There is little doubt that Toyota's implementation of a series-parallel hybrid propulsion system as used in Prius models represents a benchmark in terms of efficiency, durability, and reliability. In fact, it is reasonable to say that this hybrid drive system, as implemented by Toyota, can be said to be the basis upon which all other major car manufacturers have patterned their hybrid offerings. Having said that though, Toyota has now decided to abandon their time-tested series-parallel hybrid system in favour of a purely parallel hybrid propulsion system on the latest Tundra and according to some reports, on some Lexus vehicles, as well.
Given that a) hybrid drive trains were developed specifically to reduce fuel consumption, and b) that this particular implementation of the parallel hybrid system produces only marginal fuel savings at best, it is not clear why Toyota had opted to fit this system into vehicles that have never been known for their good fuel consumption rates, but that is perhaps a topic for another time.
Fuel savings, or the lack of fuel savings aside, parallel hybrid systems do have some advantages over series-parallel systems, and in this article, we will take a closer look at how this system works in the new Tundra* trucks, and very likely, soon, in other Toyota products, as well.
* It should be noted that although the new Tundra model is only expected to become widely available in Australia sometime during 2024, there is no harm in being prepared for the diagnostic challenges that come with a new implementation of a parallel hybrid propulsion system.
Before we get to the specifics of this particular implementation, though, we need to look at the principal differences between parallel and series-parallel hybrid propulsion systems as currently used in Toyota products, so let us start with-
Although there are some minor design differences between the series-parallel hybrid systems used by the major car manufacturers, all such systems essentially work in the same way and fail for largely the same reasons since all such systems consist of largely the same components. Here is the short version of how series-parallel hybrid systems work-
In simple terms, these systems all contain two motor-generator assemblies, commonly referred to as MG1 and MG2. In almost all cases, MG1 is used as a starter motor to start the internal combustion engine when this unit is used as an electric motor. As on all other implementations of series-parallel systems, MG1 can function as either a motor or a generator simply by switching its power supply from its field coils and its armature. Note that contrary to popular belief, the polarity of these connections does not switch around; it is where the power goes in MG1 that turns it into either a motor or a generator.
In addition to the above, MG1 can also tap some power from the engine to charge the HV (High Voltage) battery pack, in which mode MG1 functions as a standard alternator via computer-controlled charging circuits.
By way of contrast, MG2 is primarily used to assist in supplying motive power to the driving wheels (to reduce the load on the internal combustion engine), using power drawn from the HV battery pack. However, during deceleration, computer-controlled circuits and switchgear turn MG2 into a generator to assist in recharging the HV battery pack using kinetic energy provided by the vehicle’s forward motion, which brings us to the-
As the word “parallel” suggests, the parallel hybrid system in the new Tundra model uses a single MG (Motor-Generator) that is sandwiched between the engine and a conventional automatic transmission. In the schematic shown above, the MG is shown coloured blue, while the objects circled in yellow and red are the MG’s dedicated valve body and control module, respectively.
The image below shows what this setup looks like installed in a Tundra truck-
This image shows the actual location and appearance of the MG, aka 1MG in Toyota-speak, as it is installed in a hybrid Tundra truck. Before we get to the specifics of how this system works, though, it is perhaps worth mentioning that this arrangement produces around 790Nm of torque, which is 135.5Nm more than a conventional, non-hybrid Tundra fitted with the same 3.5L petrol engine produces.
While this might appear to be an attractive option, the downside to the hybrid option is that, unlike series-parallel hybrid layouts that typically produce huge fuel savings, this setup only produces fuel savings of about 0.85 kilometres per litre during city driving. Nonetheless, the additional torque is particularly useful during low-speed, but high-load conditions such as during towing, so the fuel consumption penalty is perhaps a reasonable trade-off, which brings us to-
Although this particular parallel hybrid system shares some components with parallel systems on other vehicle makes, the hybrid system on the new Tundra truck contains some purpose-built components that are unique to this implementation. Let us start by looking at the-
Front module assembly
The most notable aspect of this system is the front module assembly that contains the motor/generator, a wet clutch, and a dedicated two-solenoid valve body that controls the operation and responsiveness of the clutch.
