Nobody Understands The Point Of Hybrid Cars

TITLE: Nobody understands the point of hybrid cars CHANNEL: Technology Connections DATE: 2026-05-06 ---TRANSCRIPT--- This is a 2021 Toyota Sienna. It’s a minivan. The best kind of three row vehicle out there, according to everyone who can get over themselves. For this model year, Toyota did something very daring. They dropped the V6 engine and made every single one of these a hybrid electric vehicle. In doing so, Toyota ruffled more than a few feathers, but they also released a minivan which could go more than 50% farther on every gallon of gasoline it burns than its competition. This thing has gotten a shockingly consistent 34 miles per gallon over its life, city or highway. And it’s a minivan. My first car, a Honda Civic, could barely squeeze that out on the highway and got far worse fuel economy in the city. And that thing was a small two door coupe. This is a three row minivan. It’s even all wheel drive. The thing’s quite impressive. By now, most people know that hybrid cars get better fuel economy than their non-electrified counterparts through the use of a battery pack which stores electrical energy and electric motors which use that energy to assist the engine. But to explain this video’s title, something which I think is less well understood, is that those batteries and motors actually have relatively little to do with how hybrid cars attain their amazing fuel economy numbers. See, this vehicle is not one of those newfangled plug-in hybrids. It’s a conventional hybrid like the original Toyota Prius. And that means all of the energy available to its drivetrain comes from the gasoline in its fuel tank. There is no way to charge its batteries except driving it. And that means all of the energy the hybrid system uses to make the car go? It ultimately came from the gasoline and the engine. So why does it even have those electric motors and batteries? Well, because without them, driving this thing would suuuuccccck. To explain, we need to back up a bit. I wasn’t intending on making this video yet. I’m in the middle of a series on engine management technology, and so far I’ve only covered the catalytic converter and a mechanical overview of the internal combustion engine. So skipping ahead to hybrid drivetrains sure is skipping a lot. But you see, my spidey senses keep picking up on frankly wild misunderstandings about hybrid cars and how they work, and also why they work the way they do. Now that suddenly people care about fuel economy again, I figured it would be worth providing some actual information and not just a collection of hot takes. For example, if you’ve been under the impression that a car like this has two separate drivetrains, an electric one and a gasoline one that have been Frankensteined together into a wildly complex contraption, that’s simply not true. What Toyota’s hybrid system does is replace the traditional transmission with a new device, which is in fact much, much simpler than even the six speed manual gearbox you’ll find in this Nissan cube. Let me say that again. This thing’s drivetrain is simpler than a conventional manual transmission, and in fact, it isn’t even really a transmission at all. But I’ll get back to that. The reason hybrid drivetrains were developed is that, frankly, the internal combustion engine is just not very good at what it’s supposed to do. Yes, these are very interesting devices which tickle the ‘tisms and make lots of very satisfying noises. But when you boil them down to their core job, turning the chemical energy in a fuel into mechanical energy to move a car, they’re quite terrible. You are lucky to capture just one quarter of the energy contained in gasoline using an engine, and the rest will just be wasted as heat. That’s why your car’s got such a big radiator, and why cooling system problems can quickly destroy an engine. But what I’d like to explain today is that part of the reason engines are like that is due to a compromise. You see, engines produce wildly different amounts of power depending on how fast they’re going. Each combustion event which occurs in the cylinders can only produce so much power. So when you need to produce more engine power, you need those events to happen more frequently which means you need the engine to speed up. For example, this MR-18DE found in the Nissan Cube can only produce its rated 122 horsepower when the crankshaft is spinning at about 5,200 RPM: not that far from engine redline. At any slower engine speed, it cannot produce full power, and in fact, when running at normal cruising speeds, this thing can barely put out 50 horsepower. That is why your car has a transmission. We need to be able to vary the ratio between how quickly the engine crankshaft is spinning and how quickly the wheels are spinning in order to make practical use of an engine given its operating characteristics. When accelerating, we want to allow the engine to rev up fast so it can produce more power to get the car moving, which is why we have low gear ratios. But as the car speeds up, the wheels will start spinning too fast for the engine to keep up and that would limit how fast the car can go. But with a transmission, we can change the gear ratio as we accelerate which will slow the engine back down while keeping the wheels moving as quickly as they were before. It’s the exact same principle as the different gears on a bicycle. But we don’t just want those gears to unlock the engine’s power. In general, internal combustion engines are more fuel efficient when they are running at lower speeds. There are many reasons for this which aren’t worth picking apart too much, but remember that an engine is full of metal parts sliding against each other very quickly. While the lubrication system will do its best to reduce friction, there’s still lots of it working to slow the engine down and generate heat as it runs. And the faster the engine spins, the more friction there is. There are also plenty of pumping losses which get worse as engine speed increases. But again, it’s not worth dwelling on this. What’s important is that ideally, we want an engine which can spin quickly when we need it to produce lots of power to accelerate, but which operates very efficiently at lower speeds for fuel efficient