Diesel Engine

Diesel engines are the backbone of many, buses, ships, bulldozers, heavy duty trucks, and other construction equipment. In the past, these powerhouses were considered to be dirty, sluggish, and loud. Today, these engines have been improved to the make diesel fuel more efficient, more versatile, and cleaner to burn. Diesel engines are now one of the most efficient types of heat engines, making them a good choice as prime movers for many different operations including heavy machinery, automobiles, and power generation. The diesel engine is powered by the compression ignition engine that Rudolph Diesel invented more than a century ago.

Caterpillar Diesel Engine
Caterpillar diesel engine [129].

 

Diesel Cycle

Diesel engine cutaway
Diesel engine cutaway [6].
The diesel engine is an example of an internal combustion engine, which converts chemical energy into mechanical energy by moving pistons up and down in enclosed spaces, called cylinders. Energy is released in a series of small explosions when the fuel reacts, or combusts with oxygen from the air.

A diesel engine can be designed to operate using either a two-stroke or four-stroke cycle. A full cycle in a four-stroke diesel engine consists of two complete rotations of the crankshaft. The four-strokes that make up the cycle are the intake, compression, power, and exhaust.

Animation of a Four Stroke Diesel Cycle
Animation of a four stroke internal
combustion engine [7].
In the four four-stroke process, the intake stroke starts at top dead center (TDC) and as the piston moves down to bottom dead center (BDC), fresh air is drawn into the cylinder. The air is then compressed as the piston moves back up to TDC. The fuel is injected near the end of the compression stroke where it ignites due to the high temperature and pressure. The mechanical work is generated when the piston is forced down due to the expansion of the gases that the combustion produces. The last stroke is the exhaust stroke, when the piston travels from BDC to TDC, during which the products of combustion are expelled from the cylinder. After the exhaust stroke, the cycle starts all over again with the intake stroke. Four-stroke engines are found in most automobiles, trucks, and farm equipment.

A two-stroke engine, on the other hand, completes a full cycle in one rotation of the crankshaft. The engine is constructed so that the intake and exhaust is done within the same rotation. There is no need for an intake valve because the cylinder is designed with an opening nearing BDC through which fresh air is drawn in. The intake starts at BDC where the opening in the cylinder is with the exhaust valve open. This allows a flow of air through the cylinder. The engine then compresses the air as the piston moves to TDC. The fuel is injected the same way it is in a four-stroke engine, and the piston travels back down in the power stroke. Close to the bottom of the cylinder, the exhaust valve opens to begin the release of the combustion gases.

Theoretically, the two-stroke engine can produce twice as much power as a four-stroke engine because there is a power stroke for every turn of the crankshaft instead of every other turn. In practice, the limited amount of time that is allowed for the intake and exhaust makes the two-stroke cycle less efficient and not very clean burning. Two-stroke engines are commonly used to power chainsaws, jet skis, and dirt bikes, for example. Because a two-stroke cycle can produce more power than a four-stroke cycle when it is operated properly, two-stroke engines are also used in large applications such as locomotives and big cargo ships. Laws controlling emission levels are not as strict for these vehicles, which allows for the use of a two-stroke system.

Small engines may have only one piston cylinder assembly but large engines will have more piston cylinder pairs. When multiple pistons are connected to the crankshaft, they are staggered so that the power stroke of one piston cylinder pair helps drive the rest of the piston cylinders to complete whichever stroke they are in. This generates a continuous power supply because not all of the piston cylinder sets are producing power at the same time.

How Stuff Works → Crankshaft

The crankshaft converts the linear motion of the pistons into rotational motion that is transmitted to the load, whether it be a generator or to power a vehicle.

Animation of Crankshaft
Animation of crankshaft [24].
The crankshaft is a rod that has offset sections called throws, which are attached to the pistons. The placement of these throws depends on the setup of the engine (inline or V), and they are arranged so that when the pistons travel upward and downward, the crankshaft is turned.

Because diesel engines are usually used for heavy duty equipment the crankshafts must be strong enough to handle the load without bending or twisting. They are usually made out of a one-piece casting of heat-treated alloy steel or, sometimes, cast iron, depending on the application, which allows for good mechanical strength. Crankshafts for very heavy duty usage have a hardened bearing surface to provide extra strength.

