An ethylene-fueled scramjet operating at Mach 10 was experimentally tested in the JF-24 shock tunnel and modeled using improved delayed detached eddy simulation based on up to 368.34 million cells. An in-depth analysis of the effects of thermal and chemical nonequilibrium on combustion characteristics and engine performance was conducted. The contrary effects of nonequilibrium heating and nonequilibrium cooling that occur in different sections of a scramjet were revealed. The underlying mechanism can be attributed to the delayed relaxation of thermal nonequilibrium under energy addition or deduction. The nonequilibrium case has better mixing, while the equilibrium case has higher combustion efficiency. The synchronous reductions in thrust and drag counteract each other and lead to a higher final net thrust under nonequilibrium. The net thrust increases with the global equivalence ratio, whereas the specific impulse decreases. The evolution of flamelets and reaction paths were analyzed to reveal the effect of chemical nonequilibrium, which produces an abundance of O, OH, and NO radicals through endothermic dissociation reactions and significantly alters the rate-limiting reaction paths.