A novel method to improve the cold starting ability of a low compression ratio diesel engine through recompression of the charge

摘要

Reducing the compression ratio in a diesel engine will result in lower NOx and soot emissions but will lead to problem of starting and combustion instability at cold conditions. In this work, a novel method for improving cold starting ability has been conceptualized and verified through simulations and experiments. In this method, the temperature of the charge is enhanced by ejecting the hot air into the intake manifold at the end of compression stroke. Due to the irreversibility associated with this process, the temperature of the air in the intake manifold increases and this air is again inducted into the cylinder and compressed to further augment the temperature. This cycle is repeated till the temperature at the end of compression is sufficient to autoignite the injected fuel to aid first firing. This methodology was demonstrated on a single cylinder common rail diesel engine with a compression ratio of 14:1, which indicated that the temperature in the intake manifold progressively increased by up to 75 K within 10 crank shaft rotations during cranking. In subsequent experiments with the proposed system, ignition occurred right from the second rotation, whereas there is no combustion occurred in the conventional engine at ambient conditions (28 degrees C). However, at cold condition (10 degrees C), partial combustion was initiated only after 15 recompression rotations and does not produce any positive work output. To mitigate this, the injection schedule was further modified such that the partially burned products were retained in the cylinder without opening the valves and recompressed which autoignited the residual fuel during the subsequent compression stroke. At the end of this compression stroke, the in-cylinder temperature and pressure were high enough and hence the fuel was again injected which resulted in sustained combustion and produced an indicated mean effective pressure (IMEP) of 6 bar that could result in starting even at cold conditions (10 degrees C). Hence, it was demonstrated that the proposed methodology with modified valve timing and injection scheduling has the potential to start the engine at low temperatures particularly with low compression ratios.