发动机为什么要转两圈才做一次功？Why does an engine turn twice for every power stroke?
左边是一台单缸四冲程汽油机的剖面：前半个缸体和缸盖被切掉了，露出活塞、连杆、曲轴、两个气门、火花塞和凸轮轴。前面的虚线是正时皮带。右边的圆盘把一个完整循环画成一圈，下面的图记录缸内压力和容积。拖动曲轴（或用滑块），自己找出这台机器的节奏。
On the left is a cutaway of a single-cylinder four-stroke petrol engine: the front half of the block and head is cut away to show the piston, connecting rod, crankshaft, two valves, spark plug and camshafts. The dashed loop in front is the timing belt. On the right, the dial draws one full cycle as one lap, and the chart below records cylinder pressure against volume. Drag the crank (or use the slider) and find the machine’s rhythm yourself.
01预测Predict
02转动曲轴Turn the crank
从上止点（0°）开始，一点一点往前转。盯住两个气门和火花塞：它们在哪一段动？右边圆盘的黑色指针走一圈，是一个完整循环；橙色短指针是曲轴本身。
Start at top dead centre (0°) and turn forward a little at a time. Watch the two valves and the spark plug: in which part of the turn does each act? The black hand on the dial makes one lap per complete cycle; the short orange hand is the crankshaft itself.
- 找到两个气门都关着、活塞往上走的一段Find the stretch where both valves are shut and the piston rises
- 看到火花塞点火See the spark plug fire
- 走完四个冲程：进气、压缩、做功、排气Go through all four strokes: intake, compression, power, exhaust
03看正时记号Watch the timing marks
回到 0°，三个皮带轮上的橙色记号都朝上。按一次“+360°”：曲轴转了整整一圈，它的记号回到了顶上。两个凸轮的记号在哪里？要再转多少，三个记号才会同时朝上？
Go back to 0°: the orange marks on all three belt wheels point up. Press “+360°” once. The crank has made one full turn and its mark is back at the top. Where are the marks on the two cams? How much further before all three point up together?
04解释Explain
按顺序看图上的六处，每一步只亮起一个部分。Look at six places in order; each step lights up one part.
- 曲轴转一圈，活塞下去一次、上来一次：一圈只有两个冲程。One crank turn takes the piston down once and up once: a turn holds only two strokes.
- 进气门只在第一个冲程打开，排气门只在第四个冲程打开。第二、三冲程两个都关着。The intake valve opens only in the first stroke, the exhaust valve only in the fourth. In strokes two and three both stay shut.
- 火花塞只在压缩冲程的末尾点一次火，燃气把活塞推下去：这是唯一出力的冲程。The plug fires once, at the end of compression, and the burning gas drives the piston down: the only stroke that delivers work.
- 凸轮轮的直径是曲轴轮的两倍，所以凸轮只转半速：曲轴两圈，每个气门才开一次。The cam wheels are twice the crank wheel’s diameter, so the cams turn at half speed: each valve opens once per two crank turns.
- 四个冲程 × 180° = 720°。圆盘上的长指针走一圈，曲轴短指针走了两圈。Four strokes × 180° = 720°. While the long hand on the dial laps once, the short crank hand laps twice.
- 压力–容积图：做功冲程的曲线高、压缩冲程的低，两条之间围出的面积就是一个循环的净功。The pressure–volume chart: the power curve runs high, the compression curve low, and the area between them is the net work of one cycle.
1. 一圈只够两个冲程
活塞通过连杆挂在曲轴的偏心销上，所以曲轴每转一圈，活塞只能从上止点下到下止点、再回来。四个冲程（进气、压缩、做功、排气）各占半圈，合起来是 720°，也就是两圈。
2. 为什么不能每圈都点火
活塞往上走有两种用途：压缩新鲜混合气，或者把废气推出去。往下走也有两种：吸进新气，或者被燃气推着做功。几何上两圈一模一样，区别只在气门开没开、火花塞点没点。如果每圈都点火，排气冲程末尾缸里只有废气，点了也烧不起来。
3. 凸轮轴为什么是半速
每个气门在一个循环里只开一次，而一个循环是曲轴两圈，所以控制气门的凸轮必须每两圈转一圈。正时皮带上，凸轮轮的齿数是曲轴轮的两倍，就做到了这一点。修车时“对正时记号”也是因为这个：曲轴转一圈记号就回来了，凸轮要两圈。
4. 功从哪里来
同一个容积下，做功冲程的压力远高于压缩冲程（看压力–容积图）。燃气推活塞做的功，比压缩时花掉的多，差值就是一个循环的净功。进气和排气两个冲程几乎不出力，只是在换气。
1. One turn only holds two strokes
The piston hangs from an offset pin on the crankshaft through the connecting rod, so each crank turn can only carry it from top dead centre to bottom dead centre and back. The four strokes (intake, compression, power, exhaust) take half a turn each: 720° in all, two turns.
