selection of rod ratio for i.c. engines p m v subbarao professor mechanical engineering department...
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Selection of Rod Ratio for I.C. Engines
P M V SubbaraoProfessor
Mechanical Engineering Department
Understand the strong relation Between Kinematics & in Cylinder Thermo-Fluid
Process …..
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Rod Ratio Relationships
• Short Rod is slower at BDC range and faster at TDC range.• Long Rod is faster at BDC range and slower at TDC range.
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Rod Length Changes
• Appears a minimum length change of 21/2% is necessary to perceive a change in thermo-fluid process.
• For R & D purposes it appears a 5% change should be made.
• Perhaps any change should be 2 to 3%--i.e., ignition timing, header tube area, pipe length, cam shaft valve event area, cylinder head flow change, etc.
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Rod Ratio
• The ratio between the connecting rod length and the stroke length of a motor greatly affects the way it performs, and how long it lasts.
• This ratio (normally represented by “R”) can be calculated as follows:
• Ratio “R” = Rod Length ÷ Stroke
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Creation of Constant Volume Combustion Engine
R
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LONG ROD
• Intake Stroke -- will draw harder on cylinder head from 90o ATDC to BDC.
• Compression Stroke -- Piston travels from BDC to 90o BTDC faster than short rod.
• Goes slower from 90o BTDC to TDC--may change ignition timing requirement versus short rod as piston spends more time at top.
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Long Rod : Ignition
• If flame travel is too fast, detonation could occur.
• Does a long rod produce more efficient combustion at high RPM--measure CO, CO2?
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Long Rod : Power Stroke
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• Power Stroke -- Piston is further down in bore for any given rod/crank pin angle and thus, at any crank angle from 20o to 75o ATDC less force is exerted on the crank pin than a shorter rod.
• The piston will be higher in the bore for any given crank angle from 90o ATDC to BDC and thus cylinder pressure could be higher.
• Long rod will spend less time from 90o ATDC to BDC--allows less time for exhaust to escape on power stroke.
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Long Rod : Exhaust Stroke
• Exhaust Stroke : The piston will be more in Bore from BDC to 90o ATDC and thus cylinder pressure could be higher.
• Will force more exhaust out from BDC to 90o BTDC.
• Could have more pumping loss!
• A long rod will help during peak power.
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SHORT ROD : Intake Stroke
• Intake Stroke -- Short rod spends less time near TDC and will suck harder on the cylinder head from 10o ATDC to 90o ATDC the early part of the stroke.
• Will not suck as hard from 90o ATDC to BDC as a long rod.
• Will require a better cylinder head than long rod to produce same peak HP.
• Will require stronger wrist pins, piston pin bosses, and connecting rods than a long rod.
• Short rod spends more time at the bottom which may reduce intake charge being pumped back out intake tract as valve closes.
• May permit longer intake lobe and/or later intake closing than a long rod.
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Short Rod : Compression Stroke & Ignition
• Piston moves slower from BDC to 90o BTDC; faster from 90o BTDC to TDC than long rod.
• With same ignition timing short rod will create less cylinder compression for any given crank angle from 90o BTDC to 90o ATDC except at TDC.
• As piston comes down, it will have moved further; thus, from a "time" standpoint, the short rod may be less prone to detonation and may permit higher compression ratios.
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Short Rod : Power Stroke
• Power Stroke -- Short rod exerts more force to the crank pin at any crank angle that counts ie.-20o ATDC to 70o ATDC.
• Also side loads on cylinder walls more than long rod.
• Will probably be more critical of piston design and cylinder wall rigidity.
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Short Rod: Exhaust Stroke
• Permits earlier exhaust opening due to cylinder pressure/force being delivered to crank pin sooner with short rod.
• Stroke starts anywhere from 80o to 110o BBDC in race engines due to exhaust valve opening.
• Requires a better exhaust port as it will not pump like a long rod.
• Short rod has less pumping loss ABDC up to 90o BTDC and has more pumping loss from 90o BTDC as it approaches TDC, and may cause more reversion.
