Lego straight 4 engines (2nd generation cylinder engines) . It started a couple of years ago: the creation of a new Lego cylinder engine. I originally wanted to use it as a replacement for the regular Lego cylinder engine, because I saw the whole thing became too big for the models that I wanted to build. That's why I came up with the idea to rebuild the cylinder engine in a size as compact as possible, so it could fit even in a Lego motorbike scale model. . Normally, lots of Lego Technic cars are equipped with V8 engines and motorbikes with V2 or V4 engines, but I wanted to bring a more compact inline cylinder engine. I developed a mk1 straight five engine as a model of my own, as a goal to make it smaller in comparison to any regular Lego cylinder engine, but somehow it turned out to be relatively larger. I kept the basic idea and kept developing and looking for other ways to make it smaller. While doing that I payed attention to working as compact and detailed as I could. I think it worked out and the resulted greatly.
In my quest to make the inline engine smaller, I skipped a step and instead of a five cylinder engine I created a mk2 inline four engine, trying to make it smaller. I got rid of spring feathers for saving space and switched over to Lego rubber bands. The motion accuracy was relatively good, but due to the stress on the rubber band it felt a little too sturdy. I also had to put the parts together from both ends by placing the L-shaped gray parts.
The mk3 straight engine happened to be in fact, too straight and too stiff in performance. I reduced the width by shrinking it by a Lego brick size. By doing that I made myself position the cylinders in such a way, that the crankshaft had to move all the cylinders in one round. That resulted in an way too heavy, almost sticky, tend to declining rotation and above that the engine didn't sound realistic. To make the cylinder engine both move and sound with a some ruff in it, at least one or two cylinders have to be placed in opposite position to the remaining others. That's what I learned from inline engine mk3.
Engine mk3 didn't discourage me to decrease the size further. From this point I saw parts that were actually excess and too thick, taking in a lot of space for no purpose. From the beginning of building mk4 I scrapped lots of parts and tried to dress the inner moving parts as thin as possible. Mk4 was one of the most supple rotating inline engines that I created. It also moved a tiny bit more supple than the mk6, the most functional inline engine I built so far.
Mk5 almost had the same capabilities of mk4, but I flattened it out on the height by the bringing it down by one Lego brick size. Although it was more compact than mk4, the crackshaft of mk5 moved a little less supple. The tensity of the rubber bands turned up, because I had to move the part with the rounded axle pin to another position. The red rubber band has a bigger diameter than the white rubber band, but the red rubber band wasn't tight enough to pull the outside and the top of the engine together.
First, I thought mk5 was all I could do, but then I realised that even mk5, in all it's compact build, still had too much spare parts to make it worthy as the most compact straight four engine. I cut out lots of excess parts of the assembly and simply trimmed it down even more than mk5. The result of mk6 was amazing. It had a super downsized appearance and it had the excellent rotating flexibility of the mk4 engine. One more amazing fact is that the largest rubber bands could only be placed on the most compact of the straight engines.
I made some pictures of the original Lego cylinder engine next to the mk6 inline engine to compare them both in size and techniques. As you can see, they share almost the same height, but my custom created cylinder engine is three brick lengths more compact.