Bulletin of the Transilvania University of Braşov • Vol. 9 (58) No. 2 - 2016 Series I: Engineering Sciences
DIESEL FUEL HEATER USING ENGINE COOLANT FOR COLD WEATHER OPERATION
Veneția SANDU1
Abstract: The paper studies diesel fuel heating in vehicle operation at below-zero temperatures by means of engine coolant and a helical coil heat exchanger placed in the fuel tank. A preliminary prototype of the helical coil heat exchanger was manufactured and investigated on a diesel engine truck operating in real winter condition being measured the diesel fuel and coolant temperatures. The experimental data on coolant temperature variation and heating time were used to improve the final thermostatically controlled design.
Key words: diesel fuel preheating, helical coil heat exchanger.
1. Introduction the heat released by engine hot sources such as coolant [6], lubricant or exhaust The behavior of fuel in diesel engines gas. A patent survey in this field was running at low temperatures is strongly presented in [9] showing that the most influenced by its paraffin content which spread solutions opt for electric and may crystallize and clog the fuel filter and coolant heating. block the fuel supply. The phenomenon Especially for heavy duty diesel vehicles, known as waxing may occur under 0 °C, the high demand of electricity on board so in order to avoid paraffin precipitation and the opportunity of energy harvesting the diesel cold flow performance must be led to the implementation of special managed either depressing the cold fuel purpose designed heat exchangers mounted temperatures (Cloud Point, Pour Point, either on the fuel tank or on the low Cold Filter Plugging Point) with diesel pressure fuel ducts. flow improvers or by using fuel heating. The present study reveals the design, The gradual replacement of the testing and improvement of a fuel heater petroleum diesel with renewable fuels such used in a 100 liter tank of the 7 tonne as biodiesel accentuates that difficulty due diesel truck using the heat of the engine to higher temperatures at which coolant. crystallization occurs in biodiesel blends. The need of fuel heating led to the 2. Design Demands and Phases implementation of several techniques described in literature either as electric For diesel engines fitted with mechanical heating of fuel filter, fuel pump or fuel injection systems the optimum viscosity of duct [4], [5] or heat exchangers based on the fuel in fuel filter is reached in the
1 Mechanical Engineering Dept., Transilvania University of Braşov. 2 Bulletin of the Transilvania University of Braşov • Series I • Vol. 9 (58) No. 2 - 2016 interval 20-40 °C [9]. The fuel heater was [11] of a similar truck and are indicated in intended to be placed in the fuel tank being Table 1. heated with engine coolant taken from the cabin heating system, upstream thermostat. Heat exchanger parameters Table 1 The coolant circuit passes through a hose Pure Diesel to fuel tank heater and returns in the water fuel cooling system upstream coolant pump. Inlet temperature, ti [°C] 72 5 In summer time the system is not used Outlet temperature, t [°C] 70 20 and the circuit is closed manually with a tap e Heat capacity, cp [J/kg·K] 4190 1800 or electrically with a thermostat. The system has no influence on the engine cold starting The fuel flow rate passing through the thus being performed by a glow plug. heater was around three times higher than The design was performed in three the engine fuel consumption [12] due to phases: the pumping technique which delivers 1. Preliminary design of a experimental more fuel to cool the injectors, implying prototype (heat exchanger and fuel heating some warmed excess fuel which is system); returned in the tank. 2. Prototype testing; The heat exchanger was made of a 3. Final design. cylindrical copper pipe of 0.018/0.015 mm The testing was required to evaluate in (outer and inner diameter), resistant to real operation condition during winter the corrosion, having a very high thermal efficiency of the heat exchanger measured conductivity. as fuel temperature increase and the time The calculation was based on classical needed to reach the desired temperature. heat transfer between two fluids separated by a cylindrical wall, neglecting the heat 2.1. Preliminary Design transfer of the tank with the air and considering the heat exchanger ideal. The Paradoxically, the engine coolant subscript 1 refers to the heater agent and becomes