Comparative analysis of leakage diagnosis methods of evaporative pollutant control system-ConovaWell

In order to control the pollution caused by automobiles, OBD has been introduced as a means to control the pollution emission of in-service automobiles. Among them, the United States not only carries out OBD monitoring on exhaust emissions, but also monitors the evaporative pollutant control system [1]. In the newly released GB 18352.6-2016 [2](hereinafter referred to as the National Six Standard), the evaporative pollutant control system is included in the OBD diagnosis, and the requirements of OBD diagnosis and minimum in-service monitoring frequency (IUPR) are put forward for 0.5 mm or 1 mm leakage hole.

In order to meet the requirements of emission regulation upgrading, major Oems and parts suppliers have developed a variety of OBD diagnostic methods for evaporative pollutant control systems, such as VLDS, EONV, NVLD, DMTL, ELCM, etc. This article will introduce and compare these five commonly used methods, and summarize their applicable environments.

OBD leakage diagnosis method of evaporative pollutant control system

The existing standard regulations do not mention the specific method of OBD leak diagnosis, but require that the air tightness of the evaporation system be monitored online, whatever the method, as long as it meets the standard requirements and is declared.

1.1 VLDS diagnosis Methods

Figure 1 Schematic diagram of VLDS diagnosis method

VLDS(Vacuum Leak Detection System) diagnosis method is an active negative pressure detection method carried out in vehicle operation, and it is also one of the earliest OBD leak diagnosis methods of evaporative pollutant control system [3]. As is shown in Figure 1, The Canister Purge Valve (CPV), Canister Vent valve (CVS) and Vapor Pressure Sensor were needed to realize the VLDS diagnosis method. VPS)3 necessary hardware. When CVS and CPV are closed, the evaporative pollutant control system forms a closed space. The working principle of VLDS diagnosis method is to use the vacuum generated by the engine intake process to establish negative pressure in the evaporative pollutant control system, and then ECU drives CVS and CPV to close. Observe the level of pressure rise within the evaporative contaminant control system over time to determine the level of leakage.

The general workflow of VLDS diagnostic methods consists of four parts:

1) CPV fault diagnosis. The evaporative pollutant control system closes CPV and CVS at the same time under the condition of normal pressure, and monitors the pressure change through VPS. If no negative pressure is established, the CPV is considered to have no fault, and if there is vacuum (i.e. negative pressure establishment), the CPV is considered to have a fault.

2) CVS fault diagnosis. Under the condition that negative pressure has been established in the evaporative pollutant control system, CVS is opened and CPV is closed, and the pressure changes are monitored through VPS. If normal pressure is restored, CVS is considered to have no fault; if negative pressure still exists, CVS is considered to have a fault.

3) Diagnosis of large leakage. Under any conditions, open CPV and close CVS. If the target pressure cannot be established, the evaporation pollution control system will be regarded as having a large leak.

4) Standard hole diagnosis. After the diagnosis of large leakage is passed, the CPV and CVS are closed, kept for a period of time, and the pressure changes are monitored through VPS. In the process, if the pressure reaches the target pressure value of triggering the standard hole, it is considered that there is a small leakage beyond the standard hole in the evaporative pollutant control system.

Different manufacturers will use different order for testing. The target pressure value of standard hole is related to environmental temperature, altitude, tank shape and other factors, which needs to be determined by a lot of calibration work in the early stage of development. In order to obtain a stable pressure source, the standard hole diagnosis process will be selected at idlespeed [4], and the detection time is generally about 30 s.

The diagnosis time of VLDS diagnosis method is usually selected in the starting process, waiting for the red light or idle process of traffic congestion. It cannot be completed in the smooth driving process or continuous driving process, so the IUPR of VLDS diagnosis method is not high.

1.2 Diagnostic methods for EONV

EONV(Engine Off Natural Vacuum) diagnosis method is a passive negative pressure detection method after parking [5]. Its working principle is that the fuel temperature in the tank rises due to the thermal radiation of the exhaust pipe and the heating of the fuel pump. The change of the fuel pressure during the process of the fuel temperature dropping from high temperature to the ambient temperature is used for diagnosis. The necessary hardware for EONV diagnostics is the same as for VLDS diagnostics.

