Comparison of 1cyl SI engine and CI engine using GT SUITE

Objective:

1. To compare 1cyl SI and 1cyl DI model and list down the differences.

2. To comment on following parameters:

• BSFC
• Exhaust Temperature
• Air/Fuel Ratio

3. To determine the effect of MBD 50(Main burn duration) on performance.

Introduction:

Homogenous Mixture:

In spark ignition(SI) engines a nearly homogenous mixture of air and fuel is formed in the carburettor/ Inlet manifold. Homogenous mixture is formed outside the engine cylinder and the combustion is initiated inside the cylinder at a particular instant towards the end of the compression stroke.After spark is ignited through spark plug, the flame front spreads over the combustible mixture with a certain velocity.

Once the fuel vapour air mixture is ignited, a flame front appears and rapidly spreads through the mixture. The flame propagation is caused by heat transfer and diffusion of burning furl molecules from the combustion zone to the adjacent layers of unburnt mixture.The flame front is a narrow zone separating the fresh mixture from cobustion products.The velocity with which the flame front moves,with respect to the unburned mixture in a direction normal to its surface is called normal flame velocity.

In a homogenous mixture with an equivalence ratio, `phi`(the ratio of actual fuel-air ratio to the stoichiometric fuel-air ratio) close to 1, the flame front normally in the order of 40 cm/s.However in spark ignition engines the maximum flame speed is obtained when equivalence ratio is between 1.1 and 1.2 i.e. when the mixture is slightly richer than stoichiometric.

Hetrogenous mixture:

Generally in Direct-ignition engines, heterogenous mixture is formed. The rate of combustion is determined by the velocity of mutual diffusion of fuel vapours and air, and the rate of chemical reaction is of minor importance. Self ignition and spontaneous ignition of air fuel mixture, at the high temperature developed due to higher compression ratios is of primary importance in determining the combustion characteristics.

When the mixture is heterogenous the combustion can take place in an overall lean mixture since, there are always local zones where equivalence ratio varies between 1 to 1.2 corresponding to miaximum rate of chemical reaction.Ignition starts in this zone and the flame produced helps to burn the fuel in adjoining zones where the mixture is leaner. Similarly, in the zones where the mixture is rich the combustion occurs because of high temperature produced due to combustion initiated in the zones where equivalence ratio is 1 to 1.2.

Combustion in SI engines:

In conventional SI engines, the fuel and air is homogenously mixed together in intake system. Under normal operating conditions, combustion is initiated towards the end of the compression stroke at the spark plug by an electric discharge .A turbulent flame develops following the ignition and propagates through this premixed charge of fuel and air, and also the residual gas in the clearance volume until it reaches the combustion chamber walls.

Stage 1: Ignition lag

The first stage is referred to as the ignition lag or the preparation phase in which growth and development of a self propagating nucleus of flame takes place.This is a chemical process depending upon both temperature and pressure and nature of the fuel and the proportion of the exhaust residual gas. Further it also depends upon the relationship between the temperature and rate of reaction.

Stage 2: propagation of flame:

The second stage is a physical one and it conserned with the spread of the flame throughtout the combustion chamber.The starting point of the second stage is where the first measurable rise in pressure is seen on the indicator diagram. Heat transfer to the cylinder wall is low, because only a small part of burning mixture comes in contact with the cylinder wall during this period.The rate of heat release depends largely on the turbulent intensity and also on the reaction rate which is dependent on the mixture composition.

Stage 3: afterburning

The starting point of stage 3 is usually taken at the instant at which the maximum pressure is reached on indicator diagram. The flame velocity decreases at this stage. The rate of combustion becomnes low due to lower flame velocity and reduced flame front surface. Since expansion stroke starts before this stage of combustion, with the piston moving away from the top dead centre there can be no pressure rise during this stage.

Combustion in CI engines:

Question: Why do we need swirl motion in CI engines?

Ans: In CI engines, only air is compressed through a high compression ratio(16:1 to 20:1) rising its temperature and pressure to a high value.Fuel is injected through one or more jets into this highly compressed air in the combustion chamber.Here, the fuel jet disintegrates into a core of fuel surroundedby a spray envelope of air and fuel particles.This spray envelope is created by both the atomization and vaporization of fuel.The turbulence of air in the combustion chamber passing across the jet teras the fuel particles from the core. Liquid fuel particles evaporate by absorbing the latent heat of vaporization from the surrounding air.

Hence, an orderly and controlled movement must be imparted to the air and the fuel so that a continous flow of fresh air is bought to each burning droplet and the products ofcombustions are swept away.This motion is called swirl.

In CI engine,

1. The ignition occurs at many points simulataneously with consequent rapid rise in pressure.

2. Irrespective of load, at any given speed an approximately constant supplu of air enters the cylinder.With change in load, the quantity of fuel injected is changed,varying the air-fuel ratio.

3. The overall air fuel ratio varies from about 18:1 at full load to about 80:1 at no load.

4. Engines operating with leaner air fuel ratio(low load), always gives a better thermal efficiency but imep,power output reduce.

