3.1.1. Operating Point 1
First, the thermodynamics of OME combustion and the injection system behavior are discussed in order to draw conclusions about the emissions and their formation. In
Figure 4, the drive current of the UCV (
), the pressure in the injection line (
), the cylinder pressure (
), the rate of heat release (
), and the cumulative energy (
) normalized to the total amount of energy injected are shown for two selected
at OP
as a function of the crank angle (
). The solid lines relate to a COC closer to the top dead center (TDC) (
CA), whereas the dashed lines represent an OP with late COC (
CA).
Table 4 lists the measured values of the most important variables for the shown OPs and the respective combustion centers.
Looking at the drive current of the UCV for OP
(
Figure 4), it is noticeable that the UCV for both nozzles in OME operation is controlled by about 1–2.5
CA later than with diesel, although due to the lower LHV of OME, the
for the same effective mean pressure must be selected significantly longer overall, and the same center of combustion must be maintained (
Table 4). At late COC, the UCV is actuated up to 5
CA later for OME than with diesel. The injection pressure increases proportionally with actuation time in a PLN injection system [
65]. Therefore, due to the longer
, the maximum pressure in the injection line at this OP is ≈200 bar higher with OME for the RS-nozzle and ≈250 bar higher for the P-nozzle than with diesel. In addition, the higher peak pressure and a slightly steeper increase in pressure in OME operation is due to the higher bulk modulus of OME (
Table 1).
With diesel, there is a drop in the pressure of the injection line for both COC, which is followed by a rise shortly after the UCV is turned off. In the case of OME, the pressure in the injection line decreases continuously after shutdown. The pressure curve in a PLN system depends significantly on the position of the pressure sensor, the
, the wave impedance of the injection line (diameter, length, injector geometry), the bulk modulus, the speed of sound, the density of the fuel, and many other factors [
65]. In the pipe, the pressure wave generated by the pump piston is reflected several times, causing location-dependent interference and cancellation phenomena, and thus, the injection pressure at the nozzle outlet differs from the pressure measured in the pipe, and the latter has a phase shift. This, and the operation of the RS-nozzle in the ballistic range with diesel could be a reason for the different pressure drop. The higher pressure of the P-nozzle is mainly due to a different opening pressure of the nozzle and the changed wave impedance of the injection system due to the changed nozzle geometry.
Looking at the cylinder pressure and
(
Figure 4), it can be observed that the pressure curve with diesel and at early COC positions (
CA) shows a steeper gradient, and the peak pressure is about three bar higher compared to OME. This is due to about twice the maximum heat release during the pronounced premixed combustion phase with diesel. With OME, an only moderately pronounced premixed phase occurs for the RS-nozzle, and no premixed combustion phase occurs for the P-nozzle because of a shorter ignition delay. The RS-nozzle initially shows a low heat release for OME and diesel, as in a type of pre-injection with a subsequent steep increase in the conversion rate. The reason for this is the throttled injection rate during the rate-shaping phase. With the P-nozzle, the
rises immediately. For OME, the
increases for both nozzles during the diffusion combustion phase, which is due to the rising injection pressure and thus increasing mass flow rate during injection because of the extended
. Therefore, for both nozzles, despite the lower LHV of OME, the heat release per crank angle is higher during the diffusion combustion phase than during diesel combustion. As a result, the lower LHV of OME can partly be compensated, and a reduced burn duration (
) can be observed with OME despite a significantly longer actuation time (
Table 4). The shorter burning duration is due to the higher CN and oxygen bond in the molecular structure of OME. In the normalized cumulative heating curve of the
CA point, it can be seen that more heat is initially released during diesel combustion due to the high premix fraction and LHV, but OME combustion is completed much earlier overall. A faster burnout (
,
Table 4) can be explained by the reduced soot formation with OME during the main combustion phase which results in less soot to be oxidized in the later combustion phase [
66]. Previous findings showed that oxygenated fuels burn faster due to higher oxidation rates, which is consistent with these measurement results [
67,
68]. The friction
and the
are almost the same for both nozzles and fuels.
