Why Hybrids Get Better Fuel Economy Even Though You Still Fill Them With Gas

A few weeks back, we took a look at a Chevy Bolt with a silly contraption hanging off the back. We dived into the science, and looked at why hooking up a generator to the rear wheels wouldn’t let the EV recharge itself and drive forever. That topic raised an interesting question—don’t hybrids do the same thing? Why do they work?

From the outset, it’s worth noting that yes—hybrids do turn wheel rotation into electrical energy! Now, hybrids can’t drive forever without recharging or refueling, but they are often more fuel efficient than cars that just rely on internal combustion engines.

This can be confusing. You could be forgiven for thinking that all the extra weight of motors and batteries would have a negative impact, not a positive one. You might also think that converting the chemical energy of gas into electricity would waste energy. And yet, through nifty engineering, hybrids do come out ahead. Let’s explore why!

Play To Your Strengths

Let’s start by examining a series-parallel hybrid like the Toyota Prius. It can drive the wheels using its engine or the electric motor, or both in combination. It can also charge the battery using the gasoline engine, or by using regenerative braking. Let’s ignore regenerative braking for now, and examine the Prius on this basis.

For a regular car, the chemical energy in the gasoline is turned into kinetic energy, with some losses in the process. But the Prius is more complicated. Setting aside regenerative braking, all the energy used by the car still comes from gasoline. [Ed note: I added the blink effect below to highlight the power flow discussed in each caption. – Pete]

The engine burns gas, turning it into kinetic energy to drive the wheels. The Prius also converts kinetic energy from the engine into electricity to charge the battery. The electric motor can then draw from the battery, turning that electrical energy back into kinetic energy to drive the car. Even then, it’s worth remembering—the electrical energy in the battery originally came from the gas burned by the engine.

Those extra conversion steps come with losses when the Prius turns gas into electricity. The generator in the Prius can’t turn 100% of the engine’s output into electricity. Some is lost as friction or heat. Similarly, the electric motor isn’t perfectly efficient either. It has friction losses, resistive losses, and others besides.

Converting gas into electricity and back into kinetic energy involves multiple extra steps. The losses involved are measurable. And yet, a Prius comes out as more efficient than many regular gas cars. Even when it’s carrying around the extra weight of a motor and battery! How is it even possible?

It all comes down to the quirks of the internal combustion engine. They’re most efficient in a very narrow operational range. Outside that range, they’re often not making great use of the energy in the fuel.

Hybrids get around that with a nifty trick. In those situations where the gas engine isn’t very efficient, like low-speed operation, the engine can be used as a generator instead, charging the battery. When being used to generate electricity, the gas engine can be run in its most efficient power band to get the maximum energy out of the fuel. The energy it generates can then be stored in the battery and used to run the electric motor.

This works because the electric motor is far more efficient than the gas engine in many situations, like low-speed city driving. Even though there are more energy conversions involved, the combination of engine, battery, and motor comes out ahead. This is because the engine is being run in its optimal power band to get the most out of the fuel. This strategy is more efficient than just using the gas engine in low-speed contexts where it can’t run at its most efficient speed.

As an aside, this is why automakers keep putting more gears in gearboxes. A 10-speed gearbox will allow the engine to run closer to the RPM for peak efficiency across a far greater range of road speeds. Compare that to a car with an old two-speed auto, which will be outside the engine’s most efficient RPM range at most speeds.

Diesel-electric trains take advantage of the same concept. A diesel engine isn’t very efficient when it has to run at varying speeds all the time. Instead, diesel-electric trains run their combustion engines at near-constant RPM where they are most efficient. The engines are hooked up to generators which drive electric motors at whatever speed is required to run the train. Electric motors happily run at different speeds with much less loss of efficiency than internal combustion engines. Even though there is a loss involved in converting the diesel engine’s kinetic energy into electricity, it’s more than made up for by the improved efficiency of the electric motor in different speed ranges.

Love Me Some Regen

Okay, so we ignored regenerative braking to start with. Now let’s dive in. It’s another great tool that hybrids have that helps them save fuel compared to regular ICE cars.

When you accelerate in a gas-powered car, you turn fuel into the kinetic energy of the vehicle. The faster you go, the more kinetic energy the car has. If you then allow the car to coast down, that kinetic energy will slowly bleed off through air resistance, rolling resistance, and other minor losses. But we don’t drive like that. Instead, we inevitably end up hitting the brakes. We just burnt all this gas just to get this kinetic energy, then we apply the brakes and turn it into heat. It’s gone.

