Chapter 2
HCCI COMBUSTION
2.1. INTRODUCTION
Homogeneous-charge, compression-ignition ‘HCCI’ combustion takes place, like the simultaneous one, in homogeneous mixtures once that spontaneous combustion temperature and latency time are exceeded. However, differently from simultaneous combustion, the reaction takes place gradually, since lean mixture is used and thanks to suitable solutions, commonly consisting in high levels of charge dilution by exhaust gas. However the admission of exhaust gas into the cylinder goes to detriment of engine maximum mean effective pressure. Several researches are in progress on this subject with the purpose to realize engines with low exhaust emissions of pollutants [21]. The most important advantage of HCCI combustion in respect to the diffusive one is the possibility of minimizing soot and NOx emissions. This occurs because HCCI combustion avoids the presence, in combustion chamber, both of excessively fuel-rich zones and of local temperature peaks, responsible of soot and NOx formation respectively. CO and HC emissions can decrease as well.
It is considered a very promising solution to reduce engine pollutant emissions, but so far mostly experimental prototypes have been inspired by this concept. A few months ago General Motors demonstrated gasoline HCCI combustion concept for the first time in two drivable concept vehicles, the 2007 Saturn Aura and the Opel Vectra. In these applications HCCI combustion process is achieved at low vehicle speed and in low/medium load conditions with transition to spark ignition combustion at higher engine speeds and loads. The switch between the two combustion processes is, however, still perceivable, whereas fuel efficiency is improved by 15%.
The first studies on HCCI combustion go back to the end of ’70s. In 1979, a small gasoline two-stroke engine operating with spontaneous ignition of premixed homogeneous charge was presented by Onishi et al. [22] and was applied for electric generators. This kind of combustion was adopted with the purpose to turn, from negative to positive, the effect of high residual gas level at light loads.
In order to fix some of the problems of the two-stroke engines, ATAC (Active Thermo-Atmosphere Combustion), really close to the ideal HCCI combustion concept, was realized [22]. Being the scavenging coefficient unavoidably low at light loads, the charge is diluted by large amounts of exhaust gas that, at idle, inhibit ignition in most engine cycles. In this combustion strategy a non complete mixing between residual and fresh gas is required, so that cells of residual gas, rich of thermal energy, help igniting the fresh charge. The reaction begins with
Figure 2.1. Start of combustion for SI, CI and HCCI engines.
characteristics of homogeneous combustion (not simultaneous, thanks to residual gas inhibitory effect) in several small areas and from them it moves on towards the rest of the charge as a distributed progressive combustion.
Building on the previous work in two-stroke engines, Najt and Foster (1983) [23], and later Thring (1989) [24], proved the feasibility of extending HCCI combustion concept to four-stroke gasoline engines.
In recent years HCCI combustion has become an object of great interest and countless studies, numerical and experimental, have been performed in order to solve the problems that conflict with the application of this concept in large-series production vehicles.
HCCI is feasible with fuels both for SI and for CI engines, but it still does not cover the whole engine operating field, thus the engine must be built to operate basically as a conventional engine. A basic problem to solve is extending HCCI combustion feasibility to all engine operating conditions (or at least to a wide range of them), including the least favorable ones (heavy loads and transients).
Being triggered by homogenous charge spontaneous ignition during compression, HCCI combustion timing control and optimization are not easy (in conventional SI and CI engines the control is achieved by spark timing and injection timing respectively). In particular, this can lead to high peaks in the heat release profile, causing high pressure gradients, dangerous for the engine structure.
Finally, the homogeneous charge formation represents a critical factor for the attainment of HCCI combustion. Early injection is likely to cause excessive wall impingement of the injected sprays, which deteriorates the homogeneity of the fuel/air mixture in the combustion chamber. On the other hand, late injections bring to insufficient time for fuel evaporation and charge mixing with presence of in-cylinder charge inhomogeneity as well.