The OP with a very late COC (
CA) shows a completely different combustion behavior than the OP with a center of combustion closer to TDC. The maximum cylinder pressures occurring during combustion are approximately the same, and the pressures in the injection line are slightly increased for all three nozzles and fuel combinations due to a longer
compared to the OP with an early center of combustion (
Table 4). The reason for the elongation in
is the reduced efficiency at late COC. For
, a similar pattern occurs for all fuels and nozzles, showing high conversion rates and no classical diffusion-controlled combustion phase. Due to the late injection in the expansion phase, the mixture formation process is affected because of low temperatures in the cylinder, low turbulence in the charge movement and the low back pressure, which increases the ignition delay time and results in the formation of a partially homogenized mixture in the combustion chamber. This mixture burns at a high conversion rate after auto-ignition. Diesel combustion starts earlier than OME combustion, but it rises to its maximum at a much flatter rate than OME combustion because of lower oxidation rates (no oxygen in molecule). The earlier start of combustion with diesel can be explained by the 5
CA earlier
and the associated earlier injection for the same COC. Even with a very late COC, the OME combustion is completed significantly earlier than with diesel. In addition, the total amount of energy converted is significantly lower with diesel, which can be attributed to incomplete combustion and high heat losses due to a long combustion period with a large combustion chamber surface and thus leads to a reduction of the indicated efficiency (
) by 3 percent points compared with OME. Overall, the differences in the
between the two nozzles with OME can be considered to be small. Due to the short ignition delay and the lower LHV of OME, the rate-shaping phase plays a minor role and the effect of RS injection rate on the combustion process seems to be negligible. In addition, the maximum nozzle flow rates differ only slightly.
In order to analyze the differences in burning duration, indicated efficiency, friction and heat losses in more detail,
Figure 5a–d are used.
Figure 5a shows the ignition delay for OME and diesel and the RS-nozzle at OP
as a function of
. OME shows a shorter
for all points compared to diesel. In addition, a strong dependence on injection timing relative to TDC is observed for diesel, whereas this dependence is much less for OME. If the fuel is injected very early in the compression phase, the cylinder pressure and temperature are not yet at their maximum, which means that jet breakup and fuel evaporation take more time, thus increasing the ignition delay. During the compression phase, however, sufficient charge movement due to swirl and squish flow in the piston bowl favors mixture formation, resulting in a shorter
for injections before TDC than after TDC. In the expansion phase, pressure and temperature decrease continuously, and the kinetic energy of the cylinder charge is reduced, which is why significantly longer ignition delay times occur with injections beginning ATDC. For diesel, a clear minimum of
is shown at 4–6
CA before TDC. It is assumed that the turbulence level in the piston bowl is highest there due to squish flows, whereby mixture formation is affected positively. Shimamoto and Akiyama [
69] showed for a cylindrical piston bowl and different piston bowl diameter to bore ratios that the maximum squish velocity occurs at about 5–10
CA BTDC (losses neglected). However, the mixture formation and flow processes in the combustion chamber are very complex and strongly depend on the piston bowl and combustion chamber geometry. The significantly reduced dependence of the
on the
with OME is due to the improved ignition properties resulting from the higher CN, the lower boiling point and, above all, the oxygen content in the molecular structure. Due to the intrinsic oxygen bond, the influence of the charge movement on the mixture formation processes decreases, since the oxygen required for combustion is carried along in the molecule and does not have to be supplied from the charge air by air entrainment [
70]. This enables the combustion to be located closer to the nozzle tip, where the turbulence level is higher [
70]. Since the opening pressure of the RS-nozzle remains constant for both fuels, the injection pressure before the start of combustion is relatively similar. However, it should be noted that at the same injection pressure, OME has a lower nozzle outlet velocity and thus a lower momentum flux compared to diesel due to its higher density [
71,
72], resulting in less turbulence being introduced into the combustion chamber at this time. Pöllmann et al. [
73] observed on an optically accessible single-cylinder diesel engine with a common-rail-injection system that the ignition delay of the OME
investigated there is almost independent of the injection pressure and the nozzle geometry, but it is prolonged at low cylinder pressure at the
.
