Valved Two Stroke Engine with Extended Expansion
#1

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1.ABSTRACT
This proposal relates to conceptual design of “Valved Two Stroke Engine with Extended Expansion” . This is an hybrid of two stroke and four stroke engine.
We know that in a two stroke engine - the four strokes viz. suction, compression, power and exhaust will be completed in each rotation of crank shaft and power is produced for each rotation. Whereas in a four stroke engine the above four strokes are completed in every two rotation and power is produced once in a two rotation. Hence for same size/capacity of engine , a two stroke engine theoritically produces double the power of a four stroke engine.
The proposed engine do not have port arrangement as found in conventional two stroke engine, instead it has three valves viz. suction (shuttle), exhaust and Transfer valve. The escaping of fresh charge through exhaust port (which is an inherent deficiency of a conventional Two Stroke Engine) is prevented by a specially designed Transfer Valve arrangement.
Hence by two stroke arrangement we reduce the engine components. Say for example : a six cylinder conventional four stroke engine can be equivalently made as three cylinder two stroke engine. By the above we can save three pistons, three connecting rods, three spark plug etc. This reduces the production cost of the engine.
Secondly, by suitably modifing the engine construction, the pressure at the end of expansion stroke reaches nearly to atmospheric presssure which is refered as Extended Expansion. By extended expansion the following features are anticipated:
Increase of thermal efficiency/mileage, reduction in emission temperature and noise level.
Conventional engines are based on Otto Cycle ( for petrol engine) / Dual Cycle (for Diesel engines). The “Valved Two Stroke Engine with Extended Expansion” is based on Atkinson Cycle which is a corrolary to the above cycle.
This model will be useful for both SI & CI engines of higher capacity
2. CONVENTIONAL TWO STROKE ENGINE Vs FOUR STROKE ENGINE Vs OUR PROPOSED HYBRID ENGINE.
In a conventional four stroke engine the four strokes viz. suction, compression, power and exhaust are completed in two rotation (720⁰ crank shaft rotation). Hence for every two stroke one power stroke is produced. The four stroke engine has two valves viz. suction and exhaust valves. By the valve arrangement the escaping of fresh charge without burning is prevented.
Whereas in a two stroke engine the four strokes are completed in a single rotation of crank shaft (360⁰ crank shaft rotation). Hence for each rotation one power stroke is produced. Two stroke engines do not have valves in construction. Because of the absence of valve arrangement, some part of fresh charge escapes through exhaust without producing power and causes HC emission.
Theoretically a two stroke engine produces twice the power of a four stroke engine,
W e know that Indicated Horse power of an IC Engine ( IHP)
(IHP = Pm LAN) for two stroke engine and (IHP = Pm LAN/2) for four stroke engine.
Our project work is an hybrid of two stroke and four stroke engines. i.e like two stroke engine, the cycle is completed in a single rotation. At the same time, like a four stroke engine, our engine has valve arrangements to eliminate the escaping of fresh charge through exhaust. The engine has three valves viz. suction ( shuttle) valve, exhaust valve and transfer valve. The transfer valve forms a barrier between the fresh charge filled above the Transfer Valve and the exhaust gas leaving underneath the Transfer Valve and thus prevents the escaping/mixing of fresh charge to be carried over to the exhaust. Our engine has both the advantageous of conventional two stroke and four stroke engines.
( Refer page No.15,16 for engine construction)
 Thus the proposed engine is an HYBRID of two stroke and four stroke engine.
3.CONVENTIONAL ENGINE Vs EXTENDED EXPANSION ENGINE.
In a conventional engine, the stroke length remains same for the entire cycle. The compression ratio and expansion ratio are equal. Because the engine has to perform its cycle in a limited stroke length, the power stroke is curtailed to some extent. i.e. at the end of power stroke the gas pressure does not reach atmospheric pressure. At the end of power stroke, though the gas has some more potenial /pressure to do work , it is not utilised properly to expand upto atmospheric pressure. Because the piston has to reverse its direction to carry out its next exhaust stroke , the expanding gas during power stroke is released some where near 40₀ to 60₀ before BDC under pressure by opening exhaust valve. Notwithstanding the above, the pressure at the end of power stroke will be around 4 to 7 bar. Thus we can consider conventional engines as LIMITED EXPANSION ENGINES. This causes power loss, emit relatively high temperature to the environment, cause noise pollution. The above deficiency can be controlled by EXTENDED EXPANSION ENGINE which was first proved by Atkinson. In the extended expansion engine, the stroke length during suction and compression are equal say( ls) ; and the stroke length for power and exhaust stroke are equal say ( le) . The value of ( le) is higher than ( ls) . Because of the increased stroke length, the compression ratio ( rc) and expansion ratio ( re) are different. ( re) > ( rc ). A charge of swept volume Vs is inducted inside the cylinder , it is then ignited, burnt and expanded in a higher volume Ve. Due to this piston expands largely to reduce the gas pressure nearly to atmospheric pressure which can increase thermal efficiency. But the working model demonstrated by Atkinson was much complex in construction. It is estimated that the increase in thermal η under normal working condition will be approximately 7%. In the proposed engine we have different approach to get the extended expansion. The expansion volume (ve ) is bigger than swept volume ( vs ). It may be noteworthy to mention that an additional volume (ve - vs) = δv stands main necessity to get extended expansion. The extended expansion causes the pressure at the end of expansion to reach nearly to atmospheric pressure. Unlike the primitive model Atkinson Engine which utilises two different stroke lengths le & ls to get additional volume for expansion (δv) , in the proposed engine the δv is got by having two different diameters de and ds . ( page no.15,16 may be referrred to know the constructional detail)
We know that v = (π d2 * l) ÷ 4.
In Atkinson engine , the δv is obtained the difference in two different stroke lengths (le - ls) . Hence δv varies directly proportional to stroke length, whereas in our engine δv is got by having two different diameter sizes - de and ds . It may be noted from above formula that δv varies square of the diameter. Hence even a slightest increment in diameter can give considerable increase in additional volume δv. It may now be concluded that the size of our engine will be much smaller than the size of primitive model Atkinson engine.
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