The difference between diesel- and gasoline

Diesel engines, like their close cousins, gasoline engines, are internal combustion engines (ICE). That means fuel is mixed with air as it goes into the engine and that mixture is compressed internally, inside the engine’s cylinders. At some point, the fuel ignites (combusts), driving a piston down and turning the crankshaft, which is connected to the vehicle’s transmission and ultimately turns the wheels. The piston then moves up in the cylinder, pushing the burnt gases out of the engine and out the tailpipe as exhaust. This cycle repeats several times per second.

The more cylinders an engine has, the smoother it tends to run and the more power it can make since the combustion events occur very close together. The more cylinders an engine has, however, the more complex and mechanically inefficient the engine becomes. How the cylinders are configured also has a bearing on performance, vibration, and other factors. That’s true for both gas and diesel engines.

Where diesel and gas engines diverge is how the fuel is ignited inside the engine. In a gas engine, the air and fuel are compressed and, at a critical point in the timing of the cycle, a spark plug ignites the mixture. But in a diesel engine, there are no spark plugs. When diesel fuel and air are squeezed enough, the extreme compression generates enough heat that the mixture spontaneously combusts. This is known as “compression ignition,” and it’s the basis of how a diesel engine works. When a gasoline engine has compression ignition, we often call it “knock,” and it can ruin the engine. But diesel engines are built to take advantage of it.

Diesel fuel, which is syrup-like compared to watery gasoline, packs more of a punch in terms of energy per unit of weight than gasoline (say, per gallon or liter). Even though it often costs more than gasoline, it contains more potential energy, so less diesel is required in terms of fuel to accomplish the same amount of work (in this case, driving distance) as gasoline. That’s why diesel cars tend to get mileage in the 30s, 40s, or 50s — or more — depending on the engine and vehicle type. In terms of fuel comparative costs, it’s either a wash or a slight advantage for either fuel, depending on the price difference, vehicle, and driving style. But overall, the final cost for using either fuel over time is going to be nearly the same.

Diesel owners also have another fueling option: Biodiesel. Biodiesel is made from non-petroleum sources such as fry oil or vegetable oil. Indeed, the first diesel engines ran on peanut oil-derived diesel fuel. And while making your own gasoline at home is very complex, dangerous, and ill-advised, it is possible to safely make your own biodiesel using ready-made kits

Both gasoline and diesel engines are measured in terms of horsepower and torque. Horsepower is a measure of power, while torque is a measure of an engine’s twisting force on the driveline. Big amounts of both are great, but if you have a lot of horsepower with only a little torque, your vehicle is going to be slow to get going. It’s torque that gets things moving, which is why diesel engines are used in big trucks; they are great for moving heavy loads because they have so much torque. But diesel engines don’t rev up very high, so they make less horsepower on average than gas engines, which is why they aren’t many sports cars with diesel engines.

What car manufacturers have found is that drivers love torque, both in pickups and cars. Step on the gas and a diesel-powered car has brisk acceleration. Coupled with a capable transmission, modern diesels can get going fast in a hurry. While they don’t scream to a high redline like a gas engine, that’s not a big deal for most people, who would rather have real-world power (torque), acceleration, and good gas mileage.

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