Figure 5b shows the exhaust gas temperature (
) for OP
as a function of
. The exhaust gas temperature increases with later positions of the COC, since the heat of combustion in the working process cannot be converted into mechanical work efficiently, which significantly decreases the indicated efficiency (
Table 4). The lower exhaust gas temperature of OME is due to the higher exhaust gas mass flow because of the larger injection mass and the significantly higher heat capacity due to a higher concentration of water vapor and CO
in the exhaust gas (CO
and H
O factor in
Table 1). At later COC positions, the gap in exhaust gas temperature between OME and diesel combustion increases, since OME has a significantly shorter burning duration and faster burnout due to its oxygen content, resulting in more efficient combustion. Especially at late COC positions, a shorter combustion duration ensures lower wall heat losses. A further small effect for lower exhaust gas temperatures could be the slightly higher value for enthalpy of vaporization of OME [
18].
With OME, significantly higher oil temperatures were measured on the air-cooled engine than with diesel, despite the same ambient temperature and same fan speed (equal to the engine speed) of the engine cooling system (
Figure 5c). During the measurement campaign, the combustion chamber was regularly inspected with an endoscope to identify possible damage. It was found that during diesel operation, there were heavy soot deposits on the piston crown, whereas in OME operation, no soot deposits were visible and the piston was metallic bright. As already proven several times, very low
emissions are generated during OME combustion [
15,
16,
17,
50], which is why a buildup of soot depositions on engine components is not to be expected during OME operation. Existing soot deposits were oxidized after switching from diesel to OME operation, thereby cleaning the combustion chamber independently. The prolonged injection due to the reduced LHV leads to a longer interaction of the flame front with the piston bowl, which is expected to result in a higher heat input. An impingement of the burning spray with the piston bowl leads to higher local surface heat flux [
74]. Higher wall temperatures on the piston lead to a reduction or complete removal of soot deposits on the piston, even in diesel operation [
75]. In addition, the soot layer in the combustion chamber acts as an insulator, which is why a higher heat flux occurs in the absence of deposits and thus more heat is transferred to the piston [
75,
76]. The heat flux is strongly dependent on the composition of the depositions and therefore from the used fuel [
76]. Approximately 50% of the energy leaving the closed cylinder system goes through the piston and approximately 30% goes through the cylinder head [
77]. The energy supplied to the piston is transferred to the engine oil via the piston rings, causing the oil temperature to rise. Another effect could be that due to the higher injection pressure with OME, a longer jet penetration length occurs, and thus, the combustion takes place predominantly near the piston wall. Furthermore, more kinetic energy is introduced into the combustion chamber by the fuel system due to the higher total mass flow (lower LHV), thus increasing the turbulence level [
20], which could further increase the heat transfer coefficient and thus increase heat losses. In contrast, the absence of soot during combustion and a reduced adiabatic flame temperature of OME could reduce the heat losses through radiation, but these processes are very complex and could not be further validated in terms of this work. The differences in oil temperature between the nozzles with OME are small.
The fact that the same indicated efficiency of 45% (
Table 4) occurs for both nozzles and fuels in OME combustion at early COC despite a significantly shorter combustion period confirms that greater heat losses occur in OME operation. At first, the piston section is subjected to more stress due to the extended injection. Furthermore, more heat is extracted from the combustion chamber to evaporate the higher injection mass and due to the slightly higher values for enthalpy of vaporization of OME compared to diesel [
18]. At late COC, diesel combustion efficiency decreases more severely than OME due to inadequate mixture formation and the longer
. Surprisingly, the
at this OP is very similar for all COCs (
Table 4), although the HPP requires significantly more energy to pump the higher fuel mass and to produce the higher injection pressure. It is assumed that the higher oil temperatures occurring in OME operation reduce the viscosity of the SAE 10W-40 engine oil and thus reduce friction in the crankshaft drive. This can compensate for the higher energy demand for fuel injection at this OP.
When considering the combustion noise as a function of the center of combustion in
Figure 5d at OP
, a strong dependence on the center of combustion becomes apparent for diesel, similar to the ignition delay, which is less pronounced for OME. The combustion noise was determined from the cylinder pressure curve by means of a noise analysis using the AVL INDICOM v2.6 software. Due to the lower premixing proportion at early COC positions and the resulting smaller pressure gradient in the cylinder pressure curve, the combustion noise in OME operation is reduced by up to 6 dB for both nozzles. This corresponds to a reduction in sound intensity by a factor of four, which significantly reduces the noise level in the environment of the engine. At very late COCs, both fuels and nozzles show similar noise levels, which can be attributed to the similar
and cylinder pressure curve with small pressure gradients. For early COC, the noise emissions with the P-nozzle are slightly reduced compared to the RS-nozzle, which can be explained by the complete lack of premixed combustion phase (
Figure 4).
