[...] I'm hoping to avoid anything too dangerous [...]
too dangerous. Very bad choice
substance produces a shock wave of huge pressure that is destructive to a chamber. What you need is a slow flame front. Air+propane mixture, air+gasoline mist with proper octane rating, with luck air+methane.
Even these flame fronts create over pressures several times bigger than the equilibrium computed by the ideal gas law. Difficult to predict, use experimental data.
Also, technology limits operating pressure of piston engines, notably at seal rings. Some engines do consume only a liquid as a propellant, like H2
... In submarines, only a fraction of the produced gas is let out after expansion, the rest is compressed again, and a small amount of propellant is added. Igniting a liquid or solid propellant without much extra chamber volume
is just excluded.