How do four Stroke Engines Work
A four stroke engine's basic design is close to that of a two stroke engine. Both types can be gasoline powered (or equivalent) or diesel powered. Both can be air cooled or water cooled.Both gasoline and diesel two stroke engines do not have as high a lower end torque output as a similar sized four stroke while at the same time the horsepower may be comparable. Horsepower and torque power are really two different things. Four stroke engines develop more lower end torque (lower RPMs) while two stroke engines rely upon higher RPM to gain torque as well as horsepower.A four stroke engine is best described as an engine that exhausts on one upstroke of the piston and takes on fuel and fires on the next upstroke of the piston. Start counting strokes when piston #1 is at top dead center (exact top of it's travel in the firing position). When it travels to the exact bottom of it's travel with all the valves closed, that's stroke #1. Then it travels back up to top dead center again while the exhaust valves open to purge the cylinder of the burned fuel, stroke #2. On it's way back to bottom dead center, the intake valves open allowing fresh air in (and fuel if the engine is equipped with a carburetor), stroke #3. It again travels back to top dead center with the valves all closed (fuel is injected when the piston nears the top if true fuel injection is in the offing) where the spark plug fires just before top dead center to allow the explosion to happen exactly as the piston is at top dead center, stroke #4.A two stroke engine must exhaust on every down stroke of the piston (or somewhere during the piston's travel) and take on fuel and fire every time the piston travels to the top. The two strokes are when the piston is up at top dead center and down at bottom dead center.Some two stroke diesel engines took on air near the bottom of the piston's travel through air vents built into each liner then fuel was injected when it neared the top and it fired at the top. Exhaust was through several valves for each piston in the head and was accomplished just as the piston reached the bottom and started on it's way back up. Fresh air was both provided through the vents for the next firing and to help purge out the exhaust. These engines are largely going away and are being replaced by four stroke engines.Chain saws, weed eaters, leaf blowers, some motorcycles and four wheelers, some snowmobiles and personal watercraft as well as boat motors, etc. have two stroke engines. However more and more of these have also changed over to four stroke or electric because small two stroke engines use oil mixed with the gasoline for lubricant and smoke as a result. This smoke goes against environmentally minded folks so the two strokers are beginning to go away.Four stroke engines are wet sump engines meaning the piston and crankshaft, etc. are lubricated with an oil bath type setup (oil pan & oil pump on a car or truck). Coming into widespread use are oil coolers to cool the engine oil because of the higher operating temperatures of modern four stroke engines.Four stroke engines also tend to be favorable when alternate fuels are used such as ethanol, propane, etc. possibly because the timing can be more readily adjusted. Multi-fuel diesel engines are all four stroke engines even though two stroke engines were part of the experiments.The modern four stroke internal combustion engine has won out over many other designs and apparently will continue until a non internal combustion engine is finally developed that can completely replace it.