the heating fluid of the fuel in the subscript 2 to the heated one. preheating system. The appropriate type of On the coolant side, when the coolant heat exchanger was considered to be the passes through the circular section of the helical coiled one, with higher heat transfer coil, its velocity, , is calculated with coefficients than straight tubes due to the 1 Equation: secondary flow produced by centrifugal force induces in curved tube [3]. m 1 The helical coil heat exchanger was 1 2 . (1) designed to operate immersed in a high di 1 capacity tank according to the procedure 4 described in [2]. The fluid characteristics were approximately in the first iteration as The Reynolds number, Re, calculated for those of pure water and diesel fuel for the water viscosity at the mean temperature automotive use. between inlet and outlet, considered to be The flow rates of the agents (coolant, 70 °C, indicated a turbulent flow: diesel fuel) were provided by technical documents of the engine and auxiliaries d Re 1 i . (2) [10]; the coolant temperatures were 1 collected from test reports on cabin heating 1 Sandu, V.: Diesel Fuel Heater Using Engine Coolant for Cold Weather Operation 3
4 For Reynolds number higher than 10 it 2 1 -1 gravity g = 9.81 m/s , 0.0033 K ). is recommended the following criterial Tbl equation which correlates Nusselt number, The wall temperature on fuel side t2w Nu, with Reynolds number and Prandtl was initially considered 70 °C and the number, Pr: tbl = 32.5 °C resulting Gr2 = 196959, 2 0.25 Nu2 = 25.4 and 2 = 155 W/m K. Pr 0.8 0.43 1 . (3) By introduction of 2 in the equation of Nu1 0.021 Re1 Pr1 Prw heat transfer per unit length of pipe:
Prandtl number at the wall temperature, d11(t1 t1w) d22 (t2w t2 ) , (7) Prw, is assumed to be the same with Pr1 number of the fluid. The convection it can be calculated the temperature of the coefficient 1 from water to the coil wall wall on the water side, resulting can be written in the form: t1w = 67.8 °C. The temperature drop in the wall is considered negligible due to very 1 1 Nu1 . (4) low thermal resistence, so (t1w – t2w) 0, in di the first iteration. In the second iteration starting with On the fuel side, diesel fuel in the tank is t1w = 67.8 °C Grashof number becomes 2 supposed to be heated from 5 to 20 °C Gr2 = 189944 and 2 = 153.7 W/m K with and the flow is laminar due to high tank an error towards previous calculation less capacity and gravity, being recommended than 1%. the criterial equation which correlates By equalizing Equation (7) with heat Nusselt number with Grashof number, Gr, transfer in the pipe wall: and Prandtl number: 2 d (t t ) w (t t ) , (8) Nu 0.4 (Gr Pr )0.25 , (5) 1 1 1 1w d 1w 2w 2 2 2 ln 1 d2 3 gde (t2w t2bl ) Gr2 . (6) 2 it results the real temperature drop in the 2 wall of 0.06 °C.
For the diesel fuel, the properties were The convection coefficient from the 2 calculated at the temperature of the thermal coil wall towards the fuel is similar to Equation (4). The calculations considered boundary layer, t2bl, as arithmetic mean of t2w and t2i. water properties (mass flow rate, 3 The rate of heat Q1 transmitted from m 1 0.08 kg/s, density, 1 = 977 kg/m , water is calculated as follows: viscosity at 70 °C, = 0.42106 m2/s, 1 Prandtl number Pr1 = 2.4, thermal Q m c (t t ) . (9) conductivity 1 = 0.663 W/mK), wall 1 1 p1 1i 1e properties (copper thermal conductivity, w = 300 W/mK) and diesel fuel (density, Finally, the linear overall heat transfer, k, 3 2 = 838 kg/m , viscosity at tbl = 33.2 °C, takes into account the thermal resistances 6 2 2 = 610 m /s, Prandtl number Pr2 = 83.3, for each environment: water, copper wall thermal conductivity 2 = 0.11 W/mK, and diesel fuel: 4 Bulletin of the Transilvania University of Braşov • Series I • Vol. 9 (58) No. 2 - 2016
measure of the straight portion, Ls, k . (10) l 1 1 d 1 becomes 170 mm. ln e d 2 d d 1 i w i 2 e 2.2. Prototype Testing
