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. 2024 Mar 8;9(11):13274–13283. doi: 10.1021/acsomega.3c10037

Analysis of Various Factors’ Influence and Optimization of Low-Temperature Combustion Technology

Michal Puškár †,*, Milan Fil’o ‡, Melichar Kopas †, Daniela Kepeň Harachová †
PMCID: PMC10955711  PMID: 38524472

Abstract

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It is possible to state that presently the transport area is responsible for a considerable part of the generated greenhouse gases and that transport is one of the main polluters in urban territories. However, at the same time, it is necessary to say that vehicles equipped with internal combustion engines are always the most popular among people thanks to the undoubted advantages of the piston combustion engines. Nowadays, several hundred millions of vehicles with combustion engines within the EU and about 1.5 milliard of such vehicles worldwide are utilized. The only reasonable possibility to reduce gaseous emissions generated by the motor car transport and to keep in operation vehicles equipped with the combustion engines is utilization of new fuels, namely, synthetic fuels. A very favorable idea is the innovative technology, which is called the low-temperature combustion (LTC). The LTC technology offers high efficiency of modern internal combustion engines together with the application of new climate-neutral fuels. That is why, the LTC technology means a prospective base for the sustainable future operation of the current vehicles. This scientific article describes the dual-fuel technology, namely, optimization of combustion and also a design proposal of innovated geometric shape of the combustion chamber determined for minimization of gaseous emissions. An original LTC combustion system characterized by positive results achieved in the reduction of an emission footprint is introduced. This innovative system is also the subject of patent protection.

1. Introduction

The transport area generates a considerable portion of worldwide greenhouse gas emissions. Therefore, transport is one of the dominant polluters in urban territories. Taking into consideration these facts, a growing pressure on the automotive industry is acting to reduce gaseous emissions and to utilize renewable sources of energy. As was mentioned, vehicles equipped with combustion engines are all the time the most popular vehicles among the customers, thanks to significant advantages of combustion engines. Taking into consideration various types of engines, it is evident that the compression ignition (CI) engine is in the first place thanks to higher level of fuel efficiency.1 Globally said, the CI engines use a large amount of fossil fuels, and they emit equivalent volume of gaseous pollutants. The worldwide performed engine research is focused on fuel saving, improvement of engine efficiency, environmental protection, and reduction of engine emissions. During the previous decades was developed the innovative technology of direct injection compression combustion (DICI) and also interesting improvements in the area of CI, namely, the advanced combustion modes. The results of performed studies show that the technology of low-temperature combustion (LTC) enables to improve engine efficiency and reduce emissions.2,3 On the other side, there is a problem during application of the above-mentioned innovative combustion modes at high engine load conditions, with regard to the fact that the heat release rate (HRR) depends on chemical kinetics, which leads to high level of noise and also to high pressure rise rate.4 Bessonette et al.5 tested various fuels with a wide range of ignition ability in a homogeneous charge compression ignition (HCCI) engine. HCCI is one of the LTC technologies. Li et al.6 investigated influence of diesel/gasoline mixture ratio under different engine load conditions in a DICI engine. The result of this investigation is that pure diesel is suitable in low engine load operation, and gasoline is suitable for high engine load operation. According to the conclusions following from refs (5,6), it is necessary to apply a dual-fuel system in the LTC engine to ensure its operational stability under various loading conditions. The dual-fuel concept applied in a diesel engine is known from the year 1955.7 Old-day research focused on the utilization of gaseous fuel such as natural gas, biogas, methane, and others.8,9 Recently, the liquid fuels, e.g. gasoline10 and iso-octane,11 were also experimentally tested. Jiang et al.10 proposed the homogeneous charge-induced ignition. Inagaki et al.11 studied the dual-fuel concept created from the mixture of iso-octane and diesel fuel in a CI engine. Kokjohn et al.12 applied multiobjective genetic algorithm13 to optimize the engine operational parameters. After analysis of the above-mentioned literature sources, it is possible to say that the most complicated problem during application of the LTC technology is control of the combustion process itself. Therefore, this article presents the patented innovation called the “thermal control system of combustion process for homogeneous mixture with dual injection”, which contributes in a fundamental way to solving the problem of combustion control.

