Elsevier

Applied Thermal Engineering

Volume 121, 5 July 2017, Pages 604-616
Applied Thermal Engineering

Research Paper
Numerical study on the effects of intake valve timing on performance of a natural gas-diesel dual-fuel engine and multi-objective Pareto optimization

https://doi.org/10.1016/j.applthermaleng.2017.03.036Get rights and content

Highlights

  • The dual-fuel engine was established by using GT-POWER 1D engine simulation.

  • The effects of a natural gas-diesel dual-fuel combustion were investigated in a heavy duty engine.

  • The effects of IVC were investigated for a natural gas-diesel dual-fuel engine.

  • A multi-objective pareto optimization was performed to optimize brake power and NOx concentration.

Abstract

A natural gas-diesel dual-fuel engine is considered an attractive option for reducing the emissions of a diesel engine while maintaining high thermal efficiency. However, it is important to investigate and optimize the parameters of such an engine in dual-fuel mode. Intake valve timing is a major parameter affecting the air/fuel (A/F) ratio, which is an important factor in dual-fuel combustion characteristics. Here, a numerical study was performed to investigate the fundamentals of dual-fuel combustion and the effects of intake valve closure (IVC) changes in dual-fuel mode using a 1D engine simulation. As the natural gas energy proportion (NGP) increased, brake power decreased and nitrogen oxide (NOx) emissions decreased because of low combustion efficiency and a lower temperature in the cylinder. At each NGP, a change in IVC could increase combustion efficiency and affect NOx emissions by controlling the A/F ratio. Additionally, the start of diesel injection (SOI), a major parameter in a dual-fuel engine, and the IVC were selected as independent variables. Latin hypercube sampling (LHS) was used with these variables and a multi-objective Pareto optimization (MOP) was performed to optimize high thermal efficiency and low NOx emissions. As a result, optimal Pareto solutions were obtained.

Introduction

The diesel engine has high fuel efficiency and has been used widely in transportation and industry. However, diesel engines are a major source of nitrogen oxide (NOx) and particulate matter (PM) emission, which cause air pollution [1]. These problems have caused engine researchers to focus on increasing fuel efficiency and reducing detrimental emissions.

To reduce emissions without decreasing power performance, alternative fuels, such as dimethyl ether (DME), hydrogen, and natural gas have been examined [2]. In particular, natural gas is considered an attractive alternative fuel. One reason for this is that natural gas is distributed more widely throughout the world than crude oil [3].

Thus, many researchers have studied natural gas-diesel dual-fuel engines that can maintain the high efficiency of a diesel engine and decrease emissions with the alternative fuel. With a dual-fuel engine, natural gas can contribute to reduced carbon dioxide (CO2) emissions because of its low carbon-to-hydrogen ratio. Also, it can substantially reduce NOx emissions and at the same time produce almost zero smoke or PM because of homogeneous combustion [4], [5], [6], [7], [8], [9], which is difficult to achieve in a conventional diesel engine. To achieve this, a dual-fuel mode is used. It is important to investigate the engine parameters necessary to optimize the dual-fuel mode.

There have been many studies about operating parameters under various conditions in dual-fuel mode. Selim [10] studied the effects of various parameters on cylinder pressure and pressure rise. They observed that the maximum pressure rise rate in the cylinder decreased as engine speed increased, but increased with advancement of the diesel injection timing. The maximum pressure in the cylinder was higher than that in normal diesel mode at all loads. Imran et al. [11] investigated the effect of pilot diesel quantity on combustion characteristics and found that, as the quantity of pilot diesel increased, the ignition delay was generally shortened and the maximum pressure in the cylinder was increased. The thermal efficiency was similar to or slightly higher than that of normal diesel mode at high loads, but at low loads, it was slightly lower. Sun et al. [12] analyzed the effects of diesel injection timing on combustion performance. With earlier injection timing, the maximum pressure in the cylinder and the maximum pressure rise rate increased. The NOx emission trend showed the same pattern. Zhou et al. [13] also studied the effects of pilot diesel quantity and injection timing on combustion characteristics. They found that the ignition delay was increased by advancing injection timing and was decreased by increasing the pilot diesel quantity. Kakaee et al. [14] used computational modeling to analyze the effects of the geometry of the piston bowl on the combustion characteristics. They found that the depth of the piston bowl and the chamfered ring-land affected the emissions from the engine, especially unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions.