cruising. But here’s the problem: Life is cruel, and we can’t have that. The best we can manage is a pretty major compromise named Otto. When discussing the four stroke engine cycle, we are in fact almost universally discussing the Otto cycle. That’s O - T - T - O, named for my man Nick over here. Quick recap: The reciprocating pistons work in conjunction with intake and exhaust valves. During the intake stroke, the piston is descending and the intake valve is open, resulting in an air fuel mixture being drawn into the cylinder. When the piston is at the bottom, the intake valve closes and the cylinder is then sealed. The air fuel mixture will then be compressed as the piston travels up back towards the top, and when it’s near the top, the spark plug fires, and the conflagration of hot expanding gases forces the piston back down into the engine block producing engine power, after which the exhaust valve will open to allow the piston to push those spent gases out of the chamber as it comes back up, so the process can repeat. Cool. And the Otto cycle does a decent job of making an engine powerful when it needs to be, and also reasonably efficient when under moderate load. This is why the Otto cycle engine is what gets put in cars. It’s a decent compromise between power and efficiency, and meets the varying needs of an automobile reasonably well. But then this guy named James Atkinson realized that since the power the engine produces comes from the expansion of hot gases, the Otto cycle wastes a bit of energy by constraining how much those gases can expand. See, in the Otto cycle, the length of the intake and compression strokes are identical to the power stroke. Since the volume of gas after combustion is much greater than before ignition, Atkinson saw this as a problem. He figured at the end of the power stroke, when the piston is back at the bottom, the newly expanded hot gases are still exerting pressure on the piston. That could be captured, but in the Otto cycle, the exhaust valve will open at that point, relieving the pressure inside the cylinder and preventing us from harnessing that energy. Atkinson got pretty hot and bothered by this, so he designed… this contraption. If this looks familiar, you’ve probably spent some time taking the green stairs. Engines of the 19th century were all sorts of different from how we imagine them today. And Atkinson used two opposing pistons in the same combustion cylinder, which were being driven by an external crankshaft and some totally normal linkages to make them move like that. Yeah, this thing is quite strange and perhaps a little difficult to follow, even with this lovely animation by Michael Frey. But the key thing to notice is that the distance between the two pistons varies dramatically throughout a full cycle. This effectively enlarges the combustion chamber during the power stroke, which in turn allows the expanding gases to do more work before they’re tossed out of the cylinder. Atkinson would later redesign his engine to be slightly less steampunk, but still extremely weird. We sure did love linkages back in the day, huh? But in this design, you can see the point a little more clearly. The piston hardly moves at all in the intake and compression strokes, but it moves much farther during the power stroke, allowing those hot gases to perform more work on the piston. Now, back in 1887, we didn’t have the same valve technology we do now, so this contraption made more sense in the past. But today we can accomplish more or less the same thing with a conventional Otto cycle engine design simply through messing with the intake valve timing. If we allow the intake valve to remain open through part of the compression stroke, the ascending piston will force some of the air fuel mixture out of the cylinder and back into the intake manifold before the cylinder is sealed for compression. When done this way, it’s known as a modified Atkinson cycle engine, and that’s what you’ll find in any decent hybrid vehicle going back to the original Toyota Prius. Leaving the intake valves open as the piston begins to ascend means we can effectively shrink the size of the combustion chamber during the compression stroke and enlarge it during the power stroke, attaining the same effect that Atkinson did with all those wild linkages. The result? A modified Atkinson cycle engine can get over 40% of the chemical energy in gasoline converted to mechanical energy. Toyota claims the 2.5l engine in this Toyota Sienna can hit 41% thermal efficiency. I’ve also seen some spec sheets claim this engine tops out at 39.8%, so it might not quite hit 40. But either way, it’s quite good. For an engine: no matter what you do, the internal combustion engine remains not that great at its job, but the Atkinson cycle is a substantial efficiency improvement over a standard Otto cycle engine. So if all it takes to make an engine significantly more fuel efficient is to tweak the intake valve timing, well, you might wonder why we aren’t putting modified Atkinson cycle engines into every car on the road. Well, with the advent of variable valve timing, we kind of are a little bit. But when it comes to the engine itself, you still have to make design compromises. Remember, we’re expecting this thing to move the car from a stop and accelerate quickly. Yet we also want it to run efficiently when at cruising speed. And whenever you try to optimize for one of those two things, you make the other one worse. This is the crux of the problem. The Atkinson cycle maximizes fuel efficiency at cruising speeds, but it’s pretty lousy at delivering large amounts of power when you need it. And, well, that kind of lousy engine is exactly what’s sitting in this van’s engine compartment. While this 2.5l four cylinder can produce 186 horsepower… well, first of all, that’s not a lot for a vehicle this large or an engine that large. But also the engine delivers power in a way which many people would find incredibly annoying. So if all this minivan had was that engine, it would be a very slow turd which nobody would enjoy driving. It would be very fuel efficient! In fact, with the right transmission not far off from what this thing can manage, but it would be an unacceptable driving experience for most people. This engine