Nobody understands the point of hybrid cars [video]

### TL;DR The point of a hybrid isn't the electric motor and battery themselves—it's that they make a hyper-efficient but gutless Atkinson-cycle engine practical, drastically cutting fuel consumption. ## 2021 Toyota Sienna: A Full-Hybrid MPV The 2021 Toyota Sienna is a three-row minivan widely considered the best in its class—if you can swallow the minivan stigma. For this model year, Toyota did something bold: it dropped the V6 and made every Sienna a hybrid. That move annoyed plenty of people, but it also produced a minivan that gets over 50% more miles per gallon than its rivals. This van's fuel economy is remarkably stable, achieving 34 mpg combined in both city and highway driving. And it's a minivan with all-wheel drive. For context, my first car was a Honda Civic that barely managed that on the highway and far worse in the city—and that was a two-door coupe. ## What Hybrids Really Mean: It's Not Just Batteries and Motors Most people know hybrids use a battery pack to store electricity and electric motors to help the engine, making them more fuel-efficient than non-electrified cars. But what's less understood is that the battery and motor themselves aren't the main reason for those stunning fuel economy numbers. This car isn't a trendy plug-in hybrid; it's a traditional hybrid like the original Toyota Prius. That means all the energy available to the powertrain comes from the gasoline in the tank—there's no way to charge the battery except by driving. So every joule the hybrid system uses to move the car ultimately comes from gasoline and the engine. So why bother with the motor and battery? Because without them, driving this car would be miserable. ## The Inefficiency and Compromise of Internal Combustion Internal combustion engines aren't great at their primary job: converting fuel's chemical energy into mechanical energy to move a car. You're lucky to capture just a quarter of the energy in gasoline; the rest is wasted as heat. That's why cars have huge radiators, and why cooling system issues can quickly destroy an engine. Part of the problem is a compromise: an engine's power output varies wildly with its speed. For example, the MR-18DE engine in a Nissan Cube makes its rated 122 horsepower only at about 5200 RPM (near redline). At lower speeds, it can't produce full power; at normal cruising speeds, it barely manages 50 hp. That's why cars need transmissions: we have to change the ratio between engine crankshaft speed and wheel speed to make the engine's characteristics usable. For acceleration, we want the engine to rev up quickly and produce more power—that's what low gears do. But as speed increases, wheels turn too fast for the engine to keep up, so we change the gear ratio to drop engine speed while maintaining wheel speed. Also, internal combustion engines are more efficient at lower RPMs—more revolutions mean more friction and pumping losses as metal parts slide against each other. Ideally, we'd have an engine that can rev high for power when accelerating, yet run at low revs for efficient cruising. But in reality, you can't have both. ## Otto Cycle vs Atkinson Cycle Most four-stroke engines use the Otto cycle, named after Nikolaus Otto. In the Otto cycle, the intake and compression strokes are the same length as the power stroke. James Atkinson realized that because power comes from expanding hot gases, the Otto cycle wastes a little energy by limiting how much those gases expand—at the end of the power stroke, the still-hot gases still push on the piston, but the exhaust valve opens and releases that energy. Atkinson designed a bizarre mechanism to achieve a longer power stroke. Today, we can achieve the same effect with a conventional Otto-cycle engine by messing with intake valve timing. If we keep the intake valve open during part of the compression stroke, the rising piston pushes some of the air-fuel mixture back into the intake manifold before the cylinder is sealed for compression. This is called a modified Atkinson-cycle engine—and it's what you'll find in any decent hybrid, going back to the original Toyota Prius. By keeping the intake valve open as the piston rises, we effectively shrink the combustion chamber during compression and expand it during the power stroke. Result: a modified Atkinson-cycle engine can convert over 40% of gasoline's chemical energy into mechanical energy. Toyota claims the 2.5-liter engine in the 2021 Sienna achieves 41% thermal efficiency (some spec sheets say 39.8% at its best). Either way, that's a big improvement over a standard Otto-cycle engine. ## Why Not Put Atkinson-Cycle Engines in Every Car? If just tweaking valve timing can boost fuel efficiency so much, why not use modified Atkinson-cycle engines in every car? With variable valve timing, we sort of do. But the problem is that Atkinson cycles maximize efficiency at cruising speeds but perform poorly when you need big power. This 2.5-liter four-cylinder engine produces 186 hp—not much for a car this size or that displacement. And the way it delivers power is frustrating: it has poor low-end torque. If this minivan had only that engine, it would be a painfully slow dog that nobody would want to drive. It would be very fuel-efficient, but the driving experience would be unacceptable. ## The Real Role of Hybrids: Boost for Hard Acceleration That's exactly what hybrid cars are for. Cars only need significant engine power during acceleration. Once you're up to speed, you don't need much power at all. This minivan only needs about 30 hp to maintain 70 mph—some lawnmowers produce that. So instead of giving the car a big V6 just to make merging onto highways easier, give it a small four-cylinder optimized for cruising efficiency, then use electric motors to give the engine a boost during acceleration. Use the battery pack to store energy for those motors, then replenish that energy whenever possible, ready for the next passing maneuver. That's the whole point of a hybrid system: it's a solution to the edge case of hard acceleration, without resorting to the brute-force approach of a bigger, thirstier engine. The electric motor and battery in this car exist solely as an auxiliary energy source, one you can intelligently borrow from and replenish. Sometimes it's to increase total system power output: the engine makes 186 hp max, and the electric motor can add about 60 hp, so total is 245 hp—that's 5 more than the 3.5-liter V6 in a 2002 Honda Odyssey, a minivan that barely managed 20 mpg. Other times, during gentle acceleration, the car's computer will borrow energy from the battery. For example, if 80 hp is requested, but the computer knows the engine is more efficient at 60 hp, the car will have the engine produce only 60 hp and let the motor and battery supply the remaining 20 hp. ## Summary: Nobody Really Understands the Point of Hybrids "Nobody" is an exaggeration—the engineers who design these things certainly understand. But when the public discusses how hybrids achieve lower fuel consumption, the conversation often focuses on cool new features like regenerative braking or the ability to drive without the engine. Those are just icing. The real cake is the engine. Yes, hybrids need those electric motors so the engine doesn't feel as gutless and compromised as it really is. But only the engine consumes energy. That's why the Atkinson-cycle engine is the secret sauce—we could have used it in cars all along since James Atkinson died in 1914. But the engine itself performs terribly—until we perfected the electronics and control systems needed to develop hybrid powertrains, nobody would buy a car with an Atkinson-cycle engine. --- **Source:** [Nobody understands the point of hybrid cars [video]](https://www.youtube.com/watch?v=KnUFH5GX_fI)