Leave it to a tiny car company like Mazda to pull off something the big players have been trying to. So then you can derive the relationship between thermal efficiency and compression ratio, like MIT plotted on its web page and shown above. Pretend you have an engine whose cylinder and combustion chamber volume is 10 cc when the piston is at bottom dead center. There are two ways of calculating the compression ratio … Compression ratio is also what makes engines like Mazda’s Skyactiv-G engine so efficient. As compression ratio increases, the piston moves higher in the bore at top dead center, hence there is additional force for the expansion stroke (additional force for the same amount of fuel equals higher efficiency).. That’s the volume of the cylinder when a piston is all the way down compared to all the way up to the top. He explained that metal simply cannot withstand such high levels of stress, and a compression ratio like that would run things so hot that it’d blow up any current engine. The best answer of those came from Dr. Andy Randolph, Technical Director at ECR Engines. Drive any sluggish, wheezing, gas-sucking, old low-compression American V8 and you’ll know what I mean. What made you want to look up compression ratio? ), A compression ratio is exactly what it sounds like—a ratio where you’re compressing the maximum cylinder volume into the minimum cylinder volume. The equation is here (nu is thermal efficiency, r is compression ratio, and gamma is a property of the fluid): As you give an engine of a certain displacement a higher compression ratio, you effectively shift the P-V diagram up and to the left, and increase heat input (Qh in the diagram) more so than heat loss (Ql). Not everyone else does. It’s also important to note that the heat input and heat loss during the cycle of your engine relates to the efficiency as a function of compression ratio. The static compression ratio (sometimes called the mechanical compression ratio) is a handy reference that engine builders use to build and describe engines, but no two engines with the same CR … Post the Definition of compression ratio to Facebook, Share the Definition of compression ratio on Twitter. That’s where the cylinder volume is greatest. To reiterate: That’s bad. Build a city of skyscrapers—one synonym at a time. Now, we really should understand more about the why in addition to the how, and that means we’re going to have to venture into the realm of thermodynamics. It’s healthy and good for the soul. I was expecting an Engineering Explained video on compression ratios, and was graciously surprised and happily proved wrong! Click here to browse! Test Your Knowledge - and learn some interesting things along the way. To be more technical, in the diagram the 1-2 curve shows the compression stroke in which pressure (y-axis) rises and volume (x-axis) drops as the piston does work on the gas, squeezing it. These numbers are more precise than the ones you’d get by measuring by … You’ve heard the term compression ratio before, but have you ever wondered exactly what it means? You must — there are over 200,000 words in our free online dictionary, but you are looking for one that’s only in the Merriam-Webster Unabridged Dictionary. Basically, you want the gas that ignites when you want it to, not the kind that ignites when you don’t want it to. Put another way, you’re turning more of your input energy into work. Writer at Jalopnik and consumer of many noodles.  X Research source Tip: Many of the measurements you need are provided in the manufacturer specs. For example, for an engine with a 9:1 compression ratio, you’d say that it’s “nine to one.”.
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