Improvement of Spray Characteristics in Port Injectors

Transcription

1 Keihin Technical Review Vol.2 (213) Technical paper Improvement of Spray Characteristics in Port Injectors ポートインジェクタにおける噴霧特性の向上 Junichi NAKAMURA *1 Akira AKABANE *2 Koji KITAMURA *1 Yuzuru SASAKI *1 中村順一赤羽根明北村浩二佐々木譲 ポート噴射インジェクタより噴射される燃料噴霧は, エンジンの出力や燃焼効率に強い影響をあたえる. よって燃料を小さな油滴にする微粒化と, エンジンより受ける温度や負圧などの環境変化に依存しない正確な燃料供給が求められている. 本報では, ニードルバルブとのシート部下流の徹底した圧力損失 ( エネルギーロス ) の低減と噴孔位置の適正化による微粒化手法, 及びシート部下流のデッドボリューム削減と燃料通路長短縮による温度や負圧の変化に依存しないインジェクタを紹介する. Key Words: Fuel Injector, Atomization INTRODUCTION Lately, there have been growing demands for internal combustion engines for motorcycles and other applications to have lower emissions, better fuel economy and higher performance insusceptible to use environments. This is because of skyrocketing fuel prices and greater use of fuel injection systems under stricter global-scale emission control regulations in Various atomization techniques and flow rate stabilization techniques have been developed and put into practice. Among the currently available techniques, this development focused on and modified the structure located under the valve seat. Effects of under-seat flow and pressure were clarified in our existing injector structure so as to improve and modify a flow path from the seat to the nozzle orifice. various countries including developing countries. As part of such demands, injectors need to atomize fuel spray for lower emissions and better fuel economy and to minimize a variance in flow rate without being susceptible to changes in temperature and negative pressure so as to ensure a higher performance that is unaffected by the use environments. Our development focused on the following three improvements for injectors. (1) Atomization (2) Minimization of change in temperature and flow rate characteristics (3) Minimization of negative pressure and under-seat 1. Overview of injector for small motorcycles Fig. 1 represents the structure of our gasoline injector. When current flows through a coil, a core of injector is vacuumed and a valve train integrated with the core is lifted to open the valve as shown in the figure. Next, fuel pressure applied by a fuel pump delivers fuel through the opened seat and the fuel is sprayed through multiple nozzle orifice laid out in the plate. On Fig. 2, the under-seat flow in the existing Received 28 June 213, Reprinted with permission, from SAE paper (JSAE paper #212971). Copyright 212 SAE International and SAE of Japan. Further use or distribution of this material is not permitted without permission from SAE International or SAE of Japan. *1 Development Department 3, R&D Operations *2 Development Department 1, R&D Operations -9-

2 Improvement of Spray Characteristics in Port Injectors structure shows that fuel passes through the seat toward the center of axis, then comes down to the vertical hole area, and radially flows from the center of axis to the counter bore. After entering the counter bore, the fuel flows laterally along the plate located under the counter bore. The plate has multiple nozzle orifice laid out to deliver the required flow rate. At the nozzle orifice in the plate, the laterally flowing fuel rapidly changes its flow direction. Such rapid flow direction change separates and pulls off the fuel from the inner walls of nozzle orifice and forms a fuel film with external air trapped in the space produced by separation prior to spraying and diffusion. Since the existing structure has a long fuel flow Fuel pressure Power Supply path from the seat to the nozzle orifice and a large dead volume, fuel often tends to drip out without being atomized immediately after the valve opening and closing stages in which a full fuel pressure is not reached. Furthermore, fuel may be sprayed with an improper particle size at the start of spraying if the counter bore is filled up with fuel and fuel separation is instable, such as when a change in fuel temperature or negative pressure causes a variance in dripping rate. Fig. 4 represents a photo of the spray at the start of injection. This photo shows that there are large droplets at the start of injection. And, the graph of spray particle size vs. time plotted in Fig. 5 reveals poor atomization caused by a large particle size of fuel dripping out at the start of injection. Also, the distribution curve shown in Fig. 6 signifies the distribution of large particle sizes. Moving Core Seat, valve Fig. 1 Cross-section view of current Injector Flow direction Nozzle orifice Fig. 3 Velocity in counter bore (flowing to the nozzle orifice) Model: 1-nozzle orifice Seat Hole Counter bore Nozzle orifice Low Dead Volume Plate Separate In inner walls of nozzle orifice High Initial dripping at injection Fig. 2 Fuel flow direction and pressure distribution Fig. 4 Current dripping -1-