2. Innovative LTC Systems, Experimental Fuels, and Testing Engine

The piston combustion engine equipped with dual-fuel LTC combustion technology uses two fuels with different reactivity to create an ignition mixture in the combustion chamber. Multiple fuel injection ensures an appropriate fuel burning rate, resulting in low NOx and soot emissions, together with high thermal efficiency.14

Figure 1 offers a schematic view on the principle of the dual-fuel LTC combustion technology. The high-reactive fuel, together with the low-reactive fuel, is injected into the engine within the combustion process. The low-reactive fuel is injected using the port fuel injection (PFI) system. The injected fuel is premixed with air during piston suction stroke. The high-reactive fuel, injected directly into the combustion chamber, is used to control self-ignition and combustion of a lean premixed fuel–air mixture, which consists of low-reactive fuel, air, and eventually a small amount of exhaust gas (Figure 2). This method ensures reliable ignition of fuel mixture.15

Figure 1.

Figure 1

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Principle of dual-fuel LTC combustion technology.

Figure 2.

Figure 2

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Timing of fuel injection in dual-fuel LTC technology.

The low-reactivity fuel can also be injected by several early direct injections or by a single indirect injection into the intake manifold. Direct injection of high-reactive fuel can be performed either at once or multiple times to optimize phasing as well as according to the duration and extent of the combustion process. The possible dual-fuel combinations are as follows: gasoline–diesel, ethanol (methanol or butanol)–diesel, or gasoline–gasoline with different octane numbers. Since it is a dual-fuel system, a double-Wiebe function is proposed to calculate the mass portion of the burned fuel in relation to total fuel mass, which is based on the relation

2.

where xb is the mass fraction of the burned fuel. Variables 1 and 2 indicate combustion of fuels 1 and 2. Symbols θ01 and θ02 represent combustion beginning of the first and second fuel (Figure 2). Further, α is the fuel mass fraction burned during the first fuel combustion, which is the mass fraction of fuel burned at θ02. The burning time of the first fuel is Δθ1 = θ02 – θ01.

The measure of fuel burned in combination with dual-fuel combustion can be expressed as

2.

where xb1 and xb2 are the mass fractions of the burned fuel at each combustion phase.

2.

The result of combustion, using the reactive dual-fuel strategy, is a relatively higher thermal efficiency compared to HCCI combustion and other LTC derivatives while maintaining lower NOx and PM emissions than during diffused diesel combustion.16 At the same time, it is not necessary in the case of the dual-fuel strategy to apply some post-treatment by other methods. Therefore, it is possible to say that this concept is one of the most promising technologies.

A considerable advantage of the combustion engine, which utilizes the above-described technology, is the fact that it can be operated in a wide range of engine loads, whereby it generates almost zero level of soot and NOx emissions (Figure 3). At the same time, the LTC engine is characterized by acceptable pressure increase rate and very high indicated efficiency.17−19

Figure 3.

Figure 3

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Operational range of dual-fuel LTC technology.

Correct operation of the combustion engine in the LTC mode requires a suitable constructional modification of the combustion chamber together with adjustment of the intake and exhaust pipe system. Namely, the conventional design of the combustion chamber used in a standard single-fuel diesel engine is problematically applicable due to swirling of fuel with low reactivity. In this case, the hot spots can occur on the internal surface of the combustion chamber, and this fact excludes utilization possibility of low-reactive fuel with high octane number. The described complication requires application of additional cooling focused on such points, which can be overheated during the combustion process, e.g., the fuel injectors.20

The dual-fuel LTC combustion mode depends on stratification and the reactivity of fuel. It is difficult to define the stratification of fuel, which is associated with spray penetration and entrainment of fuel (which is directly injected) by the mixture. Reactivity is based on the amount of fuel and on the reactive properties of fuel, i.e., on the fuel cetane and octane number.21,22

Elimination of stratification or reactivity can slow the ignition timing, facilitate the rate of peak pressure rise, and reduce the HRR. These findings explain the wide operational and engine load range of combustion using advanced combustion technology.

