How Diesel Engines Work

Diesel engine is one of the two main types of internal-combustion engines used today, the other being spark-ignition engine (found in most motor cars). Invented by Rudolf Diesel, the engine makes use of compression ignition to burn the fuel. Manufactured in two stroke and four stroke versions, it initially served as a more efficient replacement for stationary steam engines. It was only in 1910s that diesel engines found their use in submarines and ships. Locomotives, large trucks and electric generating plants started making use of the engine sometime later only. Let us gather information on how diesel engines actually work.

Working Of A Diesel Engine

Each of the pistons of a diesel engine undertakes the process of compressing air in a cylinder. As fuel is injected into the engine, under high pressure - as a spray, the formation of an explosive mixture takes place, which ignites spontaneously under pressure. During the combustion of fuel, there is a conversion of the chemical energy, stored in the fuel, into thermal i.e. heat energy. With this, the temperature in each cylinder becomes as high as 2,480°C, leading to the creation of pressures of about 100 kilograms per square centimeter.

The pressure, a result of combustion and the resultant thermal energy, pushes against the tops of the pistons. This, in turn, forces the pistons, connected by a rod or some other suitable connecting mechanism, to a crankshaft they turn, to the other end of their cylinders. As the crankshaft moves, rotary power is supplied to the vehicle, or the machine, and it starts working. The power of diesel engines can be increased by supercharging, which can be described as the technique of forcing air, under pressure, into the cylinders.

Some More Information

The compressed air, if it has to ignite the fuel inside the cylinders of a diesel engine, needs to have a certain temperature. The rise in the temperature of the air, in turn, depends upon the piston, as to how well it works in compressing the air. The working of the piston is determined by the ratio between the volume of uncompressed air and the volume of the air after it is compressed. In large cylinders, the compression ratio is about 13 to 1, while small cylinders require it to be as high as 20 to 1. On an average, the ratio is 14.5 to 1.

Image Gallery: Diesel Engines

One of the most popular HowStuffWorks articles is How Car Engines Work, which explains the basic principles behind internal combustion, discusses the four-stroke cycle and talks about all of the subsystems that help your car's engine to do its job. For a long time after we published that article, one of the most common questions asked (and one of the most frequent suggestions made in the suggestion box) was, "What is the difference between a gasoline and a diesel engine?"

Diesel's story actually begins with the invention of the gasoline engine. Nikolaus August Otto had invented and patented the gasolineengine by 1876. This invention used the four-stroke combustion principle, also known as the "Otto Cycle," and it's the basic premise for most car engines today. In its early stage, the gasoline engine wasn't very efficient, and other major methods of transportation such as the steam engine fared poorly as well. Only about 10 percent of the fuel used in these types of engines actually moved a vehicle. The rest of the fuel simply produced useless heat.

Rudolf Diesel, inventor of the diesel engine.

In 1878, Rudolf Diesel was attending the Polytechnic High School of Germany (the equivalent of an engineering college) when he learned about the low efficiency of gasoline and steam engines. This disturbing information inspired him to create an engine with a higher efficiency, and he devoted much of his time to developing a "Combustion Power Engine." By 1892 Diesel had obtained a patentfor what we now call the diesel engine.

If diesel engines are so efficient, why don't we use them more often? You might see the words "diesel engine" and think of big, hefty cargo trucks spewing out black, sooty smoke and creating a loud clattering noise. This negative image of diesel trucks and engines has made diesel less attractive to casual drivers in the United States -- although diesel is great for hauling large shipments over long distances, it hasn't been the best choice for everyday commuters. This is starting to change, however, as people are improving the diesel engine to make it cleaner and less noisy.

If you haven't already done so, you'll probably want to read How Car Engines Work first, to get a feel for the basics of internal combustion. But hurry back -- in this article, we unlock the secrets of the diesel engine and learn about some new advancements.

Diesel Engines vs. Gasoline Engines

In theory, diesel engines and gasoline engines are quite similar. They are both internal combustion engines designed to convert the chemical energy available in fuel into mechanical energy. This mechanical energy moves pistons up and down inside cylinders. The pistons are connected to a crankshaft, and the up-and-down motion of the pistons, known as linear motion, creates the rotary motion needed to turn the wheels of a car forward.