is extremely compromised by being designed for fuel efficiency above all else. However, the van doesn’t just have that engine. And that, dear viewer, is the point of hybrid cars. See, here’s the thing about cars. They only need large amounts of engine power to accelerate. Sure, having 300 horses under the hood can make car go vroom real fast, but once you’re up to speed, you don’t need any where near that much power. This minivan only needs about 30 horsepower to maintain 70 miles an hour. You can get that out of some lawnmowers. So rather than give the car a huge V6 engine just to make highway on-ramps a little easier, what if we gave it a little four banger optimized for amazing efficiency at cruising speeds, and we used some electric motors to give the engine a boost when accelerating? We can power those motors using some stored energy in a small battery pack, and then, whenever we get a chance, we’ll recharge what we took from the batteries and we’ll get them ready for the next time you need to pass someone or whatever. That’s really what hybrid drivetrains have always been about. They’re an exercise in solving the edge case of rapid acceleration without resorting to the brute force option of equipping a car with a bigger, thirstier engine. The electric motors and battery pack in this thing are only there to be an auxiliary source of energy which can be borrowed from and replenished whenever it makes sense to do so. Sometimes that will be for increasing total system power output. While the engine in this minivan tops out at 186 horsepower, its electric motors can add about 60 horsepower on top of that, bringing the total to 245 horsepower whenever you floor it. Incidentally, that’s five horsepower more than the 3.5l V6 in a 2002 Honda Odyssey, a minivan which could barely manage 20 miles per gallon. But the drivetrain in this van will also borrow energy from the battery pack during mild acceleration, if it makes sense. For instance, if the throttle pedal is pushed down enough to request 80 horsepower, but the car’s computers know that the engine is more fuel efficient when limited to 60 horsepower or less… well, then the car is just going to have the engine produce 60 horsepower and fill in the remaining 20 using the electric motors and energy stored in the battery pack. Now here’s where I need to take a break and explain what I meant by the video title. Obviously, “nobody” is hyperbole. The engineers designing these things sure do get the point of hybrid cars! But when the broader public discusses how hybrid cars attain their better fuel economy, discussions tend to focus on the cool new stuff a hybrid drivetrain can unlock, like regenerative braking or the ability to move the car without using the engine. But what I’m trying to get across here is that that’s all just icing on the cake. The actual cake batter is the engine. Yes, hybrid cars need those electric motors to make the engine not seem like the wheezy, compromised slow turd of a thing that it is, but that engine is the only thing which consumes energy. And that’s why it’s the special sauce. Since James Atkinson died in 1914, we could have had this special sauce in cars the whole time. But the engine on its own is miserable. And until we had perfected the electronics and control systems necessary to develop hybrid drivetrains, nobody was going to buy a car with an Atkinson cycle engine. Not even a midwesterner. But even though an Atkinson-cycle engine is much more fuel efficient than an Otto cycle engine, it is still a reciprocating piston engine with many of the same problems. This engine still has friction losses and pumping losses, meaning it’s only at its peak fuel efficiency under certain conditions. So the other thing we can do with a hybrid drivetrain is keep the engine in those conditions under many more circumstances than would be possible without some batteries and motors to fill in usability gaps. And now here’s the other thing I meant by the video title. If you’re not familiar with how Toyota’s hybrid system works both mechanically and operationally, you should probably hold off on writing that comment explaining how you think hybrid cars could be improved. Because I promise you, Toyota is already doing that. Okay, I’ve got a lot to unpack here. First, I don’t mean to sound like a shill for Toyota. For one thing, they’ve really been dragging their feet on battery electric vehicles and are still playing with hydrogen for some reason. So, you know, there’s that. Also, their user interface software is… well, it could be better. But when it comes to the engineering of hybrid drivetrains they kind of nailed it. There’s a reason they are the standard bearer and they’ve earned that distinction. But before I explain the mechanical parts of their Hybrid Synergy Drive and dispel some of the various misconceptions about hybrid cars that are out there, let’s first look at what the thing is actually doing as you drive it. And with the help of this scan tool, I can graph some parameters which will help us make sense of it. We begin our journey at a gas station in Hammond, Indiana. I’ve set the scan tool up to log vehicle speed in blue, engine RPM in red, the accelerator pedal position in black, and the hybrid battery state of charge in green. Not graphed, but shown below is the target engine power in Watts. And for those who would like to convert that to horsepower, divide that number by 746. Before we get on the road, I’m going to take the car through a car wash, and I want you to notice that the car does not need the engine to be running in order to move. Right now, it’s simply using one of its two electric motors to spin the front wheels and drag itself along. When I started logging, the hybrid battery was at 52.5% state of charge, and by the time I had parked it in the car wash, it was down to 49%. Now the car is on this whole time. The reason the battery state of charge is slowly dropping is because the car has a lot of electronics running, including the HVAC system which has an electric air conditioning compressor. That’s one of the fun side benefits of having a hybrid drivetrain with a lot of stored electrical energy on board: if you make all the accessories electric, the car can be fully functional while the engine is shut