Experimental investigation of direct injection dual fuel of n-butanol and biodiesel on Intelligent Charge Compression Ignition (ICCI) Combustion mode

Highlights

• The n-butanol/biodiesel Intelligent Charge Compression Ignition (ICCI) Combustion mode was firstly proposed.

• Concentration and reactivity stratification can be flexible realized under the ICCI mode.

• The optimum biodiesel injection timing was −60° ATDC with a butanol energy ratio of 70%.

• The indicated thermal efficiency can reach 50.7% and NOx emissions meet the Euro 6 emission standards.

Abstract

To achieve ultra-high efficiency and low emissions over the full engine operating range, the intelligent charge compression ignition (ICCI) combustion mode was firstly proposed. In ICCI mode, by real-time control over the injection strategies of two independent direct injection systems (including fuel proportions, injection pressure, timing and duration), the concentration and reactivity stratifications of the air-fuel mixture can be flexibly adjusted. Therefore, in this paper, an experimental study was conducted on a single cylinder diesel engine to research the working mechanism of ICCI mode fueled with biodiesel and n-butanol. Experimental results showed that, the n-butanol/biodiesel ICCI combustion mode had great potential to improve engine efficiency and reduce its emissions. At medium load, the early biodiesel injection timing (SOI2) can shorten the combustion duration and improve the indicated thermal efficiency (ITE), and the maximum ITE can reach 50.7%. As the butanol energy ratio increased, the nitrogen oxides (NOx) emissions could be reduced due to the reduction of local high temperature, while NOx emissions were always at low level and it can meet Euro 6 emission standards. The butanol injection pressure had significant effect on the combustion and emission characteristics of the ICCI mode, properly increasing the butanol injection pressure can improve the ITE and reduce emissions, however, excessive butanol injection pressure deteriorates the combustion process, increased the hydrocarbon and carbon monoxide emissions. In general, the butanol-biodiesel ICCI mode can achieve stable combustion under different loads while ensuring high efficiency and ultra-low emissions with a single injection of biodiesel at low/medium loads and two injections of biodiesel at high load.

Introduction

The internal combustion engine is widely used in the production of human life due to the high energy conversion efficiency and working stability since it was invented [1]. However, the conventional diesel combustion (CDC) mode has the drawback of higher nitrogen oxides (NOx) and particulate matter (PM) emissions [2], which are extremely harmful to the environment and human health and is a major issue to limit its development [3]. Therefore, many advanced combustion technologies, which are called the low temperature combustion (LTC) modes with high efficiency and low emissions, have attracted more attention and research. Compared with CDC mode, LTC modes are characterized by the relatively lower adiabatic combustion temperature and prolonged fuel/air mixing time in the cylinder to avoid local high temperature and fuel-rich regions [4], then the NOx and PM formation can be inhibited [5].

Homogeneous charge compression ignition (HCCI) is one of the earliest LTC modes [6], the combustion occurs under homogenous lean mixture conditions that are beneficial to lower combustion temperature [7], leading to the lower NOx and PM emissions and higher thermal efficiency [8]. However, the combustion phasing and burning rate cannot be controlled because HCCI mode is mainly dependent on the chemical kinetics, which are sensitive to the temperature, reactivity and concentration of mixture in the cylinder [9]. With further research on HCCI, researchers found that the absolute homogeneous charge did not exist in the cylinder, while a proper degree of concentration and reactivity stratification had great potential in controlling the combustion phasing and burning rate [10]. Therefore, the partially premixed compression ignition (PPCI) mode with the advantage of the HCCI mode is introduced to solve these problems [11]. Researchers from Lund University [12] found that NOx and soot emissions were lower than the CDC mode, and engine loads could be extended by injecting a higher proportion of gasoline into diesel engine under PPCI mode. Benajes et al. [13] conducted an experiment on compression ignition diesel engine fueled with commercial gasoline, they concluded that NOx emissions below 0.4 g/kW h and high load conditions (>10 bar) could be achieved by using multiple injection strategies with 43.5% exhaust gas recirculation (EGR). However, PPCI mode also faced the problems of higher hydrocarbon (HC) and carbon monoxide (CO) emissions [14], insufficiency of controlling the ignition timing and combustion duration [15], and unstable combustion at high load [16].

