3.1. Engine Performance
HCCI was operated on a conventional motorcycle engine without major modifications. The operation condition was set at 2000 rpm, 40% throttle opening position with AFRs of 21.1, 22.1, 23.1, 24.1, and 24.5 for stable combustion.
In general, a leaner air–fuel mixture results in lower torque output. However, in this experiment, both
IMEP and BMEP (brake mean effective pressure) increased with the increase in
AFR (
Figure 2a). This was a compound phenomenon of the engine speed, intake air mass,
AFR, and fuel property. The overall effect of these factors was on the combustion pattern. The
IMEP calculation was based on the integration of
P·d
V. In these four cases of
P–
V diagrams, the higher
AFR resulted in a higher value of
P·d
V integration. This was contradictory to the general trend, because such kind of
P–
V diagram was a special case, which was produced by a special combustion pattern. Moreover, with an increase in
AFR, indicated thermal efficiency increased and heat loss ratio decreased (
Figure 2b). The main cause of the higher efficiency was the combination of combustion pattern and low heat transfer loss. The heat transfer loss was proportional to the difference of cylinder gas temperature and wall temperature, as shown in Equation (8). The leaner mixture produces lower cylinder gas temperature, and hence causes lower heat transfer loss. The maximum cylinder gas temperature decreased with an increase in
AFR (
Figure 3). The heat transfer loss per cycle divided by the heat input per cycle is called heat loss ratio. The summation of thermal efficiencies and heat loss ratios for various
AFRs was approximately 56% (
Figure 2b). That means the summation of work output and heat transfer loss was similar for different
AFRs. As
AFR increased from 22.1 to 24.5, the
IMEP increased 15% and the indicated thermal efficiency increased 27%.
The exhaust emissions of CO, HC, and NO are depicted with brake-specific emissions (
Figure 4). Brake-specific CO (BSCO) was small because of a lean burn. Brake-specific HC (BSHC) was relatively high, because of the low combustion temperature. Brake-specific NO (BSNO) dramatically decreased with an increase in
AFR because of the decreased combustion temperature. In general, when the combustion temperature is lower than 1800 K, NO emission is low [
40]. The maximum cylinder gas temperature of
AFR 24.5 was 1851 K, which was close to 1800 K.
3.2. Combustion Characteristics
The cylinder gas pressure is illustrated in
Figure 5a. The rate of cylinder pressure rise was sharp near the top dead center (TDC) for
AFRs 22.1 and 23.1. As the mixture became leaner, the cylinder pressure rise rate and the peak pressure decreased. The timing of the maximum pressure was delayed with increasing
AFR. The cylinder gas temperature is shown in
Figure 5b. The peak temperature was approximately 2000 K for the
AFRs of 22.1–24.1, and was close to 1800 K for the
AFR of 24.5.
Figure 6a demonstrates that the start of combustion was delayed and the combustion rate decreased with an increase in
AFR. Two-stage ignition was observed in
Figure 6a because
n-heptane is a diesel-like fuel. The first stage of ignition is the result of cool-combustion chemistry and negative temperature coefficient behavior [
33]. The HRR of the first-stage ignition decreased after it achieved its maximum value. The temperature of maximum HRR of the first-stage ignition ranged from 714 K to 726 K (
Table 4).
Figure 6b shows that the
MFB curve of the HCCI had a pattern different from that of the conventional SI engine, which can be expressed as a single-Wiebe function.
The factors affecting combustion includes pressure- and burn rate-related parameters. The burn rate-related parameters are illustrated in
Figure 7. CA50 was defined as the CA after TDC at which the
MFB was 50%. The definition of CA10 and CA90 was similar to that of CA50. MHRR1 and MHRR2 were the maximum HRR at first- and second-stage combustion, respectively. The beginning of the increase in HRR indicated the start of combustion.
θ01 and
θ02 marked the start of combustion in the first- and second-stage combustion, respectively.
Detailed combustion parameters are listed in
Table 4. For a conventional SI engine with the best torque, the maximum pressure occurs at approximately 16° aTDC [
41] (p. 375). In this study, the CA of maximum pressure varied from 5° aTDC to 22° aTDC. The relationship between the engine torque and the timing of the maximum pressure in the HCCI was more complex than that in the SI engine. The root cause was the absence of the forced trigger of ignition in the HCCI engine. The
COV and MRPR decreased with an increase in
AFR. The increased
AFR prolonged the burn duration and combustion phase; in other words, the CA of MHHR2 was large (
Table 4). If combustion occurred instantaneously at TDC, the MRPR will be extremely high.
The variations in MHRR1, and the temperature at which the MHRR1 occurred with regard to
AFR, were small (
Table 4). During first-stage combustion, when the temperature achieved a certain value (714–726 K), the chemical reaction rate decreased.
3.3. Air Cycle Simulation
The factors affecting the thermal efficiency of ICEs are complex. For a conventional SI engine, constant volume combustion at TDC achieves high thermal efficiency. A faster combustion process, relative to more moderate burning rate engines, is regarded as resulting in a direct engine efficiency gain [
41] (p. 845).
The engine test that resulted in a higher engine output with a leaner mixture was contradictory to the general concept. Therefore, air cycle simulation was performed to calculate the IMEP and thermal efficiency with a MFB similar to the experimental data.
The
P–
V diagram is depicted in
Figure 8. The solid black curve is the air-standard cycle and the other curves are the HCCI engine experimental results with different
AFRs. Causes of the deviation between air-standard cycle and engine experiment include heat transfer, finite combustion time, exhaust blowdown loss, crevice effects and leakage, incomplete combustion, pumping loss, and non-standard-air working fluid [
42]. In general, a low deviation in the
P–
V diagram from the air-standard cycle leads to high efficiency. However, the result of this study was contradictory to the general concept. Comparing
Figure 2 and
Figure 8, the higher deviation of the
P–
V diagram from the Otto cycle resulted in a higher engine output and thermal efficiency.
