FOUR-STROKE ENGINES
YANMAR TESTS RETROFITTING ENGINES WITH TWO-STAGE TCS
Image: Yanmar
Tests have shown retrofitting four-stroke auxiliaries with two-stage turbocharging is practical and effective, offering increased efficiency and output possibilities, according to Yanmar
In a paper published in March, Yoshinori Fukui of its Speciality Machinery Engine Development Department said that the experiments had been prompted by two things: the success of its four-stroke 6EY26W propulsion engine - which was launched in 2014 and has two-stage turbocharging - and the rising electrical power demands caused by a growing amount of equipment installed onboard ships. In particular, he singled out scrubbers that are being retrofitted as an alternative to using low-sulphur fuel to meet IMO’s 2020 sulphur cap. Their power demands are typically supplied either by installing additional auxiliary generator sets or uprating the existing installation. Instead, “it was thought that the ability to increase output without increasing the amount of space taken up by the engines would be a more desirable option,” his paper notes, which would make better use of the limited space available. To check whether that could be done, Yanmar set out to test the same combination of two-stage turbocharging and a ‘strong’ Miller cycle (which has an earlier inlet valve close timing than the conventional Miller cycle) that is used on the 6EY26W and apply it to a 6EY18ALW test engine. Its results were impressive: a 10 % increase in brake mean effective pressure (BMEP) and a reduction in fuel consumption of 5-10g/kWh across the entire load range. Although the paper does not express that as a percentage, a data sheet for the 6EY18ALW engine on Yanmar’s website indicates a typical fuel consumption of around 190g/kWh, which suggests the new set-up provides a saving of up to 5%. To achieve this required an additional piece of technology, however: a new waste gate valve (WGV), which Yanmar developed as part of the project. It was produced by repurposing a commercially-available valve as a WGV and using electronic control to provide linear valve opening characteristics. In fact, because there are two turbochargers, two separate
22 | JUNE 2020
8 Yanmar’s 6EY18ALW test engine fitted with two-stage turbocharging
WGVs were fitted and the researchers found contrasting results between the high-pressure WGV (HP-WPV) and lowpressure WGV (LP-WPV). In an engine with single-stage turbocharging, reductions in pressure ratio due to the WGV reduce the combustion pressure, ultimately leading to a deterioration in brake-specific fuel consumption (BSFC), their report comments. But “while the LP-WGV demonstrates this characteristic, in the case of the HP-WGV, the BSFC remains largely unaffected by changes in the pressure ratio caused by valve opening or closing,” it records. In addition, “when the results of the gas exchange work in the HP-WGV are considered, the amount of work tends to increase the greater the reduction in pressure ratio, indicating an improvement in pumping losses in the air intake and exhaust,” which compensates for the reduced combustion pressure. Overall, “the testing demonstrated a reduction in NOx emissions due to the lower pressure ratio.” It also showed that the combination of two-stage turbocharging and the new HP-WGV increases engine output but does not lead to higher maximum combustion pressure, so “the system keeps the load on pistons, cranks, and other key engine components at a similar level to that of previous engines.” There is still more work to be done, however. When fitted with two-stage turbocharging, the engines meet IMO’s Tier II NOx limits, but would need an SCR unit to meet Tier III. Future research could include investigating high-pressure SCR systems and low-NOx combustion techniques by adding water to fuel; both techniques “are already in commercial use on two-stroke engines but remain rare on four-strokes,” the paper notes. So “issues remain,” the paper concludes, but it promises that “Yanmar intends to keep working toward commercialisation of the technology in recognition of its ability to deliver added value to customers.”
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