Many consider the engine found in your average commuter or performance car to be a complicated beast. And while that may be true once you add on all of the electronics and additional mechanical features, underneath it all the engine in your car is essentially nothing more than an air pump.
Mechanical engineer Jason Fenske, of Engineering Explained, wanted to show other enthusiasts how to calculate how much air their engine consumes at wide open throttle, but also wanted to create a visual aide to help others better understand this process. Using two very large balloons, Fenske secures one around each exhaust outlet and watches as they inflate as he explains how to calculate this yourself.
Just like water, air will always follow the path of least resistance. Which in the case of Fenske's S2000 happens to be the driver-side exhaust outlet.
First, some of you might be wondering if securing a balloon to your exhaust is harmful to the engine. While we don’t ever see the need for you to do this yourself, Fenske addresses this concern by pointing out the fact that a balloon only needs slightly higher than atmospheric pressure to inflate. So the pressure inside the balloon will always remain lower or equal to what is coming out of the exhaust.
Think about it this way, you can take the same balloon and inflate it using only the power of your lungs, and in-between breathes your lungs can withstand the pressure differential between the balloon and your lungs without any conscious effort or discomfort. If your lungs can handle it without any negative effects, why would a 200+ horsepower “air pump” be any different?
If you can blow up a balloon using nothing more than the force of your lungs without any ill effect, why would an internal combustion engine fare any worse?
How Much Air Does Your Engine Consume
To calculate the estimated amount of air your engine consumes and how quickly, it requires one big assumption. We are going to assume that our engine achieves 100 percent volumetric efficiency at wide open throttle, meaning that the volume of each cylinder is perfectly filled with air, which is very difficult to achieve in reality.
Using Fenske’s 2.0-liter Honda S2000 with a 9,000 rpm redline as an example — we know that for a four stroke engine, for every two crankshaft rotations there will be one intake stroke per cylinder. Using the information above we can figure out the flow rate of our engine by multiplying our engine size (2.0-liters) by the maximum RPM (9,000 rpm) divided by two (crankshaft rotations).
Formula To Find The Flow Rate Of An Engine
Liters × (RPM ÷ 2)
2.0-liters × (9,000 rpm ÷ 2)
2 × 4,500
9,000-liters/minute or 9 cubic meters/minute
This means that our 2.0-liter engine can flow 9,000 liters per minute, or 9 cubic meters per minute if you divide the liters per minute by 1,000. Fenske then compares this to the new 8.0-liter 16-cylinder Bugatti Chiron, which takes in 60,000 liters per minute (or 60 cubic meters per minute) at peak flow!
While the air passing through the engine at idle is far less than what would be consumed at wide open throttle, it’s still a great visual aide to help better understand the concept.
Using This Data In The Real World
Think about your average two-car garage, with dimensions close to six meters in width, six meters in depth (approx. 20 feet by 20 feet), and 3 meters high. If we then multiplied the width, the depth, and the height together, this would bring the garage to a total air volume of 108 cubic meters.
If we then take that 108 cubic meter volume measurement and divide it by the volumetric flow rate of our engine (9 meters per minute), we would find that at wide open throttle and at redline it would take roughly 12 minutes for our 2.0-liter engine to consume every molecule of air in the garage. The Bugatti Chiron for comparison? The 8.0-liter 16-cylinder monster would consume the same amount of air in under two minutes!
The balloons are a great visualization of what this engine is able to do at idle in comparison to what we calculated for flow at wide open throttle. Not only is our engine rotating much slower than its 9,000 rpm redline, but the vacuum created in the cylinder at idle also drops the volumetric efficiency far below 100 percent, which is why it takes so long for one of the balloons to finally burst.