Comprehensive Guide to Diesel Engines and Fuel Systems

Basic Definition and Distinction: The diesel internal combustion engine is a compression ignition (CI) engine. It differs from the gasoline-powered Otto cycle (spark ignition) by using highly compressed, hot air to ignite fuel rather than a spark plug.
The Combustion Chamber Cycle: * Initially, only air is introduced into the combustion chamber. * The air is compressed at a high ratio, typically between $15:1$ and $22:1$ . * Pressure Dynamics: This compression results in pressures of approximately $40\,\text{bar}$ ( $4.0\,\text{MPa}$ ; $580\,\text{psi}$ ). For comparison, a petrol engine typically operates between $8$ and $14\,\text{bar}$ ( $0.80$ to $1.4\,\text{MPa}$ ; about $200\,\text{psi}$ ). * Temperature Dynamics: The high compression process heats the air to approximately $550\,^\circ\text{C}$ ( $1,022\,^\circ\text{F}$ ).
Fuel Injection Mechanism: * Timing: Fuel is injected directly into the combustion chamber at roughly the top of the compression stroke (near Top Dead Centre or TDC). * Injector Role: The fuel injector breaks the fuel down into small droplets and ensures even distribution throughout the chamber.

Vaporization and Ignition: * The heat of the compressed air vaporizes fuel from the surface of the injected droplets. * This vapor is then ignited by the internal heat of the compressed air. * The droplets continue to vaporize and burn from their surfaces, decreasing in size until the fuel is fully consumed.
Ignition Delay and Noise: * The start of vaporization causes a "delay period" during ignition. * Diesel Knock: This is the characteristic sound produced when the vapor reaches ignition temperature, causing an abrupt increase in pressure above the piston.
Power Stroke: The rapid expansion of combustion gases drives the piston downward, which supplies power to the crankshaft.
Efficiency and Compression Ratios: * High compression ratios greatly increase engine efficiency. * In spark-ignition (petrol) engines, the compression ratio is limited because fuel and air are mixed before entry; high compression would cause damaging pre-ignition (detonation). * Because diesel engines compress only air and introduce fuel just before TDC, premature detonation is not a risk, allowing for significantly higher compression ratios.

Construction: * Diesel engines generally utilize a cast iron block. * Components are much heavier and stronger to withstand higher compression ratios and the large amounts of torque generated.
Ignition System: Diesel engines do not require a separate ignition system (coils/spark plugs) as they rely on auto-ignition.
Fueling Systems: Modern diesel engines utilize extremely high-pressure fuel delivery, reaching up to $1350\,\text{psi}$ .
Power-to-Weight Ratio: Diesel engines typically have a poorer power-to-weight ratio than petrol engines because they must operate at lower engine speeds and require heavier parts to resist operating pressures.
RPM Range: Power in a diesel engine is generally delivered in a much narrower RPM range compared to a petrol engine.

Advantages: * Fuel Economy: High compression ratios lead to better thermal efficiency. Direct cylinder injection allows for a "leaner burn." * Fuel Density: Diesel fuel is more dense than petrol, containing more energy per unit volume. * Reliability: The absence of a complex electrical ignition system generally improves reliability. * Longevity: Diesel fuel acts as a better lubricant than petrol, aiding the lifespan of engine components. * Versatility: These engines can potentially run on a variety of fuel types.
Disadvantages: * Noise (Clatter): Can be mitigated by designers through methods such as indirect injection, pilot/pre-injection, adjusting injection timing/compression ratios, turbo boosting, and Exhaust Gas Recirculation (EGR). * Weight: Heavier parts are necessary to handle high torque and pressure.

Early Petrochemicals: The industry dates back to 1859 with the discovery of crude oil in Pennsylvania. Initially, the primary product was kerosene (lamp oil).
Rudolf Diesel's Innovation: Rudolf Diesel, the inventor of the compression ignition reciprocating engine, originally designed his concept to use coal dust as fuel.
Transition to Liquid Fuel: Diesel recognized that liquid petroleum by-products (which were left over from kerosene refining) were superior to coal. He redesigned the engine for liquid fuels, producing a successful prototype in 1895.
Modern Diesel Fuel: Defined as a mixture of hydrocarbons obtained from petroleum with boiling points ranging from $150\,^\circ\text{C}$ to $380\,^\circ\text{C}$ .

Variability of Crude: Crude range from thin, light-colored (brownish/greenish) low-density oils to thick, black oils resembling melted tar.
High-Gravity Crude: Contains more light products (gasoline, jet fuel) and generally has lower sulfur content.
Low-Gravity Crude: Requires more complex, expensive processing and energy to convert into lighter products.
Refinery Products: Crude is converted into transition fuels (gasoline, jet fuel, diesel) and other products including LPG, heating fuel, lubricating oil, wax, asphalt, and coke.
Industrial Processes (Atmospheric and Vacuum Distillation): * Isomerization Plant: Produces Isomerate for gasoline. * Reformer: Produces Reformate. * Alkylation Plant: Produces Alkylate. * Catalytic Cracker: Processes Vacuum Gas Oil (VGO) into Light Cycle Oil and gasoline. * Hydro-cracker: Processes Atmospheric Gas Oil (AGO). * Coker: Processes residuum into coke and fuel oil.

Comparison to Gasoline: * Diesel is heavier, oilier, and evaporates much more slowly. * The boiling point of diesel is higher than that of water. * Diesel contains longer chains of carbon atoms compared to gasoline.
Economic History: Diesel used to be cheaper than gasoline because it required less refining; however, demand has risen significantly since 2004.
Energy Density Statistics: * Diesel: 1 gallon ( $3.8\,\text{L}$ ) contains approximately $155 \times 10^6\,\text{joules}$ ( $147,000\,\text{BTU}$ ). * Gasoline: 1 gallon contains approximately $132 \times 10^6\,\text{joules}$ ( $125,000\,\text{BTU}$ ). * Impact: Higher energy density combined with engine efficiency explains the superior mileage of diesel engines over equivalent petrol engines.