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Layout of a spark-ignited, compression-ignition, exhaust-energy-recovering engine envisioned by GM engineers. (GM)

The IC future points to more downsizing, lean mixtures

Seeking to wring more from less, several novel combustion-engine innovations were discussed at SAE’s 2020 WCX Digital Summit.

Three engineers from the front lines of advanced internal-combustion (IC) engine development affirmed that despite the industry’s public promotion of a battery electric future for light-vehicle propulsion, committed research and development continues for IC. They said work is focused on incremental enhancement to IC efficiency and reduction in greenhouse-gas emissions, including oxides of nitrogen (NOx). 

The engineers participated in a recent webinar as part of the WCX Digital Summit, a series of video presentations, speeches and webinars replacing the in-person programming planned for SAE’s April WCX 2020 conference that was cancelled because of the COVID-19 outbreak.

This panel focused on technologies that have potential for improving the efficiency of  IC engines, specifically downsized boosted gasoline engines with dilute-mixture combustion and four-stroke, spark-ignited boosted engines. Also addressed was the potential for tailoring IC engines as the centerpiece for more-sophisticated hybrid powertrains – all within a framework of “reasonable cost/benefit.”

Downsizing gets aggressive
The engine downsizing movement has been underway for more than a decade, but Jeremie Dernotte, propulsion-systems research engineer at General Motors, prefers “aggressively downsized” to describe his team’s unique “disruptive engine platform:” a 3-cylinder with two 4-stroke “power cylinders” with a total displacement of 1.1L and compression ratio of 13.5:1. There is also an expander cylinder that displaces 1.33L.

In the SAE technical paper, “Downsized Boosted Gasoline Engine with Exhaust Compound and Dilute Advanced Combustion” (paper number 2020-01-0795) that details the GM engine, its exhaust-energy recovery configuration is explained: “The exhaust-compounding is the main distinction of this engine architecture, with the presence of an additional piston-expander cylinder connected to the power cylinder via insulated transfer passages. The exhaust gas from the firing cylinders is transferred during its exhaust stroke into the expander cylinder while this piston is going down (expander expansion), thus performing an additional expansion to recover the exhaust gas enthalpy into mechanical work at the crankshaft.

“As such,” the paper continues, “the expander behaves like a two-stroke engine, repeating its expansion stroke and exhaust stroke, switching the work recovery from one power cylinder to the other after every revolution.  A clutched, high-efficiency supercharger is engaged to achieve peak power with homogenous stoichiometric combustion.”

Moreover, the engine employs a sophisticated valve-timing arrangement to provide positive valve overlap (PVO) timing and negative valve overlap (NVO) to optimize the exploitation of the expander cylinder’s energy. This also facilitates the dilute low-temperature combustion – in this case, spark-ignited homogenous-charge compression-ignition (HCCI) – from which the engine derives much of its fuel-efficiency gain, while also enabling conventional stoichiometric operation when the engine is under high-load demand.

In the WCX Digital Summit presentation, Dernotte said the engine’s novel exhaust-energy recovery design contributes as much as 10% of its overall efficiency improvement. And its advanced lean-burn combustion is claimed to deliver a brake-specific fuel consumption (BFSC) gain of up to 38% compared with a normally-aspirated conventional engine with a similar power rating.

He added that the goal of the research is to make lean-burn combustion practical for a larger range of engines and vehicles. Dernotte added that although control of NOx emissions is the perpetual challenge for engineers of lean-burn engines, his GM team’s current concept has been developed with the intent for complying with all foreseeable U.S. emissions limits.

Expanding expansion
Much like engine downsizing, adopting the Atkinson cycle for extended engine expansion ratio now is a common practice, with scores of current light-vehicle gasoline engines employing some form of the Atkinson or Miller cycles (the latter employing a boosting device). But research conducted by Zhuyong Yang under the auspices of the Michigan Technological University’s Light Duty Engine Consortium (Yang last year also received his Ph.D. from Michigan Tech), seeks to take Atkinson to another level.

Yang said that if over-expansion ratio is increased, gains of around 11% in overall efficiency are possible. This allows an optimized Otto-cycle engine to advance from a potential high of around 48% efficiency to 54.1% overall efficiency. His team used a Honda engine’s Atkinson design and research conducted by Audi to study potential changes to the kinematics of a multi-link piston connecting rod and its effects on expansion ratio. They then explored variations in the  MATLAB virtual environment, progressing from a single-cylinder concept to 4-cylinder models.

He said the relative efficiency improvement over a baseline engine is 10% on the FTP drive cycle and around 13% on the US06 cycle. Some gain, he added, comes from the potential for higher compression ratios and lower exhaust temperatures expanded Atkinson operation offers, including the ability to reduce fuel enrichment at higher engine loading. Yang envisions future advances of the expanded-Atkinson concept to come from enhanced engine-size and transmission shift-optimization studies.

Boost for hybrids
“There are still going to be engines in the future,” asserted Graham Conway, who leads Southwest Research Institute’s high-efficiency gasoline research program. He summarized the research described in SAE technical paper 2020-01-0281, “Opportunities for Electrified Internal Combustion Engines.,” It  underscores his vision of an “eclectic future” for transportation in which there will be a mix of powertrains, the majority of which incorporate a combustion engine – even in the year 2050 and regardless of world region.

Conway believes  IC engines can be improved to work more efficiently in a hybrid powertrain configuration by more strategically applying torque from an electric machine in a P0, P1 or P2 configuration (the ‘Ps’ designating motor location starting from the front of the engine). His research shows that by supplementing a 1.0L engine with as little as 5 kW of electric power, the engine’s BMEP load can be reduced by around 3 bar. The electric machine’s augmentation of the engine’s low-end torque subsequently permits the engine to be designed with a higher compression and expansion ratio, “which makes the whole map more efficient,” Conway said.

But it doesn’t stop there, Conway added. Using a supplemental electric machine also means the engine’s turbocharger can be larger – thus more efficient overall – and eliminate the need to operate at high pressure ratios and low flow rates that compromise the turbo’s operation. “Things just get better and better,” he said.

Creating a hybrid arrangement in which the engine’s compression ratio is increased and the turbocharger optimized – a layout Conway dubs “Hybrid Boost,” overall brake thermal efficiency (BTE) can be increased by 2% over the baseline engine, he said. He admitted that in some drive cycles, a hybrid that simply shuts off the engine is most efficient. But he said that higher-load drive cycles or the combination of a higher-weight vehicle with a downsized engine – an increasingly popular combination in many world regions where consumers favor SUVs and pickup trucks – usually preclude the opportunity to propel the vehicle only on the electric power provided by a mild-hybrid arrangement, which makes the Hybrid Boost configuration more effective.

“We need to be optimizing our engines for hybrid applications, because we get a bigger benefit than just optimizing the engine or just optimizing the hybrid [electric] system. Rather than be ‘left behind,’” Conway said, “the internal combustion engine has a fantastic opportunity to improve in a hybrid-vehicle architecture.” He echoed the session’s other engineers in saying there are other engine-optimization techniques that new technology has made more accessible, including waste-heat recovery, “extreme” Miller-cycle techniques and advanced combustion modes.

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