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The Crank and the Flywheel: Smoothing One Bang in Two Turns
A four-stroke single-cylinder engine pushes on its crank for half a turn in every two. The rest of the time the crank must be carried round — through exhaust, intake and compression — by energy stored in a flywheel. How heavy that flywheel must be depends on how uneven the torque is and how steady the speed must stay.
This practice rung generates the torque curve of an idealised engine from the slider-crank geometry and an Otto-cycle pressure trace, sizes a flywheel from it, and builds a slider-crank model to check the piston motion by hand.
中级
About 3 hours
说明
1
1
Torque round a cycle, and the flywheel it needs
Torque round a cycle, and the flywheel it needs
正在加载 Jupyter 笔记本…
2
2
Build a slider-crank and check it by hand
Build a slider-crank and check it by hand
Cut a plywood base. Pivot a plywood crank disc on a bolt, with a crank pin 40 mm off centre. Cut a connecting rod 140 mm between hole centres, and a slider that runs in a slot or between two strips — the 'piston'. Glue a protractor under the crank disc.
Turn the crank in 30° steps and measure the piston's distance from the crank centre with the steel rule. Compare with the notebook's formula. Notice that the piston moves further during the half-turn near top dead centre than near bottom — the connecting rod's angle causes it, and it is why the torque trace is lopsided.
此步骤所需材料：
胶合板1 张
机螺钉6 个
硬木销钉1 套所需工具：
充电式电钻
钻头套装
量角器
直尺
6 英寸数显卡尺3
3
Context
Context
A practice rung: the slider-crank goes back to the rotative steam engine in this catalogue, and the flywheel to the potter's wheel and the spindle whorl. The torque trace is generated from an ideal cycle; a real engine's is less peaky because combustion takes time.
**Honest limits of the model.** No friction, no inertia of the piston itself (which matters at high speed), heat added instantly at top dead centre, and a single cylinder. Multi-cylinder engines are smoothed by overlapping power strokes; balance shafts deal with the shaking the piston's own motion causes.
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