Journal of Kones. Combustion Engines, VoIB, No 1-2, 2001

A NEW CONCEPT OF DIRECT INJECTION OF FUEL MIXTURE Wladyslaw Mitianiec Cracow University of Technology Institute ofAutomobiles and Internal Combustion Engines 31-155 Krakow, ul. Warszawska 24 Tel. (12) 6282657; Fax: (12) 6282642; e-mail: [email protected] Abstract In order to increase time required to vaporise all injected fuel, a new concept of preparation fuel mixture is proposed. Fuel is injected to separate chamber located near the cylinder and joined by two short pipes of small diameter. Connection between the cylinder and chamber are controlled by electronic valves. Hot gases are delivered to the chamber during expansion process with pressure about 3 bar. Fuel is injected to hot gases in the chamber assuring well preparation of mixture. Outflow valve of the chamber opens when exhaust port is closed to reduce fuel losses and jet of rich mixture is turned into spark plug. Only fuel dose is controlled at constant pressure and constant angle of injection process. Paper includes a description of the new system, mathematical model of exchange charge in the chamber and cylinder, injection process, simulation results of physical processes which occur in the cylinder and chamber. A diagram of the fuelling system explains a principle of work and figures show variation of gas pressure, temperature and vaporisation of fuel during crankshaft rotation. The system is designed for one-cylinder two-stroke engine, but can be applied also for multi-cylinder engines. The paper describes the first stage of work sponsored by Polish Scientific Committee.

1. Concept of rich mixture injection In modem two-stroke engines fuel is delivered by injection system to reduced fuel losses during scavenge process. However vaporising of fuel from beginning of injection to combustion process is limited by short time especially at higher rotational speed. Rich mixture injection at low pressure by use of dynamic phenomena and electronic controlled system was developed by author[8][9] and is examined at Cracow University of Technology. The system prevents outflow of fuel to exhaust system, enables better evaporation of fuel in additional chamber and injection of mixture near spark plug. Presented fuelling system enables longer contact fuel and hot exhaust gas in mixing chamber. The diagram of two-stroke engine with electronically controlled valve opening is presented in Fig.l. The similar FAST system in Piaggio engines requires additional compressor and dividing the air in two separate jets and also changes design of two-stroke engine. System of mixture preparation is a separate unit consisting of mixing chamber, low pressure injector and two electronic pneumatic valves controlling opening of exhaust gas to the chamber and outflow of rich mixture. Pneumatic valves and injector are opened as a result of electric signal from central control unit in dependence of crankshaft position and speed, throttle position and additionally temperature of cylinder wall. Mixing chamber is connected with cylinder by two short ducts and correspondent diameter to prevent pressure wave motion. In the cylinder above exhaust port from side of inlet system are the holes of chosen area giving required mass flow rate of gas and mixture. Cylinder is scavenged only by fresh air mixed with appropriate amount of lubricating oil and any fuel is entered to exhaust system. Exhaust gas is flowing into the mixing chamber from the cylinder close before exhaust port opening when pressure does not reach 5 bar. Charge in he chamber mixes with early injected fuel. High temperature of exhaust gas and long time enable evaporation of fuel. Opening of mixture valve follows after closing of exhaust port. Evaporation angle of fuel is bigger than angle of opening of exhaust port which is twice more than in direct fuel of mixture occurs in initial stage injection. Injection

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at pressure difference about 2-3 bar, but in final stage pressure in the cylinder is bigger than in mixing chamber as a result of compression process. Gas flow through cylinder holes (mixture and ...,.. plug exhaust gas) connected by ducts with mixing chamber is controlled by valves as well as by piston me crown. Injection of fuel occurs after emptying of

.

