Investigation of a high efficiency low emissions gas engine
The purpose of this project was to optimise a diesel engine converted to operate on natural gas, to suit the requirements for: low emissions, a high efficiency and sufficient power delivery within the constraints of cogeneration (combined heat and power) systems. Cogeneration Installations seek to improve the efficiency of power generation by utilising waste heat from the prime mover, as well as the production of electricity. Many small scale systems are based on open chamber gas engines, and, to reduce the payback time for the installation, the overall engine efficiency is of prime importance. Stationary engines can be subject to strict standards for emissions, the greatest challenge being presented by the control of NO emissions. The main difficulty is that the highest efficiency operating point of a spark ignition engine is also the point of maximum NO emissions. The extent of this problem was analysed by conducting tests across the entire operating map of the baseline engine at the required speed of 1500 rpm. The solution, in the form of a new high compression ratio combustion system was based on the following: An extensive literature review, the previous Brunel experience with gas engines, an evaluation of the baseline combustion and emissions performance, and the predictions of the Integrated Spark Ignition engine Simulation (ISIS) thermodynamic model. Tests were conducted on the new Fast Bum High Compression Ratio combustion system at compression ratios of 15:1 and 13:1, which demonstrated an extended lean burn capability such that an operating point was identified, that satisfied the conflicting requirements of: low emissions (less than 1g NOx/kWh or 360mg/m3), and a high brake efficiency (above 30%), as well as particular cogeneration criteria. The bmep was mostly above 6 bar. After further tuning and calibration with experimental data, the ISIS model was used to predict the engine power output, efficiency and emissions (NOx and CO) for the compression ratio of 15:1, across the entire operating map for both naturally aspirated and turbocharged configurations. The naturally aspirated results showed good agreement with the results of the experimental 15:1 FBHCR combustion system. The turbocharged engine was simulated with a bmep of 10 bar. The results identified much larger operating areas and all emissions limits were met above a brake efficiency of 36%. The conclusions are, that an open chamber fast bum high compression ratio combustion system can achieve very low emissions, particularly of NOx, and a high efficiency by having the capability of operating with lean enough mixtures. Further improvement in the efficiency is likely if other engine parameters (such as the valve timing) were to be optimised for 1500 rpm. The results from the turbocharged simulation show that turbocharging, whilst restoring the output can also achieve low emissions, and a higher efficiency than a naturally aspirated engine.