Rod ratio and dwell time... doesn't make sense

[Frankshaft]

And what do we learn from all that?

[nickpohlaandp]

And what do we learn from all that?

That I'm bookmarking that page and my initial "basic" thought of a sine wave graph for piston speed for a particular rod ratio was WAY off

[Circlotron]

And what do we learn from all that?

The simpler something appears to be, the greater the complexity that is just waiting to bite you!

[n2xlr8n]

And what do we learn from all that?

That not everyone has it all figured out, and like most good students, not afraid to ask.

[user-23911]

The example used is just an example and it's totally unrealistic.
Just like using a connecting rod of infinite length.

Real engines have rod/ stroke ratios of 1.5 ish to 2 ish and the differences are minimal.


But some really big diesel engines to it a bit different, have a 2 piece conrod to remove side loadings on the piston.
If it wasn't important, they wouldn't do it.

[David Redszus]

I ran the equations for a short rod (1.5) ratio and a long rod (2.0) ratio and began to write a description of the piston position, velocity, and acceleration curves. I decided that an explanation would be much too long and probably confusing.

So..for anyone who would like to resolve this issue once and for all, I am offering an excel spreadsheet that will allow a visual and quantitative comparison of various rod ratios. Send me your email address and I will send you the file.

The graphs of the results may then be posted for all to see.

One picture is worth a thousand words of bafflegab.

[Erland Cox]

It is all in the link I sent, calculations and graphic illustrations.

Erland

[David Redszus]

Here are a few rod ratio facts.

Rod ratio has no effect on piston position at TDC or BDC. It does affect position at 90 and 270 deg.

It has no effect on velocity at TDC or BDC. It does effect velocity at max piston speed (approx 76 deg ATDC).

At TDC, the piston acceleration curves are shaped like a sine wave. At BDC, they are not.
There is no piston dwell at TDC but there is dwell at BDC. Rod length cannot affect TDC air flow or combustion.

As the rod length increases, the differences in position, velocity and acceleration converge and have less effect.
If the rod length is long enough (not possible in a real engine) the acceleration curve becomes a sine wave at both TDC and BDC.

Rod ratio should best be ignored and forgotten, never to be revisited again.

[Walter R. Malik]

Here are a few rod ratio facts.

Rod ratio has no effect on piston position at TDC or BDC. It does affect position at 90 and 270 deg.

It has no effect on velocity at TDC or BDC. It does effect velocity at max piston speed (approx 76 deg ATDC).

At TDC, the piston acceleration curves are shaped like a sine wave. At BDC, they are not.
There is no piston dwell at TDC but there is dwell at BDC. Rod length cannot affect TDC air flow or combustion.

As the rod length increases, the differences in position, velocity and acceleration converge and have less effect.
If the rod length is long enough (not possible in a real engine) the acceleration curve becomes a sine wave at both TDC and BDC.

Rod ratio should best be ignored and forgotten, never to be revisited again.

I am just curious as to why no one seems to address any reasoning for extended time the piston spends near the bottom of the bore when the intake valve is always more open and flowing near maximum mixture velocity.

[In-Tech]

Here are a few rod ratio facts.

Rod ratio has no effect on piston position at TDC or BDC. It does affect position at 90 and 270 deg.

It has no effect on velocity at TDC or BDC. It does effect velocity at max piston speed (approx 76 deg ATDC).

At TDC, the piston acceleration curves are shaped like a sine wave. At BDC, they are not.
There is no piston dwell at TDC but there is dwell at BDC. Rod length cannot affect TDC air flow or combustion.

As the rod length increases, the differences in position, velocity and acceleration converge and have less effect.
If the rod length is long enough (not possible in a real engine) the acceleration curve becomes a sine wave at both TDC and BDC.

Rod ratio should best be ignored and forgotten, never to be revisited again.

I am just curious as to why no one seems to address any reasoning for extended time the piston spends near the bottom of the bore when the intake valve is always more open and flowing near maximum mixture velocity.

Math it out and you will see it takes a tremendous change in rod length to add even a degree of dwell, top or bottom.

[Tuner]

Look at it from the point of view of how much the combustion chamber volume changes with longer or shorter rod in the top 10 or 20 degrees, + or - 5 or 10 deg. either side of TDC.

[statsystems]

Look at it from the point of view of how much the combustion chamber volume changes with longer or shorter rod in the top 10 or 20 degrees, + or - 5 or 10 deg. either side of TDC.

That's why with a higher Rod ratio you can use less total timing and less quench before you get in trouble. The combustion chamber is smaller, longer.