The linear rate of heat between fluids is The heat exchanger designed according calculated with the formula: to values from Chapter 2.1 was manufactured and then the heating l kl (t1i t2i ) . (11) installation was assembled and tested on the facilities of Road Vehicle Institute For the intermediate values α1 = 3207 (INAR Braşov). 2 2 W/m K, α2 = 153.8 W/m K, kl = 8.22 W/mK, The heating system was adapted to the it results Φl = 633W/m. configuration of the truck AB 7120F The total minimum length, L, of the manufactured at Roman Truck company, helical coil results from the formula: series 0205, which was equipped with a 798-05 diesel engine, series 04598, derated Q to 120 HP@2500 rpm. L 1 . (12) Prior mounting, the injection pump was l demounted and readjusted on the pump test The total length results to be minimum bench, the fuel filters were replaced, the fuel tank was replaced with a newly 1.06 m and is divided in a helical length Lh modified one and the engine fuel supply and a straight length Ls: and cooling system were deaerated. The 0.5 engine coolant was made of 50% (v/v) L N (D)2 s2 . (13) h distilled water and 50% propylene glycol based antifreeze liquid, to withstand to The dimensions of the helix are: D – temperatures of 30 °C. diameter; H – height; s - pitch and, The layout of the heating and implicitly; N - number of coils. measurement system was made of 100 These parameters are dimensionally constrained by two factors: the diameter of liter fuel tank, helical coil heat the filler neck of the tank (0.105 m) which exchanger placed in fuel tank around will allow easier retrofit mounting and the fuel line supplies, T-type connectors to depth of the tank of 0.4 m. engine cooling system and coolant It was chosen D = 0.07 m with the pitch hoses, as represented in Figure 1. s = 0.04 m, resulting N = 4 and Lh = 0.9 m. The equipment included calibrated The helix was mounted inclined being instruments: data registrator, voltage positioned to reach the bottom of the tank, inverter, temperature sensors, type Pt 100, folding the fuel suction pipe. The pitch class B, in range (50 - 150 °C) with between coils has the smallest value to accuracy varying from ±0.3 °C at 0 °C and keep the height H low, thus leading to ±0.8 °C at 100 °C. total immersion of the helical coil even The diesel fuel temperature sensors were when the tank is one quarter of capacity mounted inside the lateral tank walls and full. The tank is a rounded corner right to the inlet of fuel filter; two sensors were angle parallelepiped having dimensions welded on the lateral walls, close to the 570x550x400 mm, so the minimum bottom of the tank. Sandu, V.: Diesel Fuel Heater Using Engine Coolant for Cold Weather Operation 5
Fig. 1. Layout of testing installation
The coolant temperature sensors were dry asphalt. mounted on direct and return line from The mean atmospheric temperature was - water pump to cabin heater as can be seen 2 °C,the load of the truck was 1500 kg and in Figure 2. the fuel tank full. The routing Brașov - Hărman - Prejmer The mean speed was 40 km/h and the Teliu - Brădet - Întorsura Buzăului -Vama mean values of measurements are enclosed Buzăului and return was run three times on in Table 2 [10].
a) b)
c) d) Fig. 2. a) coolant hoses to water pump and cabin and temperature sensor; b) back side tank view with coolant hoses and temperature sensor; c) front side tank view with coolant hoses and temperature sensor; d) fuel filter with temperature sensor 6 Bulletin of the Transilvania University of Braşov • Series I • Vol. 9 (58) No. 2 - 2016
The final measurements were performed The difference of fuel temperature becomes 45 minutes after the initial ones. Later on 35 °C instead of 25 °C in preliminary the testing day, at very short stops, less situation. As the fuel flow rate should be than 1 hour, it was observed that around kept constant, a solution can be the rising the helical coil subsisted a warm fuel of coolant flow rate with (35/25 = 1.4) blanket with positive effect on subsequent ratio, using a wider pipe and helical coil engine start. section, thus meaning to pass from
di = 0.015 to 0.018 m and to thin the pipe Test results Table 2 wall from 1.5 to 1 mm. Temperatures, The calculations considered coolant as a Initial Final [°C] mixture of 50% distilled water and 50% Fuel temperature in propylene glycol with the following 3.5 15 tank properties [7]: Coolant temperature - mass flowrate m1 0.112 kg/s; 66 75 3 in direct circuit - density, 1 = 930 kg/m ; Coolant temperature - viscosity at 70 °C, = 1.5 106 m2/s; 63 72 1 in return circuit - Prandtl number, Pr1 = 10.62; - thermal conductivity, 1 = 0.44 W/mK. Therefore the design was reiterated The wall properties remained unchanged aiming to lower the operation temperature and diesel fuel properties were the same, under 5 °C, to shorten the heating period just the thermal boundary layer and to consider heat loss of the tank temperature dropped from 32.5 to 30 °C. towards environment. On the coolant side the Reynolds