02-01-2013, 01:58 PM
Two Stroke SI Engine
Two Stroke SI.pptx (Size: 1.19 MB / Downloads: 90)
INTRODUCTION
This project is based on reducing the charge consumption during scavenging process in two stroke SI engine.
It will also reduce environmental pollution.
This can be achieved by using reed valve .
ABSTRACT
Fresh charge loss during scavenging process is principal reason for its high specific fuel consumption and HC emission in 2-Stroke SI engine .
To minimize this loss a new scavenging system had been developed by using read valve.
Improved Performance and reduction in exhaust emission can be achieved by optimizing the amount of atmospheric air entering through the reed valve.
Two stroke engine
A two-stroke engine is an internal combustion engine that completes the process cycle in one revolution of the crankshaft i.e. an up stroke and a down stroke of the piston.
This is accomplished by using the end of the combustion stroke and the beginning of the compression stroke to perform simultaneously the intake and exhaust functions.
In this way, two-stroke engines often provide high specific power, at least in a narrow range of rotational speeds.
SCAVeNGING PROCESS
The process of clearing the cylinder after the expansion stroke is called scavenging process.
It affects greatly on the efficiency of an engine.
When piston moves downward ,uncovers exhaust port first & then the transfer port through which charge enters in cylinder.
Deflector is a projection over piston due to which the charge is sweep up in the cylinder &push out the exhaust gas.
A small amount of fresh charge is lost & burnt gas left back in the cylinder during this process, called as short circuit.
WORKING OF REED VALVE
The reed petal is made of flexible material and it is mounted between a base and a stopper.
The base serves as a barrier for valve closure, while an upper stop, which can be curved, has a double function:
It serves as a guide to prevent excessive opening of the valve.
It optimises the shape of the open channel .
PRINCIPLE OF OPERATION
In the improved design two reed valves are fitted at the upper end of transfer duct connecting the crank case and the cylinder.
When the piston moves upward, it causes the air from the atmosphere to flow into the transfer passages through the reed valves via. control valve and the fresh charge enters the crank case in the normal way through the inlet passage.
During the down ward stroke of the piston, the pressure in the crank case closes the reed valves later open the transfer ports and air in the transfer passage enters the cylinder ahead of the fresh charge.
The air pushes the exhaust gases out and acts as a buffer screen between the two mediums, the charge and burnt gas.
Air becomes the main component that is lost to the atmosphere through the exhaust port . The charge that follows it is retained in the cylinder to a greater extend.
CONCLUSION
1. The fixing of the extra reed valves et the transfer duct in a two stroke S.I engines leads to reduction in HC and CO emission levels and considerable improvement in break thermal efficiency due to reduction in short circuiting.
2. The amount of secondary air which enters into the transfer duct to obtain better performance depends up on the throttle position and engine capacity, since it will affect the mixture quality of the trapped charge. For a small capacity engine, precious control of atmosphere air flow through the reed valves is necessary in order to obtain maximum advantage in the performance.