Fuel ignition in the case of dual-fuel LTC technology begins when the high-reactive fuel (with high cetane number) is directly injected into the combustion chamber, and then it is mixed with the low-reactive fuel (with high octane number), which is delivered into the combustion chamber through a port. The type of engine, timing, and angle of fuel injection are very important factors influencing flame propagation during the combustion process.23−25

There are two possible heat release processes (Figure 4) in the case of the dual-fuel LTC combustion. The first process is called low-temperature heat release (LTHR) and the second is high-temperature heat release (HTHR). The LTHR process depends on the high-reactive fuel with a negative temperature coefficient. The HTHR process depends on the premixed fuel, which is a low-reactive fuel.

Figure 4.

Figure 4

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Heat release rate.

2.1. Fuels for Dual-Fuel Concept LTC

The basic fuels used in the LTC mode are petroleum products such as gasoline and diesel. Many elaborated studies are mainly focused on the gasoline–diesel combinations, but in addition to gasoline, gas was also often utilized as a fuel with low reactivity.26 It can be stated that diesel or biodiesel was used as high-reactive fuel in most of the performed experiments, and only the low-reactive fuel was alternated. The characteristic properties of the most common fuels used as low-reactive fuels are described in Table 1.

Table 1. Low-Reactive Fuels Used in LTC.

type of fuel molar weight [g/mol] vaporiz. heat [kJ/kg] higher heating value [MJ/kg] lower heating value [MJ/kg] stoichiometric AFR research octane number [RON] motor octane number [MON] cetane number
gasoline 111.0 307.0 47.3 43.0 14.6 92 ÷ 98 80 ÷ 90 14 ÷ 20
NG 16.0 509.0 55.26 49.77 17.2 120.0 120.0 0
methanol 32.0 1147.0 22.54 20.05 6.5 92.0 106.0 7.0
ethanol 46.0 873.0 29.71 26.95 9.0 89.0 107.0 6.5
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CN means the cetane number and RON is the octane number determined from research of fuel. Gasoline is a commonly widespread fuel on the market, which is the main advantage for its application as a low-reactive fuel. However, there are also certain problems with petroleum-based fuels, such as formation of soot when gasoline is burned, due to its chemical composition.

Natural gas (NG) is also a nonrenewable fossil fuel. It is primarily a mixture of methane, but it also includes ethane, nitrogen, propane, carbon dioxide, etc. Compared to gasoline, it does not contain aromatics. The octane number of NG is higher than that of gasoline, which means that NG is suitable for engines with a higher compression ratio, and it can offer a greater reactivity difference in the combustion engine cylinder. Compared to gasoline, NG has higher heating values.27 Based on these fuel properties, several experiments were carried out where gasoline was replaced by NG (i.e., mixture NG–diesel). The results obtained from these experiments showed that application of NG in mixture with the diesel enables us to operate the LTC engine at higher operational load.

Alcoholic fuels (methanol, ethanol, and isobutanol) can be used as a suitable substitution of the conventional fuel (gasoline) because they are renewable sources of energy.28 According to data presented in Table 2, the alcoholic fuels are characterized by high value of octane number that is very suitable for the internal combustion engines. These fuel types reduce engine knocking and create a greater reactivity gradient. A higher heat of vaporization is another typical characteristic of alcoholic fuels. This characteristic feature causes an important cooling effect, which reduces the temperature of mixture in a cylinder. Generation of NOx emissions depends on temperature in such a way that the reduced temperature of mixture generates higher amount of the NOx emissions. The alcoholic fuels have lower heat values, which leads to higher fuel consumption.14

Table 2. High-Reactive Fuels Used in LTC.

  biodiesel [EN 14214] ULSDF [EN 590]
density, 15 [kg·m–3] 860.0 ÷ 900.0 820.0 ÷ 845.0
viscosity (at 40 °C) [mm2·s–1] 3.5 ÷ 5.0 1.9 ÷ 4.1
cetane number min. 51 min. 48
sulfur content [mg·kg–1] max. 10 max. 10
heat of evaporation [kJ·kg–1] 250.0 ÷ 290.0 282 ÷ 338
flash point [°C] 101.0 82.0
carbon content [wt %] 81.5 97.1
hydrogen content [wt %] 12.1 13.4
oxygen content [wt %] 10.8 0
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Diesel fuel (ULSDF) and biodiesel are commonly used fuels with high reactivity. Diesel is a petroleum-based fuel; i.e., it is the nonrenewable source of energy. Biofuels, which can be considered as the renewable fuels, are used as an alternative to common diesel fuel (Table 2). Recently, the biofuel attracted considerable attention during application as alternative fuel in internal combustion engines.15,16 These fuels, which are presented in Table 2, were added because of their availability and mainly because they are the mostly used.

