Example: A 125cc four-stroke engine combusts about 110cc of F/A gasses but a 125cc two-stroke engine combusts about 180cc of F/A gasses.ģ) Now the crankshaft has rotated past bottom dead center (BDC 180 degrees) and the piston is on the upstroke. The main advantage of two-stroke engines is that they can combust more volume of fuel/air mixture than the swept volume of the engine. This is the unique super-charging effect of two-stroke engines. The gasses aren't lost because a compression pressure wave has reflected from the end of the exhaust pipe, to pack the unburned gasses back into the cylinder before the piston closes off the port. Now the loop of unburned mixture gasses have traveled into the exhaust pipe's header section. The more unburned gasses you can squeeze into the combustion chamber, the more the engine will produce. That is the key to making more power in a two-stroke engine.
It is critical that the burnt gasses are scavenged from the combustion chamber, in order to make room for as much unburned gasses as possible. The gasses travel up the back side of the cylinder and loops around in the cylinder head to scavenge out the burnt mixture gasses from the previous power stroke. Meaning that the unburned mixture gasses are flowing out of the transfers and merging together to form a loop. The pressure in the cylinder must blow-down to below the pressure in the crankcase in order for the unburned mixture gasses to flow out the transfer ports during the scavenging phase.Ģ) Now the transfer ports are uncovered at about 120 degrees after TDC. The blow-down phase has started and will end when the transfer ports open. A pressure wave of hot expanding gasses flows down the exhaust pipe. At about 90 degrees after TDC the exhaust port opens ending the power stroke. This pressurizes the crankcase causing the reed valve to close. These four drawings give an explanation of how a two-stroke engine works.ġ) Starting with the piston at top dead center (TDC 0 degrees) ignition has occurred and the gasses in the combustion chamber are expanding and pushing down the piston. That is how a two-stroke engine completes a power cycle in only 360 degrees of crankshaft rotation compared to a four-stroke engine which requires 720 degrees of crankshaft rotation to complete one power cycle. There are different phases taking place in the crankcase and in the cylinder bore at the same time. Use the guide at the end of the chapter to map out your strategy for changing engine components to create the perfect power band.Īlthough a two-stroke engine has less moving parts than a four-stroke engine, a two-stroke is a complex engine because it relies on gas dynamics. Use this as a guide to inform yourself on how changes in engine components can alter the powerband of bike's engine. The most common mistake is to choose the wrong combination of engine components, making the engine run worse than stock. A myriad of different aftermarket accessories is available for you to custom tune your bike to better suit your needs. Nasa baba nabasa ko sa isang site na itinuro ni E.R.Ĭhanging the power band of your dirt bike engine is simple when you know the basics.
tingin ko segunda okey pa yan sa pinakamatarik lalo na pag may buwelo. magandang set-up na yang bike mo for the trails of mt. hindi ko pa nasusubukan ang magpalit ng reeds at carb kasi nga may kamahalan. Narinig ko sa trails okey naman daw yun big carb pero yun reeds ala ko balita lalo na doon sa dual-stage.