Combustion and emissions of a diesel engine with hydrogen addition

Abstract

Experimental investigations were conducted on a 4-cylinder natural-aspirated direct-injection diesel engine with naturally aspirated hydrogen, focusing on the effect of hydrogen addition on engine combustion, performance, regulated and unregulated emissions under a wide range of operating conditions. Two sets of investigation were adopted. First, the engine was operated at 1800 rev min-1 under five loads and hydrogen was added to provide 10 to 40% (at 10% interval) of total fuel energy at each engine load. Second, based on the Japanese 13-mode test cycle, hydrogen was added to provide 10, 20 and 30% of total fuel energy at 10 selected modes and the other 3 modes were operated with only diesel fuel. The second set of test provided information of influence of engine speed. With hydrogen addition, there is gradual reduction in in-cylinder pressure and heat release rate at low engine load and speed and corresponding decrease of brake thermal efficiency (BTE) and increase of brake specific fuel consumption (BSFC), both of which are due to the low combustion efficiency of hydrogen at low engine load and speed. Increase of in-cylinder temperature and pressure, associated with increase of engine load and speed, improves the combustion efficiency of hydrogen and hence increases BTE and reduces BSFC. At high engine load, both ignition delay and combustion duration are reduced since the combustion of diesel fuel is accelerated by hydrogen addition. A “three-stage combustion mode, composed of diesel premixed combustion, hydrogen premixed combustion and the combination of hydrogen premixed combustion and diesel diffusion combustion is observed. Meanwhile, the peak in-cylinder pressure and peak heat release rate sharply increase due to the co-combustion of diesel fuel and hydrogen. Such combustion mode could lead to abnormal combustion, such as pre-ignition, at high engine load and speed, which is harmful to the engine and have to be avoided. CO₂/CO, particulate mass, acetaldehyde (CH₃CHO), olefins (C₂H₄, C₃H₆ and C₄H₆) and cyclic hydrocarbons (C₆H₆, C₇H₈ and C₈H₁₀) could be effectively reduced at most of the operating modes. However, with high amount of hydrogen addition, total HC and formaldehyde (HCHO) increase at low engine load and speed. NOx increases at high engine load and speed due to the considerable increase of NO₂. Particle size and number concentration both decrease due to the inhibition effect of hydrogen on particle formation based on the HACA (H₂ abstraction and C₂H₂ addition) mechanism. Particular attentions were paid on the effect of hydrogen addition on the physico-chemical properties of the diesel particulate. Hence diesel particulates were sampled for off-line analysis, with the aid of TEM, TGA and GC/MS facilities. Larger primary particles are emitted at high engine load and speed which is affected by the equivalence ratio. Hydrogen addition promotes particle oxidation at low engine load and speed due to the increase of exhaust temperature, resulting in smaller particles, but it inhibits particle oxidation at high engine load due to the competition of oxygen between hydrogen and diesel fuel which results in larger primary particles. The replacement of injected diesel fuel by hydrogen inhibits the formation of soot nuclei and decreases its volume density, hence reduces the size of aggregate particles which are more spherical as indicated by an increase of fractal dimension and a decrease of radius of gyration. With increase of engine load, primary particles exhibit more graphitic structure, changing from "onion like" to "shell-core" structure. Under diesel-hydrogen dual-fuel operation, hydrogen addition promotes and inhibits primary particle oxidation at low and high engine loads, respectively, and the corresponding primary particles are "turbostratic interlayer" and "shell-amorphous" in structure, respectively. The results of recognized fringe length, tortuosity and fringe separation distance are consistent with the observed morphology as mentioned above. Hydrogen addition increases the percentage of volatile substances in diesel particulate at low engine load which is due to the interference on oxidation of volatile substances. Due to the high exhaust temperature, the volatile substances are almost fully oxidized or evaporated at high engine load and the effect of hydrogen addition on volatile substances is not significant. The oxidation reactivity is related to equivalence ratio, being higher at low engine load and speed and lower at high engine load and speed, which is indicated by the variation of activation energy and ignition temperature. The oxidation reactivity is validated to be related to the nano-scale feature parameters of primary particles. All the 16 examined particle-phase polycyclic aromatic hydrocarbons are effectively reduced at low, medium and high engine speeds under low and high engine loads. The reduction of BaP equivalent suggests a reduction of toxicity of diesel particulate with hydrogen addition. Finally, three case studies are conducted. Firstly, traditional neural networks methods and machine learning methods are successful in predicting the engine performance and tailpipe emissions of the diesel engine with hydrogen addition with high accuracy. Secondly, hydrogen is experimentally validated to be effective in reducing CO2, CO, PM in mass, number and size of diesel engine fueled with diesel-biodiesel blends and neat biodiesel with small penalty of NOx emissions. Thirdly, hydrogen addition in diesel-methane dual-fuel engine can improve the engine performance and reduce CO and total HC emission at low engine load. Methane addition in diesel-hydrogen dual-fuel engine can avoid uncontrolled combustion of hydrogen at high engine load.