Natural gas direct injection in spark ignition engines

This thesis is a study of direct injection (DI) of compressed natural gas (CNG) for spark ignited internal combustion engines. The outcome of this research can be categorised into two sections, namely, steady state engine tests and fuel injection studies. The objective of engine testing is to investigate the effect of directly injecting natural gas especially after the intake valve closure on engine performance at various operating conditions. This is accomplished on a single cylinder AVL research engine. The studies show that late injection improves the peak output (IMEPn) by about 11% at 1000 RPM to 7.5% at 2000 RPM as a result of increased volumetric efficiency. The increased peak output also marginally improves the engine thermal efficiency. At part loads too, late injection offers faster combustion as a result of injection induced turbulence and improves combustion stability, however, at the cost of increased pumping losses. Similar trends are observed for leaner mixtures as well. An extension in lean limit (λ) from 1.5 to 1.8 is achieved while maintaining excellent combustion stability (CoV of IMEPn < 3%) at the worldwide mapping point. A 5% absolute improvement in engine efficiency is achieved with stratified lean combustion at a low load of 3 bar IMEPn at 1000 RPM. Further investigation is needed to optimise the mixture formation and the ignition system in order to lower the cyclic variations and maximise engine efficiency.<br><br>Injection studies are undertaken to characterise the highly under-expanded transient jet flow of natural gas from an outward opening direct injector. Schlieren imaging technique is used at high spatial and temporal resolution to study the jet growth and mixture formation in a constant volume chamber (CVC) at a wide range of operating pressures. An outward opening direct injector 1 creates a conical area of flow that produces a hollow cone jet at the beginning of injection. The jet is headed by a toroidal vortex which collapses along the injector’s axis depending on the injection conditions. The jet growth reveals that it obeys existing correlations of penetration originally developed for single hole type of nozzle i.e. the jet penetration scales with (Mn/ρa)1/4 and t1/2. The constant of penetration is found to be 1.15 ± 0.05. The near-field shock structure is characterised at high resolution and its transient behaviour can be correlated with the needle lift profile of the injector. A 3D CFD simulation campaign is undertaken to complement the experimental data and to further predict the jet growth for different nozzle seat angle. Wider nozzle angle for a given needle lift increases the flow area and therefore results in higher mass flow rate. As a result of the CFD investigation, the proposed nozzle design is to have a wider entry angle to achieve high mass flow and an exit angle depending on the jet targeting requirements of the application.<br><br>The work presented here is expected to improve our understanding of mixture formation of directly injected gaseous fuels, particularly natural gas, for developing efficient and cleaner engines.