BHPian A.G. recently shared this with other enthusiasts:Why the Toyota Hybrid System is frugal even on the Expressway?The primary objective of the Hybrid drivetrain is to be frugal. Even at Expressway speeds (slightly less than 120 Kmph to avoid the beeps), the Hybrid system is still finding ways to maximize fuel economy, delivering 14.6 Kmpl (in fact, 0.1 Kmpl more than the XUV7OO D AT). Static Graph with legendsFor better understanding, the basic overview of the graph and setup is as below:Data points were captured at an interval of 0.5 seconds. This could have been faster but then the number of sensors would have been limited (around 30 sensors were captured from the OBD data stream).The curves with dashed / dotted lines have their value represented on the left vertical axis which are for Total Pack Voltage (V) in Blue, Engine Speed (RPM/10) in Orange & OBD Speed (Kmph)Solid line curves are represented on the right vertical axis which are for HV System Current (A) in Teal, MG2 Torque (Nm) in Brown & State of Charge (%) in Cyan.The horizontal axis is Time (T) in seconds.Adaptive Cruise was set at 118 Kmph.This was captured with all seats occupied and weekend luggage with both front & rear AC functioning.Observations:1. MG2 torque from T=125 to T=132 seconds is mostly negative, meaning it is helping to recoup energy due to a downward gradient or loading the engine to operate it near its maximum efficiency point – this in turn reduces the load on MG1 in as MG2 is generating power for the High Voltage AC Compressor & DC-DC converter to power the 12V loads of the vehicle. 2. Unlike a conventional transmission with fixed ratios where vehicle speed is a direct function of Engine speed x Gear Ratio (final drive ratio & tyre size are not considered as that would apply to Hybrid as well), the Hybrid system with its eCVT managed by MG1 is able to run the engine from as low at variable speed or even turn it off completely without any major deviation in speed (drop recorded is just 2 Kmph). This is done to operate the Engine at its maximum efficiency point based on the Torque requirement for the drive + 12V load & that is why the despite the speed being constant, the Engine RPM from T=125 to T=133 seconds is different from T=138 to T=150 seconds. 3. T=133 to T=137 seconds 3.a. With downward gradient increasing, the torque requirement drops considerably. Despite the high speed, the hybrid system starts to increase the MG2 torque and is able to turn off the Engine for maximum fuel saving. 3.b. As soon as the torque from MG2 is increased (i.e. power is drawn from the HV Pack), the voltage of the HV Pack starts to dip with a corresponding increase in current. 3.c. At T=137 seconds, the system is drawing 71.2A at 197.81V from the HV Battery Pack – therefore, the total instantaneous power output from the HV Battery Pack is 14.08KW. 3.d. Once the system is not able to maintain the set speed (as captured by the kink in OBD captured Speed), the Engine is brought back to life. 4. It is interesting to note that the Energy Balance or State of Charge of the Battery Pack is maintained as close to the start as possible automatically by the Hybrid System. This is evident from the graph below (refer to the Cyan curve on the right axis), where the SoC at start & end is the same at 50% despite being continuously used over a span of 3000 seconds or 50 minutes. This is one of the test conditions in the AIS homologation tests as well, where the delta in Energy Balance beyond a limit penalises both the Fuel Economy and CO2 emission scores via the formula prescribed in the AIS documents. 5. While maintaining the Energy Balance or SoC, the frugality of the Hybrid lies in the assistance from MG2. In the graph above, its torque in Brown is consistently assisting the Engine or driving the vehicle itself when the dashed / dotted Orange Engine RPM curve hits 0 & it recoups the extra energy when available.Concluding thoughts:A. Hybrid system is able to better operate the engine near its maximum efficiency point by loading the engine as shown by the negative torque of MG2. The energy so produced powers the electrical loads. B. Where the torque demand is low, either MG2 powers the vehicle or assists the Engine, thereby countering the low torque produced due to the Atkinson cycle. Check out BHPian comments for more insights and information.
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