How to Convert Liquid Fuel Generators to Gas Fuel Generators

 
Converting a four-stroke liquid fuel generator to run on gas fuel, especially from diesel to biogas, is a technically challenging process. Biogas, as a clean and renewable energy source, is gaining increasing attention due to its low cost and environmentally friendly characteristics. Converting diesel or gasoline generators to run on biogas not only reduces fuel costs but also decreases harmful emissions, providing an effective pathway for sustainable energy use.

Necessity and Challenges of Conversion

 
Converting a diesel generator to a biogas generator is more complex than using a liquid fuel generator directly. This is because liquid fuel generators lack an ignition system for gas combustion. Diesel engines ignite fuel by compressing air, whereas biogas requires a dedicated ignition system. The conversion process involves modifying and optimizing multiple systems to enable the diesel engine to operate on biogas.
 
For gasoline generators, the conversion to biogas is less complicated since gasoline engines already have an ignition system and a lower compression ratio that is compatible with biogas combustion characteristics. Therefore, these modifications are not necessary for gasoline engines.

Steps for Converting to a Biogas Generator

 
To achieve a successful conversion of a liquid fuel generator to a biogas generator, several key steps must be systematically followed. Each step is crucial to ensuring the performance and stability of the biogas generator. The following is a detailed overview of the conversion process.

1. Adjusting Compression Ratio and Modifying the Combustion Chamber

 
Diesel generators have a high compression ratio, which suits the auto-ignition properties of diesel but not the ignition characteristics of biogas. To use biogas, the compression ratio must be reduced, and the shape of the combustion chamber needs to be modified to better accommodate the mixing and combustion of biogas and air. This can typically be achieved by replacing the pistons or increasing the combustion chamber volume.

2. Designing the Biogas Intake System and Mixer

 
Proper mixing of biogas and air is crucial for stable combustion. Therefore, a specialized intake system and biogas-air mixer must be designed. The mixer should ensure thorough mixing of biogas and air before entering the cylinder to create a uniform combustible mixture, thereby ensuring efficient combustion and normal operation of the generator.

3. Designing the Gas Regulation System and Linkage Mechanism

 
Regulating gas fuel differs from liquid fuel and requires a more precise control system to manage the flow and pressure of the gas. A gas regulation system must be designed to ensure it operates in tandem with the generator’s governor, maintaining stable operation under varying loads. This system should quickly respond to load changes to maintain steady output from the generator.

4. Designing and Installing the Ignition System

 
For diesel generators being converted to biogas, an ignition system must be added. This system can be an electric ignition device powered by a magneto or battery and requires the installation of spark plugs to switch from compression ignition to spark ignition. This ensures the biogas-air mixture can be effectively ignited and burned in the cylinder.

Operating Principles of Biogas Generators

 
The basic operating principle of a biogas generator is to mix biogas with air to form a combustible gas, which is then ignited by a spark plug to perform work. Depending on the conversion plan, biogas generators can be classified into dual-fuel and single-fuel systems.

1. Dual-Fuel System

 
In a diesel-biogas dual-fuel system, a small amount of diesel is injected at the end of the compression stroke to ignite the biogas, ensuring its combustion. The advantage of this system is that it utilizes the high auto-ignition temperature of diesel to initiate biogas combustion, maintaining generator stability and reliability. Dual-fuel systems typically have a combustion efficiency of 22% to 25%, with a gas consumption of about 0.4 to 0.5 cubic meters per 0.736 kilowatt-hours.

2. Single-Fuel System

 
A single-fuel biogas generator relies entirely on biogas as fuel. The biogas and air mix in the mixer to form a combustible mixture that is drawn into the cylinder and ignited by a spark plug near the top dead center of the piston’s compression stroke to perform work. Due to the slow combustion speed of biogas, single-fuel systems may face issues such as incomplete combustion, high exhaust temperatures, and high thermal loads. To address these problems, improvements can be made by increasing the compression ratio, enhancing airflow turbulence, and increasing ignition energy.

Methods for Improving Biogas Combustion Performance

 
To enhance the combustion efficiency and operational stability of biogas generators, the following measures can be taken based on the combustion characteristics of biogas.

1. Turbulent Combustion Chamber Design

 
Adopting a turbulent combustion chamber design creates strong turbulence and small eddies within the chamber, effectively increasing the combustion speed of the biogas-air mixture, shortening combustion time, and improving combustion efficiency.

2. Selecting a High Compression Ratio

 
Methane, the main component of biogas, has good anti-knock properties, allowing for the selection of a higher compression ratio to improve combustion efficiency and power output.

3. Adjusting Valve Overlap Angle

 
To prevent backfire in the exhaust pipe, the valve overlap angle should be adjusted to reduce the crankshaft rotation angle by 60 degrees compared to the original diesel engine, optimizing the combustion process and reducing energy loss.
 
By following the above modifications and optimizations, a four-stroke liquid fuel generator can be successfully converted into a biogas generator, fully utilizing this clean energy source, reducing power generation costs, and minimizing environmental pollution. Depending on the fuel characteristics and application requirements, either single-fuel or dual-fuel systems can be selected to ensure efficient and stable operation of the generator under different conditions. This conversion not only promotes the use of renewable energy but also provides new technological avenues for energy diversification and environmental protection.