Reflecting on symmetry
In researching a recent article on the mirror-swirl intake on the EcoBlue Ford diesel engine, I came across this patent by Ford. The logic of this patent has a lot in common with the EcoBlue mirror-image inlet manifold.
Not as symmetrical as I imagined
When thinking of a V8 engine, in my imagination the two cylinder banks were precise mirrors of one another. It turns out in most V-engines, the heads on both banks are the same part, not mirror images of one another. The reason given for this is that it eases head assembly – all the heads with their complicated machining and valve trains can be made the same way.
The consequences of this are interesting. Take a simple two valve head on one bank of a V8 with the intakes on the V-side and the exhaust manifolds on the outside. The valves run along the length of the head, two per cylinder, in sequence: exhaust-intake, exhaust-intake, exhaust-intake, exhaust-intake. Take this exact same head, flip it around on the other bank so that the intake is on V-side and the exhaust is on the outside. Now the valves go intake-exhaust, intake-exhaust, intake-exhaust, intake-exhaust.
The end result is that the intake for one bank needs to be longer than the other, and the exhaust on one bank needs to be longer than the other. This is a nightmare for fine tuning the engine.
Disadvantages of asymmetry
The Ford patent describes the disadvantages of cylinder head asymmetry from the perspective of exhaust manifold design:
When the exhaust is inboard, it is important to ensure that the exhaust is quickly conducted away from the valley to reduce heat transfer from the exhaust runners to various components in the valley. Also, it is important that the exhaust gas entering the turbocharger, in engines so equipped, and aftertreatment devices is as warm as possible to allow fast warmup of the aftertreatment devices and to reduce turbocharger lag. Although suitable for many applications, one disadvantage of using such a single cylinder head design having a first end of the first cylinder head on the front of one cylinder bank and the first end of the second cylinder head on the back of the other cylinder bank is that the exhaust valves of one cylinder bank are displaced toward the front of the engine relative to the other, where the exhaust valves are displaced toward the rear of the engine. As a result, the exhaust manifold must conduct the exhaust gases further to the exhaust system for one of the cylinder banks.
Frankly, I am surprised that mirror image heads weren’t already a thing, but Googling around I found most comments were that the small block Chevy uses interchangeable heads (like the one pictured), i.e. one is just bolted down “backwards” so that the intakes are both on the V-side, meaning the valve configuration, combustion chambers, etc are all asymmetrical.
Why?
How did this come to be? I have a theory.
Original V-engines like the small block Chevy were all pushrod, i.e. had a single camshaft running in the V. Since all the pushrods occupied a different lobe on the camshaft, it was inevitable that intake and exhaust valves would be staggered from one bank to another – asymmetry on the axis of the camshaft was unavoidable.
The move to overhead camshafts, where each bank of the V-engine had its own cam (or two) meant that the asymmetry was no longer forced by the shared camshaft. But by then, the die had been cast (ahem) and manufacturers were accustomed to using a single part for both the left and right cylinder heads, and the asymmetry continued.
Why is this exciting?
Engine development can be a multi-billion dollar endeavour. One would think if you could cut that development cost by making the left and right banks of a V-engine identical you would. But the patent for this idea only came out in 2009!
I have items of clothing older than this patent. If you sometimes despair that all the good ideas are taken, rest assured.
We live in a manufactured landscape. The chair you are sitting in, the device you read this on, the coffee cup you are drinking from, are all the embodiment of generations of inventive thinking, to which we can all contribute.
Your turn
Does playing with symmetry give you any ideas for inventive problem solving? Principle 4 – Asymmetry
Do you know of an engine with mirrored cylinder heads in a daily driver?
