[PRESENTATION ON:-COMBUSTION IN CI ENGINE]

[FAIZ AKRAM][faizakram60@gmail.com][ROLL NO. - 120106096]

[Term-VI][Year - 2014-15]

Under guidance of[OM PRAKASH YADAV]

Department of Mechanical EngineeringSchool of Engineering and Technology

Sharda University, Greater Noida, UP, INDIA

COMBUSTION IN C.I ENGINE

• In c.i engine the air alone is compressed and raised to high pressure and temperature in the compression stroke

• The temperature of air attained is far above the self ignition temperature of the diesel fuel used

• The fuel is injected by a fuel pump into the combustion chamber under a very high pressure of about 120-210

bar at 20.-35. before TDC

• The point A represents the time at which the fuel injection starts on P-Ɵ diagram

STAGES OF COMBUSTION

• Combustion takes place in four stages

I. Ignition delay period.

II. Period of uncontrolled combustion.

III. Period of controlled combustion.

IV. Period of After burning

IGNITION DELAY PERIOD

• At the end of compression stroke the fuel injector injects the fuel in a finely atomized form into the cylinder

• These fuel molecules absorb heat from surrounding hot air and vaporized and this causing a slight drop in pressure in the cylinder as shown by curve AB

• As soon as vaporization is over the preflame reaction of mixture start. During such chemical reaction energy released at slow rate and pressure start building up.

• So the preflame reaction first start slowly and then accelerates until the ignition of fuel takes place. It corresponds to point c on diagram

• The time interval between the start of fuel injection and commencement of combustion is called the delay period.

PERIOD OF UNCONTROLLED COMBUSTION

• Once the delay period is over the air fuel mixture will auto ignite since it is above the self ignition temperature

• Flame appears at one or more location where the combustion of fuel and air mixture is optimum

• Once the flame appears the mixture in other region will either be burnt by propagating flame or it will be auto ignite because of the heat transfer from the burnt mixture

• The fuel which is accumulated during delay period is now ready for combustion and it would burnt at extremely rapid rate causing rise in cylinder temperature and pressure

• Higher the delay period higher would be the rate of pressure rise. During this period it is difficult to control the amount of fuel burning for this reason this period of rapid combustion is called period of uncontrolled combustion as represented by curve CD

PERIOD OF CONTROLLED COMBUSTION

• Once the fuel accumulated during the delay period is burnt in the period of uncontrolled combustion the temperature and pressure in the cylinder will be so high that the further quantity of fuel injected will burn as soon as it leaves the nozzle.

• Therefore the rate of pressure rise can be now controlled by controlling the fuel injection . This period of combustion is known as period of controlled combustion as represented by curve DE.

AFTER BURNING

• Theoretically the combustion is completed at the point the maximum pressure is attained during the cycle corresponding to point E few degree after TDC

• However the burning of fuel during its expansion stroke continues due to re-association of dissociated gasses and any unburnt fuel due to heterogeneous mixture

KNOCK IN C.I ENGINE

• In case of S.I engine it was the end part of gas which was responsible to produce detonation if delay period and flame speed are low

• In case of C.I engine it is the first part of the charge which is responsible to produce knock

• In case of C.I engine the fuel injected and combustion take place by way of auto ignition if the delay period of the fuel is long a large amount of fuel will be injected and accumulated in the combustion chamber

• During the period of uncontrolled combustion the fuel accumulated in the combustion chamber will suddenly be exploded which gave high pressure and temperature

• A high rate of pressure rise is equivalent to a sudden increase in load on the engine structure

• Instantaneous increased pressure of gas will give direct blow on engine structure

• A thudding sound will be heard from this impact. This noise is called knock.

• Therefore knock in C.I engine occurs if the delay period is large and knocking in C.I engine may have the same effect as in S.I engine

METHOD OF CONTROLLING KNOCK IN C.I ENGINE

• Increased in intake pressure ,temperature and compression ratio due to increased temperature and collision of molecule which reduces delay period

• Increased water jacket temperature and increases air temperature and hence delay period reduces

• Cetane no. of diesel fuel is increased and fuel autoignites at lower temperature. However, they are expensive and emit more NOx

TYPES OF C.I ENGINE

• CI engine combustion chambers are classified into two categories

I. Direct-Injection(DI)

II. Indirect-Injection(IDI)

DIRECT INJECTION

• This type of combustion chamber is also known as open combustion chamber. In this type the entire volume of the combustion chamber is located in the main cylinder and the fuel is injected into this volume.

• The main advantage of this type of chambers are minimum heat loss during compression because of lower surface area to volume ratio and hence, better efficiency. They have no cold starting problems and Fine atomization because of multi hole nozzle

INDIRECT INJECTION

• In this type of combustion chambers the combustion space is divided into two parts one part in the main cylinder and the other Part in the cylinder head. The fuel-injection is effected usually into that part of the chamber located in the cylinder head.

FUEL IGNITION QUALITY

• The ignition characteristics of the fuel affect the ignition delay. The ignition quality of a fuel is defined by its cetane number CN.

• For low cetane fuels the ignition delay is long and most of the fuel injected is burns rapidly under extreme cases this produces an audible knocking sound referred to as diesel knock

• For high cetane fuels the ignition delay is short and the heat release rate is controlled by the rate of fuel injection and fuel-air mixing – smoother engine operation

REFERENCE• EPA emission standards may be found at: http://

www.dieselnet.com/standards • J.E. Dec, SAE Trans. paper 970873 106 (3) (1997),

1319–1348.• J.E. Dec, R.E. Canaan, SAE Trans. paper 980147 107

(3) (1998) 176–204.• F. Zhao, F., T.W. Asmus, D.N. Assanis, J.E. Dec., J.A.

Eng, P.M. Navjot, Homogeneous Charge Compression Ignition (HCCI) Engines: Key Research and Development Issues, Society of Automotive Engineers, Warrendale, PA, 2003.

• M. Christensen, B. Johansson, B. SAE Trans. paper 982454 107 (4) (1998) 951–963.