Biodiesel is usually a compound which consists of acidic methyl esters or ethyl esters produced from specific raw materials, as uneatable vegetable oils, waste cooking oil, animal fats, etc., with the properties suitable for use in diesel engines. It is evident that the properties of each biodiesel type depend on the origin and quality of the biodiesel raw materials. Various experiments and tests were performed with biodiesel applied in different internal combustion engines using a wide range of operational conditions. Diesel engines with combustion of biodiesel generated increased amount of NOx emissions due to oxygen contamination.29,30

Biodiesel can be used in a dual-fuel system without constructional modifications of the engine. This claim was also proven by several experiments. Biodiesel, which is used as a high-reactive fuel, was more stable during transitions between the individual cycles in the dual-fuel LTC engine.

2.2. Fuels for Single-Fuel Concept LTC

The main advantage of the single-fuel LTC concept with additives is the installation of only one fuel tank in the vehicle. This type of fuel injection utilizes fuel with low reactivity. To obtain the required reactivity gradient, the additives increasing the cetane number are mixed into the low-reactive fuel, and then such modified fuel is directly injected into the combustion chamber as a high-reactive fuel. Examples of two typical additives improving the cetane number in the LTC engine are D-TBP (di-tert-butyl peroxide) and 2-EHN (2-ethylhexyl nitrate).31 The characteristic properties of these additives are summarized in Table 3.

Table 3. Additives.

type of additive lower heating value [MJ/kg] stochiometric AFR [in mass] liquid density [at 25 °C]
2-EHN 27.4 8.46 0.963
D-TBP 33.8 10.85 0.796
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For purposes of the experiments, both additives were used in order to achieve the best engine output characteristics.

2.3. Experimental Testing Engine

The experiments realized within the solution of this research work were performed using a 4-cylinder, 2.0 L diesel engine (Table 4). This engine was equipped with the diesel injection system common rail, like the basic engine, and also with one more injection system, which is used for injection of the second fuel to the intake manifold. This second injection system was manufactured to order. The system of intake manifold was also made especially for the purpose of the performed experiments, and it was arranged so that the intake pipes were connected individually to each of six engine cylinders. Constructional solution of the given tested diesel engine and timing of fuel injection were the same as in the case of a common diesel engine. However, the main time value of fuel injection was shortened to keep the constant level of engine torque when applying ethanol as the second fuel. The testing engine was connected to dynamometer working on the physical principle of eddy currents. An important part of the testing equipment was the water—air cooler of charge air.

Table 4. Data of Testing Engine.

type 4 cylinders, common rail injection, turbocharged
compression ratio 16.2:1
bore × stroke 81 × 95.5 mm
displacement 1968 cm3
rated output 110 kW
engine speed 3500–4000 rpm
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Engine power output and gaseous emissions data were recorded at the sample frequency of 1 Hz using the Dyne Systems Cell Assistant program, which is determined for data acquisition. Measurement of diesel fuel flow was performed by means of the mass flow sensor micro motion CMF, type Coriolis. Concentrations of the engine gaseous emissions, namely, carbon dioxide (CO2), carbon monoxide (CO), oxides of nitrogen (NOx), hydrocarbon (HC), and oxygen (O2), were measured using the gas analyzer California Analytical. The emissions of PM were monitored with the smoke meter AVL, model 415s (Figure 5). The engine operation and the fuel injection were managed using the National Instruments Labview-based Driven control unit, which is equipped with injectors for direct diesel fuel injection (DI) and gasoline PFI. The Driven system allows full control of the engine’s operational parameters. This system was programmed to enable individual control of injection for timing of start-of-injection, control of individual injections, i.e., control of injection duration for each of the DI diesel injectors, and control of injection duration for each of the PFI gasoline injectors. This setup allowed manual adjustment of the fueling duration and timing of cylinder-to-cylinder balancing. All the experimental measurements were repeated 3 times in a row to avoid random error and in order to ensure the realistic results.

Figure 5.

