Two_stroke engine for space craft propulsion (New Trend)
With the introduction of electromagnetic thrusters we found two stoke engines are suitable for generating electcity for it's high power to weight ratio.
The Internal combustion engine
To day in most applications of power generation and in transportation propulsion the power source have being the internal combustion engine. Cars, trucks, off-highway vehicles, railroad, marine, motor bikes - as well as mechanical and electrical power generators for a wide range of large and small applications. Vital parts of the IC engine are the cylinders, pistons and crank gear. The form of the working chamber is a straight cylinder with movable piston in it. Therefore the motion of the piston is in a straight line, whereas in the majority of applications the final motion required is a rotary one. So that,the straight line motion generated in the engine is converted in to continues rotary motion using crank gear and delivered to consumer.
An internal combustion engine is operated in two main cycles. They are two stroke cycle (Clerk cycle) and four stroke cycle(Otto cycle). To complete a single sequence of operation (i.e. cycle), the mobile portion or the piston has to perform number of stokes. This cycle or the sequence has to take place continuously as long as the engine works.
Engine efficiency
In most machines, including engines some part of work done is expended in over coming friction or resistance of that nature; and the remaining portion is described as useful work. The ratio of useful work to the whole work done is called the efficiency of the machine.
The degree of heat utilization in internal combustion engines is determined by different efficiencies: thermal, overall, mechanical, etc.
The thermal efficiency of the working cycle of an engine characterizes the degree of perfection with which heat is converted in to work, without account taken of losses involved in friction, cooling, heat transfer, etc. This cycle accounts only for the heat lost with the exhaust gases.
Four stroke cycle-The Otto Cycle
Each movement of the cylinder up or down the cylinder is a stroke. The most widely adopted combustion process is four stroke cycle, for its higher thermal efficiency and greater economy.
The four stroke cycle consists of an induction stroke where air and fuel are taken into the working vessel as the piston moves downwards, a compression stroke where the air and fuel are compressed by the upstroke of the cylinder, the ignition or power stroke> where the compressed mixture is ignited and the expansion forces the cylinder downwards, and an exhaust stroke where the waste gases are forced out of the cylinder. The intake and outlet ports open and close to allow air to be drawn into the cylinder and exhaust gases to be expelled. Most modern internal combustion engines use the four stroke cycle.of the four stroke combustion cycle or Otto cycle.
Two-stroke engine
Two-stroke engines do not have valves. Simple in construction.It generate a power pulse in each revolution of the crank gear. So that it's having a higher power to weight ratio.
During the working cycle, the crankshaft starts driving the piston toward the spark plug for the compression stroke. While the air-fuel mixture in the cylinder is compressed, a vacuum is created in the crankcase. The crankcase, is creating a vacuum to suck in air/fuel from the carburetor through the reed valve and then pressurizing the crankcase so that air/fuel is forced into the combustion chamber.
This vacuum opens the reed valve and sucks air & fuel from the carburetor. Once the piston leads to the end of the compression stroke, the spark plug fires to generate combustion pressure to drive the piston. The sides of the piston are acting like valves, covering and uncovering the intake and exhaust ports communicating into the side of the cylinder wall.
Two stroke engines lighter, simpler and less expensive to manufacture.They have a greater power to weight ratio.But they are lesser in efficiency and it require lubrication oil to be fed with fuel.
Problems met with conventional engine
It is evident when investigating the cycle of internal combustion
engine, only 40% or lesser amount of heat energy is converted in to useful work and the
major part of energy is dissipated as losses. Since the ideal power requirement is in rotary manner, the linear
motion has to convert to rotary motion, and it is far from ideal. The performance of the reciprocating piston
engine is limited due to its inherent characteristics of the crank gear