In practice, the wet clutch (K0 clutch in Toyota-speak) functions much like torque converter lock-up clutches, but with one significant difference. In this implementation, the clutch is not only used to “blend” the torque the MG (Motor/Generator) and the engine develop, but also to start the engine, although the engine is fitted with a conventional starter motor to start the engine when certain parts or functions of the hybrid system are not available.
If all enabling conditions are met for the MG to start the engine via the clutch, an electric pump in the transmission delivers pressurized fluid to the valve body to engage the clutch between the engine and the MG, which then cranks the engine by drawing power from the HV battery pack. This system also operates during stop/operation. It is worth noting, though, that the level of clutch lock-up the valve body provides depends on whether the engine is being started, or whether the MG is contributing torque to the engine.
In terms of servicing or repairing the front module, it should be noted that the module does not contain any serviceable or repairable parts, except for the valve body, which can be replaced separately. So, when anything in the front module (apart from the valve body) fails, stops working, or becomes defective in some way, the front module has to be replaced as a complete assembly. While this may save us, as mechanics some time in getting a customer’s vehicle back on the road, there is as yet no telling what might be involved in lodging a warranty claim when a front module fails or stops working.
One thing we might want to be careful of when servicing a new Tundra hybrid truck is not to inadvertently drain the module’s oil when we intend to drain the engine oil. The two drain plugs look similar and are not very far apart, so we will want to pay particular attention to which drain plug we remove during servicing.
HV battery pack
Unlike most other hybrid systems that use lithium-based batteries, the parallel system in this implantation uses 240 NiMH (nickel metal hydride) cells that develop a nominal voltage of 288VDC at a power rating of 6.5 Ah. In tyer288VDC and 6.5 Ah ms of battery construction, the battery pack in the Tundra truck is made up of 40 modules that each contain six 1.2-volt cells.
One notable aspect of this battery pack is that the electrolyte, which is a mixture of potassium and sodium hydroxide, has a pH of 13.5, which makes the electrolyte exceedingly alkaline. In fact, the alkalinity of this electrolyte is high enough to burn human skin in the same way strong acids do. As a practical matter, then, the correct way to neutralize electrolyte spills from this battery is to use vinegar with an ascetic acid content of at least 5 per cent. Fortunately, this is the type of vinegar we can buy from any supermarket.
NOTE: While almost any other acid, such as dilute sulphuric acid (battery acid) will also neutralize the electrolyte from this battery in the rare event of an electrolyte spill, using vinegar is recommended because there is always the risk of sustaining burns from both the alkaline electrolyte and the strong acid.
As with all other HV battery packs, the battery pack in the Tundra also needs a dedicated cooling system to ensure that the battery performs optimally. In the case of the new Tundra truck, the battery pack is located under the rear seat and is cooled by two brushless motors that each drives a high-efficiency fan that circulates air (taken from the passenger cabin) around the battery to maintain the battery's temperature between a minimum of 0.0 degrees C, and a maximum of 40 deg C.
Since the fan intakes are located in the sides of the rear seat, both intakes are fitted with filters that need periodic cleaning or replacement. However, it should be noted that as with all other HV battery temperature control systems that use circulating air, the air intakes on Tundra trucks are also susceptible to clogging with clothes, sweet wrappers, lint, dust, and other objects such as displaced floor mats.
Main system relays
Since all high-voltage systems and components are isolated from the vehicle for safety reasons, the HV battery pack is used to contain the high-voltage electric power. However, “readying-on” or activating the high-voltage circuits causes large and potentially destructive inrush currents to flow through components, so to prevent this from happening, this hybrid system uses three power-controlled main system relays that work together with a pre-charge resistor to limit inrush currents.