2.2. GASOLINE-FUELLED HCCI ENGINES
In gasoline-fuelled engines, air and fuel create a premixed homogeneous charge, as in normal SI engines; yet combustion, instead of being primed by external energy (electric spark), takes place thanks to spontaneous ignition, as in CI engines. Of course, for this purpose, spontaneous combustion temperature as well as latency time must be exceeded. To obtain a gradual combustion not acquiring the character of simultaneous reaction, appropriate solutions are necessary, which generally involve charge dilution with exhaust gas (EGR). In addition, EGR can be associated with large air-fuel ratio with low cycle-to-cycle variation. Clear advantages in thermal efficiency such as lesser decrease of specific heat ratio and reduction of dissociation, of heat transfer to cylinder walls and of pumping work occur. These advantages allow 15 to 20% improvement in fuel economy and large reduction in NOx emissions compared to conventional SI engines. This reduction occurs because the high temperature flames, typical of SI engines, are prevented by combustion extension to the whole combustion chamber volume along with EGR and air excess. CO and HC emissions can decrease as well (however not all experimental results are in agreement).
While the potential benefits of HCCI combustion are large, this combustion mode presents some problems: high hydrocarbon and CO emissions, a narrow operating range and difficult control of the combustion phasing. Compared to conventional SI engines, HCCI combustion presents also high hydrocarbon and CO emissions. Like all the homogeneous charge combustion systems, a significant portion of the in-cylinder fuel is stored in crevices during the compression stroke and escapes combustion. Unlike traditional SI engines, however, the burned gas temperature is too low to consume much of this unburned fuel re-entering the cylinder during the expansion stroke. This results in a significant increase in both hydrocarbon and CO emissions relative to conventional SI engine operations. In addition, at lower loads the peak burned gas temperatures can be too low (lower than 1500 K) to complete the CO to CO2 oxidation, with subsequent low
combustion efficiency at very light loads [24]. At higher loads there is insufficient dilution to moderate the combustion rate and pressure rise can become so large that engine noise increase significantly, and sometimes, engine damage may occur [25]. As a result, any practical HCCI engine for passenger car application will inherently operate as a multi-mode engine, as in the General Motors concept cars.
2.3. DIESEL-FUELLED HCCI ENGINES
Diesel engines have been faced with increasingly stringent emissions legislation for more than a decade. Despite substantial improvements, new standards scheduled to take effect in 2009 will require approximately further reductions in both NOx and particulate matter (PM) emissions. These new standards will likely require the development of advanced combustion systems and an improvement of the current aftertreatment operations.
Initial efforts with HCCI involved gasoline-fuelled engines, and this technology continues to be strongly pursued today. However, the need to reduce emissions from diesel engines led to investigations of the potential for
fuelled HCCI beginning in mid-1990s. There are several reasons why diesel-fuelled operation is desirable, particularly for the medium and heavy truck engine market.
Perhaps foremost is that the use of HCCI in transportation engines may be limited to engines with dual-mode combustion systems, at least for the near term [26, 27]. Due to difficulties with cold starting and in controlling the combustion rates of HCCI engines at higher loads, conventional diesel operation would be used at these conditions, while the advantages of HCCI are realized over the remainder of the operating map. Since operation at loads above the expected limit for HCCI constitutes a significant portion ok the driving cycle for most truck engine, there are significant efficiency advantages to operating as a diesel engine when not in HCCI mode.
As concerns HCCI combustion in diesel-fuelled engines, the same initial considerations about gasoline-fuelled engines avail (homogeneous mixture, spontaneous ignition, obtaining of a gradual combustion). The most important advantage of HCCI combustion in respect to the diffusive one is the possibility of minimising soot and NOx emissions. This occurs because HCCI combustion avoids the presence, in combustion chamber, both of excessively fuel-rich zones, responsible of soot formation, and of local temperature peaks, responsible of NOx formation. Also some of the problems are essentially the same, difficult combustion timing control, difficult obtaining of gradual rate of heat release, small field of existence. Although diesel-fuelled HCCI is desirable for the reasons outlined above, achieving acceptable HCCI combustion with diesel fuel can be difficult for two main reasons. First, elevated temperatures are required before significant vaporization occurs, making it difficult to form a premixed homogeneous charge. Second, diesel fuel has significant cool-combustion chemistry, leading to rapid auto-ignition once compression temperatures exceed about 800 K [28]. This can lead to overly advanced combustion phasing and/or require reduced compression ratios and low intake temperatures.
Investigations and the development of diesel-fuelled HCCI may be separated into three main categories, depending on the fuelling technique:
• premixed HCCI;
• early direct injection HCCI; • late direct injection HCCI.
