How turbochargers increase engine efficiency | Auto Expert John Cadogan

Up next: Find out how and why turbochargers increase internal combustion engine efficiency.
Welcome to another ‘What the FAQ’.
It’s self-evident that adding a turbocharger to a modern engine does increase efficiency - because all manufacturers are doing it. And they’re generally not doing it to build some fire-breathing, tyre shredding monsters (although you can achieve that via the hi-tech miracle of turbocharging if you want).
More and more carmakers are doing it on small-to-tiny engines. Hyundai-Kia’s 1.6, Renault-Nissan’s 1.2, Volkswagen’s 1.4, and Honda’s 1.5 - etc.
First up, when you add a turbo in concert with direct injection, you generally get a nice flat torque curve from something like 1500rpm to 4500rpm, delivering pleasantly rewarding, constant thrust at wide-open throttle across that range. Diesel-like mid-rpm power, right there.
Nothing peaky about it. It’s pretty strong down low and pretty satisfying to drive in the mid-rpm range. Where most people generally drive.
But now - to efficiency. Here’s a typical question, from Adam Kondic
You're an engineer I'm not even close. Please tell me how a turbo increases thermal efficiency? I understand modern turbo engines have similar compression ratios to non turbo engines but in real world driving ( not scientific i know but,) my old man reckons his fg 4 litre was better on fuel than his ecoboost.
Leaving the paternal petrol-sipping proclivities to one side: Turbochargers increase both volumetric and thermal efficiency of an engine. Let’s back up and review exactly what turbochargers are.
A turbocharger is what you get when you connect two fans to the same shaft. You drive one of the fans with hot exhaust gas, and you use that impetus to compress inlet air using the other fan.
So basically you use energy that would have been wasted in exhaust gas rushing out the pipe to jam more air into the engine than it would be able to suck on its own. More air in the combustion chamber allows more fuel to be burned.
That’s volumetric efficiency - a smaller engine doing the work of a bigger atmo engine.
And of course you get more thermal efficiency because energy that would have been wasted in the exhaust flow and essentially lost for ever is now doing useful work for the engine, vis a vis jamming extra air in on the inlet side.
When people learn four-stroke engine basics, they get taught ‘inlet, compression, ignition and exhaust’.
But I prefer: ‘suck, squeeze, bang and blow’. It’s a character flaw.
Suck and blow are clearly the most enjoyable - not to mention oddly pertinent here. The turbo gives the inlet air plumbing a ‘suck assist’ as discussed, and it gets that assist from the ‘blow’ stroke. So once again we see another example of sucking and blowing working together and making life even better.
When you drill down into engine operation, a lot of people get the mistaken notion in their heads that the upwards movement of the piston is what actually pumps exhaust gas out of the engine - like the proverbial bicycle pump, only hotter and quite a lot faster.
As nice as this idea is, it’s pretty much bullshit. That’s not what really happens.
The engine goes bang. The fuel-air mix ignites. People think it explodes, but it really just burns fast. This causes the rapid, energetic expansion of the gas mixture in the chamber. It pushes the piston down, hard, and that’s where the useful work comes from.
But then, some short time before the piston gets to the bottom of the bore, the exhaust valve opens, and what actually evacuates most of the exhaust gas is its own rapid expansion:
It literally blows itself down the exhaust pipe rather than being pumped out by the piston.
Now I can see how one might think that’s a waste - because you might think even more useful work could be extracted by the piston by opening the exhaust valve a little later, but in reality then the piston would need to do quite a bit more pumping, and any benefit from opening later would be frittered away.
Engines already experience pumping losses - they don’t need any more of that.
There’s a lot of energy in exhaust gas - especially at big throttle openings and 1500-plus rpm. Exhaust flow is pretty gentle at idle (which is when most people commonly observe it) - but it’s actually quite energetic when the engine is doing its thing. And it’s that inherent energetic-ness that the turbocharger capitalises on.