A split-cycle engine includes separate compression and expansion cylinders connected by a crossover passage. Crossover compression and expansion valves define a pressure chamber between them in the crossover passage for storing pressurized gas prior to timed delivery into the expansion cylinder. A fuel-air mixture is delivered into the expansion cylinder for ignition and expansion, developing power. One or more ignition sources, such as spark plugs, are positioned to encourage rapid combustion after ignition in the expansion cylinder but far enough from the crossover expansion valve(s) to prevent burning gases from reaching the crossover expansion valves before they are substantially closed to avoid entry of burning gas therein. A “safe distance” (“S”) between the ignition source and an opening of the crossover expansion valve is expressed by the function: S (mm)=combustion speed (mm/crank angle degree)×crank angle degrees from ignition to crossover expansion valve closing.