Q1 (16 Marks) Fuel Injection & Systems
MEKM • Written Exam

(a) Explain why pilot injection is required for a dual fuel engine when burning natural gas. (2)

(b) Describe, with the aid of a sketch, the arrangements for a dual fuel engine which is capable of burning natural gas on:

(i) The Otto cycle (7)

(ii) The Diesel cycle (7)

Appeared In: Oct 2023

Verified Model Answer (Text Solution)

Structured for DG Shipping MEO Class II examination scoring criteria.

Exam Ready
Part (a)

The autoignition temperature of natural gas, approximately 580°C, is significantly higher than that of diesel fuel, which falls between 200 and 300°C. In a dual-fuel engine, during the compression stroke, the temperature at the end of compression may not be sufficiently high to spontaneously ignite natural gas. To overcome this challenge, a method known as pilot injection is employed. A small quantity of diesel fuel is injected, and it serves as an ignition source for the natural gas. The combustion of diesel fuel initiates the ignition process for the entire mixture, allowing for a controlled and efficient combustion of natural gas in the dual-fuel engine.

Part (b)

(i) Otto Cycle:

  • The engine operates in gas mode during the suction stroke, where a lean air-gas mixture is drawn into the cylinders.
  • The cylinder head is equipped with a gas admission valve positioned in the air inlet passage, and there is a fuel injector capable of both main and pilot injection.
  • A common rail computer-operated pilot fuel injection system is utilised, providing precise control over the injected fuel. This system can easily regulate or shut off the fuel injected through the main injector nozzles.
  • During engine startup, diesel fuel is used for ignition, employing both pilot and main injection. Once combustion is stable, the engine transitions to a gas supply. This transition typically takes about one minute, during which the substitution of fuel oil by gas occurs gradually.

(ii) Diesel Cycle:

  • As a two-stroke engine uses intake air for scavenging, it's essential not to mix the gas fuel with the intake air.
  • Instead, the gas fuel is injected into the compressed air, similar to the injection process for diesel fuel.
  • Ignition is achieved by injecting fuel via the micro-pilot fuel injector, resulting in diffusion combustion.
  • This approach not only reduces CO emissions by 20% or more but also maintains low levels of unburned gas and CO emissions without the occurrence of knocking. The utilisation of micro-pilot fuel injection ensures a controlled and efficient combustion process, optimising the performance of the dual-fuel engine burning natural gas.
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