Just what I've found. YMMV?

[Stan Weiss]

Moved about a degree on just one side of BDC

Stan

Code: Select all

Pontiac 400 ci

Bore_=_4.12___Stroke_=_3.75___Rod_Length_=_6.625
Crankshaft_Degrees_at_which_Rod_and_Crank_are_90_Degrees_74.1975

Crank______Piston_____Crank_______Rod____Cylinder___Cylinder
Angle______Travel______Rod_______Bore_____Volume_____Volume
Degree_____Inches_____Angle______Angle______CI_________cc
-ATDC_
160.0000__3.668034___14.44518___5.55482__48.90100__801.34378
161.0000__3.676031___13.71316___5.28684__49.00760__803.09073
162.0000__3.683616___12.98264___5.01736__49.10873__804.74791
163.0000__3.690791___12.25354___4.74646__49.20438__806.31536
164.0000__3.697555___11.52578___4.47422__49.29456__807.79307
165.0000__3.703909___10.79928___4.20072__49.37926__809.18109
166.0000__3.709851___10.07397___3.92603__49.45849__810.47942
167.0000__3.715384____9.34977___3.65023__49.53225__811.68808
168.0000__3.720506____8.62660___3.37340__49.60053__812.80711
169.0000__3.725218____7.90438___3.09562__49.66335__813.83653
170.0000__3.729520____7.18302___2.81698__49.72070__814.77634
171.0000__3.733412____6.46246___2.53754__49.77259__815.62659
172.0000__3.736894____5.74261___2.25739__49.81901__816.38727
173.0000__3.739966____5.02340___1.97660__49.85996__817.05842
174.0000__3.742628____4.30474___1.69526__49.89546__817.64005
175.0000__3.744881____3.58656___1.41344__49.92549__818.13217
176.0000__3.746724____2.86877___1.13123__49.95006__818.53479
177.0000__3.748157____2.15130___0.84870__49.96917__818.84793
178.0000__3.749181____1.43407___0.56593__49.98282__819.07160
179.0000__3.749795____0.71699___0.28301__49.99100__819.20580
180.0000__3.750000____0.00000___0.00000__49.99373__819.25053

SBC 400 ci

Bore_=_4.126___Stroke_=_3.75___Rod_Length_=_5.565___RPM_=_6500
Crankshaft_Degrees_at_which_Rod_and_Crank_are_90_Degrees_71.3799

Crank______Piston_____Crank_______Rod____Cylinder___Cylinder
Angle______Travel______Rod_______Bore_____Volume_____Volume
Degree_____Inches_____Angle______Angle______CI_________cc
-ATDC_
160.0000__3.673997___13.38277___6.61723__49.12325__804.98587
161.0000__3.681429___12.70239___6.29761__49.22262__806.61431
162.0000__3.688476___12.02375___5.97625__49.31685__808.15830
163.0000__3.695138___11.34674___5.65326__49.40592__809.61800
164.0000__3.701416___10.67128___5.32872__49.48986__810.99353
165.0000__3.707311____9.99727___5.00273__49.56867__812.28502
166.0000__3.712822____9.32463___4.67537__49.64236__813.49259
167.0000__3.717951____8.65326___4.34674__49.71094__814.61634
168.0000__3.722698____7.98308___4.01692__49.77441__815.65641
169.0000__3.727063____7.31398___3.68602__49.83277__816.61287
170.0000__3.731047____6.64589___3.35411__49.88605__817.48584
171.0000__3.734651____5.97871___3.02129__49.93423__818.27539
172.0000__3.737874____5.31235___2.68765__49.97732__818.98161
173.0000__3.740717____4.64671___2.35329__50.01534__819.60457
174.0000__3.743181____3.98171___2.01829__50.04828__820.14434
175.0000__3.745265____3.31726___1.68274__50.07614__820.60097
176.0000__3.746970____2.65326___1.34674__50.09894__820.97451
177.0000__3.748296____1.98963___1.01037__50.11667__821.26500
178.0000__3.749243____1.32627___0.67373__50.12933__821.47247
179.0000__3.749811____0.66309___0.33691__50.13692__821.59695
180.0000__3.750000____0.00000___0.00000__50.13945__821.63844

[GerryP]

This guy does a good job of white board explanations for those of us without engineering degrees: https://www.youtube.com/watch?v=gdHQ8aTfiQQ

Yes, it's about balance but it covers the topic.

[Belgian1979]

All these comments make me wonder why usually a change in rod, requires a change in ignition timing.