What is mean by scavenging and its types with details explanation l The 3 types of scavenging l The Scavenging Process l Theoretical Scavenging Process
The Internal combustion engine At end of the expansion stroke combustion of the two-stroke engine is the left full of product combustion this is because unlike the four-stroke engine this is no exhaust stroke available to the clear cylinder of the burnt gases process of clearing cylinder after the expansion stroke is the called scavenging process this is just completed in the very short duration available between end of the expansion stroke end start of the charging process.
The efficiency of two stroke the engine depend to a great degree on the effectiveness of the scavenging process since bad scavenging the gives a low mean indicate pressure and the hence result in a high weight and the high cost per bops for the engine. With insufficient scavenging amount of the oxygen available is low so that is consequent the incomplete combustion results in the higher specific fuel the consumption.
Not only that is lubricating oil becomes the more contaminated so that it is the lubricating qualities are reduced and the results in increased wear of the system and cylinder the liners. Poor scavenging also the leads to higher mean temperatures and the grater heat stresses on the cylinder walls. The No is produce data great extent due to the high local temperatures found in the Diesel engines which are the highly dependent on the initial rise of heat release.
In the addition soot production and the oxidation are both of dependent on the mixing rate and the local flame temperatures. The injection velocity is one of the most influent parameters on the factors which are the mentioned before since it controls both the mixing process and the rate of heat release. This is the reason that injection system parameters and the nozzle geometry have been extensively studied due to the direct relation with the fuel injection rate and the fuel velocity.
To the support this it has been recognized that is characteristics of the injection system are the most important fact or sin influencing emissions and the performance of CI engines.
The present study a spark injection and the compression injection engine with the unfold valve scavenging of the cylinder and a transfer valve in the piston crown have been the described. A great disadvantage of the two-stroke engines is the ports which are made in the cylinder bearing the surface. Under the heat which it is realised during the combustion thermal extension of the range in proximity of ports and other parts of the cylinder is different and so distortion of the geometry of a cylinder liner surface force the designer to make the clearance between a piston and the cylinder liner bigger.
This is paper presents the critical review to study the effect of fuel injection timing and the scavenging using diesel on combustion and the emission characteristics of the single cylinder two stroke and the air cooled direct injection the diesel engine.
The well-known that is injection strategies including the injection timing and pressure play the most important role in the determining engine performance the especially in this scavenging the emissions. However the injection timing and the pressure quantitatively affect performance of the diesel engine.
The 3 types of scavenging
1] Inflow scavenging:-
The scavenging method one or two the intake ports are located at the sides of the cylinder and the exhaust port is located at the cylinder head. When the charge enters from both side intake ports the charge flows upwards and the after combustion the burnt gases go out of the cylinder from the exhaust valve which is located at the top of the cylinder. Less air loss occurs and the has lower fuel consumption hence gives better the engine efficiency as compared to the other methods. Good scavenging obtains during the low speed and the high speed using the inflow scavenging method. The inflow scavenging commonly is used in the large-sized two strokes engines such as in the two stroke in engines of the big marine ships.
2] Cross-flow scavenging:-
The cross-flow type scavenging has and the inlet port and the exhaust port that are located on the opposite sides to the each other. The deflector piston hump-shaped the piston head is used in this type of the scavenging. When the charge enters into the engine from the intake port due to the hump shape of a piston this incoming charge the moves upwards and pushes the upper burnt gases to the downside and the gases goes outwards through the exhaust port. Therefore hump on the piston assists in the engine’s scavenging operation. This is method of the scavenging performs well be during the low speed of the engine while bad scavenging obtains at the high speed of the engine or at full a throttle the time.
3] Loop or reverse scavenging:-
In this scavenging method the intake port and the exhaust ports are located at the same side in the engine. In this method size of the intake port is a large size to the enter a large volume of charge inside the cylinder and size of the exhaust port is small increases velocity to flow out burnt gases the quickly. When the fresh charge enters from the intake port of the fresh charge has a swirling motion as like a loop hence this is the fresh charge moves upwards and pushes back the burnt gases. Hence in the such a way these burnt gases flows out of the cylinder. This is method of the scavenging performs well be during the high speed of engine or at full throttle time while bad the scavenging obtains at the low speed of the engine.
The Scavenging Process
Before discussing the scavenging process it is useful to describe the operation cycle of the two-stroke engine with the direct injection. For this purpose an engine with scavenge and the Exhaust ports instead valves will be the considered. At the beginning of cycle when fuel Injection and the ignition have just taken place the piston is at the TDC top dead canter. The temperature and the pressure rise consequently the piston is driven down note the arrows indicate direction of the piston.
The Along power stroke exhaust ports are the uncovered open and consequently burnt gases begin to the flow out piston continues down. When the piston pasts over and the consequently opens scavenge ports pressurized air the enters and drives out remaining exhaust the gases. This is a process of introducing air and the expelling burnt gases is the called scavenging. The incoming air is used to clean out or scavenge the exhaust gases and then to fill or charge the space with the fresh air. After reaching bottom dead canter (BDC) the piston moves upward on it is the return stroke.
