Influence of hydrogen co-combustion with diesel fuel on performance, smoke and combustion phases in the compression ignition engine

Highlights

• Hydrogen as secondary fuel for the diesel engine.

• Main combustion phase and ignition delay shorten with hydrogen addition.

• Indicated thermal efficiency stays not changed remarkably.

• Hydrogen addition decreases smokiness, unburnt hydrocarbons and carbon monoxide.

• Knock occurred at hydrogen addition over 35% by energy.

Abstract

The main objective of this study was to examine impact of hydrogen addition to the compression ignition engine fueled with either rapeseed methyl ester (RME) or 7% RME blended diesel fuel (RME7) on combustion phases and ignition delay as well as smoke and exhaust toxic emissions. Literature review shows in general, hydrogen in those cases is used in small amounts below lower flammability limits. Novelty of this work is in applying hydrogen at amounts up to 44% by energy as secondary fuel to the compression ignition engine. Results from experiments show that increase of hydrogen into the engine makes ignition delay shortened that also affects main combustion phase. In all tests the trends of exhaust HC and CO toxic emissions vs. hydrogen addition were negative. The trend of smokiness decreased steadily with increase of hydrogen. Amounts of hydrogen addition by energy share were limited to nearly 35% due to combustion knock occurring at nominal load.

Introduction

The fuel consumption of whole EU transport fleet was 195,315 million tons of diesel fuel (DF) and 88,325 million tons of petrol in 2010. The forecast for 2018 were 214,344.5 million tons of DF and 72,896 million tons of petrol [1]. New diesel cars registrations increased from 23% to 51% in EU during the period of 1995–2010, while the diesel cars in EU shared 35.5% in 2010 [2]. However, diesel engines contribute the environmental pollution in significantly higher level then gasoline engines. Therefore, considerable efforts have been focused toward reducing the diesel exhaust toxic emission as it has negative effect on both the natural environment and human health. In 2015, global CO₂ emissions reached 32.3 GtCO₂, while all kinds of the transport accounted for 7.75 GtCO₂. With increasing emissions from road transport by 68% since 1990, it accounts almost for the quarter of the global CO₂ emissions – 5.8 GtCO₂ [3]. The United Nations and the EU adopted a number of legal regulations, which set the legal basis to take care of the sharpening trend in climate change. The EC took the commitment up to year 2050 to reduce the emissions by 80–95% in comparison to 1990. Whereas the transport has to reduce its emissions by 60% by 2050 in regards with 1990 [4]. To achieve and satisfy these emission obligations, progress research and development in engine technologies is required. The use of renewable fuels has the potential to reduce the emissions and, thus mitigate the effects of the environmental crisis and climate change. Among the currently available renewable fuels there are alternative biomass based biofuels. The physical properties and availability makes hydrogen quite attractive alternative fuel for road transport. Verhelst et al. [5] described the long-term scenario of hydrogen usage as an energy source including it qualitative and quantitative descriptions in order to implement the transition towards clean and sustainable energy. The authors demonstrated the importance of variety of hydrogen-based energy technologies, which enable the efficient and economical way to ensure energy needs. Although the use of sole hydrogen for combustion engines are hardly possible, the co-combustion with various fuels including renewable fuels makes objectives of research interesting. The current alternative, first generation, biomass based biofuels are produced from commonly available, edible feedstock's using well-established conversion technologies [6], [7], [8], [9]. Biofuels produced with aid of second-generation biomass conversion technologies do not compete with food production. High raw material costs is the issue, which is decisive in making biofuel processes economically attractive. The other issue associated with production of biofuels is the energy return to energy invested. This ratio should be at least 3:1 to cover expenses for infrastructure and transportation, while now for biofuels it is approximately 1.3:1. The main sources of biofuels are fatty acids of vegetable oils and animal fats. Vegetable oils consist of a mixture of triglycerides, i.e. esters of glycerol and unsaturated fatty acids. Transesterification of triglycerides with methanol gives a mixture of fatty acid methyl ester (FAME) and glycerol, which can be also considered as engine fuel [10]. The final products of this reaction are glycerol (C₃H₅(OH)₃) and RME (C₁₇H₃₁COOCH₃) [11]. The standard EN 590:2009 in accordance to the Directive 2009/30/EC defines properties of B7 diesel fuel sold at retail and limits the content of the FAME (RME) to max. 7 vol% in the fossil diesel fuel. This fuel was denoted here as RME7. These two fuels (RME7 and pure RME) with hydrogen were used in engine combustion tests presented in this paper.