Question: Why CI engines operate at leaner AFR?

Ans: At stoichiometric air fuel ratio and heterogenous mixing, certain regions within the combustion chamber is likely to be rich that some of the fuel molecules will not be able to necessary oxygen for combustion and thus produce noticeably balck smoke(more soot formation). Hence CT engines always designed to perate with excess air.

Stages in CI combustion:

1. Ignition delay: The ignition delay period is also called the preparatory phase during which some fuel has already been admitted but has not yet ignited.This period is counted from start of injection to the point where the pressure time curve separates from the motoring curve indicated as start of combustion.The dekay period in the CI engine exerts a very great influence on both engines design and performance. iT is of extreme importance because of its effect on both combustion rate and knocking and also its influence on engine starting ability and the presence of smoke in the exhaust.

The fuel does not ignite immediately upon injection into the combustion chamber.the ignition delay period can be split into two parts, the physical delay and chemical delay.The physical delay is the time betwwen the the beginning of injection and attainment of chemical reaction conditions. During this period, fuel is atomized, vaporised, mixed with air and raised to self ignition temperature.The physical delay is greatly reduced bus using high injection pressures, higher combustion chamber temperatures and high turbulence to facilate breakup of the jet and improvingg evaporation.

During the chemical delay, reactions starts slowly and then accelerates until inflammation or ignition takes place.

2. Period of rapid combustion/ Premixed combustion: The period of rapid combustion also called the uncontrolled combustion, is that phase in which the pressure rise is rapid. During the delay period, the droplets have had time to spread over a wide area and fresh air is always available around the droplets. Most of the fuel admitted would have evaporated and formed a combustible mixture with air.By this time, the preflame reactions would have also been completed.The period of rapid combustion is counted from end of delay period or the beginning of combustion to the point of maximum pressure on indicator diagram.The rate of heat release is maximum in this period. Pressure reached during the period of rapid combustion will depend on the duration of the delay period i.e. the longer the delay the more rapid and higher is the pressure rise since more fuel would have accumulated in the cylinder during the delay period.

3. Period of controlled combustion: The temperature and pressure in the second stage is quite high.Hence the fuel droplets injected during second stage burn faster with reduced ignition delay as soon they find necessary oxygen and any further pressure rise is controlled by the injection rate. The period of controlled combustio is assumed to end at maximum cycle temperature.

4. Period of after burning: Combustion does not cease with the completion of the injection process. The unburnt and partially burnt fuel particles left in the combustion chamber start burning as soon as they come in contact with oxygen.This process continues for certain duration called the after burning period. Usually this period starts from the point of maximum cycle temperature and continues over a part of the expansion stroke.

1. To compare 1cyl SI and 1cyl DI model and list down the differences

1cyl SI model:

SI wiebe combustion model:

1 cyl DI model:

DI wiebe combustion model

1. In 1cyl SI model, Fuel indolene(gasoline) is injected in the intake port to form a homogenous mixture with air when it reaches combustion chamber. Were as in 1cyl Fuel diesel is injected directly to the combustion chamber to form heterogenous mixture with air.

2. Compression ratio in SI engine are generally in the order of 8 to 12 and CI engine in the range of 16 to 20. From above we can observe that 1cyl SI model has a compression ratio of 9.5 and 1cyl CI model has a compression ratio of 16.5. Due to higher compression ratio operations, CI engines has higher themal efficiency than SI engines.Hence higher power output can be expected.

Due to higher compression ratios, higher volumetric efficiency can be obtained in CI engines.

3. Air/Fuel ratio: Generally in SI engines, air fuel ratio is maintained around stoichiometric air fuel ratio in order to achieve higher combustion efficiency. And in CI engines usually operates at leaner air fuel ratio.At stoichiometric air fuel ratio and heterogenous mixing, certain regions within the combustion chamber is likely to be rich that some of the fuel molecules will not be able to necessary oxygen for combustion and thus produce noticeably balck smoke(more soot formation). Hence CT engines always designed to perate with excess air.

In 1cyl SI model we can observe that air fuel ratio is equal to stoichiometric(14.5) and in 1cyl DI air fuel ratio is leaner(19.17).

4. Bsfc: Due to leaner air fuel ratio and higher thermal efficiency, CI engines tend to have lesser brake specific fuel consumption compared to SI engines.The same pattern can be observed in comparing the bsfc of 1cyl SI and 1cyl DI engine.

5. Exhaust temperature: Due to higher expansion ratio during expansion stroke, CI engines tend to have lesser exhaust temperature compared to SI engines. In 1cyl SI model we can oberve the exhaust temperature to be 1147 K and in 1cyl DI engine model is 1045 K.

III. Effect of main burn duration(MBD) on performance

Case:

Results:

From above results, we can observe that Shorter the main burn duration(i.e. faster combustion) results in increase in indicated mean effective pressure(IMEP). Therefore due to increase in in cylinder pressure, more power and more torque gets generated due to shorter burn duration.