Valved Two Stroke(VTS) Engine With Extended Expansion- Seminar Report

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.

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 (720^⁰ 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 (360^⁰ 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 = P_m LAN) for two stroke engine and (IHP = P_m 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. 

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  40^₀  to 60^₀  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( l_s) ;  and the stroke length for power and exhaust stroke are equal say  ( l_e) . The value of ( l_e) is higher than ( l_s) . Because of the increased stroke length, the compression ratio ( r_c) and expansion ratio ( r_e) are different.   ( r_e) > ( r_c ).   A  charge of swept volume V_s is inducted inside the cylinder , it is then ignited, burnt and expanded in a higher volume V_e. 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 (v_e ) is   bigger than swept volume ( v_s ).  It may be noteworthy to mention that an additional volume  (v_e - v_s) = 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 l_e & l_s to get additional volume for expansion (δv) , in the proposed engine the δv is got by having two different diameters d_e and d_s .

We know that    v = (π d²  * l) ÷ 4. 

In Atkinson engine , the  δv is obtained the difference in  two different stroke lengths    (l_e - l_s) . Hence δv varies directly proportional to stroke length, whereas in our engine  δv is got by having two different   diameter sizes -  d_e and d_s .  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.

  

CONSTRUCTION AND WORKING OF STEPPED PISTON MODEL

“VALVED TWO STROKE ENGINE WITH EXTENDED EXPANSION” 

Constructional Details

 a) Cylinder Block : The Cylinder Block is stepped in construction. A stepped Piston (73) reciprocates inside the Cylinder Block. There are two different chambers (spaces) formed by the cylinder block and the piston. The annular chamber and the central chamber as indicated in 82 and 84. page-16. The volume of annular chamber  V_s­­ is the Swept volume.  The volume of central chamber is V_e is the expansion volume. The engine is designed in such a way that the ratio of V_e ÷ V_s will be approximately 2 times.   The air fuel mixture is inducted and compressed in the annular chamber i.e. the suction and compression strokes are performed in the annular chamber. Whereas the power and exhaust strokes are performed in the central chamber. The compressor unit at the bottom is cooled by fins  arrangement. The expansion unit (engine part) is cooled by water jacket ) arrangement at the top as shown in the drawing.

 b)Piston: Piston is cylindrical and stepped in construction. Rings are assembled at both the minor and major diameters of the piston. The piston is driven by a common connecting  rod.

(All the valves said below are to be considered as a Revolved Section)

.c)Shuttle Valve: It is a single piece construction, ring shaped  with two spindles. Refer page No.-18.  The Shuttle Valve is a double acting type and performs as a common suction and discharge valve to the compressor part. Both the admission of air / air fuel mixture into the compressor and the discharge from the compressor are controlled by the Shuttle Valve. The valve is operated by over head cam mechanism. 

d) Exhaust Valve: It is a single piece construction, ring shaped with two spindles. The valve has large seating area. All the valves are operated by over head cam mechanisms and no special rocker arms are required. 

e)Transfer Valve:  . The Transfer Valve is a two piece construction. The central part is known as the Transfer Valve Cone/Sleeve and two spindles attached to it.  The other part is known as the Transfer Valve disc which also has two spindles attached to it. The exploded view of typical Transfer valve.The purpose of the Transfer Valve is to form a barrier between the fresh air fuel mixture admitted over the valve and the exhaust gas going out through exhaust valve ( beneath the Transfer Valve) so that the mixing of fresh air fuel mixture through exhaust is prevented. (In conventional two stroke engine such Transfer Valve arrangement is absent and hence some portion of the fresh charge escapes out through exhaust without burning/ producing power.).  All the valves illustrated are for visualizing/understanding purpose and need not match to the real construction

Working Principle

 The four strokes viz. suction, compression, power and exhaust strokes are described below: 

Suction Stroke:. Piston moves from TDC to BDC ie 0 ⁶ to 180 ⁶. During suction stroke , the shuttle valve is in the upper position and communicates with the inlet port 94. As piston moves down air fuel mixture from carburetor is inducted into the annular chamber of the compressor unit as shown by the arrows in the drawing.