The scavenge ports and the exhaust ports are closed and the air is then compressed as the piston moves to the top of its stroke. Soon before the piston reaches top dead canter (TDC) the injectors spray the fuel spark plug ignite the mixture and cycle starts again. A drawback which has a decisive influence not the only on consumption but also on power and pollution is the process of displacing the burnt gases from the cylinder and replacing t by the fresh-air charge known as the scavenging.
In ideal scavenging the entering scavenge air acts as a wedge in pushing the burnt gases out of the cylinder without the mixing with them. The Unfortunately real scavenging process is the characterized by the two problems common to the two-stroke engines in general the short-circuiting losses and the mixing. Short-circuiting the consists on expelling some of the fresh-air charge directly to exhaust and the mixing consists on fact is a small amount of the residual gases which remain trapped without being the expelled being mixed with some of the new air charge.
Theoretical Scavenging Process
Illustrates three theoretical process. These are perfect scavenging perfect mixing and the complete short circuiting.
- Perfect scavenging – The mixing air displaces product to the exhaust if you extra air is the delivered when the delivery it is not the retained.
- Short circulating – The air initially displaces all the product with in the path of the short circuit and the flows into and out of the acyl.
- Perfect mixing – The first air to enter the cylinder mixes instantaneously with the products and the gas leaving is almost all the residual for larger delivery ratio most of the gas leaving is air.
1 Delivery Ratio (Reel):-
The delivery ratio (Reel) (DR) represents of the air volume under the ambient condition of the scavenge manifold introduce per cycle and the reference volume. Delivery ratio Reel(DR) = v1/verb The delivery ratio on mass basic according to it the delivery ratio is mass of the fresh air deliver to cylinder divided by the reference mass Reel =MFAD/MCY Thus the delivery ratio is measure to air supply to the cylinder relative to the cylinder content. If Reel =1 it means that volume the scavenging that supply to the cylinder is equal to cylinder volume. Delivery ratio (Reel) (DR) usually varies between to except for the closed crank case scavenge where it is the less unity.
2 Scavenging Efficiency:-
Scavenging efficiency is define as the ratio of the volume the scavenge air which romance in the cylinder at the end of the scavenging to the volume of the cylinder itself at the moment when the scavenge and exhaust ports of the valves are fully closed. It is given by ᶯSC = v2’ /v2 and according to S.A.E. ᶯSC = MFAD/MCY Scavenging efficiency indicated to what extend the residual gases in the cylinder is replaced with fresh air. If it is equal to unity, it means that all gases existing in the cylinder at the beginning of scavenging have in swept out completely.
3 Relative Cylinder Charge:-
Relative cylinder charge is measure of the success of filling cylinder irrespective or the compression of charge and is the define as Corel = V2/ Verb It must be noted that all the volumes revered to are at slandered temperature and the pressure. Taylor however recommends the use of inlet temperature and the exhaust pressure as the reference it can be shown that is delivery ratio scavenging and the trapping efficacy are related by the following equation. Reel= Corel. ᶯsac /ᶯtrap.
4 Charging Efficiency:-
The amount of fresh charge in the cylinder is a measure of power output of the engine. The useful fresh charge divided by the displacement volume is the charging efficiency defined as the ᶯchi = Vet / Verb Charging efficiency is a measure of the success of filling cylinder with the fresh air. Naturally ᶯchi = Reel. ᶯtar.
5 Pressure Loss Coefficient (Pl):-
The pressure loss coefficient is the ratio between a main upstream and the downstream pressure during scavenging period and represent the loss of pressure to which the scavenging air is subjected when it is crosses the cylinder.
6 Excess Air Factor (Λ):-
The valve Rdel-1 is the called excess air the factor λ. For example if the delivery ratio is the excess air factor is 0.7.
Typical Timing of Two-Stroke Cycle CI Engine
The very high power the diesel engines used for the ship propulsion are a commonly two stroke the diesel engines. In fact all the engines between 400 to the 900 mm bore are loop the scavenged or unfolds type with the exhaust valve. The brake power on the single crank shaft can be to the unto 37000 kW. Nordberg 12 the cylinder 800 mm bore and the 1550 mm stroke two the stroke diesel engines develops 20000 kW at the120 rpm.
This is speed allows the engine to be directly coupled o the propeller of a ship without the necessity of gear reducer. The two stroke engine cycle is the completed in one resolution of the crankshaft. The main difference between two the stroke and the four-stroke engine is in the method of the filling fresh charge and the removing brunt gases from the cylinder. In the four-stroke engine the operations are performed by the engine piston during suction and the exhaust respectively.
In a two-stroke engine the filling process is the accomplished by charge compressed in the crankcases or by a blower. The induction of the compressed charge moves out the Product of combustion through the exhaust ports. The before no piston strokes are required for the two operations. Two strokes are sufficient to the complete cycle owner for the compressing fresh charge and the other for expression or power the stroke.