2.3.1 PREMIXED HCCI
This approach is perhaps the most immediate and simple fuelling technique to obtain the premixed homogeneous charge. The fuel is injected into the intake duct, upstream of the intake valve, similar to a conventional port fuel injected (PFI) SI engine. A pre-heating of the intake air is necessary to minimize the accumulation of liquid fuel on surfaces in the intake system.. Satisfactory results with this solution required reduced compression ratios in the range of 8 to 13:1, depending on the intake temperatures and the amount of EGR, to avoid advanced ignition and knock occurrence [29]. With intake air temperature below 130°C, soot emissions increase significantly because of the slow liquid fuel evaporation leading to a combustion occurring around large droplets. In this case also HC emissions tend to be very high. As a result of the poor combustion efficiency, reduced 10
compression ratios, and non-optimal combustion phasing, fuel consumption increased by an average of about 30% over normal direct injection (DI) diesel combustion. However, NOx emissions were dramatically reduced for HCCI operation, with levels being approximately a factor of 100 lower than those of conventional diesel combustion on a power specific basis. Smoke levels were also reduced to near zero for all HCCI points with intake temperatures in the optimal range.
2.3.2 EARLY DIRECT-INJECTION HCCI
Direct fuel injection has been perhaps the most commonly investigated approach to diesel-fuelled HCCI. Compared to premixing in the intake, by injecting the fuel part during the compression stroke, the higher in-cylinder temperatures can help the vaporization of the diesel fuel and promote the mixing. This allows cooler intake temperatures, reducing the propensity for early ignition. With a carefully designed injector, it is possible to minimize fuel wall wetting that cause low combustion efficiency and oil dilution [30, 31]. By using this strategy only one fueling system is required for both HCCI and conventional diesel operation even though not all of the HCCI DI-fueling systems are compatible with conventional diesel diffusive combustion. An injector designed for HCCI combustion should provide fast fuel atomization with limited penetration in order to rapidly create a homogeneous mixture avoiding fuel wall wetting. The main disadvantage of DI for HCCI is that less time is available for fuel/air mixing, and NOx and PM emissions can be significant if mixing is not sufficiently complete. Finally, it should be noted that controlling combustion phasing is still critical a critical issue for early-DI HCCI, since injection timing does not provide an effective means of directly controlling combustion phasing as in conventional diesel combustion.
2.3.3 LATE DIRECT-INJECTION HCCI
The late direct injection HCCI combustion technique is known as MK (modulated kinetics) developed by Nissan Motor Company. This combustion system has been adopted in the Nissan high-speed DI engine of 1998; however, the MK system application was limited to lower loads and speeds [32, 33]. In order to achieve the diluted homogeneous mixture required for HCCI, all the fuel must be injected and reasonably well mixed with the in-cylinder gases prior to auto-ignition. Thus, a long ignition delay and rapid mixing are required. The injection is retarded to 3° ATDC and high levels of EGR are adopted to decrease the free oxygen concentration to 15-16%. Rapid mixing was achieved by combining high swirl with a toroidal combustion bowl geometry. In MK mode, NOx emissions were substantially reduced without increasing PM. Combustion noise was also significantly reduced and combustion timing was controlled by injection timing, similar to conventional diesel engine. Interestingly, the thermal efficiency was slightly increased with MK combustion, despite combustion phasing being significantly retarded. This improvement has been attributed to reduced heat transfer as subsequently verified by experimental measurement. Later on, for a second generation of the MK system, several modifications were made to expand the working range to higher loads and speeds [34]. Since more fuel must be injected at higher loads, it becomes more difficult to keep the fuel injection duration shorter than the ignition
delay. A high-pressure common rail fuel system was used to provide high injection pressures at all speeds. The ignition delay was increased by reducing the compression ratio to 16:1 (from 18:1) and adding EGR cooling to reduce the intake temperature. To minimize the potential for liquid fuel impingement on the piston, the piston bowl diameter was increased from 47 to 56 mm. This change significantly reduced HC emissions under cold-engine conditions. The operating range for the second generation MK combustion system has been expanded to about half load and three quarters speed. NOx emissions are stated to be reduced of 98% and PM emissions results similar to conventional engine operations without EGR.
