MIND BOGGLING ENGINE GEOMETRY - Rod Ratio Explained

Here we have two engines. Both have the same bore and the same stroke. As you can see the only difference is the length of their connecting rods. At the same engine speed so at the same speed of rotation, the same rpm. Which engine has a faster accelerating piston?

As you can see the question I asked you was a trick question because the piston in the short rod engine accelerates faster from top dead center going down while the long rod piston accelerates faster from bottom dead center going up. So why does this happen if both engines have the same bore, same stroke and are obviously spinning at the same rpm. Well the culprit behind is obvious. It's the connecting rods, as they're the only thing different between the two engines. And this video I promise to strain your mind to the redline by explaining how something as simple as different connecting rod lengths create different piston acceleration and then using real life engine examples we will see how this impacts everything from power and torque to engine longevity, responsiveness, vibrations and even things like coolant temperatures.

So these two engines have different rod lengths, this means that they have different rod ratios. The full name is actually rod to stroke ratio. And it's the ratio of the center to center length of your connecting rod to the length of your engine's stroke which is determined by your crankshaft.

A connecting rod is essentially a fixed length line. It's absolute length obviously never changes. But the relative length of the connecting rod is constantly changing when the engine is running. In other words the connect rod length changes in relation to the piston and the crankshaft as the engine is running.

At top dead center and as you can see the connecting rod is fully upright. In this state it's at its maximum length in relation to the piston and crankshaft. Now as the engine rotates towards 90 degrees the connecting rod assumes it's fully angled position. In this position it is obviously at it's shortest in relation to the piston and the crankshaft. As we said an angled line has a shorter relative length than that same line when fully vertical.
So as the engine rotates from 0 to 90 degrees the connecting rod is becoming shorter in relation to the piston and the crankshaft. As it does so it pulls down the piston an additional distance. The piston is already traveling downward so adding distance in the same direction forces the piston to accelerate more to cover that added distance.

In fact we can observe this con-rod added distance in practice on every single piston engine ever made. Simply take any engine and rotate it to 90 degrees, or to half the stroke. Obviously at half the stroke the piston should also cover half the stroke distance? But it never does, at 90 degrees of rotation the piston of every engine will have traveled beyond half the stroke. This additional distance is the distance added by the connecting rod as it's relative length shortens.

So why does the piston in the short rod engine accelerate more? The reason is simple and it's that a shorter rod length in relation to the same stroke results in the connecting rod assuming a steeper angle against the piston and crankshaft centerline. The steeper the angle the shorter the rod becomes in relation to the piston and crankshaft.

So now we understand why the short rod piston accelerates faster away from TDC and we can use the same principles of relative rod length to understand what happens throughout the entire engine revolution.

Now let's look at the rod ratios of some real life engine examples to see how these differences in acceleration actually impact the engine.
Our first engine is the 1.6 liter Hyundai Gamma engine as found in numerous different Hyundai and Kia vehicles. As you can see this your typical daily driver engine with a modest redline, decent power and a pretty low rod ratio.
Next up we have the 2 liter Honda K20 engine. We're looking at the performance version of this engine and as you can see it makes quite a bit more power than the Hyundai engine and it also has a noticeably higher redline and also a higher rod ratio.

Our final engine is the one from the 2013 to 2018 Kawasaki ZX6R. As you can see it makes impressive power for it's very small displacement and has a redline that's almost twice that of the Honda K20. It also has by far the highest rod ratio. Somewhere around 2.2 or 2.3 is the highest realistic rod ratio for mass produced engines. Some of the highest rod ratios were found in Formula 1 cars at about 2.8

A special thank you to my patrons:
Daniel
Daniel Morgan
Pepe
Brian Alvarez
Jack H
Dave Westwood
Joe C
Zwoa Meda Beda

00:00 Piston acceleration in detail
09:40 Rod ratios of real engines

#d4a #rodratio

[ianphillips]

I have been an engineer all my life, (I'm now 57) I have to say that you explanation of what is essentially dynamic geometry, is the most straightforward and understandable that I have seen. Great video, thanks for helping others understand this fascinating field of engineering.

[andreibavaria]

4 years of automotive engineering and never seen someone explain this subject so well

[savageking]

been a racing mechanic for 5 years (im 26) and I’ve never had anyone have such a good explanation I loved this

[vernontaylor]

Almost 50 years as a mechanic and this is the best presentation I have seen - going back to my college days I remember the instructor telling us the wild angle of the short connecting rod created greater side thrust on the piston/cylinder wall; so far so obvious, but surprisingly he told us that was a good thing and set about constructing a triangle of forces to prove how the equivalent of that side thrust adds to the force of combustion and causes the engine to produce more torque. It was a long time ago and I don't remember the finer details, in fact I think I did well remembering that much, considering I have never needed to use that calculation in my entire career...

[pedersterll]

I'm not exactly an ignorant on IC engine internals, but every time I watch one of your excellent videos, I learn a lot of new stuff. What a PLEASURE !

[haze]

17 minutes and I now understand something I had no idea understanding of before, even having an above-average knowledge of how ICE's work (not saying much). This was so great. Nice job!

[insidelinemx]

Been a motorcycle mechanic and amateur motocross racer for 30 years.
I absolutely love your content. Fabulous explanations and animations. I use your content to help other techs all the time! Thank you for all the hard work you put in!

[Geijinsmash]

As someone who grew up working on cars and even went on to being an engine builder in two countries before changing careers, you taught me something today. Thank you for this 🙏🏽

[rotaxnut]

After 40 years of tuning Ford engines for club racing in the UK I can say that increasing the rod ratio gave race winning improvements to the engines power, to maximise this effect I also shortened the stroke and increased the bore diameter to restore the capacity allowed in the race class and had some great results. "Sausage" HT Racing Ltd

[electric]

Using a longer rod length not only helps to reduce secondary vibrations, but also helps to reduce higher-order oscillations that can be difficult to suppress.

