freedomCAR & vehicle technologies program U.S. Department of Energy • Office of Energy Efficiency and Renewable Energy Just the Basics Diesel Engine

Today’s direct-injection diesel engines start the explosions with Less dependence on foreign engines are more rugged, sparks from spark plugs, whereas in powerful, durable, and diesel engines, fuel ignites on its own. oil, and eventual transition reliable than gasoline engines, and use fuel much Air heats up when it’s compressed. to an emissions-free, more efficiently, as well. This fact led German engineer Rudolf Diesel to theorize that fuel could be petroleum-free vehicle made to ignite spontaneously if the air Diesel Engines Yesterday, inside an engine’s cylinders To d ay, and Tomorrow became hot enough through compression. Achieving high Diesel Fuel Ignition Diesels are workhorse engines. That’s temperatures meant producing Intake Fuel Exhaust why you find them powering heavy- much greater air compression Valve Injector Valve duty trucks, buses, tractors, and trains, than occurs in gasoline Intake Exhaust not to mention large ships, bulldozers, engines, but Diesel saw that Air Gases cranes, and other construction as a plus. According to his Cylinder equipment. In the past, diesels fit calculations, high compression Head the stereotype of muscle-bound behe- should lead to high engine moths. They were dirty and sluggish, efficiency. Part of the reason Cylinder smelly and loud. That image doesn’t is that compressing air concen- Wall apply to today’s diesel engines, trates fuel-burning oxygen. Piston however, and tomorrow’s diesels will A fuel that has high energy show even greater improvements. content per gallon, like diesel They will be even more fuel efficient, fuel, should be able to react more flexible in the fuels they can use, with most of the concentrated and also much cleaner in emissions. oxygen to deliver more punch per explosion, if it was How Diesel Engines Work injected into an engine’s cylin- ders at exactly the right time. Like a gasoline engine, a diesel is an internal combustion engine that Diesel’s calculations were converts chemical energy in fuel to correct. As a result, although mechanical energy that moves pistons diesel engines have seen vast up and down inside enclosed spaces improvements, the basic called cylinders. The pistons are concept of the four-stroke Crankshaft connected to the engine’s crankshaft, diesel engine has remained which changes their linear motion virtually unchanged for over into the rotary motion needed to 100 years. The first stroke propel the vehicle’s wheels. With both involves drawing air into a In the modern direct-injection diesel engine, gasoline and diesel engines, energy is cylinder as the piston creates space for fuel combustion is confined to a specially released in a series of small explosions it by moving away from the intake shaped region within the head of each piston. (combustion) as fuel reacts chemically valve. The piston’s subsequent upward There, diesel fuel ignites spontaneously, yet with oxygen from the air. Diesels differ swing then compresses the air, heating in a carefully controlled manner. Old-style from gasoline engines primarily in the it at the same time. Next, fuel is indirect-injection diesels were not capable way the explosions occur. Gasoline injected under high pressure as the of this precision. As a result, fuel efficiency suffered and emissions soared.

Bringing you a prosperous future where energy is clean, abundant, reliable, and affordable Just the Basics: Diesel Engine

piston approaches the top of its of as NOx) and particulate matter (PM) compression stroke, igniting sponta- have been reduced by over 90 percent neously as it contacts the heated air. since 1980, as well. Nevertheless, NOx The hot combustion gases expand, and PM emissions remain at relatively driving the piston downward in high levels. NOx contributes to acid what’s called the power stroke. During rain and smog, while adverse health its return swing, the piston pushes effects have been associated with spent gases from the cylinder, and exposures to high PM amounts. the cycle begins again with an intake of fresh air. What Improvements How Diesel Engines Have Need to Be Made? Improved Diesel engines are already more efficient than gasoline engines (45 Older diesel engines mixed fuel and percent versus 30 percent), and further air in a precombustion chamber before advances are possible (to 55-63 injecting it into a cylinder. The mixing percent). Widespread use of diesel and injection steps were controlled engines, particularly in trucks, vans, A Strong Energy Portfolio mechanically, which made it very and sport utility vehicles, therefore for a Strong America difficult to tailor the fuel-air mixture promises to substantially reduce to changing engine conditions. This United States dependence on foreign Energy efficiency and clean, led to incomplete fuel combustion, petroleum products. The stumbling particularly at low speeds. As a result, block to reaching this goal, however, renewable energy will mean fuel was wasted and tailpipe emissions remains NOx and PM emissions. were relatively high. Unfortunately, increasing diesel a stronger economy, a cleaner efficiency does not necessarily make Today’s diesels inject fuel directly these emissions go away. environment, and greater energy into an engine’s cylinders using tiny computers to deliver precisely the Three basic strategies are being independence for America. Working right amount of fuel the instant it pursued to meet the U.S. Environ- with a wide array of state, community, is needed. All functions in a modern mental Protection Agency’s diesel engine are controlled by an increasingly stringent emissions industry, and university partners, electronic control module that standards. The first concerns research communicates with an elaborate array into the diesel combustion process the U.S. Department of Energy's of sensors placed at strategic locations to better understand how soot parti- Office of Energy Efficiency and throughout the engine to monitor cles and NOx gases form. The second everything from engine speed to involves development of NO and x Renewable Energy invests in a diverse coolant and oil temperatures and PM removal technologies to eliminate even piston position. Tight electronic residual emissions in much the same portfolio of energy technologies. control means that fuel burns more way that catalytic converters eradicate thoroughly, delivering more power, emissions from gasoline engines. greater fuel economy, and fewer The third research area relates to emissions than yesterday’s diesel improving diesel fuels. The U.S. engines could achieve. Department of Energy is working with engine manufacturers and fuel Modern direct-injection diesel suppliers to develop diesel fuels engines produce low amounts of that are optimized for today’s and carbon dioxide, carbon monoxide, tomorrow’s advanced diesel engines. and unburned hydrocarbons. The new fuel formulations will enable Emissions of reactive nitrogen both high fuel economy and very August 2003 compounds (commonly spoken low emissions. Printed on recycled paper

For more information, visit www.eere.energy.gov/vehiclesandfuels or call 1-800-DOE-3732