2.4. KEY OPERATING PARAMETER FOR HCCI CONTROL AND
OPERATING RANGE EXTENSION
HCCI combustion is a process occurring gradually and is triggered by several ignition source in the whole combustion chamber. The process is really fast and, if not controlled, can change in a simultaneous reaction, dangerous for the engine structure. The maximum thermal efficiency is only realized when combustion phasing occurs within a narrow crank angle “window” where heat loss and expansion (or time) loss sum to a minimum. The location of a peak cylinder pressure in the crank angle domain would be in a range of 8-12 degrees ATDC and, since the combustion intervals are very short, departures from the idealized phasing would lead to rather abrupt efficiency diminution. Therefore, maintaining precise control of combustion phasing is very important in terms of thermal efficiency.
While making the HCCI process occur is rather straightforward, making it occur at an appropriate time in an engine cycle over a wide range of operating conditions is extremely difficult. The energy release rate from HCCI combustion depends not only on the reaction chemistry of the fuel, but also on the thermal conditions of the mixture during intake and compression processes. Many factors such as temperature, pressure, mixture concentration history and chemical kinetics can influence the auto-ignition process.
Below some key parameters for HCCI control and operating range extension are analyzed, such as:
• intake air temperature; • compression ratio;
• exhaust gas recirculation (EGR) and residual gas; • water Injection;
• boosting;
• fuel injection strategy;
• additives and fuel modifications; • engine speed.
2.4.1 INTAKE AIR TEMPERATURE
In a HCCI engine, auto ignition timing is affected by many variables, especially temperature behaviour. A higher intake temperature gives earlier initiation and higher heat release rates. It improves the start of first stage ignition (cool flame) and thus reduces main ignition delay. Historically, this has been the most widely studied HCCI control approach reported in the literature. A higher intake temperature advances HCCI combustion but the controllable range is relatively limited. Outside this range, the engine thermal efficiency are largely reduced, due to the fact that if auto-ignition is advanced into the compression stroke, it will cause a loss in thermal efficiency. Also when engine speed and load change, the auto-ignition will also vary, unless the charge temperature is varied to compensate for this. However, this compensation is generally a slow process, especially on the cycle basis due to the limited capability of intake heating under a transient condition.
2.4.2 COMPRESSION RATIO
As a replacement of intake air heating, compression ratio has been carefully investigated as an effective means to achieve HCCI combustion control [35]. A higher compression ratio can increase the charge temperature during compression stroke and effectively advance the start of auto-ignition at low loads. Furthermore higher compression and expansion ratios also contribute to higher thermal efficiency, however, a HCCI engine enabled by a higher compression ratio will encounter knock problems at higher loads with lower-octane fuels. Variable compression ratio would seem to be a potential solution to this problem, but practical variable compression ratio mechanisms are difficult to implement and no practical devices have yet been demonstrated. A study using the SAAB variable compression ratio engine prototype was reported to investigate the trade-off between inlet air temperature and compression ratio for a naturally aspirated, multi-cylinder HCCI engine. It was found the higher compression ratio can replace inlet air heating. However, the drawback with an increased compression ratio is the increased CO emissions due to faster expansion, leading to reduced reaction time [36]. As pointed out by Nait and Eng [22, 25], the choice of compression ratio (or the optimum compression ratio) is not easy and will be specific to the examined system.
2.4.3 EXHAUST GAS RECIRCULATION (EGR) AND RESIDUAL GAS
The most practical means of controlling charge temperature in an HCCI engine is through the addition of high levels of recirculated exhaust gases into the intake. In addition to the thermal contribution, the inert gases contained in the EGR can be used to control the heat release rate due to their impact on chemical reaction rate, delaying the auto-ignition time and reducing heat release rate and peak cylinder pressure. External EGR system is very simple but its thermal effect is limited due to consistent heat losses and slow response during transient operation.
In comparison, retaining the exhaust gas residuals in the cylinder through cam phasing can avoid many of the issue encountered by the external EGR approach [36]. By tuning the engine valve timing, a manageable quantity of internal
residual can be trapped in the cylinder and used to heat fresh charge. By tuning the amount of the internal residual, ignition delay and heat release rate of HCCI combustion can be adjusted.
EGR homogeneity has been found to have a significant impact on HCCI combustion. With the same EGR rate, heterogeneous EGR shows a benefit in increasing power output and lowering peak cylinder pressure (like in the ATAC technology for Two Stroke engines [21]). Heterogeneity between the fresh charge and residual gas is desired because the temperatures in EGR-rich zones are higher than those obtained in a completely homogeneous charge. The fuel-air mixture will easily auto-ignite at the boundary of these EGR islands that will represents multiple ignition sources well distributed in the whole combustion chamber.
