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 …..

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.

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.

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

Creation of Constant Volume Combustion Engine

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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.

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.

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.

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.

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.

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

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

Non-viability of Very Long rod is leading to mediocre engine performance .!!!?!?!!

How to achieve a better performance?

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How to achieve better Piston Motion at both BDC & TDC ????!!??!

Dwell Piston Mechanism

Dwell Piston Mechanism

Dwell Piston Mechanism

Piston path for Dwell four stroke cycle

Comparison of standard and Dwell four stroke Cycles

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.

• 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.

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.

Instantaneous Velocity of Dwell Piston

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PV diagram due piston dwell in TDC and BDC

Comparison of standard and Dwell four strokecycles in PV diagram