How It Works: 2016 Toyota Tacoma's 'Atkinson' Cycle

Most pickup truck fans probably know the new 2016 Toyota Tacoma will feature a new 3.5-liter V-6 engine, yet that isn’t the whole story. This new engine will use the “Atkinson” cycle, most notably found in the Toyota Prius. It promises to provide better fuel economy, but could potentially cause confusion among buyers. Here is what you need to know.

Typical ‘Otto’ Engine Cycle

Before we get to the Atkinson cycle, we should first explain that the vast majority of gasoline engines use an “Otto” engine cycle. This cycle gets its name from German inventor Nikolaus Otto and his partner, Eugen Langen, who demonstrated its use in a four-stroke model in 1876. This is the cycle most people know: intake, compression, power and exhaust. It uses fuel, air and spark plugs to fire the piston, turn the crankshaft and make the engine run.

The cycle is so popular that most people don’t think of it as the Otto cycle. They merely think of it as the gasoline engine. The Otto cycle was the basis of all gasoline engines until the Prius came along.

What Is the ‘Atkinson’ Cycle?

Simply put, the Atkinson cycle aims to use all of the energy in the cylinder. It does this by leaving the intake valve open longer, creating a shorter compression stroke.

In the Prius engine, the net result is an effective compression ratio of 8:1, while the expansion ratio is about 13:1. This differentiation results in the engine being 12 to 14 percent more efficient than the Otto cycle in terms of power output per fuel consumed.

When a typical Otto cycle finishes, there is some ambient pressure left in the cylinder. This pressure helps to “push” the spent gases out of the cylinder into the exhaust manifold and exhaust. While this system works, it wastes energy pushing the gases out rather than harnessing the energy to eventually turn the wheels of the vehicle.

With the Atkinson cycle, once the power stroke is complete (i.e., the four cycles are done) there is almost no remaining pressure in the cylinder. This is accomplished by leaving the intake valve open for a small portion of the compression stroke. As the piston head moves downward, a new, fresh fuel mixture is pumped in. At a predetermined point in the stroke of the piston, the intake valve is shut. The net effect of this is that the power stroke is shortened.

Both power and exhaust strokes remain the same in the Otto cycle, yet the Atkinson cycle uses almost the entire length of the piston travel.

A downside to the Atkinson cycle is that you still have to get the exhaust gases out of the cylinder since you aren’t using ambient pressure any longer. Early Atkinson-cycle engines had superchargers to combat this problem. However, Toyota devised a “tumble flow” intake system on the Prius to counteract the problem. This unique system uses atmospheric pressure to push the exhaust gases out.

Another downside of the Atkinson cycle is the shorter power stroke delivers a narrower rpm range. In the Prius, Toyota solved this problem with a continuously variable transmission and an electric motor. These strategies keep the engine running in the optimal range while providing a smooth driving experience.

Additionally, the Prius’ intelligent variable valve timing system adjusts the intake valve timing to maximize fuel efficiency.

We know the 2016 Tacoma will have Toyota’s D-4S technology, which features direct and port fuel injection. This system is currently used on the Scion FR-S and Subaru BR-Z. It “combines injectors that inject fuel at high pressure directly into the cylinders, together with conventional injectors, injecting fuel into the intake ports,” according to Toyota. The two systems turn on and off depending on engine speed.

The system also injects fuel during startup and at certain load points to improve combustion stability without using a restrictive intake port design that increases tumble flow.

What Does This Mean for Truck Buyers?

The Atkinson cycle is more efficient than the Otto cycle, so the 2016 Tacoma’s new engine will offer better fuel economy than other conventional V-6 engines without having to resort to turbochargers or diesel fuel.

While the Atkinson cycle does a great job of maximizing fuel economy in the Prius, it remains to be seen how the cycle will work in a pickup truck with very different duty cycles. Highway and empty-bed fuel economy will be improved, but we really don’t know how towing or payload hauling will be impacted. We assume Toyota has this figured out and that’s why it has taken so long to bring the Atkinson cycle to other vehicles (the Prius first went on sale in 1997).

In the end, the new 3.5-liter Atkinson-cycle V-6-powered Tacoma with a new six-speed automatic transmission and new rear differential is likely to have best-in-class fuel economy. How well it can handle pickup truck duty remains to be answered.

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