Recently, flexible valve actuation approaches received much attention due to their flexibility in providing a high level of internal residuals. Several approaches have been used to trap internal EGR into the engine. An interesting method relies on trapping a certain amount of exhaust gases by closing exhaust valves early in the exhaust stroke. The trapped exhaust gas is then recompressed during the final stage of the exhaust stroke until, when piston descends, inlet valve are opened and fresh charge enters the cylinder. Another strategy consists on opening the exhaust valves for the whole exhaust stroke to expel the combustion products from the cylinder. Then, as the piston induces the next induction stroke, both inlet and exhaust valves are opened simultaneously and both fresh charge and exhaust gas enter the cylinder. When comparing these two methods, the first appears to better preserve the thermal energy since large heat losses are present in the exhaust port. Then, the second strategy may be more challenging in the accurate control of the amount of exhaust gases to be trapped.
2.4.4 WATER INJECTION
The use of water injection has recently been examined as a means of controlling auto-ignition timing and slowing the heat release rate for HCCI control. Experimental tests [37] confirmed this capability with, however, increased CO and HC emissions. The NOx emissions, very low for HCCI combustion, decreased even further with water injection applications. The amount of water used was of the magnitude of the fuel quantity. Water injection has an even bigger effect on the ignition timing when higher inlet temperatures are used. This is likely due to more vaporization and a more uniform distribution of the water. Water atomization, vaporization and distribution are all important parameters for system optimization.
Water injection introduces the possibility of extending the HCCI operating range towards higher engine loads [30], with extremely low NOx emissions and almost smokeless combustion. Although the water inhibitor effect increases with advanced timing, the optimal water injection timing exists. With too advanced timings, water vaporization is delayed, which decreases the reaction suppressing effect. While the combustion suppression effect increases with injected water amount, excessive water increases HC emissions and specific fuel consumption. Therefore water injection amount should be limited to the minimum required for sufficient suppression of over-advanced combustion.
2.4.5. BOOSTING
Boosting represents an effective means to extend the HCCI operating range to higher engine loads by increasing the ratio between air + EGR and fuel. However, boosting leads to high cylinder pressure, which may limit its potential application. An excessively high indicated mean effective pressure (IMEP) leads in fact to low efficiency due to the high friction losses in the engine. The HC emissions decrease with an increase in boost pressure and engine load while the CO emissions are strongly dependent on the air-fuel ratio and on the charge preheating. When operating near the rich limit with hot inlet air, CO emissions are negligible. In all the cases, NOx emissions are extremely low.
Olsson et al. [38] studied the operating characteristics of a 6-cylinder turbocharged truck engine proving the possibility of achieving high loads, up to 1.6 MPa of brake mean effective pressure (BMEP), with ultra low NOx emissions.
2.4.6. FUEL INJECTION STRATEGY
For the obtaining of an in-cylinder homogeneous charge a minimum amount of time is necessary for the air-fuel mixing. Therefore, PFI injection could seem the most convenient strategy for HCCI combustion. However, this system does not allow to control the combustion phasing and limits the compression ratio adopted in the engine. Direct injection (DI) allows to increase the compression ratio by 1-1.5 points extending the HCCI operating range towards low load conditions. Furthermore, DI permits to control the in-cylinder injection phasing, essential step to control mixture formation and ignition timing. CFD simulations shown that different injection phasing, leading to different in-cylinder charge stratification and combustion timing, has a substantial impact on emissions generation [17]. With very advanced injection, to create an homogeneous mixture, NOx and soot emission are really small, but consistent HC emissions are generated. Oppositely, with late injection during the compression stroke, to obtain in-cylinder charge stratification, HC emissions are reduced but NOx and soot production result unacceptable. Therefore, with an injection phasing optimization, low levels of all the pollutant emissions can be achieved.
Several multiple injection strategies, sometimes combinations of PFI and DI, have been studied in the recent years in all the most important engine research centres since fuel injection strategy represents one of the fundamental parameters to control HCCI combustion and to extend its operating range to the whole engine operating conditions. An interesting strategy, called the chemical pre-activation [39], consists on injecting part of the fuel into the trapped in-cylinder residual gas, once they are recompressed in the last exhaust stroke. By so doing, chemical pre-activation reactions begin and, once the main injection timing occurs, less time is necessary for the auto-ignition process.