~- #~'.T.".,-t'::' ----,,

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mixing chamber to space where pressure is lower

r=~~~ q./ _::-j"_

than 2,5 bar and injection process is not limited by

engine speed. Time of opening both valves influences on amount of flowing exhaust gas and c flowi . Determi --'- ?'7!i r, V ~ _ owmg out mixture. ermination 0 f oneni opemng time if?'" ~ of valves and chamber capacity is most important rnIldngtask in this project. The low pressure mixture injection system enables: • better preparation of mixture for combustion, • feeding of strictly defined amount of fuel m dependence of engine power, inlol • decrease of fuel loss in scavenge process, • decrease of unburned hydrocarbons in exhaust gas, • decrease of carbon monoxide (combustion at relative fuel ratio Amore than 1), • small power of feeding fuel pump, Fig.I, General diagram oftwo-stroke engine with • simple electronic control unit, electronically controlled injection ofrich mixture • possibility of applying of the system in multicylinder engines. Mathematical model of the whole process was worked out. On the base of the model computer program was prepared and some simulation results is presented in the paper,

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2. Thermodynamic processes in mixing chamber Thermodynamic processes taking place in mixing chamber can be determined by means of three equations: mass balance, energy balance and gaseous state equation. However state of charge at inlet and outlet of the ducts follows from the conditions of boundary flows from and to the chamber at great and finite volume in comparison to pipe area, Diagram of mass and energy flow in the mixing chamber is presented in Fig.2.

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.

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==

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Mixing chamber t.m,

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Fig. 2. Thermodynamic diagram ofmixing

The mixing chamber can be heated by flow of exhaust gas around its wails or cooled by heat transfer from hot gases to the ambient medium at constant temperature To. Coefficient of heat conductivity across chamber wails is constant and depends on wail material. Amount of heat

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transferred to the ambient is dependent on thickness of chamber walls and coefficients of heat convection from inside and outside of the chamber. Charge inflow under pressure about 3 har to the chamber follows after opening of pneumatic valve during very short time. Similarly charge outflow to the cylinder after scavenge process takes place also after opening of pneumatic valve. Fuel is delivered to the chamber exclusively by electronic injector.

3. Mathematical model of mixing chamber 3.1. Mass balance of charge in mixing chamber Mass of gas inside the chamber is determiued by inflow exhaust gas or hot air from the cylinder during compression process and outflow of mixture consisting of gas and evaporated fuel to the cylinder and increase of fuel mass in gaseous state. Change of gas amount in this small volume can be defined as follows:

dmk = F; ·uI • PI -dt -F2 ,u2 ' P2 ·dt+dmp

(I)

where:

m. F1 F2 UJ, U2

- mass ofgas in the chamber, - cross section area of inlet duct which transfers hot charge from cylinder, - cross section area of outlet duct (transfer of rich mixture to the cylinder), - inlet and outlet velocity, - inlet and outlet density of charge, - mass of injected fuel in gaseous state.

3.2. Energy balance in mixing chamber At constant volume of mixing chamber and by consideration of small change of gaseous constant, one can assume that change of pressure inside the chamber takes place according to the following equation:

dpk = R k .(dQch + dQpar +(C p ) .1;. dm; Vk • c, 1

(c p)2 .1;. dm2 + (c p ) p -Tp ' dmp )

(2)

Gaseous constant R. of chamber charge should be calculate for every considered time step dt on the base of chemical species from previous calculation time step.

3.3. Dosing of fuel to the chamber During injection process fuel is delivered to the mixing chamber as a result of pressure difference

pw in electronic injector and p. in mixing chamber. Mass of injected fuel in a finite time step Lit can be obtained from the following equation:

tim p =i ,CD -F; .~2. Pp·tip· M

(3)

where pressure difference amounts !1p = Pw - Pk and i-amount of injector's holes, Fw - mean area of injector's hole, CD - coefficient of flow losses,

Pw pe

pressure of injected fuel, • pressure in mixing chamber. -

3.4. Evaporation offuel Mass of liquid fuel increases due to vaporization as a result of higher temperature of gases being in mixing chamber. Change ofliquid droplet mass in CFD' is defined by the Spalding equation [II]:

, Computational Fluid Dynamics

86

dmk =-!r.d . k v .Nu.In(I+B )

dt•

k

M

C]N

(4)

d p • diameter of fuel droplet,

where:

kv • heat conductivity of vapour, cpv • specific heat vapour, Nu - Nusselt number defined from the following dependence:

Nu = 2· (I +0,3· Reo.5.Pro.33)·F where:

