1. Introduction
Carbon-free fuels, such as hydrogen and ammonia, are promising options for sustainable propulsion, particularly for the urgent need for decarbonization in the transportation industry. Previous experimental studies on hydrogen- [
1] and ammonia [
2,
3] -fueled engines have revealed the necessity of a different approach to intake systems. Ammonia is considered to be a “hard to burn” fuel due to its high latent heat of vaporization and slow laminar burning velocity [
2]. Higher intake air temperatures were found to be necessary for the combustion process to take place. This increase in the intake air temperature affects the air-to-fuel ratio, and thus the air–fuel mixing phenomenon [
3]. It is also widely acknowledged that droplet atomization and achieving a homogenous air–fuel mixture before the start of combustion is crucial for high combustion efficiency. With the utilization of alternative fuels, further analysis of in-cylinder flow is vital as the density and viscosity of these alternative fuels are different to conventional internal combustion engine (ICE) fuels. In [
4,
5], fuel inside the combustion chamber was less susceptible to atomization due to higher momentum and longer break-up times for liquid droplets, resulting in soot production, which deposits inside the combustion chamber. It was shown that the generation of turbulence inside the cylinders improved the atomization of the fuel droplets as well as the promotion of rapid air–fuel mixing, which resulted in better performance and a reduction in hazardous emissions. In [
6], it was shown that a high swirl intensity affects engine combustion and emissions by reducing the combustion period, reducing soot generation, and CO
2 emissions. In [
7], the researchers investigated the influence of swirl flow on heat transfer in a diesel engine cylinder and reported a 4–12% increase in heat transfer due to the increased swirl ratio at different operating conditions of the engine. In [
8], it was shown that higher heat transfer was a consequence of increasing the swirl ratio, which enhanced the power performance by 5.79% and improved the fuel consumption as well. Furthermore, the influences of the following factors on swirl flows inside cylinders were investigated: the shape of the piston bowl [
9,
10,
11], the shapes and configurations of intake ports [
12,
13,
14,
15], the volume flow rates of air from the intake ports [
16,
17,
18,
19,
20], and so forth.
In a conventional engine cylinder, there are two types of intake ports, these being the helical port and the tangential port. These ports deliver intake air with different characteristics. Computational fluid dynamics (CFD) has been used to predict the flow characteristics of these intake ports [
19,
20,
21,
22]. In [
19], the results indicated that flows within the helical port have higher velocities than those within the tangential port, and flows entering into the combustion chamber from the helical port generate intense local vortical structures. On the other hand, those from the tangential port induced a significant amount of momentum with no particular vortical structure. However, airflow was diverted by the cylinder walls to generate a swirling structure on the scale of the cylinder bore. Moreover, as the various ports were throttled, there was a significant interaction between the flows from these two ports, which could result in a non-monotonic variance in swirl generation. In particular, CFD simulations revealed that the variances in swirl flow were considerably more complex when the tangential port was throttled.
In experimental analyses, there are several methods to measure gas flows inside engine cylinders, such as the hot-wire anemometry [
22] and laser Doppler velocimetry [
23], which are known for their excellent temporal and spatial resolutions. However, since these methods measure only one point in a flow field, it is difficult to measure flow velocities in the entire engine cylinder. Therefore, the particle image velocimetry (PIV) technique has garnered attention in terms of measuring flow velocities simultaneously at multiple points inside the engine cylinders without interfering with the flow fields [
12,
24,
25,
26,
27,
28,
29,
30,
31,
32,
33,
34,
35,
36,
37]. In [
30], the PIV technique was applied during the compression stroke to a light-duty optical diesel engine. They evaluated the swirl center positions (SCPs) in the
x–
y plane depending on the inclination of the rotation axis and the piston bowl’s inclination angle. They reported that the piston geometry effect might be more responsible than the intake flow effect for the tilting of SCPs. However, PIV measurements took place only during the compression stroke and in conditions where measurement planes were close to each other. Thus, it is somewhat hard to grasp the effects of the
z-axis during the piston’s reciprocating motion. In [
28], a swirl control vane was used inside the helical and the tangential ports to change the swirl ratio to 2.2, 3.5, and 4.5, and it was found that a single dominant swirl flow structure tilts with respect to the cylinder axis. Their study also focused only on the compression stroke, where the eccentricity of the swirl center also caused in-cylinder flow to be asymmetric, which lowered the swirl ratio. In our previous studies, we measured the flow velocity inside cylinders when changing the engine speed and the opening area of helical ports under both the motored- and the fired-engine conditions [
14,
15,
34,
35]. We found that by changing the helical port’s opening areas, SCPs do not shift significantly, and airflow velocities do not change with an increase in engine speed. In addition, as the helical port’s opening area was increased, the swirl ratio was reduced, which lowered the turbulent intensity. Thus, our attention shifted to the tangential port instead, which is the main focus of the present study; there being only a few studies where the tangential port’s effect was investigated experimentally. In [
19], the researchers developed a computational model to simulate intake flow and its effect on in-cylinder swirl and flow structures by using different throttle angles in both the tangential and helical ports. However, this study was mainly focused on the intake stroke. Therefore, in this study the effect of tangential port opening areas, along with their influence on compression stroke and swirl formation was investigated.