number The fuel temperature within tank has dropped under 10000 and for a laminar been increased too slowly due to high flow rate the criterial equation indicated in thermal inertia of the fuel mass. The results [1] is: were appreciated as rather fair, but the time interval of heating could be improved. d 0.065Re Pr i 1 1 L Nu1 3.66 . (14) 2.3. Final Design 1 0.04Re Pr 2 / 3 1 1 The most important data of the testing was that the coolant temperature variation In Equation (14) L is the length of the in steady state was 3 °C instead of 2 °C as pipe resulted from preliminary design, supposed in preliminary phase. L = 1.06 m. The calculations were repeated For a better accuracy of the results, the according to Equations (1), (2), (4), properties of the coolant were modified replacing Equation (3) with Equation (14). due to the presence of antifreeze agent. On the fuel side, diesel fuel in the tank is The major changes were the inlet supposed to be heated from 15 to 20 °C temperature of the diesel fuel (15 °C), the and the flow remains laminar, being used section of the coolant pipe and the the preliminary criterial Equations (5), (6). For the intermediate values Re1 = 5678, properties of the coolant. 2 Nu1 = 16.8, α1 = 411 W/m K, α2 = 159 A more rapid heating of the fuel requires 2 a higher rate of heat than in preliminary W/m K, it results Φl = 627 W/m. design calculation described by Equation The new length of the heat exchanger, (9). L , should be calculated taking into Sandu, V.: Diesel Fuel Heater Using Engine Coolant for Cold Weather Operation 7
consideration not only Q1 , but also the rate while for the final model was shortened to of heat loss of the tank towards 3.2 minutes per degree Celsius. The heat exchanger requires a temperature environment, Q : loss controller which will turn on the coolant circuit when the fuel temperature in the filter Q Q L 1 loss . (15) is below 5 °C and will turn off when is over l 20 °C. That imposed conditions prevent the overheating of the fuel in the tank and driver In the formula of the rate of heat loss: intervention at the change of ambient temperatures.
Q k St . (16) loss t 3. Conclusions
kt is overall heat transfer between the The preliminary design of the heat diesel fuel and air through the tank wall, S exchanger introduced significant errors is total area of the tank, approximated to when water properties were used instead of 0.4 m x 0.55 m x 0.57 m, in form of water-propylene glycol blend. The rectangular parallelepiped and t is the differences of fluid properties, mainly temperature difference between diesel fuel viscosity, changed the type of fluid flow and outside air. with significant modifications on Reynolds For the calculation of kt it was used the and Nusselt numbers. formula from plane wall fuel tanks: The testing of the experimental prototype provided valuable data on real operation 1 k , (17) heat loss and heating time. t 1 1 t The diesel fuel heating system using 1t t 2t engine coolant confirmed the efficiency of the on board energy harvesting solution; on long term, the advantage of saving on with 1t - convection coefficient from fuel to tank wall, 9 W/m2K; - thickness of the board energy tends to counterbalance the t constructive extension. steel wall, 0.0015 m; - thermal t conductivity of the tank wall, 50 W/mK; Acknowledgements - convection coefficient from tank wall 2t to atmospheric air at the speed of 40 km/h, The author is grateful towards Vehicle 35 W/m2K [8]. For k = 7.16 W/m2K, the t and Engine Testing team from Road rate of heat loss becomes Qloss = 381 W Vehicle Institute Brașov for their kind co- and finally the minimum length of the pipe operation and support. L 1.18 m. By applying Equation (13) at the same dimensions of the helix as References mentioned in Chapter 2.1, it results the number of coils N = 5 and the length Lh 1. Eduards, D.K., Denny, V.E., Mills, becomes 1.125 m. A.F.: Transfer Processes: An By applying the energy conservation on Introduction to Diffusion, Convection, fuel tank volume it was calculated the time and Radiation. New York, Holt, till the fuel is heated up to 20 °C; the Rinehart and Winston, 1973. heating speed of the preliminary model 2. Ștefănescu, D., Marinescu, M., Dănescu, was around 4 minutes per degree Celsius Al.: Transferul de căldură în tehnică 8 Bulletin of the Transilvania University of Braşov • Series I • Vol. 9 (58) No. 2 - 2016
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