A small portable power pack includes a fuel/air supply for mixing fuel, which is supplied from outside, with outside air, thereby providing mixed gas; a uniflow scavenging micro-engine for receiving mixed gas from the fuel/air supply and igniting mixed gas to explode; a control panel for operating and controlling the uniflow scavenging micro-engine; a capacitor battery for powering the control panel and the uniflow scavenging micro-engine. The portable power pack is easily carried and used without the restriction of spaces and sites.