![]() ![]() (Yeah, I know, but get your mind out of the gutter for a few minutes.) Things are designed so that when you're JUST starting the ignition stroke, the crankshaft has spun just enough so that when you're driven back down the cylinder the spark plug ignites and explodes that air/fuel misture, and all that power goes towards turning the crankshaft and thus powering the vehicle. Behind you is a rod which is attached to the crankshaft as the crankshaft spins the rod pushes you up and pulls you down again, over and over again, millions and billions of times over. Imagine for a moment that you're an engine piston, happily sliding up and down the cylinder. Since the air/fuel mixture is volatile even without compression, it can very easily spontaneously combust (that is, without firing a spark plug) within the hot confines of your engine. As the air/fuel mixture is compressed during that stroke, its temperature is raised significantly. Compression, if you recall your science classes, creates heat. The most important part of understanding the above in relation to your question is the second stroke, the compression stroke. At TDC the exhaust stroke ends the exhaust valve closes, the intake valve opens, and the process begins all over again. This pushes the spent gasses out through the exhaust valve and into the exhaust system. Just after BDC, the exhaust valve opens and the piston is again pushed up into the cylinder this is the exhaust stroke. The rapidly expanding ball of gas forces the piston back down the cylinder, producing power to turn the crankshaft and therefore drive the vehicle. A spark is delivered through the spark plug, igniting the compressed air/fuel mixture. Just after TDC, the compression stroke ends and the ignition stroke begins. Combustion chambers are often measured by volume ratio for example, an engine with "9:1 compression" means that for every nine units of uncompressed air/fuel in the cylinder, the engine will compress that into one unit of compressed air/fuel within the combustion chamber. This compresses the air and fuel into a much smaller space in the engine head called the combustion chamber. ![]() The intake valve closes, and the piston is forced upwards. As the piston moves downward it creates a vacuum which draws air and fuel into the cylinder.Īt BDC, the intake stroke ends and the compression stroke begins. These strokes are called intake, compression, ignition, and exhaust.Īt the beginning of the intake stroke, the piston is at TDC. A four-stroke engine therefore moves the piston a total of four times, twice up and twice down. A "stoke" refers to the distance a piston will travel from bottom dead center (BDC the lowest possible point of the stroke where the piston is not moving) to top dead center (TDC, the highest possible point of the stroke where the piston is not moving). Nearly all automobile gasoline engines are four-stroke piston engines. In order to answer this question you need to understand the basics of how and why internal combustion engines work. The short answer is, there's a middle grade in order to allow for more precise engine tunings, which in turn allows customers to get more powerful engines by way of less expensive fuel. Using a lower octane-rating than is called for can cause detonation but typically most engine-management systems will be able to compensate by retarding ignition timing but it's reactive, it has to detect detonation first before it takes measures to prevent it. Using a higher octane-rating gasoline than is called for typically will do nothing but waste your money. Typically your car will have recommended octane-rating in the manual, or printed on the gas-cap. In-general, higher octane-rating gasoline allows engines to create more power through the use of more aggressive values of the aforementioned factors (more timing, more compression, etc). This can be bad in a number of ways, and cause physical damage to the interior of the engine if it is bad enough or happens long enough.Įngines are typically built and designed (as an entire system) to expect a certain octane-rating fuel the required octane is determined by a large number of factors of the engine, such as ignition timing, compression, super/turbocharging, power-adders, etc. Detonation is self-ignition, so basically it starts to burn before the engine intends it to. Octane-rating doesn't have anything to do with the quality of the gasoline, it is a measure of detonation resistance. ![]()
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