Since the same process occurs in each cylinder, we will take a look at one cylinder to see how the four stroke process works.
The four strokes are Intake, Compression, Power and Exhaust. The piston travels down on the Intake stroke, up on the Compression stroke, down on the Power stroke and up on the Exhaust stroke.
Intake
As the piston starts down on the Intake stroke, the intake valve opens and the fuel-air mixture is drawn into the cylinder (similar to drawing back the plunger on a hypodermic needle to allow fluid to be drawn into the chamber).
When the piston reaches the botton of the intake stroke, the intake valve closes ,trapping the air-fuel mixture in the cylinder.
Compression
The piston moves up and compresses the trapped air fuel mixture that was brought in by the intake stroke. The amount that the mixture is compressed is determined by the compression ratio of the engine. The compression ratio on the average engine is in the range of 8:1 to 10:1.
This means that when the piston reaches the top of the cylinder, the air-fuel mixture is squeezed to about on tenth of its original volume.
Power
The spark plug fires, igniting the compressed air-fuel mixture which produces a powerfule expansion of the vapor. The combustion process pushes the piston down the cylinder with great force turning the crankshaft to provide the power to propel the vehicle. Each piston fires at a different time, determined by the engine firing order. By the time the crankshaft completes two revolutions, each cylinder in the engine will have gone through one power stroke.
Exhaust
With the piston at the bottom of the cylinder, the exhaust valve opens to allow the burned exhaust gas to be expelled to the exhaust system. Since the cylinder contains so much pressure, when the valve opens, the gas is expelled with a violent force (that is why a vehicle without a muffler sounds so loud.) The piston travels up to the top of the cylinder pushing all the exhaust out before closing the exhaust valve in preparation for starting the four stroke process over again.
Oiling System
Oil is the life-blood of the engine. An engine running without oil will last about as long as a human without blood. Oil is pumped under pressure to all the moving parts of the engine by an oil pump. The oil pump is mounted at the bottom of the engine in the oil pan and is connected by a gear to either the crankshaft or the camshaft. This way, when the engine is turning, the oil pump is pumping. There is an oil pressure sensor near the oil pump that monitors pressure and sends this information to a warning light or a gauge on the dashboard. When you turn the ignition key on , but before you start the car, the oil light should light, indicating that there is no oil pressure yet, but also letting you know that the warning system is working. As soon as you start cranking the engine to start it, the light should go out indicating that there is oil pressure.
Engine Cooling
Internal combustion engines must maintain a stable operating temperature, not too hot and not too cold. With the massive amounts of heat that is generated from the combustion process, if the engine did not have a method for cooling itself, it would quickly self-destruct. Major engine parts can warp causing oil and water leaks and the oil will boil and become useless.
While some engines are air-cooled, the vast majority of engines are liquid cooled. The water pump circulates coolant throughout the engine, hitting the hot areas around the cylinders and heads and then sends the hot coolant to the radiator to be cooled off.
参考译文：
因为各气缸的工作行程是相同的，所以我们就从一个气缸了解四冲程是怎样运行的。
四行程是指进气行程、压缩行程、作功行程和排气行程。进气行程，活塞下行；压缩行程，活塞上行；作功行程，活塞下行；排气行程，活塞上行。
进气行程
在进气行程，当活塞开始下行时，进气门打开，空气燃油的混合气被吸入气缸（类似于拉开注射器的柱塞，将液体吸到针管内）。
当活塞到达进气行程的下止点时，随着空气燃油的混合气进入气缸，进气口关闭。
压缩行程
活塞向上移动时，压缩在进气行程中吸入的混合气。混合气的压缩量，由发动机的压缩比决定。普通发动机的压缩比范围是8 ：1到10 ：1。
这意味着当活塞到达气缸上止点时，空气燃油混合气被压缩到原来体积的十分之一。
作功行程
火花塞点燃压缩的空气燃油混合气，燃烧使混合气迅速膨胀。其燃烧过程产生的巨大力量推动着活塞向下运行，带动曲轴而产生推动汽车的动力。每个活塞按不同的点火顺序依次点火。曲轴转两周，发动机里的气缸完成一个作功行程。
机油系统
机油是发动机的生命之源。一台没有油的发动机就像一个没有血的人。机油通过一机油泵加压，流到发动机的各个运动部件。机油泵安装在发动机油底壳的底部，并且，通过一个齿轮与凸轮轴或曲轴连接。这样，当发动机运转时，油泵也开始工作。在油泵附近有个油压力传感器，它能检测油的压力并传递给警示灯或仪表板上的油量表。当你打开点火开关，在启动车子之前，油量警示灯亮，表明此时无油压力，同时也让你知道报警系统已开始工作。一旦发动机启动，报警系统就同时启动，此时警示灯熄灭，表明此时已有油压。
发动机冷却
发动机必须保持一个恒定的工作温度，既不能太冷又不能太热。由于燃烧过程会产生大量热量，如果发动机无法自我冷却，那么发动机会严重损坏。发动机的主要部件变形从而导致漏水漏油，机油也会沸腾从而失效。
有些发动机采用空气冷却方式，大部分发动机采用液体冷却方式，水泵使冷却液在整个发动机形成循环，冷却液流至气缸和气缸盖周围发热的区域，然后水泵把热的冷却液抽到散热器使之冷却下来。
