Lets start with a little background on internal combustion and the biggest difference between gasoline and diesel engines. In simple terms, gas engines inject fuel and air into the cylinder and a spark plug is used to ignite the mixture, driving the piston down, which rotates the crankshaft, which ultimately turns the wheels. Diesel engines do not use a spark but instead rely on extremely high compression of the air and fuel mixture, which causes it to spontaneously ignite, driving the piston down, rotating the crankshaft and making the wheels on the bus go round and round. Diesel engines have the highest efficiency of any (practical) internal combustion engine and for a given amount of fuel are about 20% more efficient than gas engines. Because of their design, they also have extremely high torque, which makes them well suited to hauling heavy loads in everything from semi trucks, to locomotives to cargo ships. Diesel engines also run at lower RPMs (Revolutions Per Minute) than gas engines, which results in less wear and tear, lower maintenance costs and generally longer service lives. I’m not suggesting diesel engines are the best all around solution, but for moving heavy stuff, the more torque you have the better. If you look at how diesel engines are used; the bigger the load and the longer the haul, the more likely the powerplant is going to be diesel. There’s a reason that trucks, trains and ships are typically powered by diesel engines and fuel price is obviously not the only factor.
Torque
Diesel engines are much more suited to hauling heavy loads because of the large amount of torque they produce. Torque is twisting force, measured in foot-pounds. The V8 gas engine in my Audi makes 335 horsepower and 330 ft-lbs of torque, which is well suited to moving it’s 4400 pounds down the road in a sporty fashion. On the other hand, the in-line 6 cylinder diesel engine in my truck makes a rather paltry 255 horsepower, but generates a comparatively massive 660 ft-lbs of torque. It’s the torque that gets its 17,000+ lbs moving and allows it to tow some 40,000 lbs. To directly compare two modern truck engines, lets look at the Ford Triton V10 gas engine, which is rated at 362 horsepower and 460 ft-lbs of torque versus a Cummins X15 diesel, which makes 400 horsepower and a staggering 1650 ft-lbs of torque (at it’s lowest rating.) Each engine has it’s applications, but when it comes to moving heavy stuff long distances, diesels are generally the go-to engine.
Maintenance
Yet another factor to consider is maintenance. Will you be doing minor repairs yourself or taking it to a garage for everything beyond adding washer fluid? Anything bigger than a sprinter van is going to be difficult to fit into many garages while diesel shops are obviously built to handle big trucks. The exception to this is if your truck is based on a Ford, Chevy or Dodge platform, in which case you can usually get it serviced at the dealer, of which there are obviously many. The downside is you are going to be paying premium prices for parts and labor.
Fuel Cost and Efficiency
Diesel fuel, generally speaking, seems to cost roughly around the same as premium gasoline. People will look at that and assume it’s better to have a gas engine and fill the tank with less expensive regular gas. However, as I mentioned at the start of this post, diesel engines are more efficient than gas engines. For a given amount of fuel, say one gallon, a diesel engine converts that fuel into more work than a gas engine. It’s also important to consider that mileage (MPG) is not the only critical measurement. My truck gets 9.5 MPG, which may sound horrible compared to today’s passenger cars, until you consider that it weighs 17,000+ pounds and is only slightly more aerodynamic than a shipping container.
Like many things, it comes down to having the right tool for the job. I also just love the sound of a big diesel engine.