The concept of reactivity controlled compression ignition (RCCI) mode is a dual-fuel (low and high reactivity fuel) engine combustion technology, low reactivity fuel is injected by port fuel injection (PFI) to form the premixed mixture and high reactivity fuel is injected in the cylinder by single or multiple direct injection (DI) to achieve concentration and reactivity stratification [17]. In RCCI mode, stable combustion only can be operated at medium load, it occurs misfire due to the extreme fuel-lean mixture at low load and higher combustion pressure rise rate (PRR) at high load [18]. In order to achieve higher thermal efficiency and low emissions during the engine operation conditions, different injection strategies are used to control fuel stratification, while RCCI has certain limitations in the mixture stratification due to its injection strategies of PFI and DI, and it is impossible to achieve any degree of concentration and reactivity stratification before the injecting of high reactivity fuel in the cylinder. Hanson et al. [19], Inagaki et al. [20], Kokjohn et al. [21], and Splitter et al. [22] investigated the RCCI mode, although, they extended up to 16 bar indicated mean effective pressure (IMEP), cares also need to be taken on the higher PRR and combustion noise, resulting in higher heat transfer loss. Meanwhile, the dual fuel sequential combustion (DFSC), which was firstly proposed by Lu et al [23], also faced the similar problems as mentioned above [24].

In addition, Wissink et al. [25] directly injected low reactivity fuel under RCCI mode to expand the engine loads and decrease the incomplete combustion products, which named as direct dual fuel stratification (DDFS). However, the problem of accurately controlling the reactivity gradient still exists in DDFS, because a part of low reactivity fuel is injected near the top dead center (TDC) even after the low-temperature heat release (LTHR) [26].

In summary, in order to achieve accurately controlling the ignition timing and combustion phasing, optimize the thermal efficiency and extend engine loads, it is necessary to operate equivalence ratio, temperature and component stratification in the cylinder in the range of time and spatial scale, so the burning rate and combustion phasing can be controlled during combustion process and all operation conditions with high thermal efficiency and low emissions, then stable running from low to high load.

Therefore, a novel LTC strategy, intelligent charge compression ignition (ICCI) combustion mode, is firstly proposed based on the dual-fuel direct injection strategy in the cylinder. It uses two common rail direct injection systems that fuels covering the spectrum from low to high reactivity in different operation regimes. In ICCI mode, the injection timings of two common rail direct injection systems can be adjusted at any time during the whole engine cycle, flexible and reliable charge can be formed according to the different engine operation conditions, aiming to provide a better reactivity gradient in the cylinder that is beneficial to increase thermal efficiency and control of combustion phasing, thus it can realize the closed-loop control for all engine operation range and combustion process.

The biodiesel [27] and n-butanol [28], which are made from food raw materials such as starches and non-food raw materials such as agricultural residues respectively [29], have been widely researched in recent years because of its environmentally friendly and renewability [30], and they can significantly reduce the usage of fossil fuels [31]. In particularly, biodiesel began to be researched on internal combustion engines in the 1990s [32]. The United States [33] and Europe [34] even developed laws for the purpose of replacing a certain proportion of fossil fuels with biodiesel. However, the fuel injection strategies, such as the fuel injection timing, injection pressure and energy ratio, had significant effect on the combustion process, and what kind of fuel injection strategies was more suitable for ICCI combustion mode was necessary to be investigated, especially fueled with biofuels (biodiesel and butanol). Therefore, based on the complementarity of the physicochemical properties of butanol and biodiesel, the objective of this paper was to study the working mechanism of ICCI mode, and the effects of important control parameters, including biodiesel injection timing, injection pressure and energy ratio of butanol on performance, combustion and emission characteristics of butanol/biodiesel ICCI combustion mode were analyzed in detail.