Diesel Engine Mechanics: Valve Timing, Combustion, and Turbocharging

Valve Overlap

Valve overlap occurs when both intake and exhaust valves are open simultaneously. The duration of this overlap depends on engine characteristics and the RPM at which maximum power is developed. As engine RPM increases, the time available for gas exchange decreases, necessitating larger valve opening angles. High-performance engines utilize a broad valve overlap angle to ensure sufficient cylinder filling at high RPM.

Diesel Engine Features

In a diesel engine, only air is introduced during the intake stroke. Fuel is injected directly into the combustion chamber, where ignition occurs upon contact with air that has reached a high temperature due to strong compression.

Diesel Fuel Properties

• Density: 0.81 to 0.85 kg/L at 15°C.
• Calorific Value: Approximately 42,000 kJ/kg.
• Cetane Number: Measures ignition quality; a higher number indicates a lower temperature is required for ignition.
• Viscosity: Increases at low temperatures. Below -25°C, fuel may struggle to pass through filters and injection ducts, causing cold-start difficulties.

Mixture Formation

Due to the limited time available for mixture formation, the compressed air must be at a high temperature to facilitate fuel gasification. Additionally, significant turbulence is required to ensure the fuel mixes with the largest possible quantity of air. Unlike gasoline engines, diesel engines do not require a specific stoichiometric ratio; instead, they rely on excess air to achieve complete combustion.

Combustion Process

Fuel is injected at the end of the compression stroke. The compressed air, heated above 500°C, causes the fuel to ignite before the piston reaches Top Dead Center (TDC). In engines with auxiliary combustion chambers, glow plugs are necessary to facilitate cold starts.

Cylinder Charging

Cylinder filling is always maximized to ensure excess air for complete combustion. Diesel engines lack a throttle valve in the intake manifold, meaning there is no air restriction. Engine load is regulated solely by varying the amount of fuel injected via the accelerator pedal.

Supercharging

Supercharging is widely used in diesel engines to improve performance and increase power output without increasing displacement. It forces more air into the cylinder, allowing for more fuel to be burned during each cycle.

The Turbocharger

The turbocharger is the most common supercharging system used in four-stroke diesel engines. It utilizes the velocity of exhaust gases to spin a turbine coupled to a shaft. The other end of the shaft drives a centrifugal compressor, which increases the intake air pressure. The boost pressure varies with engine speed, and a wastegate or similar mechanism is typically used to prevent the turbo from exceeding safe RPM limits.