3.1.2. Operating Point 2
When considering an OP with higher load and engine speed (OP
) as shown in
Figure 6, in contrast to a load point with lower
, the UCV with OME is energized earlier and longer than with diesel. Additional values of important variables for OP
can be taken from
Table 5. The
must be selected significantly longer to compensate for the lower LHV of OME due to the additional fuel mass required. A much higher
and the required higher fuel mass with OME result in a significantly larger
at this OP for all COCs (
Table 5), and the injection system friction losses for the P-nozzle are larger than for the RS-nozzle. These losses can no longer be compensated by a higher oil temperature and thus lower engine oil viscosity at this OP. The decrease in
when shifting the COC toward late timings for all nozzles and fuels could be explained due to the reduced gas force on the piston rings due to the lower maximum cylinder pressure, which decreases the contact force between the piston rings and cylinder wall [
78]. In addition, reduced forces occur in the journal bearings of the crankshaft and connecting rod bearings, resulting in reduced friction losses [
78]. A detailed loss distribution calculation is only possible with a very high measurement effort and is not possible in the course of this work.
The maximum cylinder pressure is the same for both nozzles and fuels at early COC (CA), but the cylinder pressure increases with a significantly larger gradient at the start of combustion and at a later crank angle for diesel. This is due to the longer and the resulting high premixed fuel fraction in diesel operation, which results in a high conversion rate during the first phase of combustion. In the case of OME, no premixed combustion phase can be detected for both COCs and for both nozzles, which is due to the short and the described greater independence of combustion from mixture-bidding processes. The conversion rate of OME is purely limited by the fuel mass flow of the nozzle and thus by the injection system. The combustion of the small injection mass during the rate-shaping phase of the RS-nozzle can also be seen here in OME operation by a slight increase in the curve. However, the post-spray is not discernible, which could be due to a change in the closing behavior of the RS-nozzle at long actuating durations and high injection pressures (faster closing). The conversion rate increases for OME because of the rising during injection and the resulting higher fuel mass flow. For the early COC, diesel combustion starts later but overtakes the total heat conversion in OME operation, resulting in a slightly shorter and higher for diesel.
With the state of production injection system, it is not possible to inject the required amount of energy into the combustion chamber in a sufficiently short time at high engine speeds and loads. As a result, the with OME is extended to values similar to those with diesel, despite the significantly better ignition characteristics and faster burnout. The higher heat losses with OME because of evaporation (more mass and higher enthalpy of vaporization) and interaction of injection spray with piston surface leads to a reduced indicated efficiency with comparable burning duration. This disadvantage could only be compensated by nozzle adaptation in the form of an increased nozzle flow rate, which significantly reduces the whereby, despite higher heat losses, a significant increase in the indicated efficiency can be expected with OME.
At late COC (CA), diesel shows a predominantly premixed combustion with long ignition delay, similar to OP, which leads to a steep pressure rise. Due to the low temperatures and pressures during the expansion phase, the mixture formation processes are affected in a negative way, which prolongs the for diesel. The of OME shows a similar course for both nozzles at the early and late COC. Due to an earlier start of injection and the shorter ignition delay, OME combustion at late COC starts closer to TDC and thus at a higher pressure and temperature level than with diesel combustion. The fuel is continuously fed to the combustion chamber, which is why diffusion-controlled combustion is observed at approximately constant pressure. Despite the constant pressure combustion, OME shows for a late COC a shorter and a slightly higher than diesel. This is mainly due to faster burnout in the late expansion phase and more complete fuel conversion because of oxygen in the molecular structure.
The differences between the P- and RS-nozzle on thermodynamics are small, as the nozzles have similar maximum flow rates and the injection rates differ mainly in the opening and closing process. Due to the lower LHV of OME and longer actuation time, the influence of the opening and closing phases decreases, since the amount of energy supplied there takes up a smaller share of the total energy conversion. With increasing engine speed and load, the influence of these phases decreases. This leads to minor differences in heat release.
The trends for
,
,
and combustion noise of OP
are similar to OP
(
Figure 5a–d).