Hybrids, on the other hand, can capture this energy. Instead of using friction brakes alone, they can connect their drive motors to the wheels, using them as a generator instead. In this mode, the generator turns the kinetic energy of the vehicle into electrical energy to be stored in the battery. The resistance of the generator slows the car down significantly, much as the friction brakes would have. But instead of losing the energy as heat, it’s put back into the battery where it can be reused to drive the motor.

Of course, the energy you get out of regenerative braking is less than you put in. However much energy you got out of the battery or gasoline accelerating up to speed, you’ll get less back when you regeneratively brake back the vehicle. But you’ll get some, and that’s more than the zero that ICE cars are getting.

For this reason, stop-start city driving is perhaps the best example of a hybrid’s strengths. An internal combustion engine will burn lots of fuel accelerating the car from slow speed, and lots is wasted every time the car has to brake or stop. Meanwhile, a hybrid can often leave the combustion engine switched off in these scenarios. Instead, it will rely on its electric motor, which is more efficient than a combustion engine at low speeds. If it has to stop, it can use regenerative braking to recapture some of the energy it just used accelerating.

This is why hybrid cars are so good in city traffic, but less so on the open road. In highway scenarios, braking events are rare, so there’s little energy for the hybrid system to recover. Plus, in these situations, the internal combustion engine can usually run close to its peak efficiency. There’s also the need to overcome greater air resistance at high speed, where the high power output of a combustion engine comes in handy. Indeed, this is why hybrids will use their gas engines more heavily on the highway.An an aside, this is also why EVs also tend to get lower range on the highway. Their electric motors are pretty efficient everywhere, but they lose the benefits of regenerative braking when they’re driving at continuous speed.

Benefits On The Side

Hybrids come with one more main benefit compared to their ICE cousins. They can usually use smaller, more efficient engines than comparable gas-only models. The engine can be smaller because it doesn’t need to generate peak power alone; it can combine with the electric motor to hit the necessary figure. Plus, the engine doesn’t need to be tuned to deliver as much low-down torque. Instead, the electric motor can serve well in this capacity, as they generate peak torque right from zero RPM. This is also a huge boon for low-speed drivability.

It’s also worth noting that hybrids have traditionally been designed for peak efficiency across the board, in a way which regular models are not. Hybrid models often get sleeker, more aerodynamic designs, lower-rolling resistance tires, and other features to further improve their efficiency figures. These design choices have, at times, come with tradeoffs like lower cargo space or a less attractive body. But for a model that specializes in efficiency and good fuel mileage, it’s considered worthwhile.

Sum It Up

Ultimately, hybrids come in all different kinds. You get parallel hybrids, where the engine and motor work together to drive the wheels mechanically. Meanwhile, series hybrids use internal combustion engines solely as generators. The engine in these vehicles cannot mechanically drive the wheels. Then you have the more complicated series-parallel hybrids, where the engine and electric motor can work together or independently as required. The popular Toyota Prius falls into this latter category, which is perhaps the most flexible.

Then you have plug-in hybrids. Everything we’ve said about hybrids still applies to them. However, they can be even more efficient, because you can charge them with electricity from the grid. Rather than turn gasoline into electricity with your engine, you can use solar panels or your local neighborhood powerplant to fill up the battery instead. That powerplant is most assuredly more efficient than your combustion engine at making electricity.

[Ed Note: I’d like to add a note about downsizing opportunities and city-traffic-idling. Actually, I’m just going to quote the Department of Energy:

In an HEV, the extra power provided by the electric motor may allow for a smaller combustion engine. The battery can also power auxiliary loads and reduce engine idling when the vehicle is stopped. Together, these features result in better fuel economy without sacrificing performance.

I also want to note that with improved powertrain efficiency afforded by hybrid technology comes a reduction in overall system heat rejection, which means lower cooling requirements, which theoretically means a smaller required frontal cooling (grille) opening and thus improved aerodynamics.

Oh, and wow that’s a nice car above. -DT]

Whichever sort of hybrid you have, the basic concept is the same. The combustion engine is allowed to operate in its peak efficiency range more often, thanks to the electric motor helping out with drive duties. That’s how they’re more efficient than regular ICE cars. Furthermore, they are also able to recapture energy under braking, and they enable vehicles to get by with smaller, more efficient engines.

Hybrids have become a huge part of the automotive landscape. While EVs continue to improve, don’t expect the hybrid model to die out anytime soon. If anything, it could keep the internal combustion engine alive longer than you might imagine!

Image credits: David Tracy, Toyota, Honda, EPA, Ford, Laurent Jollet via Unsplash license