Both diesel engines and gasoline engines covert fuel into energy through a series of small explosions or combustions. The major difference between diesel and gasoline is the way these explosions happen. In a gasoline engine, fuel is mixed with air, compressed by pistons and ignited by sparks from spark plugs. In a diesel engine, however, the air is compressed first, and then the fuel is injected. Because air heats up when it's compressed, the fuel ignites.

The following animation shows the diesel cycle in action. You can compare it to the animation of thegasoline engine to see the differences.

The diesel engine uses a four-stroke combustion cycle just like a gasoline engine. The four strokes are:

Intake stroke -- The intake valve opens up, letting in air and moving the piston down.
Compression stroke -- The piston moves back up and compresses the air.
Combustion stroke -- As the piston reaches the top, fuel is injected at just the right moment and ignited, forcing the piston back down.
Exhaust stroke -- The piston moves back to the top, pushing out the exhaust created from the combustion out of the exhaust valve.

Remember that the diesel engine has no spark plug, that it intakes air and compresses it, and that it then injects the fuel directly into the combustion chamber (direct injection). It is the heat of the compressed air that lights the fuel in a diesel engine. In the next section, we'll examine the diesel injection process.

Diesel Fuel Injection

One big difference between a diesel engine and agas engine is in the injection process. Most car engines use port injection or a carburetor. A port injection system injects fuel just prior to the intake stroke (outside the cylinder). A carburetor mixes air and fuel long before the air enters the cylinder. In a car engine, therefore, all of the fuel is loaded into the cylinder during the intake stroke and then compressed. The compression of the fuel/air mixture limits the compression ratio of the engine -- if it compresses the air too much, the fuel/air mixture spontaneously ignites and causesknocking. Because it causes excessive heat, knocking can damage the engine.

Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder.

The injector on a diesel engine is its most complex component and has been the subject of a great deal of experimentation -- in any particular engine, it may be located in a variety of places. The injector has to be able to withstand the temperature and pressure inside the cylinder and still deliver the fuel in a fine mist. Getting the mist circulated in the cylinder so that it is evenly distributed is also a problem, so some diesel engines employ special induction valves, pre-combustion chambers or other devices to swirl the air in the combustion chamber or otherwise improve the ignition and combustion process.

Some diesel engines contain a glow plug. When a diesel engine is cold, the compression process may not raise the air to a high enough temperature to ignite the fuel. The glow plug is an electrically heated wire (think of the hot wires you see in a toaster) that heats the combustion chambers and raises the air temperature when the engine is cold so that the engine can start. According to Cley Brotherton, a Journeyman heavy equipment technician:

All functions in a modern engine are controlled by the ECM communicating with an elaborate set of sensors measuring everything from R.P.M. to engine coolant and oil temperatures and even engine position (i.e. T.D.C.). Glow plugs are rarely used today on larger engines. The ECM senses ambient air temperature and retards the timing of the engine in cold weather so the injector sprays the fuel at a later time. The air in the cylinder is compressed more, creating more heat, which aids in starting.

Smaller engines and engines that do not have such advanced computer control use glow plugs to solve the cold-starting problem.

Of course, mechanics aren't the only difference between diesel engines and gasoline engines. There's also the issue of the fuel itself.

Diesel Fuel

Petroleum fuel starts off as crudeoil that's naturally found in theEarth. When crude oil is processed at refineries, it can be separated into several different kinds of fuels, including gasoline, jet fuel, kerosene and, of course, diesel.

If you have ever compared diesel fuel and gasoline, you know that they are different. They certainly smell different. Diesel fuel is heavier and oilier. It evaporates much more slowly than gasoline -- its boiling point is actually higher than the boiling point of water. You will often hear diesel fuel referred to as "diesel oil" because it's so oily.

Diesel fuel evaporates more slowly because it is heavier. It contains more carbon atoms in longer chains than gasoline does (gasoline is typically C9H20, while diesel fuel is typically C14H30). It takes less refiningto create diesel fuel, which is why it used to be cheaper than gasoline. Since 2004, however, demand for diesel has risen for several reasons, including increased industrialization and construction in China and the U.S. [source: Energy Information Administration].

Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x106 joules (147,000 BTU), while 1 gallon of gasoline contains 132x106 joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.

Diesel fuel is used to power a wide variety of vehicles and operations. It of course fuels the diesel trucks you see lumbering down the highway, but it also helps move boats, school buses, city buses, trains, cranes, farming equipment and various emergency response vehicles and power generators. Think about how important diesel is to the economy -- without its high efficiency, both the construction industry and farming businesses would suffer immensely from investments in fuels with low power and efficiency. About 94 percent of freight -- whether it's shipped in trucks, trains or boats -- relies on diesel.

In terms of the environment, diesel has some pros and cons. The pros -- diesel emits very small amounts of carbon monoxide, hydrocarbons and carbon dioxide, emissions that lead to global warming. The cons -- high amounts of nitrogen compounds and particulate matter (soot) are released from burning diesel fuel, which lead to acid rain, smog and poor health conditions. On the next page we'll look at some recent improvements made in these areas.

Diesel Improvements and Biodiesel

During the big oil crisis in the 1970s, European car companies started advertising diesel engines for commercial use as an alternative togasoline. Those who tried it out were a bit disappointed -- the engineswere very loud, and they would arrive home to find their cars covered from front to back in black soot -- the same soot responsible for smog in big cities.

Over the past 30 to 40 years, however, vast improvements have been made on engine performance and fuel cleanliness. Direct injection devices are now controlled by advanced computers that monitor fuel combustion, increasing efficiency and reducing emissions. Better-refined diesel fuels such as ultra low sulfur diesel (ULSD) will lower the amount of harmful emissions and upgrading engines to make them compatible with cleaner fuel is becoming a simpler process. Other technologies such as CRT particulate filters and catalytic converters burn soot and reduce particulate matter, carbon monoxide and hydrocarbons by as much as 90 percent. [source: Diesel Technology Forum]. Continually improving standards for cleaner fuel from the European Union will also push the auto industries to work harder at lowering emissions -- by September 2009, the EU hopes to have particulate matter emissions down from 25mg/kilometer to 5mg/kilometer [source: EUROPA].

You may have also heard of something called biodiesel. Is it the same as diesel? Biodiesel is an alternative or additive to diesel fuel that can be used in diesel engines with little to no modifications to the engines themselves. It's not made from petroleum -- instead it comes from plant oils or animal fats that have been chemically altered. (Interesting fact: Rudolf Diesel had originally considered vegetable seed oil as fuel for his invention.) Biodiesel can either be combined with regular diesel or used completely by itself. You can read more about biodiesel in How Biodiesel Works.

Components of Combustion Process

Diesel engines have an excellent reputation for their low fuel consumption, reliability, and durability characteristics. They are also known for their extremely low hydrocarbon and carbon monoxide emissions. However, they have also been rejected by many for their odorous and sooty exhaust that is also characterized with high nitric oxide and particulate matter emissions. Since performance, fuel consumption, and emitted pollutants result from the combustion process, it is necessary first to understand the mechanisms of combustion in diesel engines if we are to improve it.

Combustion in diesel engines is very complex and until recently, its detailed mechanisms were not well understood. For decades its complexity seemed to defy researchers’ attempts to unlock its many secrets despite the availability of modern tools such as high speed photography used in “transparent” engines, computational power of contemporary computers, and the many mathematical models designed to mimic combustion in diesel engines. The application of laser-sheet imaging to the conventional diesel combustion process in the 1990s was key to greatly increasing the understanding of this process. This paper will review the most established combustion model for the conventional diesel engine to help readers gain an appreciation of diesel combustion and how it impacts performance and emission formation.

The basic premise of diesel combustion is its unique way of releasing the chemical energy stored in the fuel. To perform this process, oxygen must be made available to the fuel in a specific manner to facilitate combustion. One of the most important aspects of this process is the mixing of fuel and air, which is a process often referred to as mixture preparation.

