Do you know what boost pressure is?

Unit of boost pressure is 'Bar'.

Boost pressure (boost pressure) refers to the air pressure inside the engine piston. The unit is expressed in 'bar', where 1 bar = 1013 kPa (one atmosphere). You may be thinking, "So if all the air is inside the piston, it's 1 bar!" But actually, it's not!

Boost pressure is not absolute pressure, but relative pressure, i.e. the engine'sHow many atmospheres relative to the air outside? This shows that. See the diagram below.

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The left-hand diagram shows the engine at idling and the right-hand diagram shows the engine with the accelerator fully open. In naturally aspirated engines (NA cars), air enters the piston according to the laws of physics, because the pressure outside is higher than inside the engine. Before this entry, the air pressure is approximately -0.8 bar. When the accelerator is fully opened, a lot of air from outside enters at once, so the air pressure becomes equal to the air outside and is 0.0 bar. In this way, if the pressure is lower than the outside air, it is expressed as a minus sign, and in the previous case it was 0 bar. Incidentally, when the engine is stopped, the inside is like a vacuum, so the pressure is -1.0 bar.

What then happens to turbocharged and supercharged (SC) vehicles?

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With turbochargers and SCs, in addition to the outside air, the turbine forces exhaust gases into the piston, so if the accelerator is pressed a little harder, 0 bar will be exceeded. In the past, it was around 0.7 bar at full throttle, but now it can reach 2 bar as a matter of course. (The latest AMG A45 S is an example.) In other words, the absolute pressure is 3 bar!

If unnecessary pressure is applied to the engine, it will break down; since NA cars do not have that much pressure, boost pressure is not usually considered, and nowadays we do not say "◯◯bar". If too much boost pressure is applied, the engine may break, and a boost gauge was attached to the turbo car for the purpose of preventing it. Therefore, boost pressure is usually said in turbo/SC cars.