Quote:
Originally Posted by JohnAh
A few days I got an idea on how to improve engine balance when converting från a straight-four to paralell twin: Use a 360 degree configuration and leave the unemployed pistons in place! To reduce friction losses the pistons should be modified by cutting holes in the piston crown and removing the rings. The pumping losses are at a maximum about "halfway" between full throttle with normal valve lift and no valve lift at all. When introducing holes in the piston crown this will also introduce internal pumping losses! The holes must therefore be as large as possible. Perhaps the piston skirt should also be modified to reduce friction even more.
I have seen single cylinder motorcycle engines with a balancing device working as at counterweight to the piston, reducing the vibrations in the same way as a "perfectly symmetric" 2-cyl boxer would do. My passive modified pistons should act in a similar way. The big challenge may be how to cut the pistons for minimum pumping losses and friction while keeping the maximum weight.
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Rather than cutting holes in pistons, and still having that unwanted friction from the rings, you could just throw away the piston and rod assembly, and add weight to the crank throw. A simple bolt together steel 'split ring' that weighs the same as a piston and con rod assembly would do it.
However, it will still run rough. It's the unbalanced 'power pulses' that will be responsible for that. Lack of a power stroke from the dead cylinders. But if you deactivate 2 cylinders from a 4 cylinder engine, such that a cylinder fires every 360 degrees, it should run reasonably smooth at higher rpms. Idling, it will still vibrate like a tractor.
American car companies had been trying for years to smooth out 4 cylinder engines. Compared to a V8, most customers had historically disliked the 'feel' you experience from fewer pulses per revolution of a 4 cylinder car. Internal engine-smoothing 'balance shafts' were popular for a while, but they are not good for FE or power. Most modern 4 cylinder engines balance shafts have been replaced with these engineered HUGE vibration damper systems, formerly referred to simply as 'motor mounts'. My guess is the engine tuned mass dampers and vibration isolation systems on modern cars are mostly responsible for the smoother feel you get from a modern 4 cylinder car.
Remove 2 power pulses per revolution, and that 'system' might need some rethinking to keep the car from shaking like a Harley Davidson motorcycle
