ELECTRIC HEATERS WITH THE EFFECT OF SELF-REGULATION OF FUEL SYSTEM TEMPERATURE IN DIESEL ENGINES

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Introduction
Most of the automotive tractor machinery is operated with the use of diesel internal combustion engines (ICE) [1]. Diesel internal combustion engines are characterised by contact with the environment [2], which manifests itself in the direct dependence of the operating parameters of the internal combustion engine (oil viscosity, oil filter temperature, fuel temperature, etc.) on the temperature of the air from the intake manifold. Diesel combustion engines are characterised by increased emission of toxic gases, especially NOx [3]. It should be noted that diesel engine at idle speed is fully warmed up at approximately 650-850 rpm, and at low temperatures, the revolutions are in the range of 850-1500 rpm [4]. The high value of diesel engine rpm means that a large amount of cold air will enter from the intake manifold during this period. Exhaust emissions and fuel consumption increase dramatically when the engine is cold started but decrease when the engine is warmed up [1].
The issues of pre-start preparation of internal combustion engines of motor transport vehicles have received much attention and there are various technical solutions, which can be either stand-alone or stationary [4]. There are such aspects of diesel engine pre-start preparation as energy and economic aspects related to the possibility of using this or that method or technical solution.
 In order to save energy for starting a diesel combustion engine, a system of waste heat recovery can be used, which is about 75% of the energy produced by the combustion engine, partially lost with exhaust gases and antifreeze. The technology of storage of heated antifreeze in thermoses for quick heating of internal combustion engines is known [5], taking into account the peculiarities of the system of starting of internal combustion engines in conditions of negative temperatures [6]. The thermos device includes: a heat exchanger, which is made of ribbed plates and paraffin - material with phase transition (MFP), filling the free space. MFP is used to store heat energy coming from the internal combustion engine in the form of heat losses necessary for subsequent use in cold start conditions. Thanks to heating in less than 1 minute, the temperature of cold intake air for the internal combustion engine can vary from 0 to 30 °C. Fast heating of the main units allows to overcome the difficulties of cold start for internal combustion engines, especially in conditions with increased ice formation, and to supply pre-heated air to the internal combustion engine within 1 minute.
One of the effective technical solutions aimed at improving the start of internal combustion engines is electric heating [7], which is carried out from the battery of motor transport vehicles through the control system. Electric heaters of various types can be installed both on separate units of internal combustion engine, for example, fuel filter, and use them as a portable (portable) device. The most rational variant is a polymer-based electric heater [8], which, compared to ceramic [9], can withstand various types of vibrations and mechanical loads, and also has flexibility, which will ensure a tight fit to the heated surface. There are different types of polymer matrixes for manufacturing heaters, but the most effective and flexible solution is organocream compound [10, 11] - which is characterised by high mechanical strength and stable operation at low temperatures.
The aim of the article is to develop an electro-heating polymer composite (EHPC) with the effect of temperature self-regulation for diesel engines. In accordance with the purpose of research the following tasks were set and solved:
1.    Development of the method of obtaining electroheating polymer composite (ENPC) with the effect of temperature self-regulation.
2.    Carrying out test tests of ENPC on a filter for the fuel system of a diesel engine.
3. Optimisation of the ENPC parameters depending on the operating modes of the internal combustion engine using a microcontroller control system.

Methods and materials
Methods of obtaining polymer composite or ENPC.
The polymer composite was obtained by adding metallised MWCNTs (microwave synthesis by exposure of ultra-high frequency electromagnetic radiation to a mixture of ferrocene and graphite (1:1)) to liquid organosilicon compound (Silagerm 8030), afterwards, first to component A and then to component B (hardener on the basis of platinum catalyst), respectively. The compound was thoroughly rewet on a WiseStir HT 120DX (Korea) at 300 rpm (5 min). 
The polymer composite material was added to a special moulding vessel until the final formed heating elements were formed as flat flexible plates according to [12-14]. After that, the obtained composite was placed in a vacuum thermo-cabinet to remove volatile components contained in the polymer matrix.  
Power supply of the ENPC was carried out using a programmable power supply 

with the programmable power supply Aktakom 1351 (Aktakom, Russia) in the mode of pulsating potential from 0 to 24 V. The temperature field was investigated using a thermal imager Testo-875-1 with a 32 × 23° optical lens (SE & Co. KGaA, Testo, Lenzkirch, Germany). General view and circuit diagram of ENPC connection is presented in Figure 1.

      
a) b)

Fig. 1. Principle scheme of ENPK a: 1 - dielectric shell; 2 - current collector; 3 - heater functional material; 4 - current conducting conductors; b - general view of flat heaters. 
Fig. 1. Principle scheme of ENPC a: 1 - dielectric shell; 2 - current collector; 3 - heater functional material; 4 - current conducting conductors; b - general view of flat heaters.

