Abstract
As a researcher in GAC Motor Company Limited, I have dedicated efforts to exploring the energy efficiency evaluation of hybrid electric vehicles (HEVs). During the fuel consumption test of HEVs, the variation in electrical energy of the rechargeable electric energy storage system (REESS) significantly affects fuel consumption results. This study introduces a correction procedure based on REESS energy changes to objectively assess fuel economy. Through experimental analysis, we validated that different correction coefficients (Kfuel) can lead to contrasting conclusions in fuel consumption compliance evaluations, emphasizing the importance of precise Kfuel selection for vehicle manufacturers.
1. Introduction
With the advancement of automotive technology and the implementation of carbon neutrality policies, hybrid electric vehicles have become a pivotal solution for energy conservation and emission reduction in traditional vehicles. According to statistics from the China Passenger Car Market Information Joint Committee, the sales volume of HEVs in China surged from 550,000 in 2021 to 1.42 million in 2022, highlighting their growing market dominance .
The energy economy of HEVs is evaluated through energy consumption tests, where vehicles operate under specific cycles in both charge-depleting and charge-sustaining modes. Notably, during charge-sustaining mode tests, REESS energy variations can distort fuel consumption results: fuel energy might be stored as electricity, or REESS discharge could offset fuel use, leading to inaccurate measurements . To address this, industry standards introduce a correction procedure to adjust fuel consumption to a baseline where REESS energy variation is zero, ensuring objective evaluation .
2. Correction Procedure Requirements
When conducting charge-sustaining mode tests for HEVs, correction is unnecessary if any of the following conditions are met:
- The REESS energy variation (ΔEREESS,CS) is positive, and the cycle correction standard > 0.005;
- The cycle correction standard ≤ 0.005;
- The manufacturer proves ΔEREESS,CS is unrelated to fuel consumption . Conversely, correction is mandatory if ΔEREESS,CS is negative and the cycle correction standard > 0.005. In practice, any non-zero ΔEREESS,CS allows manufacturers to correct fuel consumption to a zero-energy-variation baseline for true fuel economy assessment .
3. Establishing the Correction Procedure
3.1 Test Methodology for Correction Coefficient
The correction coefficient (Kfuel) is determined through charge-sustaining mode tests, with at least 5 trials meeting specific criteria:
- At least one test with ΔEREESS,CS ≤ 0 and one with ΔEREESS,CS > 0;
- The fuel consumption difference between the tests with the highest negative and positive energy variations must be ≥ 0.2 L/100 km -.
3.2 Calculation and Application of Kfuel
The formula for calculating Kfuel is:\(K_{\text{fuel}} = \frac{\sum_{c=1}^{n} \left[ (E_{\text{CCS}, c} – E_{\text{CCS, avg}}) \times (F_{\text{CCS, nb}, c} – F_{\text{CCS, nb, avg}}) \right]}{\sum_{c=1}^{n} (E_{\text{CCS}, c} – E_{\text{CCS, avg}})^2}\) where:
- \(K_{\text{fuel}}\): fuel consumption correction coefficient (L/100 Wh),
- c: test sequence,
- n: total number of tests,
- \(E_{\text{CCS}, c}\): energy consumption of test c (Wh/km),
- \(E_{\text{CCS, avg}}\): average energy consumption (Wh/km),
- \(F_{\text{CCS, nb}, c}\): uncorrected fuel consumption of test c (L/100 km),
- \(F_{\text{CCS, nb, avg}}\): average uncorrected fuel consumption (L/100 km) .
To correct fuel consumption, the formulas are:\(F_{\text{CCS, c, b}} = F_{\text{CCS, c, nb}} – K_{\text{fuel}} \times E_{\text{CCS}}\)\(F_{\text{CCS, p, b}} = F_{\text{CCS, p, nb}} – K_{\text{fuel}} \times E_{\text{CCS, p}}\) where:
- \(F_{\text{CCS, c, b}}\): corrected total cycle fuel consumption (L/100 km),
- \(F_{\text{CCS, c, nb}}\): uncorrected total cycle fuel consumption (L/100 km),
- \(E_{\text{CCS}}\): total cycle energy consumption (Wh/km),
- \(F_{\text{CCS, p, b}}\): corrected fuel consumption for speed segment p (L/100 km),
- \(F_{\text{CCS, p, nb}}\): uncorrected fuel consumption for speed segment p (L/100 km),
- \(E_{\text{CCS, p}}\): energy consumption for speed segment p (Wh/km) -.
4. Experimental Vehicle and Data Analysis
4.1 Vehicle Specifications
We selected a hybrid electric vehicle model for the study, with key parameters listed in Table 1:
Table 1. Basic Information of the Test Vehicle
| Project | Parameter |
|---|---|
| Vehicle Type | M1 multi-purpose passenger car |
| Engine Type | In-cylinder direct injection, turbocharged intercooled, spark-ignition |
| Drive Shaft | Front axle |
| Transmission Type | 8AT |
| Test Mass | 2,213 kg |
| Road Resistance Coefficient | \(F = 129.47 + 0.37V + 0.05V^2\) |
| Cycle Energy Demand | 4,588 Wh |
| Declared Fuel Consumption | 5.96 L/100 km |
| Announced Test \(K_{\text{fuel}}\) | 0.0240 |
4.2 Correction Coefficient Experiments
Three production-qualified vehicles of the same model were tested following GB/T 19753-2021, using the World Light Vehicle Test Cycle (WLTC). The test data for Kfuel are summarized in Table 2, with fitting results shown in Table 3.
