Answer EKM Question 51
Q51. Name the factors an engine designer considers in the selection of the compression ratio for a compression ignition engine. Give some examples of compression ratio values.
Answer:
$\mathrm{Compression\ ratio=\frac{volume\ of\ air\ at\ the\ beginning\
of\ compresion\ stroke}{Volume\ of\ air\ after\ end\ of\ compression\
stroke }}$
Factors Considered:
(a) Compression pressure, peak pressure & power output required, higher the compression ratio higher compression pressure, Peak pressure and power output.
(b) Cylinder head, cylinder block & piston material: To withstand high compression and peak pressure the material should be adequately strong, if there is a limitation is selection of material, compression pressure and compression ratio can be reduced. (c) Size of the engine: In a small engine more heat is lost in cylinder space surface during compression than in a larger engine. Smaller engines require a higher compression ratio than longer engines.
(d) Clearance volume: Volume between the top of the Piston and cylinder head when piston is at TDC. Smaller the clearance volume greater is the compression ratio.
(e) Shape of combustion chamber & piston: If cylinder head and piston crown have a greater profile then compression ratio will reduce. An allowance has to be given for fuel valve, air inlet and exhaust valves which limits compression ratio.
(f) Greater the size of combustion chamber lesser is the compression ratio.
(g) Fuel Quality: Higher compression ratio is required to burn low quality fuel i.e fuel with low calorific value and high water content, it also depends whether distillate fuel is to be burnt or residual fuel.
(h) Cylinder head gasket size: Thicker gasket will increase clearance volume & decrease compression ratio.
(i) Camshaft design: Whether the camshaft allows overlap between the valve openings, which will affect the power developed by the engine.
(j) Ambient conditions: An engine started in low ambient temperature requires a higher compression ratio than engines started in higher ambient temperature.
(k) Engine pre-heating: If engine has provisions for pre-heating, it will require lesser compression ratio than without pre-heating.
(l) Naturally aspirated engine: Higher compression ratio than turbocharged engine.
(m) Minimum RPM required for starting: Slow speed engines require high compression ratio to ensure that the engine runs or starts on fuel with minimum rpm.
(n) Cylinder swept volume, clearance volume and piston dome and volume of combustion chamber are important factors while selecting compression ratio.
Compression ratio values:
(a) Slow speed two stroke cycle engines used for ships propulsion — 12:1
(b) Medium speed turbocharged four stroke cycle engine used for propulsion 12:1
(c) Emergency electrical generator engine: 14:1 — 16:1
(d) Small high speed, naturally aspirated 4stroke cycle automotive engine 23:1.
Factors Considered:
(a) Compression pressure, peak pressure & power output required, higher the compression ratio higher compression pressure, Peak pressure and power output.
(b) Cylinder head, cylinder block & piston material: To withstand high compression and peak pressure the material should be adequately strong, if there is a limitation is selection of material, compression pressure and compression ratio can be reduced. (c) Size of the engine: In a small engine more heat is lost in cylinder space surface during compression than in a larger engine. Smaller engines require a higher compression ratio than longer engines.
(d) Clearance volume: Volume between the top of the Piston and cylinder head when piston is at TDC. Smaller the clearance volume greater is the compression ratio.
(e) Shape of combustion chamber & piston: If cylinder head and piston crown have a greater profile then compression ratio will reduce. An allowance has to be given for fuel valve, air inlet and exhaust valves which limits compression ratio.
(f) Greater the size of combustion chamber lesser is the compression ratio.
(g) Fuel Quality: Higher compression ratio is required to burn low quality fuel i.e fuel with low calorific value and high water content, it also depends whether distillate fuel is to be burnt or residual fuel.
(h) Cylinder head gasket size: Thicker gasket will increase clearance volume & decrease compression ratio.
(i) Camshaft design: Whether the camshaft allows overlap between the valve openings, which will affect the power developed by the engine.
(j) Ambient conditions: An engine started in low ambient temperature requires a higher compression ratio than engines started in higher ambient temperature.
(k) Engine pre-heating: If engine has provisions for pre-heating, it will require lesser compression ratio than without pre-heating.
(l) Naturally aspirated engine: Higher compression ratio than turbocharged engine.
(m) Minimum RPM required for starting: Slow speed engines require high compression ratio to ensure that the engine runs or starts on fuel with minimum rpm.
(n) Cylinder swept volume, clearance volume and piston dome and volume of combustion chamber are important factors while selecting compression ratio.
Compression ratio values:
(a) Slow speed two stroke cycle engines used for ships propulsion — 12:1
(b) Medium speed turbocharged four stroke cycle engine used for propulsion 12:1
(c) Emergency electrical generator engine: 14:1 — 16:1
(d) Small high speed, naturally aspirated 4stroke cycle automotive engine 23:1.
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