It's no surprise that for most heavy-duty applications diesel engines are the preferred choice. And it certainly doesn't require huge research to find out why. Diesel engines produce more torque, and that's what mainly separates them from gas engines. The inherent ability to burn fuel more efficiently makes diesel trucks easier on the pocket as well, especially if you want to lug around a lot of load. While a petrol engine might take ages and a million engine revs to pull large items, diesel engines can do it without breaking a sweat. Okay, that's a bit of an exaggeration, but the point is you don't need to rev the nuts off the diesel engine to make it work. But what happens when you keep climbing the revs in a diesel engine, and more importantly, how far can you go without having to change to a higher gear?
What Happens in a Diesel Engine?
Before delving into that, let's understand what happens in a diesel engine. It differs from the conventional gas engine for combustion. This means it depends on a different form of ignition for the fuel and not a spark like in the case of gas engines. Diesel engines depend on compression for the same. The air-fuel mixture is compressed to a level that it ignites to produce power. The other difference is that generally, the diesel engines have a longer stroke, hence the maximum revolutions per minute is relatively smaller than petrol engines.
What's revolutions per minute, revs or RPM, you must wonder. The term RPM stands for revolutions per minute. In a four-stroke engine, each cycle has four actions: intake, compress, combustion, exhaust. In simpler words, RPM is the number of times a crankshaft rotates in the engine per minute. As long as your vehicle follows the Diesel Cycle, the mechanism remains unchanged. The cycle is simple: the engine sucks in air, compresses it which leads to combustion, followed by the exhaust. That's how a diesel engine functions.
As mentioned above, diesel engines are inherently more efficient (lower wastage of fuel, in comparison to conventional gas ones) and produce more torque. The latter makes these favorable for applications like towing, tackling extreme inclines, among other things. From the driver's perspective, the biggest difference between a turbocharged diesel engine and a naturally aspirated/turbo gas engine is the way they drive. In a diesel, you aren't revving the nuts off that thing to make progress. And since the torque is available from a fairly low RPM, it helps in everyday driving as well. That also ensures more fatigue-free driving and fewer gear shifts. Hence diesel engines are more ready for both when you want to make a quick getaway or for when you want to tow some heavy load. On the other hand, as the revs climb, while the surge in petrol-powered vehicles keeps increasing, that's not necessarily true for the diesel motors. The point here is that diesel engines don't like to rev as much as petrol ones. Which is why their maximum RPM is much less as well.
And more important than the maximum RPM is the point at which a diesel engine makes most power and torque. That comes in much, much earlier than in a petrol engine. Beyond this point, in a diesel engine, if you keep the gas pedal pinned, the truck will slowly soon stop losing power, despite the revs rising.
For example, let's talk about the 2019 Ram 2500/3500 Heavy Duty. It's one very popular truck range in the United States and comes with a perfect engine line up that handles both urban everyday driving as well as serious hauling. If you're looking at getting the petrol-powered versions, the Ram 2500 and 3500 come with a 6.4 liter Hemi V8. The engine makes a massive 410 hp and a respectable 429 lb-ft of maximum torque. The diesel-engined versions, on the other hand, are powered by a Cummins 6.7 that makes a similar 400 hp. But even with a similar size and fewer cylinders, the maximum torque is rated at an other-worldly 1000 lb-ft (for Ram 3500 HD). The maximum torque on the diesel engine isn't just higher but also produced at a much smaller reading on the tachometer. You can read more about the 6.7 Cummins powered Ram 2500 and Ram 3500 on Mechanic Guides by clicking the link here
Larger engines, on say semis, will have their peak torque made around 1000 rpm or so. The cruising rpm suggested by the manufacturer for these will be somewhere in the region of 1000 - 1500 rpm. The farther you go from this and you're bound to destroy the engine. The maximum rpm in such an engine is marked at around 2000 - 2200 rpm. Which is even lower than where a fairly decent petrol engine will begin to pick up pace. And in the case of petrol, that red line will be at around 6000 - 8000 rpm.
So there you have it: a simplified explanation of the low maximum RPM on diesel engines. It does come down to not only the kind of engine but also the size and application. For a high-revving motor, you're better off looking at petrol engines. But for something that produces more power and torque from a low rpm, there's not much that can beat diesel engines - unless we talk about electric powertrains. Those can theoretically produce the maximum torque from zero rpm onward.
