###### Abstract  
Micro reciprocating piston internal combustion engines are potentially desirable for high-energy density micro power sources. However, complex subsystem functions hinder the downsizing of reciprocating piston internal combustion engines. The homogeneous charge compression-ignition (HCCI) combustion mode requires no external ignition system; it contributes to structural simplification of the reciprocating piston internal combustion engines under a micro space constraint but has not been adequately verified at the millimeter scale. The study used a millimeter-scale HCCI reciprocating piston internal combustion engine fueled by a mixture of kerosene, ether, castor oil, and isopropyl nitrate for combustion investigation. The test engine with a displacement of 0.547 cc is the smallest reciprocating piston internal combustion engine known to have undergone in-cylinder combustion diagnosis. It is observed that the HCCI combustion mode at the millimeter scale can realize stable combustion with excellent cooperation for the thermodynamic cycle under appropriate structural and operating conditions, which is essentially not inferior to those in conventional-sized reciprocating piston internal combustion engines. This finding helps the next step of scaling down reciprocating piston internal combustion engines. micro power system; miniature engine; micro combustor; HCCI; millimeter scale; thermodynamic analysis +
Footnote †: journal: Medical Engineering  
and incomplete combustion [17; 18]. [PERSON] et al. conducted performance tests on nine miniature glow ignition reciprocating piston internal combustion engines and indicated that the thermal efficiencies decreased rapidly when the engine displacement was down-sized to 10 cc, with combustion losses accounting for 65% of the total energy loss [19; 20]. In response, the in-cylinder combustion of miniature reciprocating piston internal combustion engines was explored in depth. [PERSON] et al. tested a platinum wire glow ignition miniature internal combustion engine with a displacement of 7.4 cc and investigated the effects of nitromethane additives, air/fuel ratios, and the structure of platinum wire ignition plugs on the performance and combustion stability [21]. [PERSON] et al. performed combustion tests on a homogeneous charge compression ignition (HCCI) miniature internal combustion engine with a displacement of 6.5 cc and investigated the in-cylinder combustion at different speeds and loads [22]. [PERSON] et al. tested an HCCI miniature internal combustion engine with a displacement of 4.1 cc and investigated the effects of compression ratio, cylinder head temperature, and the shape of the combustion chamber on the in-cylinder combustion [23; 24]. [PERSON] et al. tested a glow ignition miniature internal combustion engine with a displacement of 1.5 cc and diagnosed the in-cylinder combustion characterized by the low indicated mean efficient pressure (p\({}_{\text{m}i}\)) and severe cyclic variations [25]. [PERSON] et al. diagnosed the in-cylinder combustion of a glow ignition miniature internal combustion engine with a displacement of 0.99 cc, pointed out the imperfection of lagging combustion time, long combustion duration, and sizeable cyclic variation, and tried to improve in-cylinder combustion by fuel additives (nitromethane, hydrogen peroxide, etc.) and platinum wire glow intensity [26; 27; 28]. Miniature reciprocating piston internal combustion engines are fueled by the methanol-nitromethane mixture in the glow ignition combustion mode and the ether-ketosene combination in the HCCI combustion mode. Both blended fuels add castor oil as a lubricant. Under the HCCI combustion mode, the air-fuel mixture is ignited simultaneously at multiple points; thus, the combustion rate is higher. In addition, no flame quenching happens due to the absence of flame propagation, and no external ignition needs to be installed. These characteristics make the HCCI combustion mode more adaptable to a tiny space [29; 30; 31].