expansion and T_h the hottest temperature (before expansion). Temperatures given in K (Kelvin), so 100% if you manage to get T_c to absolute zero temperature.

Just think in terms of the P-V diagram, you want the pressure increase curve to be as vertical as possible to maximize the area enclosed by the cycle

This doesn't help at all. Diagrams are diagrams, what does it actually mean physically?

Think of it as similar to hi compression automobile engine (race car) being more efficient vs a low compression engine (tractor engine). Not exactly correct but a way of starting to think about it

Explosions release more energy in a shorter time, which leads to a higher peak temperature and thus higher kinetic energy in the products. This directly results in more pressure/thrust.

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