In the world of automotive and light truck propulsion, internal combustion engines are the most popular choice. Many cars and trucks employ gasoline and diesel engines to move, but DIESEL ENGINES are different from gasoline engines in many ways. Before the full-size next-generation TITAN^® XD arrives and we get a chance to look under the hood, let’s take a look at some diesel basics.

What makes gasoline and diesel engines different?

Compression Ignition vs. Spark Ignition

• The diesel internal combustion engine differs from the gasoline-powered engine in that it uses highly compressed hot air to ignite the fuel rather than using a spark plug.

• Only air is initially introduced into the combustion chamber in a diesel engine. 

• The air is then compressed with a compression ratio typically between 15:1 and 22:1, resulting in compression pressures typically from 300-500 psi compared to 120-200 psi in a gasoline engine.

• This high compression causes the air temperature in the cylinder to become very hot.

• Just before the piston reaches the top of the compression stroke, fuel is injected directly into the compressed air.

• The compressed air temperature is hot enough (typically above 1000ºF [538ºC]) to spontaneously ignite the fuel (which is sometimes called compression-ignition or self-ignition).

Glow Plugs

• Glow plugs (one per cylinder, located in the combustion chamber)

• Compression ignition works best when the combustion chambers (cylinders) are warm or hot.

• Glow plugs are used to help start diesel engines by supplying additional heat to ignite the fuel, which is most helpful in cold conditions or with a cold engine.

• The colder the ambient temperature or the colder the engine the more need there is for glow plug assistance when starting a diesel engine.

• Most glow plugs used today heat very quickly to temperatures of 1000ºF (538ºC) or more.

Efficiency

• The addition of a turbocharger or supercharger to a diesel engine greatly assists in increasing fuel economy and power output.

• Boost pressures can be higher on diesel engines than on gasoline engines because gasoline engines have a greater susceptibility to detonation (often called spark knock).

• Diesel engine efficiency is significantly higher than that of its gasoline burning counterpart due to the leaner running characteristics of the diesel engine and higher compression ratios.

Note: The engine efficiency of an internal combustion engine is the relationship between the total energy available in the fuel (100%) and the energy that ends up being used for work (driving the wheels). Typical engine efficiencies: gasoline engines 25-30% vs. diesel engines 36-45%.

Fuels

The most common type of diesel fuel is a specific fractional distillate of petroleum fuel oil, but alternatives that are not derived from petroleum, such as biodiesel, Biomass To Liquid (BTL) or Gas To Liquid (GTL) diesel, are increasingly being developed and adopted. Many of these alternative fuels are not approved or approved only in a blend with petroleum-based fuel. Power and fuel economy are reduced with biodiesel use.

• Sulfur content in diesel fuel

– High levels of sulfur in diesel fuel are harmful for the environment because they prevent the use of catalytic diesel particulate filters to control diesel particulate emissions as well as more advanced technologies, such as nitrogen oxide (NOx) absorbers (still under development), to reduce emissions.

– Sulfur in the fuel is oxidized during combustion, producing sulfur dioxide and sulfur trioxide that, in the presence of water, rapidly convert to sulfuric acid, one of the chemical processes that results in acid rain.

– The process for lowering sulfur reduces the lubricating properties of the fuel. Additives must be put into the fuel to give it the required ability to help lubricate engine combustion chamber components (cylinders, pistons, and valves).

– Ultra-Low Sulfur Diesel (ULSD) is a standard for defining diesel fuel with substantially lowered sulfur content.

– As of 2006, almost all of the petroleum-based diesel fuel available in the UK, Europe and North America is of a ULSD type. As of December 1, 2010, all diesel fuel sold in the U.S. for highway use (any public road) must be ULSD.

• Diesel fuel viscosity increases (thickens) as temperature decreases. If the temperature is cold enough, the diesel fuel could change to a gel that cannot flow in the fuel systems. Many fuel stations sell additives to prevent diesel fuel from becoming gel in low temperatures. Fuel suppliers may also put additives into their diesel fuels, often called “winter diesel.” Make sure to check with local diesel fuel suppliers when operating in cold weather.

Fuel Delivery

• A vital component of all diesel engines is a mechanical or electronic governor which regulates the idling speed and maximum speed of the engine by controlling the rate of fuel delivery.

• The speed and power output of a diesel engine are not controlled by an air intake throttle (although some diesel engines are equipped with a throttle valve[s] for other purposes); they are controlled by fuel delivery (timing and volume) into the combustion chambers.

• Most modern automotive and light truck diesel engines use a series of fuel pumps that supply fuel constantly, at high pressure through a common rail, to each injector.

• Diesel engine fuel injection systems operate at much higher pressures than gasoline engines.

– Standard gasoline injection systems: Up to 60 psi

– Gasoline direct injection: Up to 2,000 psi

– Common rail diesel injection: Over 10,000 psi

• Fuel contaminants such as dirt and water are often more problematic in diesel engines than in gasoline engines. Water can cause serious damage, due to corrosion, to the injection pump and the injectors. Dirt, even very fine particulate matter, can damage the injection pumps due to the close tolerances to which the pumps are machined.

• All diesel engines will have a fuel filter (usually much finer than a filter on a gasoline engine) and a water trap. The fuel filter must be replaced much more often on a diesel engine than on a gasoline engine.

• Fuel injection introduces potential hazards in engine maintenance due to the high fuel pressures used. Residual pressure can remain in the fuel lines long after an injection-equipped engine has been shut down. This residual pressure must be relieved before opening the fuel system. If residual pressure is relieved by external bleed-off, the fuel must be safely contained. To avoid the risk of personal injury or fire, drain diesel fuel into an approved container and safely dispose of in accordance with local environmental regulations.

• Depending on the circumstance, fuel is flammable. When inspecting or performing service or repairs on the fuel system, to reduce the possibility of fire and resulting severe personal injury, death, or property damage, never smoke or allow sparks or flames (such as pilot lights, electrical switches, or welding equipment) in the work area.