Most people know that, in any piston engine, the piston's secondary motion can be broken down into a primary component (one that varies at the same speed as the crankshaft) and secondary component (varying at twice the engine speed). However, what most people don't realise is that the secondary vibration itself is not perfectly sinusoidal and will have _harmonics_ (whole number multiples of the fundamental frequency).

The secondary motion becomes more "pointed" at mid-stroke (less sinusoidal) with lower rod ratios, and as the secondary motion gets more and more "pointed" both the secondary oscillation itself and its harmonics (4th order, 6th order, _et cetera_ oscillations) increase in amplitude.


The peak-to-peak amplitudes of the secondary oscillation's fundamental and harmonics can be calculated by:

A = 2 / π * ∫(cos(nθ) * √(L^2 – (S sin(θ) / 2)^2) dθ, –π to π)

Where _n_ is a non-zero even number, L is the rod's centre-to-centre distance, and S is the stroke length.

This effect is most prevalent on large two-stroke low-speed marine Diesel engines, where the rod ratio is often around 1 or less than 1. -(to the point where they need crossheads to take up the lateral thrust).- The reason is that these engines have extremely long strokes, so by using crossheads and short rods they can make the engine shorter in height.

Most of these engines also lack balance shafts, so they transmit almost all of their external forces and external rocking moments to the ship (for a marine application) or foundation (for stationary land-based power generation). This problem is aggravated by the fact that the components are all very heavy, which makes the forces even greater.

Take a look at MAN Diesel's project guides for their two-stroke engines (K-model, L-model, S-model, and G-model engine families, though as of 2022 they don't make K or L engines anymore), and on the section where they list the firing orders of each of the variants they will also list not only the first- and second-order vibrations, but also the second- and third-order harmonics of the secondary vibrations (fourth- and sixth-order vibrations). For example, although the 12G90ME has perfect primary balance, they do _not_ have perfect secondary balance. Although there is no second-order rocking moment, the second harmonic of the secondary vibration is still significant and produces a _fourth-order_ rocking moment of 724 kN•m (534000 lb-ft) at 84 RPM (which is conveniently twice that of the 6G90ME's fourth-order rocking moment of 362 kN•m).

The greatest offenders are the secondary rocking moments on five- and six-cylinder engines, which is why they sometimes _are_ fitted with balance shafts.

Large six-cylinder engines also have a sixth-order vertical shake. The heavy components and short rods already make fourth- and sixth-order oscillations a problem, and the 60° spacing between crank throws doesn't help either, as the sixth-order forces (the secondary vibration's third harmonic) for each cylinder all point in the same direction. The sixth-order vertical shake on the 6G90ME has a magnitude of about 32 kN (7200 lbf) at 84 RPM. The same is true for four-cylinder engines, as regardless of the firing order the secondary vibrations' second harmonics all point in the same direction, resulting in a fourth-order vertical shake.

You'll also notice that the 10- and 11-cylinder engines have non-zero net external forces. This makes me suspect that the 10- and 11-cylinder engines are odd-firing, because if they were even-firing the net forces should all be zero.

The reason you only have to deal with primary and secondary vibrations on most engines you'll come across is because their rod ratios are relatively high and their components are relatively light. So not only are the secondary vibrations smaller, they are also much closer to being sinusoidal, which means the higher-order harmonics of the secondary vibrations are negligible.

[dcshores]

I learned more from the 20 minutes of this video that 4 years plus of study! Not only a very professional graphic presentation that displayed it all so accurately but you were able to explain it all and speak in a manner that us lowly wannabes could easily understand. Thank you!

[madsciencegary]

I don't care about how engines work really, yet I find his explanation so clear and compelling that I not only watched the whole thing, I was actually interested.

[superdesultory]

This was awesome dude! I really liked your animations. I don't think people gave high school geometry class enough respect - little changes in engine geometry seem to be what make the difference between average engines and great engines. Before your channel i hadn't really learned much about engines (even with a mech. Engineering degree lol), but you have some really great info

[jaredkennedy]

I used to put a lot of effort into building performance Ford 300-6 engines. My pinnacle build was going to be a twin turbo with decently high compression, and one of the things I played around with was rod ratio. Luckily, I could mix and match off the shelf parts to get a decent boost here. Rods from the 240-6 were more than 1/2" longer, 6.2 vs 6.8". Combined with pistons intended for a stroker 347 V8, they fit and gave a compression ratio of about 9.2:1, with the head I was planning on using. Unfortunately, I got married, moved a bunch of times, and the project never got beyond the parts collection stage. I did later build a high compression torque monster 300 for my van, but that's a whole other story.

[Chitus]

You literally make things easy to understand, you’ve got a gift, thank you for sharing it with us

[KingdomAuto]

Huge respect for the time and effort you put into your content! Keep up the great work!

[robertlhommedieu]

I have an above average understanding of engine dynamics, but every time I watch one of your videos, I learn some nuance that I didn't understand before. Congratulations on the very good videos you produce. Keep up the good work! I hope others appreciate your expertise.

[luukscholten]

I just found this channel, and I absolutely love this guy. While I'm not an engineer, listening to your videos is awesome and the explanations are extremely well made!

[stephencummins]

Brilliant explanation, although simple in principle, the comparative dynamics become complicated and this guy takes us through the whole process so coherently, we’ll done, thank you man.

[johngoode]

Was aware about the affect of rod length on piston acceleration after studying Kliens construction in my engineering course and playing with high performance 2 stroke engines all my life but the way this guy explains it with the graphics etc is fantastic...great work mate.