The air/fuel (A/F) ratio is a major parameter affecting dual-fuel combustion. Papagiannakis et al. [15], [16], [17] analyzed the effect of the natural gas mass ratio (NMR) on combustion characteristic at various loads and speeds. They found that as NMR increased, ignition delay and combustion duration were increased and the maximum pressure in the cylinder and NOx emissions were decreased. Also, thermal efficiency decreased as NMR increased, because of the low total relative A/F ratio. That is, the natural gas-air mixture had an influence on the A/F ratio, which could change the thermal efficiency and combustion characteristics. Thus, it is important to control the A/F ratio, such as by adjusting valve timing at various natural gas ratios. Indeed, variable valve timing is a candidate technology that, although widely used in spark ignition engines to improve performance and fuel economy, has received relatively little attention for diesel applications [18], [19], [20]. Furthermore, there are few reported studies about the effects of valve timing in a dual-fuel engine.

Regarding valve timing, it is difficult to determine the optimum operating point due to a large range of factors affecting valve timing, engine performance, and emission gases. Moreover, under conditions of various loads and natural gas proportions, many factors affect engine performance. However, these factors have only been partially confirmed by actual tests. To address this problem, we performed simulations in which we varied a range of parameters. We assessed the impact of a number of input and hardware factors using a design of experiments (DoE) method. This enabled us to obtain the optimum operating conditions and design parameters, as outlined in previous studies [21], [22].

In this study, numerical analyses were conducted to analyze combustion characteristics and engine performance in a dual-fuel mode. The effect of variable valve timing on the dual-fuel engine was investigated by changing the intake valve closure (IVC) angle under various loads and natural gas proportions. The main parameters, i.e., start of diesel injection (SOI) and IVC, were selected as independent variables for the multi-objective Pareto optimization (MOP). As a result, optimum sets of parameters were found using multi-objective Pareto optimization. The main goal of this study was to identify suitable designs by investigating the effects of different parameters on engine performance and emissions. The overall process of the numerical analysis is shown in Fig. 1.

Section snippets

Target engine and modeling

Table 1 lists the specifications of the target engine. This is a heavy duty engine intended for power generation, so it has slightly different characteristics compared with a normal vehicle engine. It has a larger displacement, longer stroke, and lower operating rpm, which result in different operational characteristics. The numerical analysis was conducted with an engine model established using GT-POWER, a 1D engine cycle simulation based on thermodynamic analyses. GT-POWER can be used for

Model validation

The engine model was established with many input data points, such as the hardware specification, and then a validation was carried out to confirm the reliability of the simulation model. The simulated data were compared with experimental data under the same engine operating conditions (Table 2). The numerical values of engine parameters, such as brake power, boost pressure, injected fuel mass, max pressure, and NOx concentration, corresponded well with experimental data (within 4% error; Fig. 3

Conclusions

In this study, the effects of IVC were investigated for a natural gas-diesel dual-fuel engine. A numerical analysis was performed using a 1D simulation model. Also, MOP, for high brake power and low NOx emissions, was achieved by adjusting the IVC and SOI. The major conclusions are as follows:

  • (1)

    A 1D engine model of a heavy duty diesel engine was constructed based on experimental data, for example on brake power, max pressure, fuel mass, NOx emissions, and boost pressure. The combustion model used

Acknowledgements

This work was supported by the Power Generation & Electricity Delivery Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (No. 20131010176B), and granted financial resources by the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20131010176B).

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