In Toyota-speak, these relays are designated SMRB, SMRP, and SMRG, but the finer details of how these relays work fall outside the scope of this article, so we will quote a short section directly from an official Toyota repair manual* that describes this process briefly-
“…The SMRs (System Main Relays) are the relays that connect or disconnect the high-voltage power system in accordance with commands from the power management control ECU. There are 3 SMRs and 1 pre-charge resistor. SMRB, SMRP, SMRG, and the pre-charge resistor are located in the HV battery junction block assembly in the HV battery pack.To connect to the high-voltage power system, the vehicle will first turn on SMRP and SMRB to charge the vehicle through the system's main resistor. Then, SMRP will be turned off after SMRG is turned on. To shut off the high voltage power system, SMRB and SMRG are turned off.” Source:http://manualespurdy.cisdigital.com/Lexus/RX450h/rm2270e/MANUAL.HTM/rm2270e/repair2/html/frame_rm000002caj02cx.html
* Although the above section is taken from a repair manual for Lexus vehicles, the implementation of this parallel hybrid system is identical on Lexus vehicles and Tundra trucks.
Inverter/converter assembly
The inverter assembly, which is located in the engine compartment, contains the MG control module, and the DC/DC converter, but unlike most other hybrid systems, this implementation does not use a conventional boost converter. Instead, this system uses six insulated gate bi-polar transistors that function in pairs to convert DC battery power into the three-phase electric current that drives the U, V, and W phases of the MG assembly.
The DC/DC converter works in the same way as DC/DC converters work in all other hybrid drive trains. This component is essentially a series of step-down transformers, rectifiers, and smoothing or filtering capacitors that collectively, convert the high-voltage current from the HV battery pack into 14VDC at 150 Amps to power the low-voltage accessories on the vehicle. These include, but are not limited to the power windows, wipers, audio system, and lighting. The DC/DC converter also functions as an alternator to charge the 12V auxiliary lead-acid battery that is also located under the rear seat but note that this battery vents any gases to the outside of the vehicle via dedicated vents and ducts
In addition to the above, the inverter assembly is also equipped with a dedicated cooling system that uses a separate radiator and a dedicated cooling fan to allow for effective cooling of the inverter assembly during times when the engine is not running, which brings us to-
When the MG is used to start the engine, the wet clutch in the front module engages the MG to crank the engine with power drawn from the HV battery pack. Once the engine starts and the state of charge of the HV battery is below a pre-defined threshold, the wet clutch automatically locks the engine to the MG to charge the HV battery.
However, if the HV battery pack's state of charge is above the minimum threshold when the engine is idling, the transmission is engaged, and the accelerator is pressed lightly, the wet clutch disengages to allow the MG to propel the vehicle without assistance from the internal combustion engine.
Note, though, that at a road speed of 40km/h, the wet clutch engages again to lock the engine and transmission together. In this mode, the engine supplies the most (almost all) motive power, but depending on the HV battery's state of charge, some of the engine's torque might be tapped off to drive the MG to recharge the HV battery pack. In addition, in this mode, the MG can contribute significant amounts of torque under some conditions through a complex series of interactions between various control modules based on the engine load/speed, throttle position, and the HV battery's state of charge, among other parameters.
Note that during deceleration, the wet clutch will disengage automatically to allow the drive train to drive the MG via the transmission to recharge the HV battery. This process, known as regenerative braking, generates a significant braking force and can be strong enough to slow even a heavy vehicle such as a Tundra truck down safely without assistance from the hydraulic brake system.
We mentioned elsewhere that the new Tundra hybrid truck has STOP and GO functionality, so let us look at how this works in this implementation. Let us start by listing the-
As with all other vehicles that feature Stop and Go systems, the system will only work when certain enabling conditions are met or satisfied. On the new Tundra truck, these conditions include the following-
The points listed above represent the minimum enabling conditions for the STOP and GO system, and provided these are all met, the system will work as expected on the current production vehicles. However, this was not always the case, because early models suffered from severely annoying issues, even if no fault codes were present that could prevent the STOP and GO system from working as designed. With this in mind, let us look at some issues that a) may or may not have been corrected on early models, and b) could present us with major diagnostic challenges if they were not corrected. Let us start with-
Delayed starting
While the engines on conventional vehicles with STOP and GO functionality typically start in only a few degrees of crankshaft rotation when the STOP and GO system is in operation, it took significantly longer for the engines in early iterations of the Tundra to start. In practice, the process of building pressure in the wet clutch’s control system to engage the clutch to crank the engine typically took longer than one second, to which time must be added the time taken to sometimes rotate the engine through one complete engine cycle before it would start.
