Here's another in my series of engines that would probably cause more trouble than they're worth.

I've often thought that inline 3 cylinder engines were a bit odd and not very smooth. Well, to make up for the lack of a 4th cylinder, you could make the middle cylinder twice as big as the outside ones (in volume, not diameter). This might smooth out some of the vibration and power delivery, or it might just magnify this problem.

Mostly, I just would like to try out this arrangement to see what would happen. I know that there have been several 3-cylinder engines in the past with odd configurations -- there was one 2-stroke motorbike engine with 2 cylinders inline and the middle one sticking out at 45 degrees -- but I'm not sure if anyone has tried an engine with different sized cylinders.
-- discontinuuity, Nov 21 2005

There have historically been some engines with two smaller cylinders and one larger ones, but those were 'five-strokes'. Basically, the two smaller cylinders work in tandem, each running a four-stroke cycle (so on every upstroke, one or other will be exhausting). Rather than having the exhaust vented, however, the exhaust drives the big cylinder. Since the combined volume of the big cylinder and the small cylinder that's exhausting into it will increase during the small cylinder's upstroke, useful work can be done.

The net effect is an engine which has less power than if all cylinders were operating in conventional fashion, but greater efficiency. Although today's higher compression ratios mean that the expansion cylinder is no longer as beneficial to efficiency as it once was, it would still offer some benefit.
-- supercat, Nov 22 2005


Hmm...That reminds me of a triple-expansion steam engine I once saw. Perhaps if you injected water into the bigger cylinder along with the hot exhaust gasses, you could get more expansion and efficiency.
-- discontinuuity, Nov 22 2005


Several motorcycles are triples. Triumph, Suzuki, others.
-- bristolz, Nov 22 2005


I don't remember which motorbike had the 3rd cylinder sticking out like that. Something old and tw-stroke.

Many small cars and motorbikes have 3 cylinder engines, but I've never heard of any with different-sized pistons.
-- discontinuuity, Nov 22 2005


Sticking out? The Triumph is made today and it's an inline triple.
-- bristolz, Nov 22 2005


There's a custom Harley-Feuling W3 engine with that extra cylinder, looks like three fingers spread. I like the efficiency take on this, though -- reminds me of the new Nissan VQ engine that doesn't need an EGR system.
-- reensure, Nov 22 2005


Not sure how relevant this is, but I understand that 3-cylinder engines are used in place of small fours to improve the cylinder surface area/volume ratio and thus reduce heat loss through the walls of the cylinder. Apologies if this is a bit tangential; feel free to delete it if you don't think it adds anything.
-- david_scothern, Nov 22 2005


Regarding the original idea, with one cylinder larger than the others, and therefore producing more power, would that not make things less smooth?

What is the reasoning behind the proposal that this would make things smoother?
-- Texticle, Nov 22 2005


Well, it wouldn't run, you see?
-- bristolz, Nov 23 2005


Yes, I'm not sure whether this would just make the engine completely worthless or not. I wanted to explore different applications of this layout. Basically, I just think it would be fun to make something so much more complex than it needs to be.
-- discontinuuity, Nov 23 2005


//Basically, I just think it would be fun to make something so much more complex than it needs to be.//

Welcome to the half-bakery [rasberry re-tart]! ;)
-- Minimal, Nov 23 2005


After looking under the hood of several cars, I believe there are a lot of automotive engineers who share [rasberry]'s mindset.

A number of different steam "compound locomotives" (q.g.) used the multiple expansion idea as you said; with an oversize third cylinder mid-frame and results ranging from very good to crazed.
-- lurch, Nov 23 2005


Honda's cvcc (compound vortex controlled combustion) engine of yesteryear used a similar principle in its bell-shaped combustion chamber. A 3 bbl carb provides a small but rich charge in the smaller top part of cyl and the other 2 barrels provided a [too lean to burn on its own] larger charge in the lower part. The stratified charge approach worked fairly well in producing a mileage champ. EFI made it obsolete, but mebbe this head needs revisited.
-- keithturtle, Feb 05 2006



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