Why Do Diesel Engines Have To Use Turbo?

Actually, it doesn't have to be; there are lots of NA diesel engines in circulation and use, but most modern ones are turbocharged to produce more power with the same or smaller engine capacity.

One common knock against diesel engines is their perceived lack of horsepower compared to gasoline engines. Diesel engines typically have lower RPM ranges, meaning they need more displacement (engine size) to achieve the same peak power as a gasoline engine. However, where they shine is in torque, offering more pulling power at lower revs. Think of it like lifting a heavy weight; a diesel engine would be like a steady, strong crane, while a petrol engine might be faster but have less torque.

I saw some Japanese diesel engines of the 1980s and '90s, often with hefty 2.5–3-liter capacities, that only managed to cough up 80–90 HP. Several factors likely contributed to this, ultimately leading manufacturers to embrace turbos. Size isn't everything, after all, and sometimes regulations or other constraints limit capacity increases. The addition of a turbo, then, became the key to unlocking more power while keeping the familiar engine footprint.

Take the Mitsubishi 4D56 engine (as shown above), for instance. It comes in two flavors: naturally aspirated (NA) and turbocharged. The NA version chugs along with a modest 74 horsepower, while its turbocharged cousin flexes its muscles with 99 horsepower. But that's not the end of the story. In its latest iteration, this engine pumps out a whopping 178 horsepower at 4000 rpm, a far cry from its humble beginnings. Now, the difference between the NA and turbo versions isn't just about brute force; I suspect there's some technological wizardry at play too.

There's a reason why many diesel engines, especially in commercial vehicles, pack a turbocharger under the hood. It all boils down to power. On their own, they can feel a bit sluggish in their naturally aspirated (NA) state. So, to give them the extra muscle needed for hauling or heavier tasks, manufacturers turn to the trusty turbocharger. It's a compact and relatively straightforward addition, making it a popular choice for boosting the diesel's oomph.