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Rod Ratio vs. Intake Efficiency
• A “R” value of 1.75 is considered “ideal” by some respected engine builders, if the breathing is optimized for the design.
• Except for purpose-built racing engines, most other projects are compromises where 1.75 may not produce the best results.
• The “R” value can be used as a correction factor to better “match” the intake to the manifold.
• Low “R” numbers (1.45 - 1.75) are produced by short rods in relation to the stroke.
• High “R” numbers (1.75 - 2.1) are produced by long rods in relation to the stroke.
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Angle Limitation & Engine Durability
• Sine of Rod Angle = Stroke ÷ (Rod Length * 2)
• The angle of the rod at 90° ATDC is a good indication of how much stress the piston and cylinder wall will be subjected to with a specific rod/stroke selection.
• Angles beyond 17° promote excessive wear at the piston major thrust surface, and piston breakage could be the result.
• Piston selection will be critical for the life expectation of the engine; maximum skirt length below the pin is desired.
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Rod Angle
“R” Ratio
Examples Comments
13½° 2.142-1 High speed motor with small ports. Best breathing with small ports
14° 2.067-114½° 1.997-1 Long rods for good breathing with small
ports15° 1.932-1 Long rods to help breathing with small
ports. Responds well to stroke increases (“n” value too large for intake port size)
15½° 1.871-1 Responds well to stroke increases (“n” value too large for intake port size)
16° 1.814-1 Mopar 383/400 Approximate “ideal” compromise between stress & breathing (1.81-1)
16½° 1.760-1 Chevy 327 Good choice for motors with good breathing
http://victorylibrary.com/mopar/rod-tech-c.htm
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17° 1.710-1 Mopar 360 Ford 302, 351W, 460
”Safe” limit for thrust angle. Approaching practical limit for street motors
17½° 1.663-1 Approaching practical limit for street motors
18° 1.618-1 Chevy BB 396/427
Approaching practical limit for street motors. Good power due to large intake port
18½° 1.576-1 Limited street use
19° 1.536-1 Chevy BB 454 Good power due to large intake port
19½° 1.498-1 Not practical for street use due to short pistons
20° 1.462-1 Chevy SB 400 Poor peak power. Longer rods are used in any serious application
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Non-viability of Very Long rod is leading to mediocre engine performance .!!!?!?!!
How to achieve a better performance?
How to promote low quality renewable fuels?
How to achieve better Piston Motion at both BDC & TDC ????!!??!
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Dwell Piston Mechanism
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Dwell Piston Mechanism
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Dwell Piston Mechanism
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Piston path for Dwell four stroke cycle
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Comparison of standard and Dwell four stroke Cycles
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Longer Dwell at TDC
• The benefits of a longer dwell at TDC are myriad.
• When the piston is stationary, the combustion region stays small longer in the crankshaft's arc of rotation, containing the flame as it tries to expand.
• Meanwhile, the cylinder pressure is rising, because the mechanically locked piston cannot move until the angle of the crankshaft allows it to.
• During this time, not only will the cylinder pressure rise, but by keeping the combustion region small longer in the crankshaft's rotation, the odds of abnormal combustion occurring are reduced.
• The small, tight and confined flame does not allow as readily for a rogue ignition event to occur, quenching the possibility of detonation.
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• Abnormal combustion can be qualified as either pre-ignition or post-ignition and is identified as anything other than the normal singular flame front initiated by the arcing of the spark plug.
• The longer the piston dwell period at TDC, the more powerful and octane-tolerant the engine becomes.
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Longer Dwell at TDC
• Holding the piston at maximum volume (BDC piston dwell) provides additional time for the cylinder to fully charge before closing the intake valves.
• At the end of power stroke longer dwell at BDC will reduce the pressure during exhaust stroke and minimize pumping losses.
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Instantaneous Velocity of Dwell Piston
d
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dSS p
60
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PV diagram due piston dwell in TDC and BDC
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Comparison of standard and Dwell four strokecycles in PV diagram