In diesel engines, fuel is often injected into the engine cylinder near the end of the compression stroke, just a few crank angle degrees before top dead center [Heywood 1988]. The liquid fuel is usually injected at high velocity as one or more jets through small orifices or nozzles in the injector tip. It atomizes into small droplets and penetrates into the combustion chamber. The atomized fuel absorbs heat from the surrounding heated compressed air, vaporizes, and mixes with the surrounding high-temperature high-pressure air. As the piston continues to move closer to top dead center (TDC), the mixture (mostly air) temperature reaches the fuel’s ignition temperature. Instantaneous ignition of some premixed fuel and air occurs after the ignition delay period. This instantaneous ignition is considered the start of combustion (also the end of the ignition delay period) and is marked by a sharp cylinder pressure increase as combustion of the fuel-air mixture takes place. Increased pressure resulting from the premixed combustion compresses and heats the unburned portion of the charge and shortens the delay before its ignition. It also increases the evaporation rate of the remaining fuel. Atomization, vaporization, fuel vapor-air mixing, and combustion continue until all the injected fuel has combusted.

Diesel combustion is characterized by lean overall A/F ratio. The lowest average A/F ratio is often found at peak torque conditions. To avoid excessive smoke formation, A/F ratio at peak torque is usually maintained above 25:1, well above the stoichiometric (chemically correct) equivalence ratio. In turbocharged diesel engines the A/F ratio at idle may exceed 160:1. Therefore, excess air present in the cylinder after the fuel has combusted continues to mix with burning and already burned gases throughout the combustion and expansion processes. At the opening of the exhaust valve, excess air along with the combustion products are exhausted, which explains the oxidizing nature of diesel exhaust. Although combustion occurs after vaporized fuel mixes with air, forms a locally rich but combustible mixture, and the proper ignition temperature is reached, the overall A/F ratio is lean. In other words, the majority of the air inducted into the cylinder of a diesel engine is compressed and heated, but never engages in the combustion process. Oxygen in the excess air helps oxidize gaseous hydrocarbons and carbon monoxide, reducing them to extremely small concentrations in the exhaust gas.

The following factors play a primary role in the diesel combustion process:

The inducted charge air, its temperature, and its kinetic energy in several dimensions.
The injected fuel’s atomization, spray penetration, temperature, and chemical characteristics.

While these two factors are most important, there are other parameters that may dramatically influence them and therefore play a secondary, but still important role in the combustion process. For instance:

Intake port design, which has a strong influence on charge air motion (especially as it enters the cylinder) and ultimately the mixing rate in the combustion chamber. The intake port design may also influence charge air temperature. This may be accomplished by heat transfer from the water jacket to the charge air through the intake port surface area.
Intake valve size, which controls the total mass of air inducted into the cylinder in a finite amount of time.
Compression ratio, which influences fuel vaporization and consequently mixing rate and combustion quality.
Injection pressure, which controls the injection duration for a given nozzle hole size.
Nozzle hole geometry (length/diameter), which controls the spray penetration as well as atomization.
Spray geometry, which directly impacts combustion quality through air utilization. For instance, a larger spray cone angle may place the fuel on top of the piston, and outside the combustion bowl in open chamber DI diesel engines. This condition would lead to excessive smoke (incomplete combustion) because of depriving the fuel of access to the air available in the combustion bowl (chamber). Wide cone angles may also cause the fuel to be sprayed on the cylinder walls, rather than inside the combustion bowl where it is required. Fuel sprayed on the cylinder wall will eventually be scraped downward to the oil sump where it will shorten the lube oil life. As the spray angle is one of the variables that impacts the rate of mixing of air into the fuel jet near the outlet of the injector, it can have a significant impact on the overall combustion process.
Valve configuration, which controls the injector position. Two-valve systems force an inclined injector position, which implies uneven spray arrangement that leads to compromised fuel/air mixing. On the other hand, four-valve designs allow for vertical injector installation, symmetric fuel spray arrangement and equal access to the available air by each of the fuel sprays.
Top piston ring position, which controls the dead space between the piston top land (area between top piston ring groove and the top of the piston crown), and the cylinder liner. This dead space/volume traps air that is compressed during the compression stroke and expands without ever engaging in the combustion process.

It is therefore important to realize that the combustion system of the diesel engine is not limited to the combustion bowl, injector sprays, and their immediate surroundings. Rather, it includes any part, component, or system that may affect the final outcome of the combustion process.