The EONV diagnostics workflow is simple. It simply requires shutting down the CPV and CVS after shutdown and monitoring the changes in tank pressure over time. However, the standard hole target Pressure value of this diagnostic method is sensitive to such factors as altitude, oil amount in the tank, Reid Vapor Pressure (RVP) and ambient temperature, etc., so a lot of calibration work should be carried out in the development stage of the vehicle model to determine it [6].

The EONV detection time is generally about 1 800 s [7], and there are many cases where the parking time is longer than that, and the IUPR rate is relatively high. Because EONV diagnostics use the same hardware as VLDS solutions, they can be used together to improve IUPR.

1.3 Diagnostic Methods for NVLD

FIG. 2 Schematic diagram of NVLD temperature detection curve

NVLD(Natural Vacuum Leak Detection) is a passive negative pressure detection method after parking [8]. The working principle of NVLD diagnostic method is similar to that of EONV diagnostic method, which is also used to detect the pressure changes caused by the waste heat inside the tank after the vehicle is turned off. The difference is that NVLD diagnostic method collects the surface temperature of the tank instead of the temperature of the fuel inside the tank (the two trends are the same, as shown in Figure 2), thus greatly reducing the workload of calibration. Figure 3 shows the hardware layout of the NVLD diagnosis method. It requires intelligent modules to be arranged on the surface of the tank for sensing the temperature of the tank surface, and basic modules to be arranged at the position of the air vent of the carbon tank. The basic module has a solenoid valve and a vacuum switch. The solenoid valve is similar to CVS. After closing, the evaporative pollutant control system can form a closed space. The vacuum switch is used to determine whether the leakage level is up to standard.

Figure 3 Hardware layout of NVLD diagnosis method

The trend of external ambient temperature, fuel temperature in the tank and surface temperature of the tank after vehicle flameout is shown in Figure 2. In the first 90 minutes, the slope consistency between the surface temperature of the tank and the temperature of the fuel in the tank was poor, so diagnosis could not be made. However, in the 2 hours after 90 minutes, the temperature difference was sufficient and the slope consistency between the surface temperature of the tank and the temperature of the fuel in the tank collected by the intelligent module was good. Therefore, the diagnosis time of NVLD diagnosis method is selected from 90 minutes after vehicle flameout, and the diagnosis is completed within 2 hours.

During detection, the electromagnetic valve of the ECU drives the NVLD basic module is closed,so that the evaporative pollutant control system forms a closed space. The diagnosis process is as follows:

1) If the NVLD intelligent module detects that the temperature difference on the surface of the tank reaches -8 ℃ within 2 hours of detection, it is regarded as valid detection; otherwise, it is regarded as invalid detection.

2) If the vacuum switch is triggered, the air tightness is good; If the vacuum switch is not triggered, it is regarded as leakage.

3) If two consecutive leaks are judged, the MIL lamp will be lit.

The NVLD diagnosis method requires at least 1.5 hours of stopping time to complete the diagnosis. For example, refueling, sudden change of ambient temperature, or restarting of the vehicle before the detection is completed may result in invalid diagnosis. Therefore, the theoretical IUPR of NVLD diagnostic method is inferior to EONV diagnostic method.

1.4 DMTL diagnostic methods

FIG. 4 Schematic diagram of DMTL hardware layout

The Diagnostic method of DMTL(Diagnostic Module Tank Leak) is an active positive pressure detection method carried out after parking [9], and DMTL should be arranged outside the air vent of carbon tank, as shown in Figure 4. DMTL mainly consists of air pump, solenoid valve, heating unit and standard small hole components, as shown in Figure 5.

(a) Reference model

(b) Diagnostic mode

The working principle of DMTL is to continuously pump and pressurize the evaporative pollutant control system through its internal air pump. The feedback current through the air pump motor is compared with the calibration current to determine whether there is leakage in the evaporative pollutant control system. The diagnostic process is as follows:

1) After starting the vehicle, the heating unit will heat the DMTL to ensure that the internal components of the DMTL are kept dry during the diagnosis process.

2) After the vehicle stops, the ECU controls DMTL to enter the reference mode for detection, and the solenoid valve is switched to the position as shown in Figure 5(a). The air pump pumps air and collects the internal preset 0.5mm standard keyhole path current.

3) After the reference mode diagnosis is completed, ECU controls DMTL into the diagnostic mode, and the solenoid valve is switched to the position as shown in Figure 5(b). The air pump pumps air to the evaporative pollutant control system, collects the air pump current, and compares it with the pre-calibrated value to determine whether there is leakage.

