If You Understand Volumetric Efficiency You Understand Engines
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Published 2023-08-06
At idle and other low load scenarios inside the intake manifold we will find vacuum, or air pressure below that of the atmospheric air pressure outside the engine. This occurs because the throttle plate is closed and prevents entry of large amounts of outside air into the intake manifold while at the same time the engine is running and the downward motion of the pistons is rapidly creating a void or absence of air above it. The same air is then mixed with fuel, compressed and burned. In other words it’s consumed. So the engine is consuming more air than the throttle plate is allowing inside the manifold, meaning that we actually have less air per unit of volume inside the manifold than in the ambient atmosphere outside the engine. In other words a cubic inch or cubic centimeter or cubic whatever of air inside the intake manifold at idle actually contains less molecules of air than that same cubic inch or cubic centimeter of ambient atmosphere air outside the engine. Because we have less air we have less air pressure, or in other words, a vacuum inside the intake manifold.
But as the throttle plate opens more and more outside air is allowed into the engine and pressure gradually increases, it transitions from vacuum to atmospheric pressure. The engine of course can’t consume the entire atmosphere and therefore the pressure inside the intake manifold becomes atmospheric when the throttle is fully opened. But notice that on our map atmospheric pressure is displayed as zero. Below zero is vacuum. Above zero is boost. A naturally aspirated engine will never go significantly beyond this point whereas a turbocharged or supercharged engine will venture into this area and how high it will go depends on how much boost is generated.
Speaking of what the engine is capable of doing we must ask what do the numbers in the table itself actually mean? 87 what? Well, this is volumetric efficiency of the engine at that particular intersection of manifold pressure or engine load and engine rpm which means that this is not 87 of some unit, it’s 87 percent. 87 percent of what? 87% of the volume of the engine, or it’s displacement. In other words 87 means that 87 of the engine’s displacement has been filled with air. 100% would mean that all of the engine’s displacement has been filled with air. As you know the displacement of an engine is actually the volume of the cylinder and the combustion chamber above the piston. 100% volumetric efficiency means that the engine has managed to ingest enough air to completely fill this space with air. What does a volumetric efficiency of 110 mean? As you can see this occurs at boost, in other words a forced induction device, aka turbo or supercharger is forcing more air into the engine than the engine would be capable of aspirating naturally. Due to the action of the forced induction device the volume of air inside the cylinder is greater than the volume of the cylinder and so the pressure of air inside the cylinder and consequently the intake manifold increases.
What’s interesting to observe is that volumetric efficiency actually starts dropping off as RPM increases. Shouldn’t the engine be ingesting more air the faster it spins? Well yes, but up to a point and this table very accurately reflects the anatomy of the engine. At low rpm we have low piston speed and because the pistons are moving slowly the velocity of the air entering the engine is also reduced so we’re unable to fill the cylinder. As piston speeds increase air velocity increases and we reach a point where we achieve maximum volumetric efficiency. But as piston speeds increase even further the intake and exhaust valves are open for ever shorter amounts of time because the opening and closing of the valves is synced to the speed of the piston via a cam chain or cam belt. At some point the valves are no longer open long enough for enough air to enter the engine and volumetric efficiency starts falling off. The engine simply can’t breathe fast enough to match the rpm.
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All Comments (21)
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100 cars that changed the world: amzn.to/3DGXDf6 Stem engine model: amzn.to/3Y5iZLv 1-2-3 count with Car parts: amzn.to/3jxP7Z6 Patreon: www.patreon.com/d4a Motivation: www.youtube.com/channel/UCt3YSIPcvJsYbwGCDLNiIKA
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I accidentally added this to my sleep playlist so for the past week or two I've been randomly waking up this man explaining what a volumetric efficiency is.
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This map contains all the fundamentals of the engine operation. It doesn’t matter if it’s car, motorcycle or truck it’s all the same for internal combustion engines. Proceeds to show a Tesla, and a Rivian. Lol. That had me laughing. Thank you 🙏
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This was genuinely the best explanation of VE and Lambda I've ever seen. This put everything into such perspective. Makes me realize an aftermarket ECU might be far less complicated than I initially though to tune for.
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I kinda just assumed that ECUs were mysterious black boxes that ran some secret software specific to each car. After watching this video, I now want to learn more about ECUs.
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Yes modern technology is great, but explaining this to us is awesome ❤
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One of the most useful tuning videos on the internet today. I'm midway through modern EFIing a 70s car, and I'm glad to see my decision validated here. Wideband, and e-throttle should massively improve emissions and driveability.
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I self learned tuning about 15-20years ago. From dizzy/webbers to dizzy/fuel analog ecu and finally full digital ignition/fuel. I street tuned with innovative wide band and listening for knock . You’ve explained this beautifully. Kinda miss the cold nights on concrete floor tuning days
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You are by far the best at explaining all of the science behind these different tuning technologies. I watched your boost school series all the way through, this tuning series, and so many other videos on your channel. You've given us so much knowledge, and for that, I thank you. I really hope you enjoy making these videos, because you're excellent at it.
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would love a tuning series. your videos on the physical aspects/concepts of engines are so informative, and as somebody just getting into tuning from scratch, it would be awesome to see
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You forgot to say that even in NA engines the VE can go above 100% thanks to the help of exhaust pulse scavenging combined with valve overlap which helps pull in more air and have more than 100% VE despite engine being naturally aspirated
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For those who don't know, it is very beneficial to tune in actual Lambda vs AFR. If your target is 1.0 Lambda and you're getting .9 lambda, you know you're 10% off. Just pull 10% off your VE table and you're GTG. No need to guess and check based on the error and no complex calculations to perform.
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I learned about volumetric efficiency in the 70’s while still at school, messing around with carburettors etc. This video has brought so much enjoyment to me, thank you
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Nice touch at 0:42 visualizing how VE map applies to all engines in cars, motorcycles and trucks ; )
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You are a treasure to the car community, sir. By the way, what is shown at 14:10 is called "Closed Loop Controller" or "Closed Feedback Controller" (it was not mentioned during the video I think) and it is massively used at basically all industrial processes I can think of: You measure the output of the process and input it back as one of the variables doing the calculation. One of the most common examples of the math doing the magic to let the system know how much to vary the input to adapt to the measured output is the PID Algorithm (Proportional-Integral-Derivative). Even 3D printers use it on the firmware.
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This can teach someone how engines work who has never worked on a car that’s how good it is. Bravo
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What a fantastic video. After watching a ton of tuning videos, this is the first time it’s all come together in a concise way. This should be the first tuning video anyone should watch.
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Perhaps one of the most complicated subjects I've seen you do a video on but paradoxically also one of the most clearly and simply explained. You're a great teacher.
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As a retired Auto Shop teacher, I want to say that this really hits the mark. I wish I could've had access to this video back when I was teaching. The mapping and feedback systems are so far advanced from the old school carburetors and centrifugal/vacuum advance ignition systems that it's like a different world. The funny part is that the principles have remained the same, just that now the ideal values can be achieved, whereas in the 'good old days' we were happy to get close! 😁😁 Keep up the good work!
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This video was legit the best I’ve ever watched on the matter. Super clear yet in depth, very well shot… All your videos are great but this one I enjoyed particularly. Big thanks for the amazing content, your channel deserves the million subs.