To investigate this phenomenon, the air cycle with heat input and heat transfer was simulated. The heat input was calculated from MFB. A double-Wiebe function was proposed for simulating the MFB curve.
A functional form often used to represent the
MFB versus CA curve is the Wiebe function [
41] (p. 390):
where
θ is the CA,
θ0 is the start of combustion, and Δ
θ is the total combustion duration (
xb = 0–1);
a and
m are adjustable parameters.
The double-Wiebe function, a combination of two-stage combustion, is referring to [
32] and expressed as:
where
xb is the
MFB. Subscriptions 1 and 2 indicate the first and second stages of combustion, respectively, and
θ01 and
θ02 represent the start of first- and second-stage combustion, respectively (
Figure 7). Furthermore, α is the fraction of fuel mass burned during the first-stage combustion, which is the
MFB at
θ02. The combustion duration of the first stage is Δ
θ1 =
θ02 −
θ01.
The rate of
MFB combined with two-stage combustion can be expressed as:
where
xb1 and
xb2 are the
MFB of each stage of combustion.
By combining Equation (15) with Equations (19)–(21), the rate of heat input in air cycle simulation can be calculated.
The parameters of the double-Wiebe function were determined by the curve fitting from the experimental
MFB data. They are listed in
Table 5.
θ0, Δ
θ, and
α were identified from the experimental data, and a and m were mainly dependent on the fuel type and did not vary with
AFR. In this study of
n-heptane HCCI,
a1 = 3 and
m1 = 1 for first-stage combustion, and
a2 = 10 and
m2 = 1 for second-stage combustion. The start of combustion (
θ01) was delayed by 5° CA, and the total combustion duration (Δ
θ1 + Δ
θ2) was lengthened by 22° CA, as
AFR increased from 22.1 to 24.5.
The simulated
MFB by using the double-Wiebe function was close to the experimental result. The comparisons of
MFB curves between simulated and experimental results are illustrated in
Figure 9, wherein the dashed and solid lines indicate simulated and experimental results, respectively.
The heat input rate in air cycle simulation was calculated from the double-Wiebe function (Equation (19)).
The double-Wiebe functions for
AFR, 22.1, 24.1, and 24.5, represented three different combustion patterns, which were classified as combustion patterns 1, 2, and 3, respectively. The parameters of the double-Wiebe function for the three combustion pattern are listed in
Table 6. The combustion pattern represents combustion phasing. The start of combustion was delayed from combustion patterns 1 to 3. The combustion rates became slower from combustion patterns 1 to 3, and the combustion durations, Δ
θ1 and Δ
θ2, increased.
Among the three combustion patterns, pattern 3 possessed the highest
IMEP and thermal efficiency (
Figure 10 and
Figure 11) because of low heat loss and a special case of
P–
V diagram. The detailed data of the air cycle simulation results are shown in
Table A1,
Table A2,
Table A3,
Table A4,
Table A5 and
Table A6, which include the heat loss per cycle (
Qht). The slow combustion rate caused low combustion temperature and, hence, the heat transfer from the cylinder gas to the wall decreased. However, for prolonged combustion duration, such as Δ
θ1 = 25 and Δ
θ2 = 45, the
IMEP and thermal efficiency decreased.
A combination of diesel- and gasoline-like fuels can be used in dual fuel for controlling the combustion phase, by using the gasoline-like fuel as the ignition suppressor and the diesel-like fuel as the ignition improver. Several studies have reported that the addition of a high octane number fuel in an HCCI engine delays the start of combustion and prolongs the combustion duration [
12,
28,
43,
44]. EGR delays the autoignition timing and reduces the combustion reaction rate [
45,
46,
47,
48]. Therefore, various combustion patterns can be achieved by adjusting
AFR and the rates of dual fuel and EGR, along with the proper type of fuel.
Another method to reduce the heat loss is to increase the wall temperature. The combustion temperature of HCCI is much lower than that of SI. Thus, the capacity of the cooling system of an HCCI engine can be decreased more easily than that of the conventional SI engine. Moreover, ceramic coating on the surface of the combustion chamber can decrease heat loss. Reducing the cooling capacity or using a ceramic coating causes an increase in the wall temperature. The
IMEP and the indicated thermal efficiency increased with increasing wall temperature, as shown in
Figure 10, and was obviously caused by the low heat loss.
The effects of
AFR on
IMEP and thermal efficiency are indicated in
Figure 11. No fuel was used in the air cycle simulation;
AFR was used for calculating
Qin rather than the fuel input. Decreasing
Qin, and by extension, increasing
AFR, reduced
IMEP, and the indicated thermal efficiency increased by a small margin. The effects of decreasing
Qin on
IMEP and thermal efficiency were not as pronounced as those on combustion pattern. The change in
AFR caused variation in the combustion pattern. In this study, three combustion patterns were achieved by adjusting
AFR.
In the air cycle simulation, the CA of MRPR depended only on the combustion pattern. The faster the combustion rate, the more advanced the CA of MRPR (
Table A1,
Table A2,
Table A3,
Table A4,
Table A5 and
Table A6). The advanced CA of MRPR caused higher MRPR. Furthermore, more heat input led to higher MRPR. A study [
49] set a limit of 6 bar/deg for the MRPR to decrease the combustion noise and avoid engine damage. All of the simulations of combustion pattern 1 reached an MRPR of >6 bar/deg because of fast combustion.