Turbochargers and Aftercoolers

Turbocharger
Turbocharger cutaway.
To increase an engine’s efficiency, turbochargers and aftercoolers are installed on the generators. The turbocharger draws in exhaust air, which still contains energy and the energy is extracted when it is sent through a small turbine. The turbine subsequently powers a small compressor placed before the air intake.

The compressor increases the pressure of the intake air going through the cylinders. Unfortunately, compressing the air also heats it slightly, so it must then be sent through an aftercooler. The aftercooler lowers the temperature of the air, making it more dense, resulting in pre-compressed and denser air. Because the exhaust waste is used, less work is needed during the compression stroke for the air to reach a critical temperature.

For diesel vehicles, the term intercooler is usually used in place of aftercooler, which had been used before more recent developments in combustion engine design. The meaning of intercooler has changed to specify a cooling stage in between two heating stages, while aftercoolers are the final cooling stage before the engine.

Transportation

Transportation

Most of the major manufacturers that produce personal vehicles with diesel engines are based in Europe. Diesel cars are more popular in Europe where 54.9% of registered cars are powered by diesel engines [88]. Comparatively, only about 3% of cars in the U.S. are diesels [89]. Because gasoline is much less expensive in this country most vehicles produced in the U.S. run on gasoline engines, even though diesel engines are up to 40% more efficient than gasoline engines [89].

Semi trucks

Semis are the vehicles that most people associate with diesel engines. Because 18 wheeler trucks have to haul extremely heavy loads (up to 80,000 lb maximum weight [90]), they need the extra torque that is produced by a diesel engine. As of 2012, 68.5% of all freight shipped in U.S. are transported using diesel trucks [91]. Most 18 wheelers have engines that are between 400-600 horsepower, which is about three times greater than the horsepower for the typical family vehicle (150-200 hp). Semi trucks also generally produce over 1200 lb-ft of torque with some of the most powerful semi truck engines producing over 2000 lb-ft of torque [90].

Power Generation

The combination of a diesel engine and an electrical generator is referred to as a diesel generating set or diesel genset. The engine produces

Caterpillar Diesel Engine
Caterpillar diesel engine [129].
mechanical energy that is transformed into electrical energy by the generator. The engine and the generator work together to produce an electrical current. Depending on the size of the components, a diesel generator can produce anywhere from 8 kW for a home application to 2000 kW for an industrial complex [56].

Diesel generators are used in many applications, including backup or standby power sources in remote locations where access to a power grid is not possible. Diesel generators are commonly used on construction and mining sites to power lights and other electrical tools. Sites such as schools, hospitals, and military bases usually have diesel gensets in case of the electricity outages.

Diesel vs. Gasoline

Diesel engines have a much higher compression ratio, lean burn operation, and direct injection, and are therefore more energy efficient and have more torque than a spark-ignition gasoline engine of the same displacement. In addition, diesel fuel contains about 11% more energy by volume than gasoline, increasing the diesel’s fuel efficiency ratio [92].

In a gasoline engine the air and fuel are mixed before entering the cylinder. It is much more difficult to compress an air/fuel mixture than it is to compress air alone. In a diesel engine, the compression problem is solved, which greatly increases the mileage and efficiency of a diesel car. Over the diesel's operating range, the average thermodynamic efficiency is in the mid-40% range, which is about 15% better than a gas engine [93].

A diesel engine is less complicated than a gasoline engine because it does not need an ignition system, but diesel engines can be noisy and may require more maintenance than gasoline engines.

Diesel engines can tow or carry heavier loads for longer distances because they produce more torque, so most large trucks, pickups, and vehicles that transport large amounts of cargo or have a high towing capacity have diesel engines.

Selecting correct engine

A buyer should assess many different design characteristics before purchasing a diesel engine, depending on what is needed for a particular job. One of the first things to evaluate is the amount of energy or torque the engine can generate; matching the size to the work needs is critical. The size and weight of the engine will affect vehicle performance. If the engine is going to be used as a backup or standby unit, the potential noise level should be assessed. Fuel efficiency of the unit is also important, as well as the cost depending on the application.

Here is a video animation showing how a diesel engine works.
Want to know more on how engines work? Click here.
Want to learn more about turbocharging? Check out this MTU article.
How can you make a semi-truck more efficient? Click here to find out.
How do diesel powered cars compare to unleaded gas powered cars? Read this CarsDirect article to find out.
How do diesel cars compare to gas and electric powered cars? Read this article.