Figure 5

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Experimental test engine.

3. Results and Discussion

3.1. Proposal of Combustion Area Optimization

The combustion process, together with the mixing of fuel and air, is significantly influenced by the shape or geometry of the combustion chamber. The fuel mixture of the LTC engine is premixed with air inside the intake pipe.32,33 This fact is fundamentally different from the classic diesel engine, where the pistons are designed for fuel homogenization directly in the combustion space. The geometry of the combustion chamber influences the combustion process also due to heat transfer because modification of the combustion chamber geometry can reduce or increase heat transfer. Experiments performed by several authors recorded an increase of thermal efficiency from 37 to 40% at 2600 rpm thanks to the modified combustion chamber geometry. The second advantage was keeping emissions below the limit values defined by the EURO standard without a necessity to install a catalytic converter or DPF filter. Figure 6 illustrates the innovative design of the optimized combustion chamber geometry applied within the performed experiment.

Figure 6.

Figure 6

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Comparison of pistons used in gasoline and diesel engines with the newly designed and tested piston shape.

The geometry of the combustion chamber specified for the LTC engine must be significantly different from the usual shape of the combustion chamber used in a classic diesel engine. The swirl ratio of the air–fuel mixture must be higher; the area of the combustion chamber can be much smaller, and the overall geometrical shape is so to say “softer”. Injection of high-reactivity fuel in the LTC engine is different from the conventional diesel engine. High-reactive fuel, which is injected directly into a cylinder in the classic diesel engine, requires a combustion chamber situated in the piston. The presented innovative combustion chamber is designed for only one type of fuel. The dual-fuel mode requires a different shape of the combustion chamber. The low-reactive fuel fulfills the whole cylinder space above the piston, which means that the high-reactive fuel is delivered into the already filled area. This fact means a problem, which cannot be solved only by changing the design of the injector. It is necessary to modify the design of the combustion area for the given specific case; i.e., the shape of combustion area must be tailor-made. The suitable swirl ratio of the air–fuel mixture is also very important to ensure a proper combustion process and to eliminate harmful emissions.

3.2. Emission Analysis Using the New Designed Combustion Chamber

So, the LTC process is influenced by the geometry of the combustion space, which is illustrated in Figure 8. Application of the LTC technology can reduce generation of NOx and PM emissions, but it is not able to reduce CO and HC emissions compared to the conventional diesel engine.34−36 Generation of CO and HC emissions can be caused by a lower combustion temperature but also by the volume of a crevice space, where the unburned fuel can be trapped. The main purpose of the proposed design was to reduce the generation of HC emissions by minimization of the crevice spaces. Application of the new piston design resulted in reduction of HC emissions compared to standard pistons used in the conventional diesel engine, as it is shown in Figure 7.

Figure 8.

Figure 8

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Dual-fuel LTC engine conception.

Figure 7.

Figure 7

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Influence of the modified combustion chamber shape (dashed line) and standard combustion chamber shape (solid line) on engine output characteristics.

Combustion temperature influences the generation of CO emissions. In the case of the LTC engine, these emissions decrease with increasing engine load. At the highest combustion chamber temperature, the level of CO generation is the lowest. It was found that HC emissions are not as sensitive to combustion temperature as CO emissions and are mainly a product of unburned fuel. The LTC engine can be operated in a wide range of engine loads, whereby the optimized combustion chamber offers lower NO and PM emissions as well as efficiency benefits compared with the conventional diesel engine.

It is possible very positively to evaluate the advantage of the applied new geometry of the combustion chamber for the combined efficiency parameters. It is evident from Figure 7 that the combined efficiency increased significantly in the whole engine load spectrum.

3.3. Emission Analysis Using Various Fuel Combinations

Emission analysis at various fuel combinations was carried out according to the technical scheme illustrated in Figure 8. The low-reactive fuel was injected into the intake port, and the high-reactivity fuel was directly injected into the combustion chamber.

Comparison of engine emissions at combinations of various fuel types is presented in Figure 9. The following fuel combinations were applied: E85 with diesel, gasoline with di-tertiary butyl peroxide (D-TBP), and the last combination was the classic fuel gasoline and diesel.37−39 Since the operational conditions and fuel phasing are the same or similar, the main influence on combustion is derived from the chemical effects of the fuel.

