Nissan Says It Has Reached A Breakthrough In Engine Thermal Efficiency

Image: Nissan (Other)

Many automakers are on a mission to make their internal combustion engines better at turning fuel into energy. Friday, Nissan says that it reached a breakthrough to reach 50 percent thermal efficiency through its e-Power hybrid technology.

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This latest development is the next generation of the e-Power hybrid system first launched in 2016. The system uses the gasoline engine not to drive wheels, but solely to charge the batteries for an electric drive system. It’s a series hybrid that does not offer plug-in capability.

Illustration for article titled Nissan Says It Has Reached A Breakthrough In Engine Thermal Efficiency
Image: Nissan (Other)

Nissan says that e-Power allows its engines to be more efficient because they’re only being used as a generator for the battery, which can act as a sort of buffer between the internal combustion engine and the driver’s speed and torque demands. This allows the engine to remain in a relatively narrow range of engine speed and load points (called operating points) that yield highest efficiency (see plot above). Nissan describes how the system works, writing in its press release:

Conventional internal combustion engine (ICE) vehicles demand power and performance from an engine under a wide range of speeds (RPMs) and loads. This fundamental requirement means conventional engines cannot perform at their optimal efficiency at all times. However, Nissan’s e-POWER system utilizes an on-board engine as a dedicated electricity generator for the system’s e-powertrain. Operation of the engine is limited to its most efficient range, appropriately managing the engine’s electricity generation and the amount of electricity stored in the battery.

With this dedicated approach, and the evolution of battery technology and energy management techniques, Nissan has been able to improve thermal efficiency beyond current levels. Development of the next generation e-POWER system continues this path of efficiency through Nissan’s design and development of an engine exclusively for e-POWER.

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Image: Nissan (Other)

SAE defines engine thermal efficiency as:

A heat engine’s thermal efficiency is the ratio between the useful output of a device and the input, in energy terms. The thermal efficiency must be between 0% and 100% when expressed as a percentage. Due to factors including friction, heat loss, etc., thermal efficiencies typically are much less than 100%.

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SAE also says that the typical automotive engine only operates at around 25 percent thermal efficiency, though we’ve been seeing around 40 percent in a number of applications lately. Automakers are always chasing higher efficiency so that their engines get better use out of the fuel that they burn. Hitting 50 percent efficiency is a big deal.

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To get to that seemingly magical number, Nissan says it developed the STARC — strong, tumble and appropriately stretched robust ignition channel — concept. That’s a confusing name, but the marque says the concept increases thermal efficiency by improving flow of the air-fuel mixture in the cylinder, and by using a high energy ignition system that allows for more reliable ignition of the diluted burn. (Nissan calls a diluted mixture one that is leaner or has more inert EGR in the mixture. EGR is exhaust gas recirculation, i.e. exhaust gases being pumped into the cylinder.)

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Nissan provides the visual below to drive home the importance of stable, precise fluid flow across the spark plug in the nucleation of a strong initial flame front, which is necessary for complete combustion, especially of diluted mixtures and especially with high compression ratios (both of which allow for improved efficiency, per the slide above).

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Below you can see how Nissan modeled fluid velocity in the cylinder during the compression stroke. Maintaining tumble all the way until the end of the compression stroke, Nissan says, is key in creating a stable fluid velocity across the spark plug, which yields a c-shaped ignition “discharge channel” that ultimately allows for complete burn of the diluted fuel mixture without knock.

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Typically, achieving such precise flow is achieved through optimizing the shape of the intake ports/valves and piston crown. Here’s a look at combustion chamber fluid velocity vectors:

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To get a better idea of how the new design creates stable fluid velocity across the plug, the company has provided this animation, which also shows a more conventional design for comparison:

Nissan says that, in its internal testing, was able to achieve a thermal efficiency of 43 percent using exhaust gas recirculation and 50 percent by operating the engine at a fixed rpm, fixed load, along exhaust gas recirculation.

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With all of this said, Toshihiro Hirai, senior vice president of powertrain and EV engineering at Nissan, makes clear that this technology will take some time. From Automotive News:

It took 50 years to increase thermal efficiency (of conventional engines) from 30 percent to 40 percent,” Hirai said.

But with e-Power, we can increase it to 50 percent in several years. That has been the target for the engineering community,” he said, describing that level as the “ultimate, challenging goal.

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Still, this is a pretty cool development and it will be awesome to see where Nissan takes it.

For the full technical breakdown, watch this nerdiness:

We’ve reached out to some powertrain engineers to get their takes on this design.

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(Some context added by David Tracy)

Staff Writer at Jalopnik and learning pilot. Loves all vehicles! Smart Fortwo (x4), Honda Beat, Suzuki Every, AmTran Bus, VW Jetta TDI (x2), Audi TT, Buell Lightning, Triumph Tiger, Genuine Stella...

DISCUSSION

teamtestbot
teamtestbot - the David Tracy of Vans

The series hybrid is criminally underrated and underloved for automotive use. I dunno why anybody is treating this as breaking news and fancy tech at this point besides puffery and marketing. It should have been every hybrid vehicle from the start, and has been dominant in commercial use for decades. Trains, ships, mining trucks, industrial machinery like manufacturing processes that use fuel solely to generate power for giant motors, etc.

Isolating the heat energy extraction from the kinetic energy conversion is how you achieve efficiency in both processes. Heat engines being used as prime movers in varying speed and load conditions conflates the two and is why the well-to-wheels efficiency of gasoline is so horrid.