Study On Miniaturization Limit Of Micro Reciprocating Internal Combustion Engine

Power MEMS, as micro power generators, have the function to achieve high power and energy density in small scale applications, which is very suitable for micro aerial vehicle(MAV) or human portable devices to realize more size reduction and longer running time. Since the specific energy of hydrocarbon fuels(on average level of 105 k J/kg) are much greater than the traditional chemical batteries(on average level of 102 k J/kg), miniature heat engines that use hydrocarbon fuels appear to be a natural choice for high energy density power units. Because of simple structure, the micro reciprocating internal combustion engine is one kind of these generators attracting increased attention, and is very suitable as a miniaturized power source. Therefore, in this work, the miniaturization limit of the micro reciprocating internal combustion engine was investigated. The current miniature engine with displacement of 0.99 cc is experimentally investigated and thermodynamically analyzed here and feasibility of scaling down the miniature internal combustion engine from small scale to meso scale was inspected. The main contents are as follows:(1) Based on the principle of torque combination method, micro engine experimental bench was setup and performances of a miniature engine with displacement of 0.99 cc were investigated and compared with that of ordinary full-size engines according to similarity criterion. Then the technical features of micro internal combustion engine were analyzed. Results show that the micro engine has significantly lower brake mean effective pressure(BMEP), higher fuel consumption rate, poor thermal efficiency and low heat load. However, the miniature engine can achieve higher operational speed because friction mean effective pressure(FMEP) seems to be less sensitive to the engine speed, so that it achieves a superior specific power.(2) Due to structural limitations, the micro reciprocating internal combustion engines use glow-ignition mode. So a combustion test platform was setup and combustion diagnosis of the miniature engine with glow-ignition was investigated. Combustion diagnosis results show that this mode has lower rate of heat release and longer combustion duration. Especially, the rapid combustion period seems longer about 20 degrees than conventional full-size engines with spark plug ignition. Since the glow ignition cannot give stable ignition timing, and thus the start of combustion(SOC) varies considerably from cycle to cycle, which results the serious cycle-by-cycle variations. Therefore, to develop micro engines by scaling-down reciprocating internal combustion engines, efforts should be pay on improving processes of combustion in cylinder. Thus, the experiment of compression ignition for micro engine was designed and tested and compared with glow-ignition mode. Result show that the performance characteristics of compression ignition were much better than glow-ignition. But at compression ignition mode, it has serious cyclic variations. It is differential for micro engine to achieve stable compression ignition combustion through thermodynamic process. So the combustion mode of glow-ignition combine with compression ignition was proposed.(3) Combustion diagnosis results show that the micro internal combustion engine with glow-ignition has higher level of cyclic variations, lower rate of heat release and longer combustion duration. In order to improve the characteristics of glow-ignition at micro space condition, the experiment of adjust fuel component was tested for character diagnosis of the combustion. First, adding additives of nitromethane and hydrogen peroxide were investigated. With the additive of nitromethane, the start of combustion(SOC) is advanced, the combustion velocity is improved and the coefficient of variation is reduced. However, the combustion characteristics become poor when the hydrogen peroxide is added to the fuel. In order to analyze the effects of nitromethane on the combustion, experiments of adding different proportions of nitromethane to methanol fuel and ethanol fuel were investigated. Under the glow-ignition condition with alcohol fuels, the nitromethane affects its indicated mean effective pressure(IMEP) and characteristic of cyclic variation by the adjustment of SOC. Adding nitromethane makes the SOC advance and let the combustion duration shorten that lead to the decrease of cycle-by-cycle variation. While with high proportion of nitromethane, the SOC tends to become more dispersed and the cycle-by-cycle variations become more severe, which result in performance degradation.(4) In order to scale down the combustion space and improve the combustion characteristics of glow-ignition, the experiment of increase the hot strength of platinum wire was tested. Combustion diagnosis results show that increasing the hot strength of platinum wire, the start of combustion is advanced, but its cycle-by-cycle variations became more severe. The combustion duration trends to shorten, but the rate of heat release decreases. However, under the condition with high misfire rate, it can reduce misfire obviously. Therefore, the combustion stability of the miniature engine can be improved by adjust the hot strength of platinum wire.(5) To analyze the working process of the micro internal combustion engine, cycle simulation method was introduced. It reveals that the low volumetric efficiency leads to the low BMEP and the main reasons for the lower thermal efficiency are rich mixture, incomplete combustion, and fuel loss in the scavenging process. Consequently, approaches of further minimizing micro engine are to increase engine speed and improve charging process and combustion in cylinder. Also, compression ratio and gap width between cylinder and piston have an important influence on the performance of micro engine. Higher compression ratio will result in more leakage, and then decline engine performance.(6) In order to deduce the minimal possible size of the micro reciprocating internal combustion engine, “Scale Down” procedure of miniature engine was simulated and the thermodynamic phenomena and its variation were analyzed. As the miniature engine is scaled down from small scale step-by-step to meso scale, charge characteristics become more favorable for high speeds. But the ratio of clearance area to cylinder volume, as well as the surface-volume ratio of combustion chamber becomes higher, so that mass leakage and heat loss in cylinder tend to increase, and then specific power and indicated mean effective pressure(IMEP) is distinctly decreases. It is thermodynamically deduced that the miniaturization limit of the scaled-down engine cylinder bore is between 4 to 5 mm, and the engine operating speed is around 40000~50000 r/min. By reducing the compression ratio of cylinder and crankcase, and shorten the duration of scavenging and exhaust phase, the minimum limit of cylinder bore size can still be reduced to 4 mm or less.