down. The car let the state of charge get down to just below 40%, and then it decided to start the engine. This happened just before the end of the car wash. But I want you to notice as I pull out that it’s not really working to charge the battery. Instead it simply maintaining its state of charge around 39%. I will explain why it’s not charging the battery in due time. But now that I’m actually moving, I want you to notice that the engine speed is entirely decoupled from the vehicle speed. This cars engine can be used as an electrical generator, to push the car forward with the front wheels, or indeed both at the same time, which is actually the only way the engine can push the car forward. But more on that later. As I accelerate to this traffic light, you’ll see the engine speed up to produce more power to accelerate. But then the light was turning red and I had to let off the gas so the engine revved right back down. And notice that here the battery state of charge went up rapidly, but only as vehicle speed was decreasing. That’s because the car was using regenerative braking to charge the battery pack. Again, I’ll explain that more later. But now the light turning red was a problem for me because I need to take that entrance ramp up there and I’m in the wrong lane. So I had to practically floor it to get around this truck next to me, and I felt very bad about that maneuver. But look at the data we got out of it! Here we see the hybrid battery state of charge quickly drop back below 40% as the car used some of its stored energy to assist the engine during this sudden need for power. But as soon as I let off the accelerator, the state of charge stopped dropping. I was nowhere near highway speeds yet and still had to do quite a lot more accelerating. But now I don’t need to exceed what the engine can do by itself. So the car simply lets the engine do all the work. In fact, the hybrid battery state of charge is increasing slightly, indicating that the engine is producing more power than is actually required to move the car right now, and it’s using that excess power to charge the battery back up. Now here’s something which is very important. It is unusual that this is happening and the car doesn’t ever want to do this if it can be avoided. It’s only using the engine to charge the battery pack right now because the battery fell below 45% state of charge from the car sitting in the car wash parked for a good five minutes. 45% is as low as the car ever wants the battery to be when it’s being driven, because if it’s lower than that, the car can’t sustain the power boost function of its electric motors for very long at all. When getting around that truck, we saw it lose three percentage points in just six seconds and I wasn’t even flooring it. So now that the car is in drive and in motion, it wants to quickly get the battery back to its target minimum charge to prevent an apparent loss of available power to the driver. But, and this is something I really want to make sure everyone out there understands, in all other circumstances, the car goes out of its way to avoid charging the battery pack using the engine, and for a very good reason. If you’ll permit me, one of the things I’d like to unpack is what I have found to be a very common, but I would argue misplaced fixation on the concept of the diesel-electric locomotive and how that technology could potentially be deployed in hybrid cars like this. I’m not going to get too deep into this, because what follows will hopefully explain why I believe the fixation is very misplaced. But one of my goals with this video is to help more people realize that it is not good to convert one source of energy to another unless you actually need to do that. Now, I know this might sound odd to the many people out there who know that a diesel-electric locomotive does just that: They use a generator to convert the mechanical output from their diesel engine into electricity, which will then power electric motors to move the train. But what I’ve never seen discussed is that efficiency is not the point of that process. And in fact, trains would be more fuel efficient if they weren’t doing that. See, no electric generator is 100% efficient. And likewise, no electric motor is 100% efficient. By converting the mechanical energy from the diesel engine to electricity and then back to mechanical energy, a two-step conversion process is happening in which a significant percentage of the power output from the engine gets wasted in conversion losses. Those losses mean that more diesel fuel gets burned than if that two-step conversion process wasn’t happening. But that’s the thing. Locomotives need to get tons of material moving from a dead stop by themselves. That requires a gargantuan amount of torque, an amount which a piston engine is not capable of producing on its own. But it can do it in a roundabout way if you… do precisely what a diesel electric locomotive does. Locomotives actually need to do that two-step conversion in order to perform their function. But it is not the most fuel efficient way to move a vehicle using an engine. Now, what I think is a pretty major source of confusion here is that many people know trains are very fuel efficient, but that’s nothing to do with the drivetrain. Trains are inherently fuel efficient because they have steel wheels running on steel rails and a middle finger to the concept of aerodynamics. They’re so energy efficient, by virtue of being trains, that the conversion losses of the diesel-electric drivetrain essentially don’t matter to that use case. In short, what I want to stress is just because it’s a good idea for a train does not mean it’s a good idea for a car where aerodynamics do matter a lot, and it has rubber tires running on a road surface. And if you’re thinking, “Well, fine, but trains don’t have that battery pack to store energy. So what if the engine could stay in its most efficient operating profile no matter what, and whenever it’s producing more power than is needed to move the car, we’ll just shuffle that power over to the battery pack to use later?” Well, that sounds like a good idea, but charging a battery is another kind of energy conversion, which is also inherently lossy and thus should also be avoided. That is why