2.4.7. ADDITIVES AND FUEL MODIFICATIONS
Fuel selection is an important aspect of HCCI engine development. Both fuel volatility and auto-ignition characteristics are important parameters. The fuel must have an high volatility in order to easily form an homogeneous charge. Chemically, 15
fuels with single-stage ignition are less sensitive to change in loads and speed and this can ease the requirements of an HCCI control system over a wide range of engine operating conditions. In order to obtain high fuel efficiency, the auto-ignition temperature of the fuel is critical for an optimal selection of engine compression ratio. In addition to meeting all of the requirements for HCCI operation at lower loads, the fuel must also meet performance criteria for full load operation. For example, to execute gasoline HCCI combustion, the fuel must have a low octane rating to readily auto-ignite. In comparison, the gasoline fuel has to have a high octane rating to sustain the conventional SI flame propagation at high loads without knocking problems. This contradicting fuel requirement makes HCCI development extremely difficult. Obviously, there is no universal fuel that is specific to HCCI application. The optimal fuel depends on the combustion control strategies used and also the operating conditions. It is appropriate to say that HCCI engines can be operated with any fuel, if this fuel has been built up for that specific engine.
Some chemical components have the ability to inhibit or promote the auto-ignition heat release process. Therefore, HCCI auto-auto-ignition can be controlled by modifying the fuel, by adding an ignition promoter or inhibitor, so that it is more chemically reactive or inhibitive. For example, using natural gas as fuel, even a small amount of NOx present in the trapped charge may be important in HCCI control. Varying NO2 concentration by adding some chemical fuel additives may
even offer a control opportunity, although the impact on the engine out NOx emission needs to be evaluated [40]. This suggests that whether it is possible to control the reactivity of the components of a fuel mixture it will be easy to control HCCI combustion over a wide range of operating conditions. This range will be wider if wider is the difference in reactivity of the mixture components [41]. Combining two fuels with very different octane numbers to form a fuel mixture, for every load and speed condition, it will be possible to find an optimum combination of the two fuels to control HCCI combustion. The higher the octane number difference existing between the blending fuels, the wider the operating range that can be achieved. Furthermore, for the mixture composition it is recommended to use as much high-octane fuel as possible and as low as low-octane fuel as possible to obtain a wide range of engine operation [42].
2.4.8. ENGINE SPEED
Theoretically, the ignition delay of HCCI combustion depends largely on mixture chemistry, and it is relatively independent of engine speed. However, the ignition time of HCCI combustion relative to the engine crank angle will be retarded when the engine speed increases. When ignition occurs before TDC, the temperature rise from compression will compensate the relative ignition retardation at high engine speed. If ignition appears after TDC, the relative ignition delay caused by high engine speed will be further retarded by expansion, which slows the temperature rise. In order to achieve the optimal combustion phasing, the ignition delay must be compensated through other means such as increasing intake temperature while engine speed increases [43]. By doing this, the stable combustion zone shifts towards high temperature conditions, with the risk of generating simultaneous reaction and reducing the maximum load for which it is possible to operate in HCCI mode.
2.5. CO AND HC EMISSIONS AND THEIR CONTROL
Usually HCCI combustion operations lead to high levels of CO and HC emissions, particularly at light loads. This occurs mainly because temperatures are so low that fuel near the wall does not burn and also the combustion temperature are too low to complete the CO oxidation reaction. As with all the homogeneous charge combustion systems, a significant portion of the in-cylinder fuel is stored in crevices during the compression stroke and escapes combustion. The problem is compounded by low exhaust gas temperatures resulting in low aftertreatment efficiency levels. Wall insulation, catalyst coating and reduced piston crevices are useful instruments to prevent the wall quenching. Supercharging and increasing intake air temperature can reduce HC emission. Lower exhaust gas temperatures with HCCI combustion due to ultra-lean operation, fast combustion (shorter duration), and large expansion ratio is the major obstacle to use an oxidation catalyst. Ensuring a higher exhaust gas temperature without losing efficiency and increasing NOx emission is fundamental. Selection of the proper materials for low-temperature catalysis is also an important factor [44].
