DIESEL ENGINE

Like a gasoline engine, a diesel is an internal combustion engine that converts chemical energy in fuel to mechanical energy that moves pistons up and down inside enclosed spaces called cylinders. The pistons are connected to the engine’s crankshaft, which changes their linear motion into the rotary motion needed to propel the vehicle’s wheels. With both gasoline and diesel engines, energy is released in a series of small explosions (combustion) as fuel reacts chemically with oxygen from the air. Diesels differ from gasoline engines primarily in the way the explosions occur. Gasoline engines start the explosions with sparks from spark plugs, whereas in diesel engines, fuel ignites on its own. Air heats up when it’s compressed.
This fact led German engineer Rudolf Diesel to theorize that fuel could be made to ignite spontaneously if the air inside an engine’s cylinders became hot enough through compression. Achieving high temperatures meant producing much greater air compression than occurs in gasoline engines, but Diesel saw that as a plus. According to his calculations, high compression should lead to high engine efficiency. Part of the reason is that compressing air concentrates fuel-burning oxygen. A fuel that has high energy content per gallon, like diesel fuel, should be able to react with most of the concentrated oxygen to deliver more punch per explosion, if it was injected into an engine’s cylinders at exactly the right time. Diesel’s calculations were correct. As a result, although diesel engines have seen vast improvements, the basic concept of the four-stroke diesel engine has remained virtually unchanged for over 100 years. The first stroke involves drawing air into a cylinder as the piston creates space for it by moving away from the intake valve. The piston’s subsequent upward swing then compresses the air, heating it at the same time. Next, fuel is injected under high pressure as the piston approaches the top of its compression stroke, igniting spontaneously as it contacts the heated air. The hot combustion gases expand, driving the piston downward in what’s called the power stroke. During its return swing, the piston pushes spent gases from the cylinder, and the cycle begins again with an intake of fresh air. 

Older diesel engines mixed fuel and air in a precombustion chamber before injecting it into a cylinder. The mixing and injection steps were controlled mechanically, which made it very difficult to tailor the fuel-air mixture to changing engine conditions. This led to incomplete fuel combustion, particularly at low speeds. As a result, fuel was wasted and tailpipe emissions were relatively high. Today’s diesels inject fuel directly into an engine’s cylinders using tiny computers to deliver precisely the right amount of fuel the instant it is needed. All functions in a modern diesel engine are controlled by an electronic control module that communicates with an elaborate array of sensors placed at strategic locations throughout the engine to monitor everything from engine speed to coolant and oil temperatures and even piston position. Tight electronic control means that fuel burns more thoroughly, delivering more power, greater fuel economy, and fewer emissions than yesterday’s diesel engines could achieve. Modern direct-injection diesel engines produce low amounts of carbon dioxide, carbon monoxide, and unburned hydrocarbons. Emissions of reactive nitrogen compounds (commonly spoken of as NOx) and particulate matter (PM) have been reduced by over 90 percent since 1980, as well. Nevertheless, NOx and PM emissions remain at relatively high levels. NOx contributes to acid rain and smog, while adverse health effects have been associated with exposures to high PM amounts.