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Delong'S Speed Is Limited. It'S The Culprit!

Nowadays, multi valve technology is widely used in engines. The increase of intake valve can significantly increase the intake volume, and multi exhaust valve can significantly increase the cross-sectional area of exhaust flow, reduce the exhaust resistance, and reduce the generation of exhaust back pressure of speed is limited.

Delong'S Speed Is Limited. It'S The Culprit!

In the free exhaust time, a large amount of exhaust gas is discharged, and the forced exhaust time piston can go up. Because the effective work consumed by exhaust is also reduced, the engine torque is high and the power performance is improved. Let’s share a case related to exhaust today of speed is limited.

Delong'S Speed Is Limited. It'S The Culprit! (2)

User description: the vehicle has a recent fault with a speed limit of 80km / h, and the fault light is not on. Not long after the failure, I went to the service station for maintenance without results. Then I cleaned the exhaust pipe, but the speed limit still exists. I’ll come here when I get back to Jiaozuo (Master Wei Zhuzhu) of speed is limited.

Troubleshooting Steps Speed Is Limited:

  1. According to the owner’s statement, use the Turner C80 diagnostic instrument to read the fault code and display that the system is normal. Then follow the owner’s test run and find that the vehicle accelerates in place, the engine speed is normal, the driving speed is the same as that described by the owner, the speed limit is 80km / h, no fault light is on and the power is insufficient.
  2. Use C80 to perform the override test to check whether the urea injection is normal. After execution, the nozzle atomization is normal. Next, test whether the nitrogen and oxygen sensor is also normal of speed is limited.
  3. According to this situation, consider whether the vehicle has set the maximum speed. Use the diagnostic instrument to calibrate the system, read the current maximum speed and display 130km / h. you can eliminate the maximum speed setting here. Read the idle data flow with the diagnostic instrument and found that the intake pressure was low. Next, check whether there was air leakage or blockage in the intake system, and no fault was found of speed is limited.
  4. Considering that there is no problem with the intake system but the intake pressure is so low, the possible causes of intake blockage and air leakage do not exist, leaving the supercharger with low working efficiency and the exhaust back pressure exceeding the upper limit. Due to the maintenance time and cost of the supercharger, we decided to check the exhaust back pressure first, install the exhaust back pressure gauge and start the vehicle. It was found that the idle speed was normal, However, when the engine accelerates to 2000 rpm, the exhaust back pressure has exceeded the upper limit value (0.2bar), which obviously has the trend of meter explosion. Therefore, after cleaning the exhaust pipe with the consent of the owner, the vehicle returns to normal troubleshooting of speed is limited.
Delong'S Speed Is Limited. It'S The Culprit! (3)

Fault Summary:

Through such faults, we can learn that the problems handled by others are not necessarily OK (such as exhaust pipe cleaning). When repairing the exhaust back pressure, we should understand what symptoms will occur to the vehicle when the actual value exceeds the range (the upper limit of exhaust back pressure shall not exceed 0.2bar, and when there is no exhaust back pressure, it means that the exhaust pipe is removed) of speed is limited.

Knowledge Sharing

  • Exhaust back pressure has an important impact on engine economy, power and sound quality. It is one of the important indexes in the development and design process.
  • The increase of exhaust back pressure will increase the pump Qigong loss, resulting in increased mechanical power consumption and reduced mechanical efficiency. At the same time, too high back pressure increases the residual exhaust gas coefficient (similar to high-temperature EGR to a certain extent), resulting in insufficient combustion, reduced combustion work and deterioration of fuel economy.
  • Excessive residual exhaust gas coefficient will also cause exhaust temperature rise, smoke increase and emission deterioration. If turbocharger is installed, its working environment will become worse and the intake pressure of diesel engine will be reduced accordingly.
  • A reasonable exhaust back pressure can eliminate most of the exhaust gas in the free exhaust stage and reduce the exhaust power consumption of the piston in the forced exhaust stage. Therefore, reducing the exhaust back pressure can reduce the exhaust resistance and improve the engine power.
  • Exhaust back pressure has a great influence on diesel particulate filter (DPF). Generally, when dealing with the exhaust problem in this regard, the back pressure is the key consideration.
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