2. Why it can’t fire every turn
A rising piston can do two jobs: squeeze fresh mixture, or push exhaust out. A falling piston can also do two: draw in fresh mixture, or be driven by burning gas. The two turns are geometrically identical; what differs is whether the valves are open and whether the plug fires. Fire on every turn and the spark at the end of the exhaust stroke meets only spent gas, with nothing left to burn.
3. Why the camshaft runs at half speed
Each valve opens once per cycle, and a cycle is two crank turns, so the cams that push the valves must turn once every two crank turns. On the timing belt, each cam wheel has twice the crank wheel’s teeth, which does exactly that. It is also why mechanics “line up the timing marks”: the crank mark comes back after one turn, the cam marks only after two.
4. Where the work comes from
At any given volume the power stroke runs at far higher pressure than compression (see the pressure–volume chart). The gas pushes the piston harder on the way down than the piston pushed the gas on the way up, and the difference is the cycle’s net work. Intake and exhaust do almost no work; they only swap the gas.
ncam = ncrank / 2
power strokes per second = (rpm / 60) / 2
二冲程发动机呢？
摩托艇、链锯上的二冲程机确实每圈做一次功：它们不用气门，而是在活塞接近下止点时打开缸壁上的气口，一边排废气一边灌新气。代价是有一部分新鲜混合气会直接跑掉，所以更费油、更脏。
What about two-stroke engines?
Two-strokes on chainsaws and outboard motors really do deliver one power stroke per turn. They have no valves; ports in the cylinder wall open near bottom dead centre and swap exhaust for fresh mixture at once. The price is that some fresh mixture escapes straight out, so they burn more fuel and run dirtier.
“四冲程发动机转两圈才做一次功，因为 ___。凸轮轴要用半速，因为 ___。”“A four-stroke engine needs two turns per power stroke because ___. The camshaft runs at half speed because ___.”
迁移：多缸发动机Transfer question
05独立练习Independent practice
练习时，控件会锁住，冲程、气门和压力的读数会隐藏，圆盘上的冲程名称也会去掉。提示一题比一题少。核对之后，读数再显示出来。
During practice the controls lock, the stroke, valve and pressure readouts hide, and the stroke names leave the dial. Each case gives less help than the one before. After you check, the readouts come back.
这个模型的适用范围Where this model applies
气门正时按理想情况画：进气门正好在 0°–180° 打开，排气门正好在 540°–720°。真实发动机的气门会提前打开、推迟关闭，进排气门在上止点附近还会同时开一小段（气门重叠），点火也在上止点之前几度。
压力是理想奥托循环：进气、排气按大气压附近的常数，压缩和膨胀按多变指数 1.3，燃烧当作在上止点瞬间完成，峰值压力取压缩末压力的 3.2 倍。数值只用来比较大小。
尺寸是虚构的（缸径 84 mm、行程 72 mm、压缩比 10），不对应任何真实型号。剖面是正交投影，零件按比例；皮带画成前方的虚线，以免挡住剖面。
Valve timing is idealised: intake open from exactly 0° to 180°, exhaust from 540° to 720°. Real engines open valves early and close them late, keep both slightly open together around top dead centre (valve overlap), and fire a few degrees before top dead centre.
Pressure follows an ideal Otto cycle: intake and exhaust near atmospheric, compression and expansion with a polytropic exponent of 1.3, combustion treated as instant at top dead centre, and a peak of 3.2 times the end-of-compression pressure. The numbers are for comparison only.
Dimensions are invented (84 mm bore, 72 mm stroke, compression ratio 10) and match no real model. The cutaway is an orthographic projection with parts to scale; the belt is drawn as a dashed phantom in front so it doesn’t hide the section.
离线可用。全部控件都是原生单选框、滑块、输入框和按钮，可以用 Tab、方向键和空格操作。没有快捷键会干扰你的写作。三维零件由 Anatomy 工具包的回转体生成。Works offline. Every control is a native radio, slider, input or button, reachable with Tab, arrow keys and Space. No keyboard shortcuts override your writing. The 3D parts are turned solids generated with the Anatomy kit.
- 进气Intake
- 压缩Compression
- 做功Power
- 排气Exhaust
- 正时记号Timing mark