the car doesn’t charge the battery with its engine unless the battery is too low. When the car is parked as it was in the car wash, it does use the battery pack like a buffer to power its accessories, and will run the engine for a few minutes at a time to charge it back up when it gets low. In fact, here’s exactly what that looks like: with the vehicle stationary, once the battery pack dips below 40%, the engine switches on and charges the battery up to about 48%, a process which takes three minutes. And then it will shut the engine back off and use the energy it just stored until it’s too low again. But that’s just a fancy kind of idling this drivetrain can do thanks to its fully electric suite of accessories. Otherwise, when the vehicle is being driven, the battery pack is charged almost exclusively with regenerative braking. For those who may not know what regenerative braking is, well, the electric motor which can propel the car forward via the front wheels does that when we put electric current through its stator windings, which in turn generate a rotating magnetic field which spins the motor. But if the motor is spinning because of something else, it creates a rotating magnetic field, which we can draw power from using those same stator windings. That was a long winded way to say “electric motors are also electric generators,” and when using the motor as a generator, it will actually slow down the rotational speed of whatever it’s attached to. In this case, the wheels of the car. In other words, it functions like a brake when generating electricity. And that’s what regenerative braking is. It’s a way to capture energy from the vehicle slowing down and charge the battery pack. And since you need brakes anyway, this is a free source of energy. And when the energy is free, conversion losses don’t matter. And so as I drive on the open road where I’m not using the brakes much at all, the battery pack is just sitting there at about 45% state of charge. The car doesn’t want to dip into that energy because there’s no point! It all came from the engine anyway. There are only two reasons for this car to use that stored energy: One, the driver has floored it and wants all 245 horsepower this system can offer. Or two, the current load on the system causes the engine to go so far outside of its efficiency band that it would save fuel to borrow from the battery pack and replenish what was borrowed when the engine is no longer under such a heavy load. But, because of conversion losses, that second scenario is much rarer than people think it is. The engine has to get way outside of its efficiency band for borrowing energy from the battery pack to make any sense, and that’s why the car is not really using the energy in there at all when it’s cruising on the highway, or even during mild acceleration. If the engine is able to do it by itself efficiently, that is always the best course of action. Around town is a completely different story though. When you’re on the highway, a car is constantly working to push itself forward, but off the highway, you’re going to keep switching back and forth between accelerating and braking. In a traditional car, friction brakes reduce vehicle speed simply by converting energy into heat. That works great for stopping the car, but that’s all it does. The battery pack in this car is able to absorb what would be wasted as heat through regenerative braking, and since that process doesn’t use the engine at all, whatever energy it can absorb from the vehicle slowing down is free. So when regen breaking gets the battery pack above 45%, then the car will use that energy to… help the engine. This is what I keep getting back to, and what I’d really like to hammer home today. If you’ve been imagining hybrid cars to use their battery packs something like a bank account which gets paid into when there’s excess energy and borrowed from when there’s excess power demand, this is only kinda sorta of true. The gasoline in this thing’s fuel tank and that engine remain the only source of input power this system has. So the engine is always going to be the primary propulsion device. And that’s why this minivan almost always starts the engine just after you begin moving. You have to treat it very gently if you don’t want the engine to start because there just isn’t that much energy or power to be had from its small battery pack sitting underneath the front seats. That battery is not really there to push the car forward when it’s got a full charge. It’s just there to augment this engine’s output so the engine itself can stay entirely within its most efficient operating range in as many circumstances as possible. In that way, this thing is like the diesel-electric drivetrain many people imagine hybrid cars should emulate. And that’s why I said “Toyota’s already done that.” But the most efficient way for an engine to run is for it to not. So when the car has an opportunity to shut the engine off, it almost always takes it. For example, pretty much whenever you let off the accelerator, the engine shuts off. You’re no longer commanding forward power, so there’s no point burning any gasoline. If the car has enough free energy stored in its battery pack from regen braking and you’re commanding only light throttle, then it might decide to keep the engine off for a good while. But once your accelerator request exceeds the power output of the battery pack, the car has to start the engine to keep up. So it does. Still, if there’s free energy left in the battery pack, the car will sometimes kind of flip the script and use the engine to augment the limited power from the electric side of its drivetrain. Here you can see the battery state of charge dropping even though the engine is running. It’s doing that because it’s got some free energy to use up, so it might as well do that and take some load off the engine, which has the side bonus of keeping the engine in a really energy efficient operating condition even though the vehicle is accelerating and climbing a hill. If you’d like to see in data and hear from a microphone what the powertrain of this vehicle does under a wide variety of circumstances, you can check out this Sights and Sounds video I’ve released on my second channel. I put an audio recorder in the