Compression stroke: Pistonmoves from BDC to TDC ie from 180˚ to 310˚. During compression stroke,   the shuttle valve moves downward and the port position changes to 90. Compressed air/ charge is discharged out through transfer manifold (90-91) and temporarily gets filled above the transfer valve. During compression stroke, the transfer valve (79) rests in its lower position w.r.t. to cylinder block .  The filling of air fuel mixture above transfer valve takes place up to 50˚ before TDC.

Transferring operation:  Around 50˚ before TDC, as the piston still moves near to the TDC,  the exhaust valve  closes, cutting the venting of exhaust gas of previous cycle. Simultaneously, the transfer valve cone  suddenly moves up from its seat of  TV disc causing a  wide gap in between them.  As the piston  still continues moving up and also due the pressure built up above the transfer  valve during the period of 180˚ to 310˚,  the charge which was held above the transfer valve disc gets  transferred into the central chamber through the gap between disc and cone and mixes with residual exhaust gas trapped. At 330˚ the TV disc also moves up compressing the left over fresh charge above it and transfers into the central chamber.  Now the transfer port 91 is blocked by the top seating of Transfer valve disc at 340˚.

Ignition and combustion : Around 20˚ before TDC , the  charge  is ignited by the spark plug (80). Temperature as well as pressure increases abruptly during combustion.

Power stroke:  The brunt of the combustion products over the piston causes the power stroke and piston moves down from TDC to BDC. At the end of power stroke the pressure inside the cylinder will be reduced to that of atmospheric pressure as the expansion volume (V_e) is greater than the annular volume or swept by the compressor (V_s) following Atkinson Cycle. This volume difference is suitably designed so that the gas can expand nearly to atmospheric pressure during expansion stroke by which additional work can be extracted. 

It may be noted that while power stroke is in progress in the central chamber, simultaneously suction stroke is effected in the compressor unit for the next cycle.

Exhaust cum compression stroke:    When piston moves upward from 180˚ to 360˚, the exhaust valve is opened and the products of combustion is discharged out to atmosphere as shown by the arrows in the drawing. The exhaust valve is closed at 50˚ before TDC. The valve timings given above are only exemplary data and may vary with the actual valve timing.

It can be seen that during this upward movement of piston, simultaneously compression stroke /filling of charge is effected from the annular chamber for the next cycle.

The cycle gets repeated.

All the four strokes are completed in one complete rotation i.e in 360˚ without any carry over fresh charge with the exhaust . The engine is two stroke engine. It may be noted that a smaller volume V_s of charge is burnt and expanded in a higher volume V_e so that the pressure during expansion stroke expands until its pressure reaches nearly to atmospheric level following Atkinson Cycle which is referred as the Extended Expansion.

** Alternatively, disc valves which are used in a conventional reciprocating air compressors may also be replaced instead of the shuttle valve. The disc valve can perform the function of air admission and discharge from the annular chamber. The disc valve operates on the differential pressure between valve plates and hence no cam arrangement is required.

 TWO CHANNEL FUEL SUPPLY.

In a S.I. engine, increase of compression ratio increases the thermal efficiency. However by increasing the compression ratio beyond some value will lead to detonation and hence the value of compression ratio is optimized.  In conventional engine, all the four cycles viz. suction, compression, power & exhaust are performed in a single chamber/cylinder. At the end of compression stroke (i.e. before ignition) the temperature of the charge remains almost uniform in the clearance volume. Before igniting the charge, it will be practically difficult to achieve different temperatures of charge inside the clearance volume.

In the proposed engine, it is possible to achieve two/more different temperature zones in the clearance volume. i.e. we can get an hot charge to the vicinity of spark plug and a cold charge enveloping it. By this arrangement, the flame front issuing from the spark plug can travel till the cylinder wall without causing detonation effect.  As the temperature of the charge nearer to wall of the cylinder is fairly cool,   the auto ignition / detonation can be averted.

The charge that is pumped out which is hot is branched into two lines. One line goes via. a counter flow intercooler and the other in bypass to it. The hot charge which bypasses the cooler reaches the port 91 and then 91 B. The charge cooled by the cooler passes via. port 91 A. The mixture strength in the hot line can be varied by Carburetor C1&C2 and the flow can be varied by valves V1& V2. The transfer valve has a barrier ring attached to the disc which is cylindrical in shape. Two operating spindles are attached to the barrier ring.