The authors have focused on the utilization of ammonia as a carbon-free fuel in conventional ICEs. The aim is to develop an ICE that can be powered solely with ammonia. However, due to its chemical properties, ammonia is considered to be hard-to-burn fuel in conventional ICEs. Thus, the authors have conducted co-combustion experiments using an ammonia–gasoline fuel mixture [
3]. However, the results have shown that the percentage of ammoniain the fuel mixture can only be increased up to 67% without losing significant power output. Thus, in order to improve ammonia’s flame retardancy and increase its ratio in the fuel mixture, the authors shifted their focus on the intake system to investigate the formation of swirl flow and turbulence inside the cylinder. The investigation focused on the intake port opening areas, which is considered to be an effective method resulting in minimal changes to a conventional ICE. In the present study, the PIV technique was applied to a commonly arranged intake–exhaust port four-stroke engine, at three different measurement planes to investigate the influence of the tangential port’s opening area on swirl flow generation. The PIV measurements took place during the intake and compression strokes, in order to obtain a broader perspective on swirl flow generation. The helical port’s opening area was set to 100% throughout the experiments, whereas the tangential port’s opening area was changed using several kinds of gaskets. The data obtained from the PIV experiments were used to calculate spatially averaged velocities and TKE. Three-dimensional coordinates of SCPs were calculated, where the tilting of SCPs became clear with the piston’s reciprocating motion and the tangential port’s opening areas. It was found that swirl flows were successfully formed when the tangential port’s opening area was 25% or more. In addition, a preliminary relationship was found between the calculated TKE and swirl flow generation, where low variances of TKE and SCPs in in-cylinder flows led to the successful formation of swirl flow.
4. Conclusions
For a four-valve diesel engine, the effects of the opening area of the tangential port on the in-cylinder swirl flow were investigated, where the opening areas were changed to 0%, 25%, 50%, 75%, and 100% using different gaskets. The present study measured the velocities in a transparent engine cylinder in three different measurement planes, at z = −10 mm, −20 mm, and −30 mm, by using the PIV technique. The gathered experimental data were used to evaluate velocity vectors, streamline maps, spatially averaged TKE, and SCPs in three-dimensional coordinates. Important conclusions obtained from the present study are summarized below:
The velocity vector and the streamline maps were evaluated using the obtained velocities through PIV measurements. In the case of the 0% opening area, during intake stroke, complicated flows were observed at z = −10 mm, −20 mm and −30 mm, where variances of TKE and SCPs were large. During the compression stroke, complicated flows were also observed at z = −10 mm and −20 mm measurement planes. At the latter period of the compression stroke, swirl-like flows started to form at z = −30 mm.
In the case of opening areas of 25% or more (flows from both ports), during intake stroke, similar tendencies were observed in the cases where only the helical port was used, where no swirl flow generation was observed. After the first half of the compression stroke, swirl-like flows were observed at z = −10 and −20 mm and swirl flows were successfully formed at z = −30 mm.
Swirl ratios were calculated from the center of the engine cylinder for each tangential port opening area. Calculations showed that, during the intake stroke, a sinusoidal pattern was apparent in the swirl ratios, meaning proper swirl flow was not formed. On the other hand, during the compression stroke, as the tangential port opening areas increased, swirl ratio also increased and reached to a steady level for cases when the tangential port opening was 25% and above.
The spatially averaged TKE and its variances were evaluated using the obtained velocities. During the compression stroke, large differences in TKE were not observed, thus a comparison cannot be made. However, during the intake stroke differences in the TKE were apparent depending on the measurement planes and the size of the opening areas. It was concluded that, during the intake stroke, as the variances of TKE became larger, complicated or swirl-like flows were formed. As these variances started to become smaller, swirl flows started to form.
SCPs and their variances were evaluated using the obtained velocities. SCPs were not clear during the intake stroke. However, for cases with an opening area of 25% or more, SCPs were observed clearly during the compression stroke. A tilting motion of SCPs was also observed in the x–y planes in the z direction during the compression stroke. The SCP variances under the conditions that form complicated or swirl-like flows were larger than those under the conditions that form swirl flows. It was concluded that swirl flows were successfully formed when the variances of SCPs were relatively low.
As the next step of this study, the know-how on the swirl flow formation will be applied to a modified diesel engine with a high compression ratio, which is fueled by an ammonia–gasoline mixture. In this setup, the fuel mixture of ammonia–gasoline will be used in a port injection method. Subsequent experiments will be conducted to validate the outcomes presented in this paper through ammonia–gasoline co-firing.