3 Keihin Technical Review Vol.2 (213) As a long fuel path from the seat to the nozzle orifice causes severe pressure loss, as shown in Fig. 7 resulting in a drop in fuel pressure applied by a fuel pump before reaching the nozzle orifice, the particle size of sprayed fuel eventually becomes large. This is due to the lower effectiveness of air entrapment and diffusion during fuel separation from the hole wall as a consequence of a rapid change of flow direction in the nozzle orifice. Frequency [%] Fig Time [ms] Fig. 5 5 Change in particle size over time Particle size distribution of current model Flow rate change [%] Flow rate change [%] Meanwhile, there was an issue of a change in the temperature and. After the valve closes, a lower pressure in the dead volume boils the fuel. As the fuel cubically expands it is pushed out. At higher fuel temperatures, a large dead volume under the seat causes a variance in injection rate in the event of a change in temperature (Fig. 8). A change in negative pressure could also cause a variance in the injection rate due to the aforesaid effects of dead volume. At a higher negative pressure from an engine, fuel flows out from the dead volume (Fig. 9). 15.% 1.% 5.%.% -5.% -1.% -15.% 1 C 3 C 5 C 7 C 9 C Fuel temperature [ C] Fig. 8 4% 2% % Change in the temperature and flow rate characteristics % Pb [mmhg] Pressure (kpa) 3 25 Current-Pressure Current-Velocity 2 1 Velocity (m/s) Fig. 9 Change in the negative pressure and flow rate characteristics 2. Approach to resolve issues 2 Fig. 7 Seat Valve seat hole measurement position above holes Pressure loss and velocity of under-seat In light of the existing issues described under section 1, an approach was selected and determined based on the three mechanisms of injectors. (1) Atomization: Shorter flow path from the seat to nozzle orifice; minimized pressure loss and -11-

4 Improvement of Spray Characteristics in Port Injectors facilitated separation in the nozzle orifice (liquid film forming) (2) Minimization of change in temperature and flow rate characteristics: Smaller dead volume (3) Minimization of change in negative pressure and : Smaller dead volume 2-1. Atomization mechanism Generally, fuel injected by an injector splits in the process shown in Fig. 1. While the mechanism shown in Fig. 1 is a generally known droplet mechanism, instability of this process could be a possible cause of failed atomization. As shown in Fig. 2, the flow in the cylinder called a counter bore goes into the nozzle orifice but the pattern of such flow into the nozzle orifice varies depending on their layout. This may prevent the optimum liquid film from forming. As fuel separation in the nozzle orifice depends on the intensity of the lateral flow above the nozzle orifice, instability of one flow direction obstructs the separation. Such instability means it is hard to ensure a stable particle size in each nozzle orifice as the liquid film forming is a passive process that induces separation in the nozzle orifice and uses air trapped in the separation area in the nozzle orifice. As a method to ensure stable atomization, it was determined to form an active flow that would induce liquid film forming on the inner walls of nozzle nozzle orifice. of gasoline. This graph signifies that gasoline can remain in a liquid state at an atmospheric pressure of 11.3 kpa and starts vaporizing at its vapor pressure of 53.3 kpa or lower even under atmospheric pressure. It is also shown that gasoline starts vaporizing at a temperature of 38 C or higher even in the atmosphere. This suggests that the characteristics can be improved by setting a pre-spray pressure under the injector valve seat through the nozzle orifice to meet the requirement stated below. The distribution of under-seat flow pressure in an existing injector is shown below. As seen in Fig. 11, the pressure decreases in the dead volume under the seat and in the larger-diameter counter bore to or below the aforesaid requirement. Apparently, reducing the pressure loss through an improved dead volume is effective as a countermeasure Method of approach and effectiveness Atomization Fig. 12 represents the improved under-seat layout of this development. The developed layout shortens the flow path from the seat to the nozzle orifice and reduces the dead volume, which minimizes fuel pressure loss before it reaches the nozzle orifice in the plate. It also allows fuel flowing from the seat 11.3kPa Mechanisms of temperature/negative pressure and under-seat Fig. 11 represents a vapor pressure curve Vaper pressure (kpa) kPa 24 C Liquid film forming (Thin film) Liquid column forming Liquid droplet forming (Atomization) Temperature ( C) Fig. 1 Process of atomization Fig. 11 Vapor pressure curve of gasoline -12-