engine compartment of this thing on my way to Hammond, and being able to hear the engine and motors so clearly revealed some interesting things about how the system makes decisions and how it works in general. But to wrap this video up, now it’s time to talk about the mechanical details of Toyota’s hybrid system, because what I’ve talked about so far might very well make it sound fragile and dauntingly complex, when in fact the system is so mechanically simple it almost hurts to think about. So, here’s the wildest thing about how Toyota designed their hybrid system: The engine in this vehicle is not actually connected to the front wheels. And yet, the engine in this vehicle is permanently coupled to the front wheels. To explain that apparent contradiction, let’s put the Cube up on the lift and let its engine spin the front wheels while it’s off the ground. With the engine running and the car in gear, both front tires are turning at equal speed. But because the car has to navigate turns where one wheel will be rotating faster than the other, the power coming from the engine and transmission is delivered through what’s called a differential. It’s not important to know the mechanical details of that, but the differential is designed to split the power output of the engine between the two wheels, while also allowing them to spin independently. And the ordinary differential has a bit of a quirk: I can easily stop one of the two wheels with my hands, and now that I’ve done that, the other wheel has sped up. Note that the engine didn’t get any faster, it’s still just idling. But by preventing one wheel from moving, the other wheel is now spinning twice as fast. But it gets even weirder. If I start to rotate the wheel I’m holding backwards, the free spinning wheel on the other side spins even faster! And if I then start pushing the wheel forward again but faster than the engine wants to spin it, the wheel on the other side starts to slow down. In fact, if I can spin the tire fast enough, I can get the other tire to come to a complete stop. What’s this got to do with the Sienna? Well, believe it or not, this phenomenon is at the heart of Toyota’s hybrid system. But rather than use the engine to spin a pair of wheels through a differential, the engine spins the rotors of two electric motors. I’m only going to give you a very surface-level explanation of this, but look. That’s it. Watch this video from the Weber Auto YouTube channel if you want the full explanation. But those parts on the table are the whole thing. That is Toyota’s Hybrid Synergy Drive. It’s just two electric motors and a planetary gearset. That’s it. The only parts not shown are the stators and wiring which actually drive the electric motors and the case which holds all this stuff together. Oh, and also in reality, this would all be covered in oil for lubrication and cooling. But when it comes to the spinny bits which connect the engine and motors to the wheels of the car, there are literally just three: the parts of a planetary gearset. The crankshaft of the engine spends the planet carrier of the planetary gear set, a small electric motor generator unit known as MG1 spins the sun gear of that gearset, and a larger electric motor, MG2, spins the ring gear of that gearset, which is itself coupled to the wheels of the car through a conventional differential. Okay, it’s time to get out the whiteboard because I need something to point at as I explain the details of how this works. This drawing is not at all accurate when it comes to how these three parts are arranged and fit together, but that’s kind of the whole problem with describing Toyota’s system. What each part by itself does is very simple, and how they’re coupled together is also very simple. But the choreography of the dance they’re performing to make the car go is not. Here is my best (and revised) attempt at painting the complete picture for you. So here’s the engine. Here’s MG1 and here’s MG2. What connects this all together is that planetary gearset which acts to split the output power from the engine between the rotors of MG1 and MG2. What I think is the most confusing thing to wrap your head around here is that all three of these devices can move under their own power. MG1 and MG2 are both electrical generators, but they are also both electric motors. The engine burns fuel to spin and the motors use electricity to spin, but the car is in direct control of all three of these things and can spin them at whatever speed it desires. Now for a moment, I want you to forget that these are motors, because you can think of this arrangement as equivalent to a diesel electric drivetrain. But rather than connect an engine to a single electrical generator, it’s been connected to two of them at the same time through that planetary gearset. We refer to that gearset generically as a power splitdevice, because that’s what it does. And it operates exactly like the differential in a typical car. That is why I used the Cube as a demo. And just as I can make one of the Cube’s tires speed up just by stopping the other one, a Toyota Hybrid can make one of its two generators speed up just by slowing the other one down. Now, that might seem trivial, but what I haven’t drawn into this diagram yet is that one of those generators, MG2, is also coupled to the wheels of the car. So this one is not actually free to spin at whatever speed it wants to. Its rotational speed depends on how fast the wheels and thus the car are going. And in fact when the car is stopped, this can’t spin at all. But that doesn’t mean the engine can’t spin. MG1 is always free to rotate. So if MG2 cannot, what will happen is that all of the engine’s rotational output gets sent to MG1 and this will spin really fast. It’s just like me holding this tire stationary. The engine output is all going to the other tire or in a Toyota hybrid, if the wheels are stopped and MG2 cannot spin, then all of the engine output will go to MG1. But of course, MG1 isn’t a tire, it’s an electrical generator. And MG2 isn’t just an electrical generator, it is also a motor. So whenever a Toyota hybrid wants to move forward, it will simply use the engine to spin the rotor of MG1 really fast, which will generate electricity. And then it will shuttle that electricity