Compared to the mid-90s, diesel vehicles have made a serious comeback. The increase in popularity is largely the result of a steady string of advancements and improvements to the diesel engine. New engine designs, noise- and vibration-damping technologies, and improvements like electronic engine control have spawned a new generation of engines that are more powerful and 30-35% more fuel efficient than similar-size gasoline engines, all while running quieter and smoother than diesel engines of the past. Cold-weather starting has been improved. Noise levels are down. Reliability has been improved. Tail pipe emissions are down. Most importantly, horsepower and torque are up significantly! Many of today’s diesel engines that come as an option in the medium-duty trucks are now advertising over 800ft-lbs of torque. Those numbers were unheard of 20 years ago.
To help understand today’s advanced diesel engines, it may help to understand the diesel engine in general. The main difference between a diesel and gasoline engine is the absence of an ignition system to ignite the fuel mixture. Diesel engines are compression-ignition engines as opposed to spark-ignition, so there are no spark plugs, wires, or ignition coil. Instead, heat generated from compression ignites the air/fuel mixture in the cylinders. They also have a much higher compression ratio than gasoline engines, which allows them to compress air into a tighter space in the cylinder to create the high temperatures needed to ignite the air/fuel mixture. Let’s not forget the fuel, either. The type of fuel used in gasoline and diesel engines contributes to the power produced by the engine. Compression-ignition engines burn fuel evenly, which allows diesel engines to have very good fuel mileage while producing more power than gasoline engines for the amount of fuel used.
Due to the combustion and high compression ratio inherent in diesel engines, diesel fuel system components need to be able to withstand high internal pressure. That’s why diesel components are made of high-grade metals. But the strongest, heaviest metal can’t do much for a diesel component that has clearances down to six-millionths of an inch. These tight tolerances are the reason why heavy-duty engines are still vulnerable to issues. For example, the smallest amount of dirt can wreak havoc on the engine’s internal components and fuel injection system, while water (which has poor lubrication properties) can cause corrosive damage and accelerate wear.
Before you start a diesel repair, it's important to remember this crucial first step in diesel diagnosis:
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