Toyota Not Ready to Jump on Small Displacement, Turbo Engine Bandwagon
October 16, 2013 9:26 AM
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Larger displacement Atkinson cycle engines may be in the future for Toyota
A number of automotive manufacturers have begun to move away from larger displacement naturally aspirated engines to smaller displacement turbocharged engines. The general idea was that smaller and lighter engines would use less fuel while offering the same sort of performance thanks to the addition of a turbocharger. However, in the real world many drivers have discovered that turbocharged small displacement engines are often
unable to deliver on their fuel efficiency claims
Toyota is considering bucking the industry trend and rather than going with smaller turbocharged engines, is considering larger naturally aspirated engines to improve fuel efficiency. Senior managing officer in charge of drivetrain R&D for Toyota Koei Saga recently said that Toyota believes gasoline engines could benefit more from upsizing capacity in conjunction with Atkinson combustion cycles than going smaller with turbochargers.
Atkinson engines today are typically only used in hybrid vehicles like the Prius,
Ford Fusion Hybrid
Hyundai Sonata Hybrid
Honda Accord Hybrid
Increasing the displacement of an engine using the Atkinson cycle would deliver a specific output less than that of similarly sized conventional combustion cycle engines, but fuel economy would be better. Toyota believes that fuel economy would be better than the smaller engines they replace.
Toyota has offered no timeframe for bringing larger displacement Atkinson cycle engines to market and hasn't hinted at which models might get the Atkinson cycle engines.
Mazda experimented with similar “delayed valve”
Miller Cycle technology
over a decade ago in the
Millenia midsize sedan
. But instead of using electric motors to make up for the reduced power density like today’s Atkinson-engine hybrid vehicles, the Millenia used a supercharger.
Saga also talked a bit about the
next generation Toyota Prius
saying that the vehicle will use a mixture of battery technology including lithium-ion and nickel batteries. The reason for mixing battery types is that lithium-ion batteries are better for performance, but the durability and lifespan is better for nickel batteries.
Green Car Reports
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RE: Low-power operation
10/16/2013 4:06:34 PM
A big source of low power efficiency loss is pumping loss. Because the intake operates near stoichometric air-fuel mixtures, the only way to reduce output is by lowering the total molecules of this mixture entering the cylinder. Traditionally, this is done by a throttle valve, which literally restricts the intake, thus wasting energy as air/fuel squeezes by.
The Atkinson engine tries to reduce pumping losses by letting air/fuel flow freely into the cylinder, but then let some of it flow back out before shutting the valve and compressing. That free flow is the difference between a regular cycle with reduced throttle and an Atkinson cycle with an equal rate of fuel consumption.
Cylinder shut-down would achieve a similar goal: while they still have friction, there's no air/gas flowing in and out. It may not be as effective as Atkinson, though, because compressing and expanding gas in a closed cylinder will have some losses.
Small displacement turbos go after frictional losses instead of pumping losses. I don't have any data to say which approach is better, but it's nice to see progress on both fronts.
RE: Low-power operation
10/16/2013 7:34:05 PM
I believe that most of the benefit of the Atkinson cycle engine is an improvement in the thermal efficiency of the engine. The Otto cycle engine sucks a volume of air into the cylinder compresses it and heats it by burning fuel in it to increase the pressure in the cylinder (heat plus combustion products) it then expands the gasses to the same volume as prior to compression before exhausting the gasses. This means that the still heated gasses are going to be exhausted while considerable pressure energy remains meaning that pressure energy is no longer able to do useful work in the engine. That remaining energy is effectively dumped via the exhaust system. A true Atkinson cycle engine has a shorter compression stroke than power stroke and so extracts more of the pressure energy available before exhausting the gasses. This would improve the thermal efficiency of the engine (more of the heat from burning of fuel is converted to work) while at the same time reducing its power density.
While the engine would probably spend more time with its throttle further open due to its reduced power density if the engine was built with a larger displacement to compensate, that would have the effect of normalizing the throttle position and pumping losses would be greater due to induction air passing into and out of the cylinder on the intake side. Pumping air into the cylinder past the intake valve and then pushing some of it back out again past the intake valve. This would be an extra double pass of the intake valve for gasses that are back flowed out of the cylinder on the intake side with a small pressure drop or energy loss for each pass. A larger displacement could allow larger intake and exhaust valves for an easier breathing engine. It would also mean bigger pistons and heavier crankshaft and consequently bearing surfaces with a larger area and increase sliding velocities for greater frictional losses. Also the increased engine weight would negatively affect the vehicles overall efficiency.
RE: Low-power operation
10/17/2013 6:58:57 AM
A shorter compression stroke does the exact same thing as putting less air-fuel mixture into the cylinder. You get the same number of molecules of air/fuel in there compressed to the same volume and expanded to the same volume after ignition.
The big difference is that Atkinson does that more efficiently. Air doing double passes though open valves isn't nearly as bad as air going across a throttle that is more closed. The pressure drop is clearly far less with the former than the latter.
You're right that there are negatives with a larger engine, but it's a matter of magnitudes to see whether they are worth it.
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