Methods of studying the structure of MWCNTs
The morphology of the MWCNT surface was investigated on a scanning electron microscope (SEM) "TESCAN LYRA 3" (TESCAN, Czech Republic) at 5 kV. A confocal microscope based spectrometer ("Spectra", NT-MDT SI) was used to measure the com- bination light scattering spectra. 100× objective with NA = 0.7, semiconductor laser (λ = 532 nm, excitation power about 50 MW).
Figure 2 shows the scanning electron microscopy of the surface metallised MWCNTs and the Raman spectrum.
      
a) b)
Fig. 2. a: Scanning electron microscopy of metallised MWCNTs; b: Raman spectrum of MWCNTs.
Fig. 2. a: Scanning electron microscopy of metallised MWCNTs; b - com-bination scattering spec-trum of MWCNTs.
MWCNTs are bundles of carbon nanotubes, which are intertwined, and the D/G peak ratio characteristic of multilayer carbon nanotubes follows from the CR spectra.

Methodology of connection of ENPC to the power supply system of YAMZ 238 internal combustion engine.
The obtained ENPCs were connected in parallel, combining them in sections with the formation of thermal contact between the composite and the fuel filter (due to the flexibility of the heater - a dense thermal contact was formed). To limit current surges in separate sections (Figure 3), each of them was sequentially connected to a polymer fuse.
 
1 - fuel filter; 2 - ENPC; 3 - filtering element; 4 - element for elimination of high currents.
Fig. 3. a) location of ENPK in the fuel filter (FTO);
b) circuit diagram of ENPC switching on
Fig. 3. a) location of ENPC in the fuel filter (FTO);
b) circuit diagram of EFPC switching on

Results and discussion
The control system of ENPC, presented in Fig. 4, allows to stabilise the fuel temperature and the power of the heating element during the heating process, as well as to change the number of heating sections depending on the fuel supply mode. At the same time, the system has both direct and feedback connections between the individual units. 
 
Fig. 4. ENPK management system
Fig. 4. ENPK management system

ENPKs are directly regulated through the control system, which is realised by means of a programmable microcontroller. The signal from the primary temperature meter (Figure 4) is fed to the control system and converted into an ADC (analogue-to-digital converter), which in turn determines the value of the supply current of the ENPK by changing the number of heater sections switched on through a DAC (digital-to-analogue converter). The ENPC temperature is constant (in steady state) and depends on the value of the supply voltage, while the current consumption depends on the temperature of the fuel filter and fuel and is characterised by a non-linear dependence. As a result, an adapted heating of the fuel system or the so-called temperature self-regulation effect is ensured.
Figure 5 shows the dynamics of ENPC heating during 5 min. At the same time, the presented characteristics have a non-linear character of heating. The power supply mode corresponds to 12 V voltage, which is close to the battery power supply mode.
      
a) b)
Fig. 5. Crankcase thermograms with engine oil and fuel filter
Fig. 5. Crankcase thermograms with engine oil and fuel filter

According to Fig. 5a, the temperature of ENPK reaches a plateau after about 250 s, i.e. temperature self-stabilisation occurs (Fig. 5a), which follows from the reduction of current to the operating value (Fig. 5b). This indicates that the processes occurring in the ENPC are characterised by the interrelation of thermal and electrical phenomena. The basis of the pro-process of electric heating of composite material at the flow of electric current is polarisation of polymer matrix and tunnelling of elementary charge in MWCNTs.
Based on the thermal balance equations for the fuel thermal management device with ENPC, the system of differential equations (1) with initial conditions and variation parameters is obtained:
                                            (1)
initial conditions: τ > 0;   
variation parameters: .
where Ren - power of ENPK, W; K1-2 , K2-3, K1, K3 - heat transfer coefficients from ENPK to fuel, from ENPK to external environment, from fuel system elements to external environment, respectively, W/(м2С); TEN, TT, Tokr - temperature of ENPK, fuel and external environment, respectively, °С; Fen, F1, F2, F11, F3 - areas of ENPK in the place of contact with fuel, ENPK with the external environment, ENPK with fuel, fuel pipelines and FTO respectively, m2; Sen, ST - heat capacity of ENPK and diesel fuel, J/(кгС); hen - height of ENPK, m; VT - volume of fuel, m3; ρen, ρT - densities of ENPK and fuel, kg/m3; τ - time, s.
It should be noted that the current consumed by the heaters is directly proportional to the fuel temperature (2).

                                                                          In~Tt (2)

However, it is necessary to take into account the transient mode of the heater operation, which is characterised by the starting current and in this case the expression (1) is supplemented by a coefficient taking into account the starting mode. For selection of heaters it is expedient to take into account the peculiarities of inrush current flow. The mathematical dependence[15] is used to find the inrush current during the operation of ENPK:
                                                (3)
where ST - heat capacity of fuel, J/(kg∙С); DT-fuel flow rate, m3/s; R(t1) - electrical co-protection of ENPC, Ohm; ρ20 - density of diesel fuel at 20 °С, kg/m3; N-adjustment factor; ΔT-temperature increment of fuel during ENPC operation, С; τ-time, s.
When analysing (3) and the temperature dependence during heating, it follows from Figure 5 that the ENPK operates in the temperature self-regulation mode, which is associated with changes in ∆T and ∆DT in the fuel, which causes a change in the current I(R(t1)) for the ENPK. 
The block diagram of the thermoregulated diesel fuel heating algorithm (Figure 6) is used in the control system.  Thermal regulation of fuel through controlled heating is carried out due to the fact that the algorithm of functioning of the ENPC is based on the control and active heating to the temperature when compared with the set point Tz in each section T2 and T1, as well as the current consumption in the ENPC. 
 
