Engine key in fuel economy as researchers aim for 45% by 2020 with practical HCCI

Jun 23, 2014
Yoshiro Tsuruhara, Nikkei Automotive Technology

The Crown Hybrid announced by Toyota Motor Corp of Japan in December 2012 hit 38.5%, and the Accord Hybrid from Honda Motor Co, Ltd of Japan in June 2013 was 38.8%: while gasoline engines are generally thought to offer peak thermal efficiencies of about 33%, already the numbers have risen this far.

Until recently, Completely Built-up Unit (CBU) manufacturers in Japan have concentrated on improving fuel economy through electrification as in HEVs, the use of continuously variable transmissions (CVT), and idling-stop mechanisms, all of which in fact waste the driving force generated by the engine. Efforts to boost the efficiency of the engine itself have pretty much been inexpensive tweaks such as cutting frictional loss, introducing Atkinson cycles, or increasing the compression ratio. In general, these improvements do a better job of reducing cost than of improving engine performance.

Fuel economy competition has been intensifying rapidly in HEVs, however, and it has become clear that the efficiency of the engine itself must be raised. Outside Japan, market position demands better engine mileage.

The engines in the above Accord and Crown use pretty much the same constituent technologies used in other Japanese-manufacture cars, such as Atkinson cycle and high compression ratios. As discussed in Part 2, however, these vehicles also incorporate a variety of technologies that have been passed over until now because of high cost, such as an Exhaust Gas Recirculation (EGR) cooler to boost the compression ratio, and reducing pumping loss (Fig 1).

Fig.1 Roadmap to Higher Thermal Efficiency
Manufacturers are hoping to improve thermal efficiency to 45% by about 2020, in time for the CO2 95 g/km emission regulation set to take effect in Europe that year, through technologies including downsizing and HCCI. This might be boosted as high as 50% by about 2025 through the use of thermal insulation, waste heat recovery systems, and other means.
[Click to enlarge image]

In markets like Europe where there is considerable high-speed driving and minimal improvement in HEV fuel economy, or where cost is a major consideration such as in developing nations, mileage will have to be improved for non-HEV designs. CBU manufacturers in Europe and America have already begun to introduce turbocharge downsizing, cutting displacement to boost mileage and making up for the lost power through turbocharging. The design delivers fuel economy close to that of HEVs at lower cost.

Some Japanese manufacturers have adopted the same approach. Fuji Heavy Industries Ltd (Subaru) of Japan plans to ship the Levorg in May 2014 with a newly developed 1.6-L horizontally opposed 4-cylinder direct-injection turbo engine. It was developed with Western turbo downsizing in mind, and delivers the same horsepower and torque as a 2.5-L naturally aspirated engine at 20% better fuel economy.

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