Figure 9.

Figure 9

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Emission characteristics using various fuel combinations: E85 with gasoline (solid lines); gasoline with 1.75 D-TBP (dashed line); and gasoline with diesel (dotted–dashed line).

Within the range of engine load between 10 and 12 bar, the emissions were minimal in the case of applied gasoline with 1.75 D-TBP (dashed line) and gasoline with diesel (dotted–dashed line). Vice versa, in the case of applied E85 with gasoline (solid lines), the minimal emissions are at the engine load level between 14 and 16 bar.

In the case of PM emission analysis, the mixture of gasoline with diesel (dotted–dashed line) has the best parameters. If E85 is applied with gasoline (solid lines), the emissions are stable mainly at higher engine load between 10 and 16 bar.

The dual-fuel system gasoline with diesel significantly reduces the NOx emissions. On the other hand, the worst characteristic has E85 with gasoline, mainly at a higher engine load between 10 and 16 bar. The CO emissions are lowest using gasoline with diesel, mainly at an engine load between 12 and 14 bar. Gasoline with diesel (dashed line) has the best emission parameters between 10 and 12 bar.

3.4. Proposal of a New LTC Combustion Control System and Emission Analysis

Within the performed research activities, an innovative system of thermal control determined for the process of homogeneous mixture combustion with dual injection was developed. This system applies not only the thermal principle but also the advanced fuel injection system. It represents a relatively new principle with significant improvements compared to the dual-fuel LTC, and, at the same time, it is the subject of a patent application (Figure 10). The given system consists of a combined intake pipe and a throttle valve. The intake pipe is joined with a turbo-compressor and connected to the engine intake tract. The combined intake pipe is also connected to the intake recirculation pipe. The low-pressure injector, installed in the recirculation pipe, is connected to the low-pressure regulator, low-pressure pump, and fuel tank. The combined exhaust pipe is connected to the turbocharger and to the engine exhaust tract. The other important constructional parts of this system are as follows: the low-temperature exhaust pipe with exhaust gas reservoir, return pipe, recirculation cooler, ERG valve, high-pressure injector, fuel heater, high-pressure regulator, high-pressure pump, and the supply pump with the regulator connected to the fuel tank. The principle of the presented invention consists of such a constructional arrangement of the above-mentioned components that the intake recirculation pipe is connected to the intake tract of one engine cylinder, and the low-temperature exhaust pipe is connected to the exhaust tract of one engine cylinder. In the low-temperature exhaust pipe, the exhaust gas reservoir is installed, connected by a return pipe to the recirculation cooler and to the ERG valve, which is joined with the intake recirculation pipe.

Figure 10.

Figure 10

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Thermal control system of combustion process for homogeneous mixture with dual injection. 1—combined intake pipe, 2—turbo-compressor, 3—throttle valve, 4—combustion engine, 5—intake recirculation pipe, 6—low-pressure injector, 7—low-pressure regulator, 8—low-pressure pump, 9—fuel tank, 10—combined exhaust pipe, 11—low-temperature exhaust pipe, 12—exhaust gas reservoir, 13—return pipe, 14—recirculation cooler, 15—ERG ventil, 16—high-pressure injector, 17—fuel heater, 18—high-pressure regulator, 19—high-pressure pump, 20—supply pump with regulator, and 21—engine cylinder.

The most important aspect of an engine equipped with dual-fuel LTC combustion technology is fuel control. The fuel control has a relevant impact on the combustion process because stratification of fuel reactivity is very important. The dual-fuel LTC strategy is based on mixing of two different fuels in order to create differences in fuel reactivity.40 However, the dual-fuel strategy requires two fuel tanks installed in the vehicle to store two types of fuel with different reactivity. From this point of view, the single-fuel LTC strategy is more advantageous because there is no need for a second fuel tank. In this case, the different fuel reactivity is achieved by different temperatures (Figure 11).

Figure 11.

Figure 11

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LTC thermal control system using dual injection of the same fuel.