When the transfer valve cone/sleeve is opened at 50^° before TDC, the hot charge will be discharged through the gap between disc and the sleeve, moves inwards and flushes away some residual burnt gas of previous cycle. Thus the vicinity of spark plug is surrounded by a layer of hot charge (the mixture strength and flow can be altered according to the engine requirement by adjusting carburetor and valve). Succeeding to it when the disc lifts up, the cold charge is compressed and flows outwards and moves in the inner chamber. Thus before ignition takes place there will be two temperature zones inside the inner chamber. i.e. Hot charge to the vicinity of spark plug and a cold charge enveloping it. Now if the charge is ignited the flame front can travel up to the wall of the cylinder without causing detonation.  By this arrangement we can increase compression ratio and hence the operating cycle temperature without causing detonation.  

For a CI engine, there will no barrier ring. While fuel injection starts, the transfer valve cone lifts first.  As the piston nears to TDC, the air residing above the transfer valve disc gets released thro’ the gap between transfer valve disc and cone and enters into the inner chamber & pressure above transfer valve disc reduces.  When the fuel injection continues, the transfer valve disc also lifts up and directs a fresh stream of air towards the injector. Thus there is a continuous supply of fresh air which mixes continuously to the fuel injected and no induced turbulence is needed.

Advantages of  “Valved Two Stroke Engine with Extended Expansion”

   1)    By the “valved Two Stroke Arrangement”:  Number of working components can be reduced. As said earlier   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 /injector etc.

2)                        By the “ Extended Expansion Arrangement” : 

a)We can increase thermal efficiency / increase mileage. b) Emission Temperature can be reduced.

c) Exhaust noise can be reduced.& d) Pumping loss and blow down can be reduced.

      3)    By the split up construction of engine:

              a) The valves have wider seating area when compared to that of conventional engine. Hence volumetric efficiency can be increased.

              b) Unlike a conventional engine, the exhaust valve is located in a lower elevation than that of Transfer valve.  Hence the fresh charge to be ignited resides in an upper stratum while trapped exhaust gas occupy lower stratum.  This promotes for a good burning of the charge.

            c) In a conventional engine, at 720 ⁰ position, both the suction and exhaust overlap with each other.  Hence during suction stroke, the effective mass of charge inducted per cycle is reduced.  Whereas in our proposed engine, suction and exhaust occur at two different regions and hence no over - lapping occurs and   does not reduce the mass inducted / volumetric efficiency.

         d) 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 l_e & l_s to get additional volume for expansion (δv) , in the proposed engine the δv is got by having two different diameters d_e and d_s . ( page no.15 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    (l_e - l_s) . Hence δv varies directly proportional to stroke length, whereas in our engine  δv is got by having two different   diameter sizes -  d_e and d_s .  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.

         e) By the two channel fuel supply arrangement, we can increase the thermal efficiency and operating cycle temperature i.e. ( T_max.) without producing detonation.

LINER TYPE MODEL

Apart from the stepped piston model as discussed above, a conceptual design of Liner Type Model is also proposed and discussed below. In the stepped model we have piston and cylinder in stepped construction. Whereas in the liner model, the piston will have a groove extending from piston head. A  liner acts as a guide for the travel of grooved piston. The liner projects downwards from the cylinder head, bifurcates the annular chamber (of volume V_s) the central chamber (of volume V_e) as indicated in the following figures. Page No. 25 , 26,27&28. Similar valves are assembled to the Liner model also.

 Unlike the Transfer valve described for the stepped piston model which is two piece construction, the transfer valve shown in the liner type is of single piece construction. In two piece construction, first the transfer valve cone/sleeve lifts up at 50^° before TDC causing a wide gap between the disc and the cone. Through this gap,  the pressure acting above the Transfer Valve gets relived into the central chamber and the differential pressure acting  above and below  sides of Transfer Valve disc gets equalized. Hence the force required to lift the Transfer Valve gets balanced and becomes less.  In a single piece construction, the force for lifting will be greater as the valve is not geometrically  balanced and some work will be wasted for opening this valve. The spindles attached to the disc portion are operated by over head cam. The working of the liner type model is similar to that of stepped engine and hence self explanatory.  

The leak through the transfer valve spindles with respect to head/ valve guide during filling can be boxed up by providing mechanical seal arrangements.

Conclusion:

   Comparing with the available reports on earlier invented IC Engines, it may be found that the proposed engine will be simpler, has many salient features & will be much useful to engine industries.