Design and Development of VCR Mechanism
4.4 METHOD OF ATTACHMENT OF NOVEL VCR MECHANISM ON THE LARGE VCR ENGINE
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Fig 4.16 Assembly of VCR HONDA engine
The procedure laid in section 4.5 for converting the constant CR engine to variable CR Honda engine as shown in Fig.4.16. The experiments are then performed on the VCR Honda engine with variation of CR by novel mechanism only. The range of CR obtained are CR 4.91, CR 5.02, CR 5.27 and CR 5.4 over fixed CR 4.81. This range represent the minute increment of CR because for the spark ignition engine, it will be beneficial to keep the compression ratio variation with small magnitude for safety of the engine as well as possible knock.
4.4 METHOD OF ATTACHMENT OF NOVEL VCR MECHANISM ON THE
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VCR engine is having large bore diameter 87.5 mm to carry the high CR. The design of the novel VCR mechanism consist of secondary cylinder, secondary piston, ball screw and nut, cylinder piston rings, secondary cylinder head. Following the Eq.4.1 to Eq.4.12, the design of the novel VCR mechanism is presented. The specifications of the novel VCR mechanism are tabulated in Table 2. This additional secondary mechanism is also integrated with the engine for change in compression ratio.
Fig. 4.17 The procedure of mounting novel VCR mechanism on the engine head following procedure
(a) The provision made for mounting VCR mechanism; (b) the drill of 31.8 mm diameter with thread inside over 40 mm depth; (c) the novel VCR mechanism fixed over engine ; (d) the assembly of engine head with novel VCR; (e)the space consumed on the convention engine; (f) the inside structure of the engine head occupying suction, exhaust valve and third is secondary piston arranged very precisely; (g) the secondary piston displacement of 5 mm inside combustion chamber;(h) the secondary piston displacement of 2 mm inside combustion chamber;(i) the secondary piston displacement of 0 mm inside combustion chamber.
This top-up mechanism (Fig.4.17: a- i) consists of secondary cylinder and piston mounted on the engine head. The method of operation of top up mechanism has been discussed in section 4.2. The unique feature of this mechanism is that the spark plug is mounted inside the secondary piston which also moves coaxially with the piston.
As per this technique discussed in article 4.6, the geometric clearance volume (CV) is calculated using Eq.4.14. Any kind of alterations in this geometric CV allows to increase or decrease in compressionratio.
The theoretical compression ratios have been found from
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V
VC' c ;
' '
C D C
V V
CRV (Eq-4.14)
Where, VC' is the clearance volume of the cylinder;Vis the volume of secondary piston protruded inside clearance volume; VC is the geometric clearance volume as specified by the manufacturer of the engine; VD is the swept volume of cylinder, and CR is the compression ratio of the engine
The diameter of secondary piston is 31.10 mm and secondary cylinder diameter is 31.8mm.
The possible combinations of CR and secondary piston displacement are coordinated as displayed in Table.4.3.
4.4.1 Simultaneous use of two VCRs
In addition to the tilting block VCR arrangement of large VCR engine, when the top up mechanism (Novel VCR mechanism) is added, the engine becomes versatile and there are range of CR can be attained depending on the secondary piston displacement in or out of combustion chamber. However one important point here is that the increase in CR is limited by the engine structure capacity.
Fig. 4.18 Top up VCR mechanism and its location on engine head
Considering this, as the engine structure is light in weight and subjected to lower pressure comparative to diesel, it is beneficial to vary the compression ratio over minute level, so that there could be no chance of engine gets damaged. The other octane fuels also need a minute increase or decrease in CR over petrol fuel.
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Fig. 4.19 The secondary piston movement in clearance volume of SI engine.
Knowing this fact, the topup mechanism is designed to vary the compression ratio online to a minute level as depicted schematically in Fig. 4.19 and tabulated in Table 4.3. As shown in Fig. 4.19, for CR10, the depth 12.2 mm is available to move the secondary piston inside combustion chamber. Similarly, if CR9 is maintained by tilting block mechanism then the same will be 13.7 mm and for CR8, it will be 15.7 mm. So whatever be the displacement of secondary piston inside combustion chamber, it will reduce the clearance volume by an amount equal to protrusion volume of secondary piston which ultimately causes rise in CR (Eq.4.14).
Table 4.3 The theoretical VCR for novel mechanism To
maintai n CR8
Displace ment of pivot, mm
To maintain CR9
Displace ment of pivot, mm
To maintain CR10
Displace ment of pivot, mm
auxiliary piston displacem ent, mm
Above CR10
Above CR9
Above CR8
7.89 6.8 8.86 4 9.82 1 2 10.18 9.15 8.11
7.73 7.5 8.65 4.2 9.56 1.8 5 10.48 9.38 8.29
7.48 8.8 8.33 5.2 9.16 2.8 10 11.02 9.8 8.6
Table 4.3 shows the change in magnitude of CR with the displacement of secondary piston.
The rise in CR of the engine above fixed CR8, the possible increment is CR8.11, CR8.29 and CR8.6 similarly at fixed CR9, the increment is CR9.15, CR9.38 and CR9.8 and finally for fixed CR10, the CR increased to CR10.18, CR10.48 and CR11.02 for displacement of 2 mm, 5mm and 10 mm of secondary piston respectively. It is very interesting to observe the effect of such minute change in compression ratio over performance, combustion and emission analysis.
In order to compare the performance of the engine with top up VCR attachment and that with conventional VCR engine, can be justified only if the working conditions are constant. This includes the fixed compression ratio, fixed ignition timing and air/fuel ratio.
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The schematic diagram of original tilting block assembly from Fig.3.5 for large VCR engine is considered. The CR adjustment lever length is 150.1 mm between two pivots represent CR10. If this length increased then CR keeps on reducing. After installation of top up mechanism on this engine block and to keep compression ratio fixed, it needs to coordinate both the tilting block VCR and top-up VCR simultaneously such that the final compression ratio is CR8 or CR9 or CR10 (Fig. 4.20-a).
(a)
(b) (c)
Fig. 4.20 (a) Simultaneous adjustment of tilting and top up VCR for fixed CR10; (b) Simultaneous adjustment of tilting and top up VCR for fixed CR9; (c) Simultaneous adjustment of tilting and top
up VCR for fixed CR8.
The method of simultaneous coordination of two VCR mechanisms can be achieved as demonstrated in Fig. 4.20(a-c). A shown in Fig.4.20, if top up VCR mechanism becomes operational by 5mm protrusion inside combustion chamber, then scenario will be changed and displayed in Fig.4.19.
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There is reduction in clearance volume equal to volume occupied by protruded secondary piston inside combustion chamber. In such situation, the pivot distance needs to be increased causing increase of clearance volume and ultimately maintain constant CR. To maintain CR10 with 5mm protrusion length of secondary piston, the pivot distance of 151.8 mm must be adjusted (Table.4.3), Similarly for same protrusion length corresponding length of 154.3 mm and 157.5 mm should be adjusted to operate engine at CR9 and CR8 respectively (Fig.4.20 b-c).
4.5 METHOD OF VARYING SPARK PLUG LOCATION IN LARGE VCR