over to MG2 in order to make it spin. Since MG2 is connected to the front wheels, that’s going to push the car forward or indeed backwards. Reverse gear is accomplished simply by spinning the rotor of MG2 backwards. But, when the car is moving forward… well, you may notice that what I’ve just described here is a diesel electric locomotive. And earlier I went on a tirade about why that is a silly way to move a car because of conversion losses. Well, that is the brilliant thing about the powersplit device. If the engine is running at a fixed speed and one of these two motors slows down, then the powersplit device will mechanically force the other one to speed up. Now, to explain why this is important, well, remember how regenerative braking works? When we use a motor as a generator, It creates an opposing mechanical force on the thing which is spinning it. And that force acts to slow the spinning thing down. During regenerative braking, the slowing rotor of MG2 slows down the wheels of the car, acting like a brake. But the same thing happens as MG1 generates electricity using the engine. As the stator windings surrounding MG1’s rotor harness power from the rotating magnetic field the engine is producing by spinning this rotor, an opposing torque is generated which will try to slow the rotor down. But because of the powersplit device coupling all of this together, even if the engine is running at a constant speed, MG1 slowing down causes MG2, and thus the wheels of the car to speed up. The fact that the relative rotational speed of the engine and wheels can be altered on the fly simply by speeding up or slowing down the speed of MG1 is what makes this system functionally equivalent to a continuously variable transmission. And that’s why Toyota calls it an eCVT. Personally, I don’t think they’re doing themselves any favors using that term, since most people who know the term CVT associate it with horrible things which are mechanically fragile when this is anything but. The powersplit device has no gears to change, or clutch packs to wear out, or weird belt chain things to move between two cones. It’s as mechanically simple as you could possibly imagine, and all of its parts are permanently coupled together. Yet it does all the stuff we’ve been talking about. And what’s really remarkable to me about this system is that it all fits into a package which doesn’t even look that different from a traditional transmission. That’s why nothing under the hood of this car looks all that out of the ordinary, except for the fact that the gearbox has some beefy wires coming out of it. Now, in reality, and this is another thing which makes holding all this together in your head pretty difficult, We don’t actually need the rotational speeds to change in order to transmit power from the engine to the wheels. What matters is the opposing torque MG1 is putting on the engine crankshaft, and that torque can be constant while the speeds are constant, too. Remember, it’s really easy to stop this spinning tire because an ordinary open differential will send all the engine power over to whichever tire has the least traction. It’s kind of a bummer, really, and that’s why limited slip differentials are a thing. But luckily for us, a similar thing happens with the planetary gearset. So if MG1 is producing an opposing torque on the engine, rather than actually slow the engine down, that torque just gets shuttled over into MG2. And thus the act of using MG1 as a generator actually causes the engine to push directly on the wheels of the car. This is why the Toyota hybrid system, in my opinion, can’t neatly be classified as either a series hybrid or a parallel hybrid. It’s actually somewhere in the middle. While there is a mechanical connection between the engine crankshaft and the wheels, the engine can’t actually push the wheels forward unless the hybrid system is doing something to slow down MG1. Otherwise, it’s equivalent to a car with one tire off the ground. All the engine will do in that case is spin that tire really fast and the car won’t actually move. Now, this means there are some conversion losses happening in Toyota’s system, but how much and exactly where they’re occurring is way above my pay grade. My mental model suggests the car doesn’t have to do very much work to cause the engine to mechanically contribute, so the losses are minimal, but I have no real basis for that other than the stellar real world fuel economy most Toyota hybrids have historically attained. Now, that was the first time I’ve even mentioned the terms series hybrid and parallel hybrid in this video. And that’s because to the end user, the difference doesn’t really matter. But at the same time, the difference is kind of the heart of the point I’m trying to make in this video. Toyota’s hybrid system is a parallel hybrid system, because the engine can mechanically contribute to the movement of the car, and I hope by now you understand why this is a good thing. Series hybrids are different. Series hybrids have an engine which can only power an electrical generator, and the electricity which is generated that way will be used to charge a battery or power electric motors to move the car. But unless you have a very good reason to design a system that way, this is not a good idea. Diesel electric locomotives have a very good reason to decouple the engine from the motors: for torque. And in theory, a future extended-range electric vehicle might benefit from separating the engine and generator from the rest of the car. But I promise, such a future vehicle is not going to benefit from that from a fuel efficiency standpoint. There could very well be a good packaging reason to design the vehicle that way. But if you’re insisting on using the engine primarily as a generator for any other reason, I would argue you’re making a mistake. And we have a perfect example of such a mistake with my previous car, a Gen1 Chevy Volt. For reasons which are unclear but probably rhyme with General Motors, that car was designed explicitly to operate as a series hybrid in most driving conditions. Well, after its battery charge had been depleted. The whole point of the Chevy Volt was to be an electric car first with a small electric range, but which also had a gas powered