The principle of single-fuel LTC operation consists in the fact that the system applies dual injection of the same fuel by means of two different injection systems at two different injection temperatures and pressures.41 In this case, the CI of an extremely lean mixture is initiated by the injection of a small amount of heated fuel. Both injection systems can work with a single or multiple injection. Thus, ignition of the air–fuel mixture is not achieved using a different fuel than it is in most current LTC designs. This fact is a significant advantage because it enables us to use the LTC technology also for burning of biofuels and for all types of standard vehicle combustion engines.

According to performed preliminary analyses of the patented solution, it can be assumed that the main advantage of the presented innovative design proposal is a possibility of engine operation in a wider range of loading regimes, whereby the engine should generate minimal emissions of soot and NOx. At the same time, the single-fuel strategy is characterized by faster high-temperature response compared to the dual-fuel strategy. The differences are not so fundamental due to fact that only one cylinder worked in a single-fuel strategy, in accordance with Figure 12.

Figure 12.

Figure 12

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Emission characteristics at dual injection of the same fuel (diesel) using two different injection systems (solid lines) and dual-fuel strategy gasoline–diesel (dotted–dashed lines).

As follows from the HC analysis, the single-fuel strategy produces less emissions at low-level engine load (up to 8 bar). After that, there is an increase in the middle engine load level, and consequently, above approximately 11 bar, the HC emissions are again more favorable at the single-fuel strategy.

In the case of PM emissions, the proposed and patented single-fuel strategy is evidently more efficient in terms of emissions, which proves the whole course of emission production, excepting the area at 10 bar engine loading, where the emission values are identical with the dual-fuel strategy.

Concerning the NOx emissions, both fuel strategies are comparable to each other. The single-fuel strategy has a certain advantage at engine load between 6 and 9 bar. The CO emissions are almost identical in the case of both strategies, but the single-fuel strategy has moderately lower emissions.

4. Conclusions

The article describes the principles of the LTC technology, the influence of various factors acting on the combustion process, and also the patented innovative solutions that could contribute to the LTC area. In the text, the different fuel combinations are analyzed with the aim to achieve the highest gradient of reactivity, to optimize engine power output, and to reduce the amount of the engine gaseous emissions. The single-fuel additives were also tested, and they demonstrated their ability to be utilized in the LTC engines. The innovative geometrical shape of the combustion chamber, which was designed, patented, and tested within our scientific research work, is essential. Based on the results obtained from the performed experiments with the LTC strategy, it is possible to state the following achievements:

  • The above-described optimized piston with the reduced surface area compared to the standard piston shape, minimized heat-transfer losses in the engine, and improved the engine combustion efficiency. The new piston shape design also helped to reduce the HC emissions. Based on the results obtained from the performed experiments with the LTC strategy, it is possible to state the following achievements:

  • The analyzed studies demonstrated a relevant fact that the LTC technology is a suitable strategy to fulfill the current and future emission standards without after-treatment of NOx and soot.

  • The LTC technology, together with optimized piston geometry, significantly improved the thermal efficiency of the tested engine in a wide range of the engine operational loads.

  • The experiments performed with the testing engine demonstrated reasons of the improved engine power output using the LTC combustion mode compared to the conventional diesel combustion. The engine efficiency was improved mainly thanks to the reduced heat-transfer losses. The LTC combustion also very significantly reduced the NOx emissions and soot emissions compared to the standard diesel combustion without application of the EGR.

  • The LTC experiments using the single-fuel strategy, using mixture gasoline/gasoline enriched with a small amount of D-TBP/2-EHN, generated almost identical emissions like in the case of the dual-fuel strategy, using mixture gasoline/diesel, mainly concerning the CO emissions. Despite this fact, the patented single-fuel strategy is beneficial in the solution of the LTC operational technology.

  • The LTC operation with low-level emissions and without after treatment of NOx and PM was successfully applied in a common multicylinder engine equipped with the OEM diesel injection and with turbocharging in the range of engine speed and engine load typical for light duty vehicles.

Acknowledgments

This work was supported by the Slovak Research and Development Agency under contract no. APVV-19-0328. The article was written in the framework of grant projects: VEGA 1/0318/21 “Research and development of innovations for more efficient utilization of renewable energy sources and for reduction of the carbon footprint of vehicles” and KEGA 007TUKE-4/2023 “Transfer of innovations and advanced technologies, determined for more ecological and more efficient vehicle drive systems, into the educational process”.

The authors declare no competing financial interest.

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