Putting diesel in your regular gas-guzzling automobile isn’t just a waste of money—it can also cause some problems for your car. For semis and other trucks, however, diesel is just what the doctor (or mechanic, rather) ordered. And that’s because those types of vehicles typically have diesel engines, rather than gasoline engines.
One big difference between the two engines is how they turn fuel into energy. According to Engineering Explained, gas engines usually use a spark plug to ignite the mixture of fuel and air in the cylinder. But diesel engines compress the air until it reaches a high enough temperature that the fuel will ignite as soon as it gets injected into the cylinder. In order to heat the air to that point, a diesel engine needs an especially high compression ratio: the cylinder’s volume when the piston is at its highest point compared to the volume when the piston is at its lowest point. In other words, diesel engine pistons cover more distance than gas engine pistons. (If gas engines had such high compression ratios, the air/fuel combo would ignite itself before the spark plug could do it.)
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Those higher compression ratios—and the fact that the fuel combusts right when it enters the chamber—are part of the reason diesel engines are more fuel-efficient than gas engines. They also help give diesel vehicles more torque: the amount of force applied to an object multiplied by the distance at which it’s applied. Though torque is related to horsepower, it’s not the same thing; horsepower is the rate at which work is done (i.e. how fast your engine is transferring energy to your vehicle to keep it moving). If you want a really fast ride, you want an engine with lots of horsepower. But if you want a vehicle that can cart heavy loads, you need extra torque. Because of inertia, it’s tough to go from being motionless to being in motion when you’re laden down with extra weight—high-torque trucks have the leverage they need to make it easier.
So while you could say that semis and other load-bearing vehicles use diesel because drivers get a better bang for their buck, it’s hard to talk about the merits of diesel fuel without discussing diesel engines overall.
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