Model steam engines typically use hexamine fuel tablets, methylated spirits (aka meths or denatured alcohol), butane gas, or electricity to heat the boiler. Cylinders are either oscillating (single-acting or double-acting) or fixed cylinder using slide-valves, piston valves or poppet valves (normally double-acting). Spring safety valves and steam whistles are other common features of model steam engines. Some stationary engines also have feedwater pumps to replenish boiler water, allowing them to run indefinitely as long as sufficient fuel is available.