5 Keihin Technical Review Vol.2 (213) to meet the flow returning from the center of the axis above the nozzle orifice, and drags fuel against the inner wall of the nozzle orifice, forming a void in the center of hole and facilitating liquid film forming in the nozzle orifice. In addition, a smaller dead volume allows for the aforesaid under-seat flow immediately after spraying, which can minimize initial and end dripping. Fig. 12 illustrates a flow line of under-seat flow as identified through fluid analysis of this layout. As seen in Fig. 12, liquid-film-formed fuel at the entrance to the nozzle orifice can actively form a liquid film of fuel sprayed and induce atomization. In addition, the pressure decrease point is located at a post-spray point. These achievements minimize the effects of variance in gasoline remaining in the dead volume, which results in minimization of any change or variance in the flow rate in comparison with the currently achievable level even at high temperature and/or negative pressure. Moreover, a smaller dead volume reduces the time required to fill the dead volume with fuel and contributes to both minimum (Table 1) initial dripping and better injection responsiveness (Fig. 15) Temperature/Negative pressure and under-seat And as shown in Fig. 13, 14, the developed injector can keep the under-seat pressure at a vapor pressure of gasoline or higher and thus neither a Pressure (kpa) 3 25 New-Pressure Current-Pressure New-Velocity Current-Velocity 2 1 Velocity (m/s) pressure decrease nor boiling occurs in the injector. 2 Seat Valve seat hole measurement position above holes Fig. 14 Comparison of pressure and velocity Table 1 Countermeasure Fig. 12 Low High Cross-section of new injector under seat and flow direction Dead volume Length of flow path CURRENT 1.2mm 3 3.9mm NEW.5mm 3 1.2mm seat seat Valve seat hole Valve seat hole Above nozzle orifice (a) Above nozzle orifice (b) (a) (b) Fig. 13 Comparison of layout Fig. 15 Initial dripping at injection -13-

6 Improvement of Spray Characteristics in Port Injectors 3. Result of countermeasures This section describes the Result of countermeasures incorporated in the developed injector. The developed layout was more effective in the areas listed below (Table 2). This can also be observed in distribution (Fig. 16, 17). Temperature/Negative pressure and under-seat improvements also minimize changes in temperature and, and in negative pressure and under-seat flow rate characteristics (Fig. 18, 19). Flow rate change [%] 15.% 1.% 5.%.% -5.% -1.% -15.% 1 C 3 C 5 C 7 C 9 C Fuel temperature [ C] Fig. 18 Comparison of change in the temperature and Table 2 Atomization (S.M.D) Temperature Negative pressure Result of countermeasure NEW CURRENT RESULT 64µm 85µm Improvement 25% 2% 9% Improvement 77% +11.7% +28% Improvement 58% Flow rate change 4% 2% % Current model -2% Pb [mmhg] Fig. 19 Comparison of Change in the negative pressure and CONCLUSIONS Frequency [%] Time [ms] Fig Fig. 17 Comparison of change in particle size over time Comparison of change in Particle size distribution This developed injector can reduce any variance in flow rate in the event of a change in temperature in different engine use environments or a change in negative pressure in different engine running modes and can improve the atomization of fuel to be sprayed. Under changing market situations in the future, manufacturers will need to develop and offer environmentally friendly products at a moderate price particularly in developing countries where sales of motorcycles and automobiles are expanding. And along with a full-fledged growth of FI for motorcycles, these advantages can make a great contribution to lower emissions and better fuel economy, not only in developed but also in developing countries. -14-

7 Keihin Technical Review Vol.2 (213) REFERENCES (1) Daisuke Matsuo, Akihiko Haramai, Kazuhiko Sato, Minoru Ueda: Development of a Small Low-cost Fuel INJECTOR to Overcome Diversification of Requirements in Global Markets, SETC Paper (2) Mitsutomo Kawahara, Kenichi Saitoh, Kazuhiko Sato: Reduction of operation noises of Injector for small motorcycle. SETC Paper Authers Junichi NAKAMURA Akira AKABANE Koji KITAMURA Yuzuru SASAKI We sincerely appreciate all support we recieved from everyone. We continue challenging the further new technology so that it can contribute to lower emissions and better fuel economy. (NAKAMURA) -15-