Taken together, these actions sometimes took as long as two to three seconds, which was unacceptably long for many drivers. On current models, however, the ECU stores the position of the crankshaft when the engine switches off, meaning that with the position of the crankshaft already known when the engine cranks again, the engine will now start on the first “available” cylinder, as opposed to always starting on cylinder number 1. This means that if cylinder number 1 is just past its ignition point, the engine will start on the next cylinder in the firing order. While this programming upgrade reduces start-up times significantly, even the latest iteration of the 3.5 V6 engine still takes about one second to start during STOP and GO operation.
Transmission fluid pressure issues
When the engine is not running during SDTOP and GO operation, the mechanical pump in the 10-speed transmission cannot maintain the line pressure required to engage either gears or the wet clutch that starts the engine.
To overcome this problem, the transmission also contains an electrically operated pressure pump to supply the required line pressure when the engine is not running. However, while this system works well when it works, this system sets frequent "LOW FLUID PRESSURE" trouble codes as a result of-
Regardless of the cause of such codes, the STOP and GO system will a) not work as expected, or b) not work at all, as this writer discovered a few weeks ago when he encountered a brand new Tundra truck with a host of fault codes and an intermittently inoperable STOP and GO system. Since this truck had only a few hundred kilometres on the speedometer, this writer did the right thing: he took the easy way out and referred the vehicle to the dealer that sold the vehicle to the unhappy customer.
We need not delve into the details of this case here, but suffice it to say t it took the dealership almost three days to diagnose a badly crimped wire termination as the cause of the problem. Anyhow, the point is that the implementation of a parallel hybrid drive system on the new Tundra trucks represents what is essentially a new design that can only be diagnosed with fully updated Techstream equipment, and then only if you have access to original (complete) service and repair information from official Toyota sources, which brings us to-
The 12-volt starter motor
Unlike most, if not all other hybrid drive trains, the Tundra truck comes with a conventional 12V starter motor with which to start the engine if the MG is not available to start the engine for whatever reason. However, the main reason why this system comes with a conventional starter motor is that in cold climates, such as in much of the North American market, sub-zero temperatures could interfere with the operation of the wet clutch, hence the need for an alternative way to start the engine. This is obviously not a concern in the local environment.
So, even if the starter motor won’t be used much in Australia and other markets with warm to hot climates, the starter motor does have a service life. Moreover, even though the starter motor is rated to undergo a ridiculous 384 000 starting cycles, the ECU will issue a warning that the starter motor has to be replaced after it had started the engine 384 000 times. Additionally, the control module that manages the ECU will disable the STOP and GO system until the starter is replaced- even if it still works as designed, but note that a starter replacement also requires a reset of the starting counter with a suitable and capable scan tool.
Finally, the STOP and GO control module also has a service life; this module has to be replaced after it had started the engine 1 million times. It is hard to image either the starter motor or the STOP and GO control module ever reaching their programmed limits in terms of starting events before the vehicle falls apart through long use, but is nevertheless something to keep in mind if you ever come across an old and ramshackle Tundra hybrid truck that suddenly won't start, which leaves us with this-
Although we will likely not see the Tundra trucks with parallel hybrid drive systems in our bays in the immediate future, there is no doubt that hybrid systems will become more common on pickup trucks made by all the major manufacturers. In fact, Stellantis is now introducing its 48V mild hybrid Ram trucks to the global market, and other manufacturers will no doubt follow this trend in market segments that prioritise increased power delivery through hybrid drive trains over fuel economy.
Thus, having said the above, it is inevitable that we will start seeing hybrid trucks in our bays within the next few years. This means that unless we begin to prepare ourselves for the diagnostic challenges these vehicles will bring now, we might not be able to catch up on the new systems in time to remain relevant and/or competitive in a rapidly changing repair environment. Are you up for the challenge?
