Atkinson Variable Displacement V8 Coming To The Tundra
A note from the Editor: with the new Tacoma coming to market with an Atkinson cycle engine, we thought it made sense to republish this older post that explains how it works.
When rumors of a V8 Lexus coupe with “variable displacement” hit the automotive press a few months ago, I was more than a little skeptical. Quite a few Toyota employees have said that the typical variable displacement technologies employed by GM and Chrysler-Fiat are “not in the best interests of the consumer,” which is Toyota-speak for “variable displacement is a stupid gimmick,” at least when GM and Chrysler-Fiat do it.
Considering all the problems GM trucks have had with their active fuel management systems (which can consume 1-2 quarts of oil between changes), Toyota’s stance makes sense. Yet a number of sources claimed Toyota’s new V8 Lexus coupe would have variable displacement technology, and I found myself a bit dismayed. Would Toyota succumb to the allure of variable displacement systems that raises a vehicle’s EPA rating without any real-world benefits?
Fortunately, the answer is no. The new Lexus RC F comes with a 5.0L V8 that can operate both as a traditional Otto cycle engine and as an efficient Atkinson cycle engine, which – in a manner of speaking – means the engine is variable displacement. When you combine this news with today’s announcement from Toyota about a “new series of gas engines,” it’s safe to say that the Tundra will be getting a variable displacement V8. Probably in the next 2 years.
The 5.7L and 4.6L Are Reaching End of Life
Toyota’s typical pattern is to produce an engine for about 10 years. The Tundra’s 5.7L came out in 2007, and since the 4.6L is basically a down-sized variant, it’s probable both of these engines will be replaced or substantially upgraded in the next couple of years.
But the question is, replaced with what? Diesel rumors aside, designing a new V8 just for the Tundra doesn’t seem probable. New engines cost money, and the existing 5.7L and 4.6L V8s are adequate as-is. With some “tweaking,” they could both be considerably more efficient…which brings us to the Atkinson cycle.
Atkinson vs Otto – What’s The Big Deal?
There are a lot of explanations about the function of the Atkinson cycle as compared to the standard 4-stroke “Otto” cycle we’re all familiar with (if you’re not familiar with the standard Otto cycle, read this – http://www.animatedengines.com/otto.html), but my favorite explanation of the Atkinson cycle goes like this:
- When the power stroke is completed on an Otto cycle engine, the cylinder has some ambient pressure. While this pressure helps to “push” the spent exhaust gases out of the cylinder, it’s also wasted energy. After all, this is “left-over” pressure that could have been used to drive the piston a little further.
- When the power stroke is complete on an Atkinson cycle engine, there is no (or almost no) remaining pressure in the cylinder. Every bit of pressure from the combustion process is used to drive the piston, and that is very efficient.
- You can convert an Otto cycle engine into an Atkinson cycle engine by leaving the intake valve open for a portion of the compression stroke. This “shortens” the length of the compression stroke, which means the power stroke is essentially longer.
- To make an Atkinson engine work, you have to figure out how to get the exhaust gases to leave the cylinder without the benefit of ambient pressure. In the old days, all Atkinson cycle engines had superchargers to combat this problem. Toyota, however, invented a “tumble flow” intake system on the Prius that uses atmospheric pressure to “push” the exhaust gases out. This system is also used on the RC F’s 5.0L V8.
The “variable displacement” system in the new Lexus RC F (which, for the record, is a beautiful car) works as described above.
NOTE: Holding intake valves open during the compression stroke sacrifices total power output about 30% (give or take). A 30% power sacrifice is not something truck or sports car owners want during acceleration, towing, etc…which is why switching between the Otto and Atkinson cycles is so valuable. The engine’s maximum efficiency improves without sacrificing peak output.
What Engine(s) Will We See in the 2016/7 Tundra?
If the Atkinson technology used in the Lexus RC F were brought over to the Tundra’s 5.7L and nothing else was changed, we’d see no loss of peak torque or horsepower, yet about a 10% improvement in fuel economy. If the transmission was updated to 7 or 8 speeds, we’d see another 5-10% increase in gas mileage. Add these two improvements up and the next-generation Tundra 5.7L might be able to hit 22mpg on the highway, while the 4.6L might hit 23 or 24mpg.
Throw in some other fuel economy enhancements (electric power steering, variable valve lift, an ‘eco’ driving mode that reduces power to save gas), and it’s conceivable that the next Tundra powertrain could hit 25mpg+ highway, and 17-18mpg in the city.
If these numbers don’t impress you, consider:
- The engines won’t have to sacrifice any power or displacement to achieve these improvements
- The fuel economy gains won’t rely upon any complicated or expensive “tricks” like turbocharging, replacing steel with aluminum, or direct injection
Going this route, Toyota would be able to keep the costs of the Tundra’s new powertrain low and improve fuel economy without sacrificing any power.
To be clear: The fuel economy ratings offered above are just informed speculation. I also have no confirmation on the notion that Toyota will keep the 5.7 and 4.6L V8s. However, it says here that Toyota will be bringing a variable displacement V8 to the Tundra at some point in the next 2 years, using the same system found on the RC F.
Filed Under: Tundra News