A naturally-aspirated engine is limited by how much air it can pull in on its own. The piston descends, atmospheric pressure pushes air in behind it, and that is the whole supply. Burn rate is set by how much oxygen is in the cylinder, so the amount of air is the amount of power.
A turbocharger changes the supply. It puts a compressor in the intake and drives it with a turbine sitting in the exhaust — using energy that was already on its way out of the tailpipe. That is the entire idea. Everything people argue about afterwards, lag and boost and wastegates and intercoolers, is the cost of making it work without wrecking the engine.
Two wheels on one shaft
A turbocharger is two small radial machines back to back. Hot exhaust gas leaves the cylinder head with pressure and heat still in it, and is routed through a spiral housing onto the turbine wheel, which it spins. A shaft carries that rotation to the compressor wheel in a mirror-image housing on the intake side, which draws in ambient air, flings it outward, and delivers it to the engine at above atmospheric pressure.
Both wheels are on one shaft, so they always turn together, and they turn fast — a small car turbo passes 150,000 rpm in normal use. That is why the bearing housing between them is fed with engine oil, and why letting a hot turbo idle for a moment before switching off is not superstition: the shaft is still spinning when the oil pump stops.
What boost actually buys
"Boost" is the pressure above atmospheric that the compressor delivers, quoted in bar or psi depending on where you are. It matters because packing air more tightly puts more oxygen in the same cylinder — and more oxygen means more fuel can be burnt in it.
| Boost (gauge) | In psi | Absolute pressure | Air in the cylinder |
|---|---|---|---|
| None (N/A engine) | 0.0 | 1.01 bar | 1.00× |
| 0.5 bar | 7.3 | 1.51 bar | 1.49× |
| 1.0 bar | 14.5 | 2.01 bar | 1.99× |
| 1.5 bar | 21.8 | 2.51 bar | 2.48× |
One bar of boost is very close to 1.99× the air, because you have roughly doubled the absolute pressure — 1.01325 bar becomes 2.01. That is where the folk claim "one bar doubles your power" comes from, and as a first approximation it is fair. Since manufacturers quote boost in bar, psi or kPa depending on the market, a pressure converter saves a surprising amount of arithmetic when comparing two engines.
The catch is temperature. Compressing air heats it, and hot air is less dense — so some of the density you just paid for is given straight back. That is what the intercooler is for: a radiator between the compressor and the engine that cools the charge back down before it goes in. The table above assumes equal temperature, which is exactly what an intercooler is trying to achieve and never quite does.
Turbo lag, and where it comes from
A turbo makes no boost until there is enough exhaust flow to spin the turbine, and at idle or low load there is not. Open the throttle and three things have to happen in order: more fuel burns, more exhaust flows, and the turbine and compressor — real objects with real inertia — accelerate to speed. The delay between your foot and the boost is turbo lag.
That shape explains most of how a turbocharged engine feels. Below the threshold it drives like a smaller engine than it is; through the ramp it gains power quickly enough to be startling; above it, the wastegate is holding boost flat and the engine behaves normally again. Small turbos spool early and run out of breath at the top; large ones lag badly and then pull hard. A twin-scroll housing, or two turbos of different sizes, is an attempt to have both.
The wastegate: why boost has to be capped
Left alone, a turbo is a positive feedback loop. More boost means more fuel burnt, which means more exhaust, which spins the turbine faster, which makes more boost. Nothing in that chain stops on its own, and the end of it is detonation and a hole in a piston.
The wastegate is the valve that breaks the loop. Once boost reaches the target, it opens a path that lets some exhaust bypass the turbine entirely, so the turbine stops accelerating and boost holds flat — the ceiling in the chart above. A blow-off valve does the opposite job on the intake side: when you lift off the throttle, the compressor is still spinning into a closed passage, and that valve vents the trapped pressure rather than letting it stall the compressor.
What it costs
Turbocharging recovers energy that was being thrown away, so it genuinely does improve efficiency — it is why almost every modern engine has one. It is not free:
- Heat. A turbine housing glows in hard use. Everything near it needs to tolerate that, and the oil feeding the bearing needs to survive it.
- Compression ratio. Forced induction raises cylinder pressure, so turbocharged petrol engines usually run a lower geometric compression ratio to keep detonation away — giving back some off-boost efficiency.
- Complexity. An intercooler, its plumbing, a wastegate, boost control, and an oil feed and drain, all absent from a naturally-aspirated engine.
- Response. Even a well-matched turbo is not as immediate as an engine that makes its power without waiting for anything to spin up.
That trade is why a supercharger — a compressor driven by a belt off the crankshaft instead of by exhaust — still exists. It responds instantly because it is geared to the engine, but it burns engine power to make its own boost, where the turbo runs on waste.
See the engine it bolts to
All of this is happening on top of an ordinary four-stroke cycle, and it is much easier to follow once you can watch that cycle run. Our 3D four-stroke engine simulator renders the whole 720° — pistons, rods, crankshaft, camshafts and valves — with intake charge and exhaust gas drawn as coloured particle streams. The intake stroke it shows is the one a turbo is feeding, and the exhaust stroke is where the turbine takes its cut.
The turbo itself has a simulator too: the 3D turbocharger simulator cuts a turbo open, spools it from idle so you can watch the lag, holds the boost with a working wastegate, and makes it surge on a lift-off, with its operating point moving across a compressor map.
If you want the cycle itself explained first, our guide to two-stroke versus four-stroke covers what those four strokes are doing and why the count matters.