Table 2. Test Data for Fuel Consumption Correction Coefficient
| Vehicle No. | Test No. | Total Energy Variation (Wh) | Mileage (km) | Energy Consumption (Wh/km) | Fuel Consumption (L/100 km) |
|---|---|---|---|---|---|
| Sample 1 | 1 | 445 | 23.14 | 19.23 | 6.450 |
| 2 | 163 | 22.95 | 7.10 | 6.054 | |
| 3 | 118 | 23.06 | 5.12 | 5.903 | |
| 4 | 165 | 23.08 | 7.15 | 6.136 | |
| 5 | -107 | 23.24 | -4.60 | 5.729 | |
| Sample 2 | 1 | 465 | 23.24 | 20.01 | 6.290 |
| 2 | 183 | 22.98 | 7.96 | 5.896 | |
| 3 | 126 | 23.16 | 5.44 | 6.125 | |
| 4 | 134 | 23.18 | 5.78 | 5.744 | |
| 5 | -96 | 23.22 | -4.13 | 5.862 |
Table 3. Comparison of \(K_{\text{fuel}}\) for Three Vehicles of the Same Model
| Vehicle | \(K_{\text{fuel}}\) | Intercept |
|---|---|---|
| Announced Vehicle | 0.0240 | 5.88 |
| Sample 1 | 0.0310 | 5.84 |
| Sample 2 | 0.0179 | 5.86 |
5. Differences in Correction Coefficients
The experimental results revealed significant variations in \(K_{\text{fuel}}\) among different vehicles. The maximum difference between \(K_{\text{fuel}}\) values (0.0310 and 0.0179) reached 42%, demonstrating that vehicle-to-vehicle differences in production consistency or state can drastically affect \(K_{\text{fuel}}\) .
Notably, the intercepts of the fitting curves for the three vehicles (5.88, 5.84, 5.86) had a standard deviation of only 0.016, indicating that after correcting for REESS energy variations, the fuel consumption consistency among the vehicles was high. This confirms that \(K_{\text{fuel}}\)-based correction effectively reflects the true fuel economy of HEVs .
6. Impact of Correction Coefficients on Fuel Consumption
6.1 Case Study with Different \(K_{\text{fuel}}\)
Five production vehicles of the same model were tested, and their fuel consumption was corrected using different \(K_{\text{fuel}}\) values. The results are presented in Table 4:
Table 4. Fuel Consumption Results Before and After Correction with Different \(K_{\text{fuel}}\)
| Test Vehicle | Energy Variation (Wh) | Cycle Mileage (km) | Uncorrected Fuel Consumption (L/100 km) | Corrected Fuel Consumption (L/100 km) | ||
|---|---|---|---|---|---|---|
| \(K_{\text{fuel}}=0.0240\) | \(K_{\text{fuel}}=0.0310\) | \(K_{\text{fuel}}=0.0179\) | ||||
| Sample 3 | 280 | 23.21 | 6.09 | 5.80 | 5.71 | 5.87 |
| Sample 4 | 233 | 22.97 | 6.19 | 5.94 | 5.87 | 6.00 |
| Sample 5 | 307 | 22.97 | 6.18 | 5.86 | 5.76 | 5.94 |
| Sample 6 | 307 | 23.13 | 6.21 | 5.89 | 5.80 | 5.97 |
| Sample 7 | 336 | 23.22 | 6.21 | 5.87 | 5.77 | 5.95 |
6.2 Implications for Compliance Evaluation
All five vehicles had uncorrected fuel consumption exceeding the declared value (5.96 L/100 km). When corrected with \(K_{\text{fuel}}=0.0240\) and \(0.0310\), all results met the declaration, but with \(K_{\text{fuel}}=0.0179\), two vehicles failed (6.00 L/100 km and 5.97 L/100 km) .
This highlights that using an inappropriate \(K_{\text{fuel}}\) can lead to erroneous compliance conclusions. Vehicle manufacturers must therefore consider the representativeness of \(K_{\text{fuel}}\) and establish safe margins for declared fuel consumption values to avoid misassessment .
7. Conclusions
- For HEV fuel consumption tests in charge-sustaining mode, correcting with \(K_{\text{fuel}}\) eliminates the impact of REESS energy variations, providing an objective evaluation of fuel economy. The correction procedure ensures that test results reflect real-world vehicle efficiency .
- Different \(K_{\text{fuel}}\) values can significantly alter correction outcomes, even reversing compliance evaluation conclusions. Manufacturers must prioritize the representativeness of \(K_{\text{fuel}}\) and incorporate safety margins when declaring fuel consumption to ensure accuracy and reliability .
- Production variations cause substantial \(K_{\text{fuel}}\) fluctuations among vehicles of the same model. Directly applying a generic \(K_{\text{fuel}}\) risks invalidating compliance assessments. It is recommended that manufacturers conduct vehicle-specific \(K_{\text{fuel}}\) tests before correction to guarantee precision -.