How Fuel Oil is Injected and Ignited in a Heavy-Oil Two-Stroke Engine
In a heavy-oil two-stroke internal combustion engine, such as a MAN B&W or Sulzer engine, fuel oil (FO) is injected into the cylinder by a fuel injection pump. There is one pump for each cylinder. The pump's plunger (or ram) is driven on its pumping stroke by a cam and returned by a spring. This system works at a constant stroke. The amount of fuel delivered is controlled by varying the point at which the pressure side of the plunger is put into communication with the suction side. When this communication is established, the pressure drops suddenly, and injection stops.
The fuel is then delivered to a fuel injector in the cylinder head. The injector contains a spring-loaded needle valve. When the pressure of the fuel oil overcomes the spring pressure, the needle valve lifts, and fuel is forced through tiny nozzle holes into the cylinder. These holes are designed to create a fine spray pattern, which is crucial for atomization and penetration.
Atomization is the process of breaking the fuel into very fine droplets, increasing their surface area for better mixing with the air. It is directly proportional to the pressure difference between the fuel and the cylinder. The fuel-air mixture auto-ignites due to the high temperature of the air, which has been heated by compression. This is how ignition occurs in a compression-ignition engine; no spark plug is needed.
$$Atomization\:\alpha\:\frac{P}{\mu.m.A}$$
Where:
- P = Pressure difference between fuel oil and cylinder
- m = Mass flow rate
- μ = Fuel viscosity
- A = Cross-sectional area of nozzle hole
- Due to atomization, the fuel is broken into fine droplets with large surface area, which mix efficiently with air.
- In the presence of high temperature (from compression) and injection, this air-fuel mixture undergoes auto-ignition, initiating combustion.
Timing Diagram
In a heavy-oil two-stroke engine, fuel injection typically begins around 10 to 15 degrees before top dead center (BTDC) and continues until approximately 10 degrees after top dead center (ATDC). The exact timing can be adjusted. This timing ensures that fuel is injected into the highly compressed air charge in the cylinder at the optimal time for efficient combustion.
Timing of Injection
- Injection Starts: 10-15° BTDC
- Injection Ends: 15~25° ATDC
Effect of Advancing Injection Timing
Advancing the injection timing means that fuel is injected earlier in the compression stroke (e.g., at 15° BTDC instead of 10° BTDC). This has significant effects on engine performance.
Advancing the injection timing leads to an increase in power output, which reduces the specific fuel consumption (SFC), or fuel per brake horsepower hour. Since injection and ignition occur earlier, the combustion process is initiated closer to the top dead center, allowing for a more complete and efficient expansion of the hot gases, which generates more power. This is similar to giving the engine a "head start" on its power stroke.
Advancing the timing allows for a more complete combustion process. Since burning starts earlier, there is more time for the fuel to combust fully before the exhaust valve opens. This reduces the amount of unburned or partially burned fuel leaving the cylinder, which lowers the exhaust gas temperature. Less "after-burning" in the exhaust manifold occurs.
Advancing the injection timing causes the peak cylinder pressure to occur earlier and to be higher. This is because combustion starts when the piston is still moving upward, compressing the gases. The rapid pressure rise from combustion is added to the already increasing compression pressure, resulting in a higher maximum pressure. This is a primary reason for advancing timing—to achieve greater power and efficiency, although excessive advancement can lead to engine damage.