range extender to give it unlimited range for road trips. But once you had run out of charge and it started up its engine, you discovered that the car wouldn’t keep the engine running when you were cruising at moderate speeds. Instead, it would run the engine in spurts where it was producing way more power than the car actually needed to drive, say, 40 miles an hour, and then it would send the excess power into the battery pack to use later. So after a while it would shut off the engine and use up that energy. But because there were four steps of energy conversion going on there: engine to electricity, excess electricity to charge the battery, discharging the battery to get electricity out of it, and then using that electricity to spin an electric motor, It got surprisingly mediocre fuel economy. The Gen 1 Volt, on premium gas to boot, could only manage 35 miles per gallon city 40 highway. So that car, which was a lot smaller than a minivan, got basically the same fuel economy as this minivan. Worse, from a dollars perspective, if you actually used premium fuel. That’s pretty terrible! But given how that car was designed to operate, that’s to be expected. Even with the flexibility of electric motors and fancy electronics, series hybrids are just less efficient than parallel hybrids. If you can use the engine to push the wheels of the car, you should be doing that to avoid conversion losses. Nissan, for some reason, is supposedly about to release a new series hybrid drivetrain with the Nissan Rogue, and I don’t really understand why they’re doing that, but we’ll see what kind of fuel economy they can get out of it. My guess is, like the Volt, it’s going to be decent, but far from exceptional. By the way, it’s very possible there’s a patent reason that GM chose to build the Volt the way they did. Their Voltec drive system was actually remarkably similar in many ways to Toyota’s hybrid system. It, too, fit two electric motor generator units into a package which replaced the conventional transmission, and in a layout which was really similar to what’s in this minivan. They could very well have gotten into legal trouble had they made the design any more similar. And Ford ended up licensing Toyota’s hybrid technology because Ford also ended up making a very similar design. In fact, the hybrid Ford Maverick is basically a Prius, but truck shaped. By now, most of the original Toyota patents will have expired, so I don’t know how much this matters in 2026, but I wanted to mention it. I’m about to wrap up, but in case any of you out there need another reason to see this as a really cool way to do things well, remember how this van is all wheel drive? Toyota made that happen in a really fascinating way, which is only possible because this is a hybrid electric vehicle. They just slapped a third electric motor on the rear axle. This thing’s not very powerful. It’s only about 40 horsepower. But that’s plenty to get the car unstuck or to give it some extra traction in wet, slippery conditions. And because this thing normally just tootles along without doing anything, there’s virtually no efficiency penalty to having it. The EPA rating between the front wheel drive and all wheel drive versions of this car varies by exactly one mile per gallon, so doing it this way is pretty clever. And Toyota’s hybrid drivetrain design has another pretty huge trick up its sleeve: It can be tweaked to make a car a plug-in hybrid trivially. This van’s main electric motor, MG2, can actually produce 180 horsepower. The reason the hybrid system can only provide about 60 horsepower of boost power to the engine is a limitation of the battery pack it’s been equipped with. If it had a larger battery pack capable of delivering more power, this thing could operate in all-electric mode with pretty decent power on tap. And that’s why many of Toyota’s plug-in hybrids really are just their standard hybrids. But with a larger battery pack which can be recharged using off board power. However, something I want you to know is that plug in hybrids only make sense for those that can reliably charge them at home or at work with cheap electricity, because when plug in hybrids switch to gasoline power, they have a pretty significant fuel economy penalty compared to their non plug in counterparts. This is mostly due to the fact that they’re carrying more weight around with them, since they have a much larger and heavier battery pack, though changes to the drivetrain design to prioritize engine-less operation can also make fuel economy worse. So if you’re not able to charge your car every day, either at home or at work with cheap electricity, don’t consider a plug in hybrid. There’s no point having those extra batteries unless you can actually use them. Personally though, as I’m sure many of you already know, I’m over the internal combustion engine. Since I can charge my car at home, I’d much rather that car just have more batteries and not have such things as a catalytic converter or a transmission, or a fuel tank, fuel pump, fuel injectors, or fuel. But while we are still figuring out how to undertake the monumental task of getting wires from buildings to parking lots, well, charging a car will be less convenient than refueling for many people. So if you’re going to have a car with an engine, why not make the smarter choice and get one with an engine designed to burn as little fuel as possible? Gas costs money, you know, and sometimes it gets very expensive very quickly. Making choices which lower your ongoing costs, I think, should always be in fashion because your future self is worth treating, too. ♫ losslessly smooth jazz ♫ whoop And because this thing normally just tootles along without doing anything, There’s virtually no penalty to having it. The miles per gallon - Oh, I’m looking at my face. That’s silly. There are still, woo. hoo. hoo! And I backed the teleprompter up too much. Farts. Farts. Farts! …replenished whenever it makes sense to do so from an efficiency perspective. Why did you add that line? I should have written that differently and I just figured out how. That. Why’d you stop? You weren’t supposed to stop, you turd. this line could use a revision. I’m doing it on the fly. This may be a mistake. with a larger battery pack capable of.