Diesel Engines

Diesel engines represent tested, proven technology with a long history of success. Invented by Dr. Rudolph Diesel, a German engineer, and first marketed in 1897, the diesel engine is now the dominant power plant in heavy-duty trucks, construction equipment, farm equipment, buses, and marine applications. Diesel engines in cars and light trucks will become more common soon.

There are many reasons for this, one of which is that low-sulfur diesel fuel will be available in the United States. Diesel vehicles are very common in Europe and other places where cleaner fuels are available.

The operation of a diesel engine is comparable to a gasoline engine. They also have a number of components in common, such as the crankshaft, pistons, valves, camshaft, and water and oil pumps.

They both are available as two- or four-stroke combustion cycle engines. However, diesel engines have compression ignition systems. Rather than relying on a spark for ignition, a diesel engine uses the heat produced by compressing air in the combustion chamber to ignite the fuel. The compression ratio of diesel engines is typically three times (as high as 25:1) that of a gasoline engine. As intake air is compressed, its temperature rises to 1,300°F to 1,650°F (700°C to 900°C). Just before the air is fully compressed, a fuel injector sprays a small amount of diesel fuel into the cylinder. The high temperature of the compressed air instantly ignites the fuel. The combustion causes increased heat in the cylinder and the resulting high pressure moves the piston down on its power stroke.

---

Injector, Distribution Pipe (Rail), Rail pressure sensor, High-pressure piston pumps, Fuel pump; Fuel temperature sensor Pressure regulating valve; Pressure limiter; Fuel filter; Tank; Other Sensors

-Reference mark, Engine speed

-Accelerator pedal position, Loading pressure

-Radiator and air temperature sensor

ECU controller

+++A common rail fuel injection system.

-- Construction

Diesel engines are heavier than gasoline engines of the same power. A diesel engine must be made stronger to contain the extremely high compression and combustion pressures. A diesel engine also produces less horsepower than a same-sized gasoline engine. Therefore, to provide the required power, the displacement of the engine is increased.

This results in a physically larger engine. Diesels have high torque outputs at very low engine speeds but don’t run well at high engine speeds.

On many diesel engines, turbochargers and inter coolers are used to increase their power output.

Diesel combustion chambers are different from gasoline combustion chambers because diesel fuel burns differently. Three types of combustion chambers are used in diesel engines: open combustion chamber, precombustion chamber, and turbulence combustion chamber. The open combustion chamber is located directly inside the piston. Diesel fuel is injected directly into the center of the chamber. The shape of the chamber and the quench area produce turbulence. The precombustion chamber is a smaller, second chamber connection to the main combustion chamber. On the power stroke, fuel is injected into the small chamber. Combustion is started there and then spreads to the main chamber. This design allows for lower fuel injection pressures and simpler injection systems. The turbulence combustion chamber creates an increase in air velocity or turbulence in the combustion chamber. The fuel is injected into the turbulent air and burns more completely.

Fuel injection is used on all diesel engines. Older diesel engines had a distributor-type injection pump driven and regulated by the engine. The pump supplied fuel to injectors that sprayed the fuel into the engine's combustion chamber. Newer diesel engines are equipped with common rail systems. Common rail systems are direct injection (DI) systems. The injectors' nozzles are placed inside the combustion chamber. The piston top has a depression where initial combustion takes place. The injector must be able to withstand the temperature and pressure inside the cylinder and must be able to deliver a fine spray of fuel into those conditions. These systems have a high pressure (14,500_ psi or 1,000_ bar) fuel rail connected to individual solenoid-type injectors.

+++ The high-output Cummins turbo diesel I-6 engine used in Dodge Ram heavy-duty trucks.

The injectors are controlled by a computer that attempts to match injector operation to the operating conditions of the engine. Newer diesel fuel injectors rely on stacked piezoelectric crystals rather than solenoids. Piezo crystals quickly expand when electrical current is applied to them. The crystals allow the injectors to respond very quickly to the needs of the engine. With this new-style injector, diesel engines are quieter, more fuel efficient, cleaner, and have more power.

Diesel engines are also available in two-stroke cycle models. Most diesels generally use the four stroke cycle, while some larger diesels operate with the two-stroke cycle. Two-stroke diesels must use forced induction from either a turbocharger or a supercharger. These engines are ideal for some applications because they provide high torque for their displacement.

Advantages

When compared to gasoline engines, diesel engines offer many advantages. They are more efficient and use less fuel than a gasoline engine of the same size. Diesel engines are very durable. This is due to stronger construction and the fact that diesel fuel is a better lubricant than gasoline. This means that the fuel is less likely to remove the desired film of oil on the cylinder walls and piston rings of the engine. Diesel engines are also better suited for moving heavy loads at low speeds.

Disadvantages

The primary disadvantages of using diesel engines in passenger cars and light trucks include:

¦ Low power output

¦ Difficult cold weather starting

¦ Noise

¦ Exhaust emissions

Many diesel engines are fit with a turbocharger to increase their power. Combining turbochargers with common rail injection systems have resulted in more horsepower.

+++ A catalytic converter and particulate trap for a diesel engine.

+++ The Honda Civic Hybrid has a 1.3-liter gasoline engine and a 20-horsepower electric motor.

In cold weather, diesel engines can be difficult to start because the cold air cannot become hot enough to cause combustion, in spite of the high compression ratios. This problem is compounded by the fact that the cold metal of the cylinder block and head absorbs the heat generated during the compression stroke. Some diesel engines use glow plugs to help ignite fuel during cold starting. These small electrical heaters are placed inside the cylinder and are used only to warm the combustion chamber when the engine is cold. Other diesels have a resistive grid heater in the intake manifold to warm the air until the engine reaches operating temperature.

A characteristic of a diesel engine is its sound. This noise, knock or clatter, is caused by the sudden ignition of the fuel as it’s injected into the combustion chamber. Through the use of electronically controlled common rail injector systems, manufacturers have been able to minimize the noise.

Emissions have always been an obstacle for diesel cars and new stricter emissions standards will go into effect shortly. Cleaner, low-sulfur, diesel fuel has been available in the United States since 2007. With new technologies and the cleaner fuel, the emissions levels from a diesel engine should be able to run as clean as most gasoline engines. Many diesel vehicles have an assortment of traps and filters to clean the exhaust before it enters the atmosphere. Some diesel engines have diesel particulate filters and catalytic converters. Particulate filters catch the black soot (unburned carbon compounds) that is typically expelled from a diesel vehicle's exhaust. Most diesel cars will have selective catalytic reduction (SCR) systems to reduce NOx emissions. SCR is a process wherein a substance is injected into the exhaust stream and then absorbed onto a catalyst. This action breaks down the exhaust's NOx to form H2O and N2.

Others will use NOx traps. Diesel engines produce very little carbon monoxide because they run with an abundance of air.

Next: Other Automotive Power Plants

Prev.: Engine Measurement and Performance