Pr F

I F=-.!n(I+BM

BM

BM

(5)

• laminar Prandtl number for continuous phase, • correction ofFrossling coefficient during mass exchange: (6)

)

• dimensionless number of mass exchange (Spalding number) between liquid and

gas phase defined in (11). However in the calculation process very simple model was applied based on the global approach, in which characteristic time of evaporation of droplets having the same mean diameter is determined. The first droplets of fuel delivered to the cylinder begin evaporate from the start of injection to the mixing chamber, because initial evaporation temperature of gasoline amounts about 320 K. In zero-dimensional model velocity of fuel evaporation is defined by dependence: dmft = -E:...-,-,--'-'mfj> -mft __ dt "v(t)

where:

(7)

"v' characteristic time of fuel evaporation with droplet diameter d, tv - index representing total mass of fuel vapour,

fp . index representing total mass of fuel Characteristic time of fuel evaporation is determined from the following formula [I]: t:

[I

(

I(

32 hp ·T,(Pc) )] [s], (t)= Pf ·D . -·c -In Tc(t)-Tf ) +-. v 4.A.f 3 Pi Tc(t)-T,(PI) 2 Tc(t)-T,(pc)

(8)

where:

D32 • mean Sauter diameter of fuel droplet, cpf - specific heat offuel (for gasoline c pf =2,4 kJ/(kg K) atp=1 bar i T=298K),

Tf

. temperature of delivered fuel in K,

r. . saturation temperature of liquid fuel at current pressure in chamber,

Tc - temperature of charge in chamber, hp

•

A.f

• fuel heat conductivity [W/(m·K)].

evaporation heat offuel (350 kJlkg for gasoline),

At known temperature and pressure in the mixing chamber and fuel temperature process of fuel evaporation can be determined.

4. Calculation process On the base of mathematical model a computer program was written by the author in Pascal by use of previous units which enable to simulate various physical processes in internal combustion engine. Program includes also unsteady gas flow through inlet, transfer and exhaust systems. In combustion process Vibe formula was used with chosen exponents. During simulation process one can change the main control parameters such: opening angles of pneumatic valves, volume of chamber, throttle opening, time of fuel injection to the chamber and engine speed. Simulation

87

process considers to the engine specified in Table I. Table 1. EnPine snecification

In the paper only some results are presented at engine speed 5000 rpm and full throttle opening. In the enclosed figures are presented some engine parameters such as power, fuel consumption, air excess, residual gas and chamber parameters such as: pressure and temperature, evaporation ratio of fuel. The results was obtained at change of initial inlet phase in the chamber from 100 - 80 deg CA BTDC, but with conservation of constant angle of opening valve 30 deg CA and constant phase of delivery of rich mixture 40 deg CA ABDC. Also chamber volume was changed from 6 to 16% of engine capacity. E~

DEZAMET 154 Bore 52mm Stroke 58mm Connected rod lenlrtil 1I0mm Geometric comnressionratio 8.5 Geometric compression ratio of the 1.35 crankcase Be-;;;-nninu of exhaust eort open 77 dee:BBDC Bee:innine: of inlet nort oeen 60 dee:BTDC Exhaust--;;;;-rt width 33mm Main diameter of carburettor 20mm

5. Discuss of simulation results 5.1. Influence of inlet angles on engine parameters Filling of the chamber depends mostly on initial inlet opening angle, when hot gases and higher pressure from cylinder flows to the chamber. Higher pressure and temperature in cylinder causes an increase of chamber pressure an temperature. Variation of pressure in mixing chamber strongly depends on inlet angles, which is presented in Fig.3 for three cases 80, 90 and 100 deg CA BIDC for constant chamber volume. n-5OOO rpm V m-9 ,8% V

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300

360

Fig. 3. Variation ofpressure in mixing chamber ofconstantvolume Retarding of inlet angle influences on increase of engine power and fuel consumption (FigA). However at early opening of inlet valve also increase of fuel consumption is observed but with small power. Results were obtained at the same chamber volume 9,8% of engine capacity.

88

n=5OOO rpm

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