Horsepower is fun in its own way, but torque can be just as entertaining. If you want to rip stumps out of the ground, you'll want a whole lot of torque. That also means you'll likely prefer a diesel engine. Compared to their gasoline-swilling counterparts, diesel engines are the torque kings. Why is that?
Jason Fenkse from Engineering Explained knows why, and he's here to offer up five reasons why a diesel engine makes more torque than a comparable gasoline engine.
First up is compression ratio. With a gasoline engine, the piston stops a bit short of the top of the cylinder as it moves up and down during the combustion cycle. By comparison, a diesel piston runs all the way out to the edge of the cylinder. The diesel engine needs to do this because it lacks spark plugs and closing that gap increases compression and therefore superheats the air-fuel mixture to create combustion. Jason does a great job of providing a visual explanation of this with a naked piston and cylinder.
Next, we move on to the speed of combustion. With its combustion at the top of the compression stroke, the diesel engine creates immediate power as the piston travels back down the cylinder. With a gasoline engine, the piston has already started to move away from the top of the cylinder but combustion is only just occurring so it doesn't travel as quickly. The diesel goes faster, and it results in increased torque.
Bore and stroke size are another consideration. This involves some meaty math as torque equals force multiplied by distance. In the video you can see how a gasoline engine might have a wider bore but the piston doesn't travel as far, so its stroke length is shorter. With the diesel, however, the stroke length is longer (partially due to that extra travel at the end of the piston stroke), and that increases the distance in the torque equation. All of that force travels farther, thus we often have more torque.
As a fourth point, Jason moves on to turbocharging. This is a technology often found on modern diesel engines, as the diesel engine requires a healthy amount of airflow. Diesels are built to handle turbos. Add more air, and you make more power. The same holds for a gas engine, but they aren't turbocharged as often.
Finally, diesel fuel itself packs more energy density than gasoline. Jason says it's between 10 and 15 percent. More energy also means more power.
Jason does an excellent job of walking us through the basics of diesel engines and the torque they produce. Click play and enjoy some great engineering clearly explained.