What is CNG (Compressed Natural Gas) Engine?
CNG, or Compressed Natural Gas, is natural gas that is compressed and stored in a gaseous state in containers. In addition to natural gas, compressed natural gas can also be used with bio-gas (mainly methane) produced artificially. As its name suggests, a CNG engine is an engine that uses CNG as its fuel (mainly used in large and passenger vehicles).
The working principle of CNG engine
High-pressure compressed natural gas flows out of the storage cylinder, filtered through a natural gas filter, and enters the high-pressure regulator through a high-pressure solenoid valve. The function of the high-pressure regulator is to reduce the pressure and heat the high-pressure compressed natural gas (working pressure 200bar-30bar) to adjust the pressure to 7bar-9bar. During the decompression process, high-pressure natural gas needs to absorb a large amount of heat due to decompression expansion. To prevent the decompressor from freezing, the gas needs to be heated by using engine coolant in the decompressor.
After being decompressed, natural gas enters the electronically controlled pressure regulator (EPR), which precisely controls the supply of natural gas based on the engine’s running status. Natural gas and air are fully mixed in the mixer and enter the cylinder of the engine, where they are ignited by the spark plug for combustion. The spark-plug ignition time is controlled by the ECM, while the oxygen sensor instantly monitors the oxygen concentration of the exhaust after combustion, calculates the air-fuel ratio, and the ECM adjusts the natural gas supply in a timely manner based on the feedback signal from the oxygen sensor.
Natural Gas Engine Control System
Turbo-Charger Control
The turbocharger provides a variable intake charging pressure corresponding to the engine load, while the waste gate valve of the turbocharger can reduce the charging pressure by releasing the exhaust pressure at the turbine. This is done through a diaphragm that pushes a lever to open the waste gate valve. The ECI system uses the waste gate control valve to control the pressure to the waste gate actuator.
The installation of an exhaust bypass control valve is used to prevent pressure from reaching the exhaust bypass valve actuator. The pressure supplied to the exhaust bypass control valve should be between 22 and 25 psig. The exhaust bypass control valve has a pressure sensor that monitors the pressure of the actuator, which releases or sends pressure to the actuator based on the WGP command size calculated by the ECM.
Load Control
ECM receives a variable voltage signal from the throttle pedal with IVS, representing the load command, and then converts this position command into a load percentage command. The ECM uses the load command to calculate the MAP and TIP targets necessary to achieve this load.
When the load on the engine increases towards the MAP target, the electronic throttle and exhaust bypass valve opening are continuously adjusted to achieve the MAP pressure value. Then, the electronic throttle is further adjusted to achieve the desired pressure difference before and after the throttle, while continuously adjusting the opening of the exhaust bypass valve during the throttle adjustment process to maintain the MAP pressure value consistent with the target value.
Fuel Control
The electronically controlled pressure regulator (EPR) is a pressure regulator driven by a high-speed and high-power motor controlled by a microprocessor. The microprocessor in the EPR receives fuel pressure instructions from the ECM through CAN communication (the difference between the fuel outlet pressure and the air pressure at the mixer inlet), and sends information from the EPR back to the ECM. Then, the motor in the EPR drives the regulator diaphragm to adjust the pressure difference at the EPR outlet, so that it meets the ECM’s instruction requirements.
Composition of CNG engine gas system
1. High pressure fuel cut-off valve
Working principle: The valve core is driven by a coil, and its opening and closing are controlled by the ECM. In the stopped state, it remains closed.
Function: To timely cut off or restore fuel supply.
2. High pressure reducer
Working principle: Overcome the spring resistance through a pressure diaphragm, drive the lever, adjust the flow area of the throttle hole, and thereby control the pressure of the decompressed natural gas. Function: By throttling and heating, high-pressure compressed natural gas is decompressed to low-pressure natural gas at 7bar-9bar.
3. Low-pressure fuel cutoff valve
Working principle: The valve core is driven by a coil, and its opening and closing are controlled by the ECM. It is in a normally closed state during shutdown.
Function: Timely cut off or restore fuel supply.
4. Electrical control pressure regulator component
Working principle: There is a control chip inside this part, which receives control instructions from the ECM and controls the amount of natural gas through a high-speed solenoid valve to effectively control the air-fuel ratio in real time.
Function: Control the supply of natural gas.
5. Engine mixer
Working principle and function: Mix natural gas and air after cooling to make the combustion more complete and gentler. It effectively reduces NOx emissions and exhaust temperature.
6. Electronic throttle
Working principle and function: By controlling the opening of the butterfly valve, the amount of mixed gas entering the cylinder is controlled, thereby controlling the engine speed and load. The driver transmits the power demand to the ECM through the throttle pedal. After receiving the throttle pedal signal, the ECM controls the opening of the electronic throttle based on the engine operating conditions. By controlling the opening of the butterfly valve, the idle speed and speed regulation characteristic curve are controlled.
7. Electronic control module
Function: The core of electronically controlled CNG engine management, which monitors the running conditions of the engine through various sensors and controls each actuator based on the running conditions and control MAP of the engine.
Working Environment: 1) Temperature: (-40-105)℃ 2) Maximum Shock: 8G@10-1000HZ Working Voltage: 16-32 V DC
8. Exhaust bypass control valve
Working principle and function: By controlling the duty cycle of the exhaust bypass control valve, the outlet pressure of the exhaust bypass control valve is controlled, thereby controlling the turbocharged pressure of the engine. This technology can effectively improve the low-speed torque and power response of the engine, meeting the working requirements of frequent starts for buses.
9. Anti-surge valve
Function: When the engine suddenly decelerates, the low-pressure pressure behind the throttle valve is transmitted to the pressure feedback joint of the anti-surge valve through the surge valve’s ventilation hose. This opens the one-way check membrane of the surge valve, balancing the pressure of the compressor before and after the turbocharger, avoiding surge in the turbocharger, and protecting the turbocharger.
10. Electronic throttle pedal
The driver drives and controls the operation of the engine through the electronic throttle pedal, reflecting the driver’s actual power demand.
