Back in 1882, British engineer James Atkinson patented an engine design. That old technology is new again, and you’re going to be hearing a lot more about it as automakers take advantage of its fuel-saving abilities.
Atkinson engine technology is currently used mostly in hybrid vehicles, but Toyota has put it in the redesigned Tacoma pickup truck, and with a slightly different twist than that of a hybrid: it’s used alongside conventional engine operation.
As you know, engines make their power when pistons move up and down to turn the central crankshaft. Each engine cycle consists of four piston strokes: down to draw in gasoline vapour, up to compress it, down when the vapour ignites to make power, and then up to expel the exhaust.
Most engines use an Otto cycle, named for its developer Nikolaus Otto, in which the lengths of the power and intake strokes are always the same. In an Atkinson cycle engine, the valve timing is altered to essentially reduce the length of the piston’s intake stroke in comparison to its power stroke, in turn reducing fuel consumption.
The piston’s actual mechanical length of travel within the cylinder doesn’t change because it’s fixed, says Mike Sweers, chief engineer for Toyota trucks. “We’re holding the intake valve open during the initial compression stroke,” he says. “In a normal Otto cycle, the valve closes and you’ve got a vacuum in there. The crank turns for its upward stroke, and you have friction as it has to overcome the vacuum. With Atkinson, the crank pushes but there’s no vacuum. The valve then closes before combustion.”
Since the valve stays open longer, the piston’s upward movement forces some of the fuel-air mixture back out of the cylinder. It remains in the intake manifold and goes back in the next time the intake valve opens.
Because it doesn’t have to work as hard to overcome vacuum, the engine is more efficient. The downside is that an Atkinson engine isn’t as powerful as an Otto-cycle engine, which is why it gradually fell out of favour with car manufacturers. It can be enough when cruising at a steady speed, but it won’t give the burst of power that drivers expect when they want to accelerate.
The modern hybrid powertrain initially helped bring the Atkinson back. The battery works alongside the gasoline engine, providing additional power when needed and making up for the reduction in torque. Virtually every hybrid uses Atkinson technology, taking advantage of that lower fuel consumption.
However, Toyota has gone a step beyond with the new Tacoma, which combines Atkinson and Otto technology. James Atkinson used unique mechanical linkages to achieve his goal back in the day, but modern versions of his engine use electronics. That allows engineers to vary the valve timing in this dual-cycle engine, keeping the intake valve open longer when an Atkinson cycle is desirable, or closing it sooner for an Otto effect.
“It’s torque demand,” Sweers explains. “At constant speed, we’re running Atkinson. When you ask the engine to produce more torque and do more work, it changes over. The change is entirely in the valve timing. We have a brand-new variable valve timing system where the exhaust and the intake are completely independent of one another.”
This isn’t the first use of the technology. The V8 in the Lexus RC F also switches between the two, but this is the first time it’s been used in a truck. Like the cylinder deactivation used by other manufacturers to reduce fuel consumption, the transition between Atkinson and Otto operation is seamless and undetectable.
“The efficiency is in internal friction improvements,” Sweers says. “The thermal efficiency is always the same, but with an Atkinson cycle, it’s changing the compression ratio but keeping the expansion ratio. It’s less fuel to do the same amount of work, and that’s how we get the efficiency out of the engine.”