Why do turbos need time to “spool up” before they can have a significant power boost?
In: Engineering
Each pound of boost it produces helps produce the next pound of boost. Accumulating and so on. Five pounds help make the sixth. Making the oxygen denser allowing for a more efficient combustion rate.
A turbo is a fan inside a small volume chamber. It is powered by the exhaust gases of the engine. If it is turning slowly, it is filling and emptying that chamber slowly. when it is turning a few hundred or a few thousand rpm as the car idles it is pumping very little air into the engine. When the turbo is turning many thousands of rpm, it is moving a lot of air into the engine. The more air the engine has, the more fuel it can burn, and the higher the pressure of the exhaust gas turning the turbo fan. When the engine accelerates, it takes the engine some time to rev up, and to start generating a lot of exhaust gas to turn the turbo fan. As the turbo starts turning faster, the engine exhaust helps it to turn the turbo even faster, which helps to turn the engine faster, etc.
But it takes a bit of time for the turbo to take advantage of the initial increase in engine exhaust.
A turbo is driven by the exhaust gas. At idle the exhaust gas Is comparatively low pressure. As the rpm increases the pressure and flow of exhaust increases. This spins the turbo which spins the intake side. Increasing boost.
It’s because they spin so fast. The turbo in your car has a typical operating speed of 20,000rpm, and it takes time to spin the assembly up to those sort of speeds. You can make that process faster by making the turbo smaller, but that also makes the turbo less effective, so there’s a compromise in the design there.
(Note some cars use two turbos–a smaller one that spins up quickly to give more immediate power delivery, and a larger one that takes time to spin up to provide maximum power over a longer time).
Turbos are wheels of shovels in the air intake pipe that hinder the intake air from flowing. Unless they spin fast and forcefully enough to literally press the air into the combustion chamber. The force that drives the wheel is the dynamic pressure of the exhaust gas. At low RPM this pressure is too low to deliver sufficient thrust to the turbo charger. At higher RPM there are more cumbustions per timeframe, thus more exhaust gas in the same volume of the exhaust pipe, and thus more pressure driving the turbo. At some RPM there is a break-even point where the exhaust pressure drives the turbo just right to deliver the amount of air at which the engine would work like without a turbo. Below that the turbo costs power, above that the turbo delivers power.
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