A 0.5mm small hole is arranged inside the DMTL as a reference for comparison. Leakage of other aperture standards needs to be obtained through preliminary calibration and calculation simulation, as shown in Figure 6.

FIG. 6 DMTL diagnostic current curve

For the well sealed evaporative pollutant control system, the diagnosis can be completed in about 2 minutes after the shutdown. For the leakage system, it only takes about 10 minutes to complete. Therefore, the theory of DMTL, IUPR, is the highest among all diagnostic methods.

1.5 Diagnostic methods of ELCM

FIG. 7 Schematic diagram of the working principle of ELCM

As for ELCM(Evaporative Leak Check Module), the diagnostic method is an active negative pressure detection method for after parking [10]. Its layout position is the same as DMTL, and its working principle is similar to DMTL. The difference is that ELCM adopts the method of vacuuming outwardly for detection and arranges pressure sensors on the diagnostic path, as shown in FIG. 7. The diagnosis time of ELCM diagnosis method is also after parking, and the diagnosis time is consistent with that of DMTL. Therefore, the theoretical IUPR of ELCM is also the highest among all diagnostic methods, equal to DMTL.

2 Applicability Analysis

The above five diagnostic methods are developed on the basis of ordinary gasoline models, so they are also suitable for ordinary gasoline vehicles with atmospheric pressure evaporative pollutant control system. However, with the continuous strengthening of national laws and regulations on energy conservation and emission reduction [11], the corresponding configuration of vehicle models will become more and more abundant. The adaptability of five diagnostic methods in start-stop function and high-pressure evaporative pollutant control systems is analyzed below.

1) Analysis of adaptability of start-stop function. Start-stop function is a configuration that shuts down the engine at idle speed to achieve the purpose of energy saving and emission reduction [12].

Since VLDS diagnosis is carried out under idle conditions, for models with start-stop function, start-stop function needs to be shielded at least once during each running process. Ifdiagnosis is not completed for the first time, the start-stop function will still be shielded at the next idle speed, which will mislead drivers of vehicle start-stop function failure to a certain extent. Therefore, it is not recommended to use VLDS diagnosis method on vehicle models with start-stop function. The four diagnostic methods, EONV, NVLD, DMTL and ELCM, will not interfere with the start-stop function because they are diagnosed after stopping, and can be applied to the models with start-stop function.

2) High-pressure evaporative pollutant control system. Evaporative pollutant control system is divided into atmospheric pressure type and high pressure type. The atmospheric pressure evaporative pollutant control system controls the evaporative pollutant in the process of day and night ventilation and refueling at the same time. The tank is not resistant to high pressure, and sufficient engine desorption is required to maintain its working capacity. The high-pressure evaporative pollutant control system is equipped with oil tank barrier valve (FTIV), which can withstand high pressure. Evaporative pollutants breathing day and night can be kept in the oil tank by increasing the pressure of the oil tank to prevent them from entering the carbon tank [13]. Therefore, the high-pressure evaporative pollutant control system only needs to control the evaporative pollutant during the refueling process.

Plug-in hybrid electric vehicles (PHEVs) are unable to provide adequate desorption capability due to the long period of pure electric operation (engine not running). If atmospheric pressure evaporative pollutant control system is used, the carbon tank will fail due to long period of desorption. As a result, most PHEVs use high-pressure evaporative pollutant control systems [14], resulting in continuous high pressure in the tank (which is only opened to release pressure in the case of refueling). Based on the above analysis, the three methods of VLDS, EONV and NVLD cannot diagnose the tightness of high-pressure fuel tanks, so they are not suitable for high-pressure evaporative pollutant control system. However, DMTL and ELCM diagnostic methods are not sensitive to the pressure in the tank and only need to open the FTIV for detection. Therefore, they can be used in high-pressure evaporative pollutant control systems.

3 Closing remarks

The leakage diagnosis of evaporative pollutant control system is a part of the diagnostic requirements of National VI Standard OBD, and it is also a necessary measure to monitor the level of evaporative pollutant emission of gasoline vehicles. With the continuous development of the automobile industry and the national competent units on energy conservation and emission reduction requirements more and more strict, high-pressure evaporative pollutant control system and stop and other functional configuration will become the basic configuration of future models, in the process of the selection of evaporative pollutant control system leakage diagnosis, need to choose the most suitable diagnosis method according to its characteristics.