To be honest, the standard Lego Technic cylinder engine model is a bit too big for my building scale. Therefor I built my self designed cylinder models. These are compact enough to even fit them into a Lego motorbike scale model.
About this creation
A lot of Lego Technic cars are equipped with V8 engines and motorbikes with V2 or V4 engines. I developed a mk1 straight five engine to make it a more compact. It turned out to be actually larger and from that point I kept downsizing my model. I payed attention to the details and worked as compact as I could. I think it worked out pretty well.
Instead of developing the five cylinder engine further, I created a smaller mk2 inline four engine. I replaced the spring feathers by Lego rubber bands to save space. Due to a lot of stress on the rubber band it felt too sturdy when in motion. The L-shaped gray parts hold the rubber bands and the upper layer of the engine together.
The mk3 straight engine was too straight and too stiff in performance. I reduced the width by shrinking it by a Lego brick size to position the cylinders in such a way, that the crankshaft could rotate all the cylinders in 360 degrees. That resulted in a way too heavy, almost sticky and tend to declining rotation. Above that the engine didn't sound realistic, but forced. To make the cylinder engine both move and sound with a subtle ruff in it, at least one or two cylinders have to be placed in opposite position. That's what I learned from inline engine mk3.
Engine mk3 didn't discourage me to downsize further. From this point I saw more excess and too thick parts then I saw on mk1 and mk2. I scrapped lots of parts and mk4 became one of the most supple rotating inline engines that I created so far. I must admit that mk4 also rotated a bit more supple than the mk6.
Inline cylinder engine mk5 has almost the same capabilities of mk4, but I flattened it out on the height by one brick size. The flattening causes a bit more tensity in the rubber bands. Although mk5 is more compact than mk4, the crankshaft of mk5 rotates a little less supple. The red rubber band was too big in diameter, so it couldn't hold together the top of the engine. Instead of that I used the white one.
First, I thought mk5 was the best I could create, but then I realised that it still has too much parts. I rebuilt, then downsized it. Compared to mk5, mk6 has a super downsized appearance and it has the excellent rotating qualities of the mk4 engine.
I took some pictures of the original Lego Technic cylinder and compared it to my self-assembled inline engines. As you can see, they share almost the same height, but my most downsized cylinder engine mk6 is three brick lengths more compact.