As known from literature survey, there are several works presenting impact of hydrogen on diesel fuel combustion in the compression ignition (CI) engine. Szwaja et al. [14] carried out tests with amounts of 5% hydrogen by energy share to fossil diesel fuel and revealed the shorten ignition lag and decrease in the rate of combustion pressure rise. With a hydrogen energy share (HES) of 5–15%, the entire combustion duration did not change significantly, but with hydrogen share of 15%, peak combustion pressure p_max increased. With HES of 17% the combustion knock occurred and with HES of 25% the fast combustion was accompanied by severe combustion knock. Labeckas et al. [12] conducted experimental study on a CI engine fueled with 5–15 vol% ethanol blended DF and additionally blend (E15B) consisted of ethanol (15 vol%), DF (80 vol%) and RME (5 vol%). They observed that oxygen content in the mixture reflects the auto-ignition delay caused by use of E15B blend more predictably than the Cetane Number (CN) does. The auto-ignition delay for oxygenated blend E15B was 15.4% longer than for DF and the indicated specific fuel consumption (ISFC) was increased. Yang et al. [15] studied influence of hydrogen addition on the performance of the CI engine and determined the best ratio for H₂ addition. They found hydrogen enrichment decreases particulate matter (PM) emissions and provides optimal results for maximum heat release rate (HRR) at 17% H₂ by volume. In the investigation by Rocha et al. [16] performed on a diesel generator, hydrogen was supplied at HES of 5–24% to the diesel – biodiesel (7%) blend (B7). With increase of HES, both CO₂, CO and HC emissions decreased. However, the NO_x increased due to increase of in-cylinder temperature. There was also both increase in the peak pressure and heat release rate noticed, since ignition delay was reduced due to increase of HES. Experiments carried out on the CI engine [17] with fossil DF blended with 20 vol% RME and HES of 0–5% revealed the lower engine performance, efficiency and toxic emissions (CO, HC) except NO_x, which slightly increased. As they observed, addition of hydrogen did not affect auto-ignition delay. No significant increase in NO_x was also observed during testing the CI engine with EGR [18] running with DF blended 7% FAME. However, HES of 25% caused the reduction of p_max and reduction of CO₂ emission by 22%. Chelladorai et al. [19] investigated the grapeseed oil as a fuel substitute obtained from biomass waste from winery industry. He studied effect of hydrogen addition to a CI engine fueled the grapeseed oil. At full load, the maximum indicated thermal efficiency (ITE) with diesel, grapeseed biodiesel (GSBD) and neat grapeseed oil (NGSO) increased from 32.34%, 30.28% and 25.94% to 36.04%, 33.97% and 30.95% for HES of 14.46%, 14.1% and 12.8%, respectively. Ignition delay increased with hydrogen induction as a result of reduced oxygen concentration in the in-cylinder charge. Based on studies by Zhou [23] on the CI engine with various amounts of hydrogen added, it shows that emissions and performance parameters are dependent on the parameters as follows: injection timing of DF, its combustion duration, HES, BMEP and engine speed.

As known, the lower flammability limit (LFL) of the hydrogen-air mixture changes with change of temperature and pressure. The experiments performed by Schroeder and Holtappels [24] show that LFL decreases with increase in temperature. The temperature in region of the actual start of combustion (SOC) was 370–412 °C with both fuels during the experiment performed by authors. At this temperature, the LFL decreased to 1.5% hydrogen by volume. However, this decrease of LFL does not cause hydrogen ignition due to temperature is still too low (370–412 °C) and not sufficient for hydrogen auto-ignition. Moreover, increase of pressure has opposite effect to temperature. The LFL increased from 4% to 5.6% with increase of pressure up to 50 bar and further increase of pressure has no influence on LFL as reported from Refs. [24], [25]. At ignition point the pressure in the cylinder was 3.5–3.9 MPa, while the LFL was 3.0–3.1% hydrogen by volume. Therefore, one concludes that hydrogen can be effectively co-combusted with injected biofuels if only LFL was achieved in the engine cylinder. Before that, the lean mixture of air – hydrogen with biofuel burns incompletely and hydrogen does not make positive effect on the combustion duration and engine performance [5], [26].

As reviewed, there are some gaps in knowledge dealing with hydrogen assisted combustion in the CI engine fueled various fuels. This paper presents results of investigation on hydrogen co-combusted with RME7 and pure RME on both performance, combustion phases and toxic exhaust emissions from the CI engine. It was observed, that emissions and engine performance are dependent on the following: HES, injection parameters as timing and its duration, and equivalence ratio. The novelty of this work deals with relatively high hydrogen content in the entire amounts of fuels combusted at in the CI engine at the hydrogen knock onset. Contribution to the state-of-art is realized by knowledge extension in the field of thermodynamic analysis of the diesel engine working in the dual-fuel mode and use hydrogen as the secondary fuel at amounts up to 44% by energy.