Is your oil level low betweenoil changes? If your car's engine is operating as it should, there will be no need to add oil. Unfortunately older engines rarely enjoy this luxury. As the engine wears, oil makes its escape. A little oil added now and then is nothing to worry about, but if you're adding a quart or more between oil changes, you may have a fixable problem in there. Your engine may be burning oil thanks to worn piston rings. Your engine could also be leaking oil thanks to a bad gasket or cracked part. Or you could be losing oil through the head gasket into the cooling system. This can be an expensive repair. <P> Check the following symptoms to see what your oil consumption problem might be.<P>

No Smoke in Exhaust

Symptom:The car uses more oil than normal, but there is no trace of smoke from the exhaust. The oil level is low between scheduled oil changes . You never noticed it before and it doesn't appear that the oil is being burned by the engine. There is not a trace of smoke in the exhaust.

Possible causes:

The PCV system is not working properly.
The Fix: Replace PCV valve.
The engine may have mechanical problems.
The Fix: Check compression to determine engine condition.
The engine's valve seals may be worn.
The Fix: Replace valve seals. (Generally not a DIY job)
The engine's gaskets and seals may be damaged.
The Fix: Replace gaskets and seals as required.

Coolant Brown and Foamy

Symptom:Engine is using more oil than normal. Coolant appears brownish and foamy. Your car seems to be losing oil someplace, but there aren't any obvious leaks, and no smoke from the exhaust. You check your coolant and it looks like foamy root beer

Possible Causes:

Blown head gasket.
The Fix: Replace head gasket.
Cracked cylinder head.
The Fix: Remove and repair head, or replace cylinder head with new part.
Leaking oil-to-water cooler. Some oil coolers circulate oil inside a chamber that is filled with coolant. This allows for the exchange of heat between the two systems. Sometimes a leak in the oil line inside this chamber can cause oil to leach into your cooling system.
The Fix: Repair or replace oil cooler.

Oil Puddles or Drips Under Car

Symptom:Engine is using more oil than normal. Oil puddles under the car when parked. The oil level is low between oil changes. You see puddles of oil under the car. Obviously you have an oil leaks. You may or may not see smoke or smell oil burning when you stop at a light, stop sign. or park the car. You should make sure the engine always has the proper oil level.

Possible causes:

The PCV system is not working properly.
The Fix: Replace PCV valve. Check and repair PCV system as required.
The engine's gaskets and seals may be damaged.
The Fix: Replace gaskets and seals as required. Finding them is the trick, and visual inspection is the best way.
Oil filter may not be tightened properly.
The Fix: Tighten or replace oil filter. Sometimes the fix is far more simple than you would have imagined!

Smoke in Exhaust

Symptom:Engine uses more oil than normal, and there is some smoke from the exhaust. The oil level is low between oil changes. It appears that the oil is being burned by the engine because of the smoke in the exhaust. You may or may not notice the engine doesn't have the same power as it used to.

Possible causes:

The PCV system is not working properly. A clogged PCV system can cause major oil blowback, which means that oil is actually being sucked back into the engine through the air intake.
The Fix: Replace PCV valve.
The engine may have mechanical problems.
The Fix: Check compression to determine engine condition. An engine with poor compression may be a simple fix, but it could also have major leaks in the rings, head gasket, or other places.
The engine's piston rings may be worn. A worn piston ring causes engine oil to slip past. This means that engine oil will be found on the wrong side of the rings. This can be due to a worn ring, or in a worst case scenario, a grooved and worn cylinder wall.
The Fix: Replace piston rings. (Generally not a DIY job)
The engine's valve seals may be worn. Similar to worn piston rings, a worn valve seal will let oil slide through where it should not.
The Fix: Replace valve seals. (Generally not a DIY job)

How Diesel Engines Work

How Diesel Engines Work

Diesel Engines vs. Gasoline Engines

Diesel Fuel Injection

Diesel Fuel

Diesel Improvements and Biodiesel

Combustion in Diesel Engines

Components of Combustion Process

Troubleshooting Engine Oil Consumption

No Smoke in Exhaust

Coolant Brown and Foamy

Oil Puddles or Drips Under Car

Smoke in Exhaust