Figure 6. Algorithm of heating of diesel fuel with thermal regulation
Fig. 6. Algorithm of diesel fuel heating with thermal regulation
Figure 7 a shows a section of the fuel filter for diesel fuel with ENPC on the side surface (3D model obtained in the Blender 4.1.1 programme) and a thermogram of the fuel filter as a result of the thermal effect of the heating element Figure 7b. The thermal effect on the side surface of the fuel filter is shown in Fig. 6 c, and the heat flux from the heater surface in Fig. 6 d.
      
a) b)
 
c) d)

Fig. 7. a - fuel filter; b - thermogram of the internal cavity of the fuel filter; c - distribution of the temperature field at the lateral contact with the filter; d - thermogram of the heater.
Fig. 7. a - fuel filter; b - thermogram of the internal cavity of the fuel filter; c - distribution of the temperature field at lateral contact with the filter; d - thermogram of the heater.
From the analysis of the thermogram, it follows that the lateral heat flow from the heater allows heating the fuel filter with diesel fuel in the inner cavity up to 45,9 ºC. 
In order to create the software of the control microcontroller (control system) of the ENPC, an equation of the form [12] was used, which allowed to estimate the change of power depending on the temperature:
                                               (4)
The parameters of equation (4) for the YAMZ 238 internal combustion engine are presented in Table 1.

Table 1- Parameters of Equation 4

No. Internal combustion engine operating mode Coefficients of ap-proximation Correcting
correction factor (Gti)
        a b c c e Gt1
T4<-10 Gt2
T4 < -20 Gt3
T4 < -40
1 Start 120 196 0,1 0,1 0,1 1,1 1,2 1,27
2 Idling 100 110 0,05 0,6 1 1,1 1,1,2
3 Operation under load
(25% of nominal value) 140 150 0.11 1.1 0.9 1.2 1.4
4 Operation under load
(50% of rated value) 150 170 0,12 1,2 1,2 1,2 1,4 1,4 1,4
5 Operation under load
(75% of rated value) 170 190 0,14 1,3 1,3 1,3 1,5 1,6

The correction factor (Gti) allows to take into account the possibility of reducing the ambient temperature with the subsequent increase in the power of ENPK due to the inclusion of a large number of ENPK sections in the temperature range from minus 10 to 40 °С. For the formation of software for the control system of the ENGC 5 operating modes are used, which include start-up, idling and modes under load (25, 50, 75% of the nominal value of the full load of the internal combustion engine).
Conclusion
The polymer composite was obtained by adding metallised MWCNTs (synthesised by microwave method) to the organosilicon compound. The use of electric heaters on the basis of polymer composite (ENPC), allows to improve the temperature regime for fuel, and accordingly the conditions of starting diesel engine. At the same time their energy efficiency is significantly improved due to the effect of temperature self-regulation provided by metallised MWCNTs with a given structure. 
The process of electric heating of composite material under the flow of electric current is based on polarisation of polymer matrix and tunnelling of elementary charge in MWCNTs. A system of differential equations was obtained in the process of ENPC heating. As a result, the control system of ENPC was developed using a microcontroller.
Realisation of fuel heating by a new type of composite heater ENPK with the effect of temperature self-regulation and control by a microcontroller working according to a given algorithm was provided by approximation of the solution of the system of differential equations. The developed algorithm for controlling heaters in the fuel supply system took into account both the fuel temperature and the supply current and voltage of the heater. As a result, the temperature of diesel fuel was maintained with a given error in the modes of starting the engine, idling and under load (50% of the nominal value), respectively.
Controlled heating of fuel with a given temperature allows to reduce the load on the battery, which significantly expands the potential of electric heating technologies for motor transport vehicles.

The research was carried out under the Russian Science Foundation grant No. 24-29-00855, https:// rscf.ru/project/24-29-00855/.

About the authors

Alexandr Shchegolkov

Email: Energynano@yandex.ru
ORCID iD: 0000-0002-4317-0689
SPIN-code: 4893-5232
Scopus Author ID: 57992653500
Russian Federation

Alexey Viktorovich Shchegolkov

Moscow Polytechnic University

Author for correspondence.
Email: alexxx5000@mail.ru
ORCID iD: 0000-0002-1838-3842
SPIN-code: 4929-5059
Scopus Author ID: 57205443030

 к.т.н., доцент

Russian Federation, Moscow Polytechnic University, Moscow, Russia, 107023, Moscow, 38, Bolshaya Semyonovskaya St.

References

Supplementary files