Recently at a Cafe, a young couple sitting next to me were taking about turbos (what are the odds!). Thinking that this conversation was right up my alley, I got talking with them and they asked me a question that caught me off guard: why are there no naturally aspirated diesel engines?
Honestly, diesel engines are something I never think about. As someone who is focused on road racing, diesels are something that simply doesn’t have a place there. Before you say, “But Audi won Le Mans with a diesel”, know that there is probably nothing more political than sanctioning bodies like the FIA, SCCA, or any other group that manages a popular entertainment product. Rules can be crafted to favor or bias anything they want, and I do not believe that diesels would have been able to win without the supporting rules.
So anyway, the question was totally outside the realm of my normal thinking. Sadly, I couldn’t give them an answer right off the cuff and even a quick scan of a web search didn’t reveal any definitive info. On my way home, I thought about it for a while and the answer became clear: it’s because of volumetric efficiency.
Also, there is naturally aspirated diesels in service, but they are not in automotive applications; they are used in generators. I’ll touch a bit more on that later. But this was something I never would normally think about.
The simple definition of volumetric efficiency (VE) is how much air is in the cylinder. If you can fill a cylinder 100% with air, then you are at 100% volumetric efficiency. When an engine is stopped and the piston is at bottom dead center, the atmosphere has all the time in the world to fill that cylinder and it will be 100% full. But sadly, when the engine is running, the entire induction system is in the way of the cylinders being totally filled so the VE will be lower.
The magic of a gasoline engine is that at high RPM, the intake air and exhaust gasses start moving so fast that they gain a lot of momentum. We can then design intake manifolds and exhaust systems to use this momentum in our favor to help fill the cylinders, even surpassing 100% VE! The diesel, with its low RPM range, can’t really take full effect of this magic.
Even though diesel fuel is energy packed, the nature of how it burns limits these engines to low RPMs. Because of this, the VE of a diesel is going to be low and the engine will not produce enough horsepower to get out of its own way. Therefore, to make accelerating with traffic or merging onto a busy highway even possible, we have to supplement the VE of a diesel with forced induction.
The beauty of the diesel is when its cruising down the highway with a full load in an 18-wheeler. The long stroke in the engines and slow burn of the fuel can keep that load moving efficiently, making them better suited to this task than a gasoline engine. The turbo is only needed when the truck is accelerating or climbing.
Here’s a different way to think about it. The diesel engine is good at providing a constant force but not move through an rpm band. A perfect application for an NA diesel would be powering a generator, and it is not uncommon to see them used this way. In this application, the engine can run at a targeted RPM and just stay there. They do make turbo diesel generators as well, because who doesn’t want more power or use the generator at high altitude? But NA diesels definitely have a use in this application.
And there you have it! Diesels are so alien to me that it was really interesting to think about this and how it all works. I hope after reading this, you can be better prepared than I was for what actually is such a simple question.