FOUR-STROKE ENGINES
SOLVING THE CLEAN-UP CONUNDRUM
Image: ABB Turbocharging
Two-stage turbocharging has been around for quite a while, but there's recently been a resurgence of interest. However, the targets differ from those of automotive applications which focus on dynamic behaviour, writes Stevie Knight
Firstly, as Michael Willmann of ABB Turbocharging points out: “Any step that helps to minimize the engine footprint, such as increasing the power density through the use of higher receiver pressure, is desirable.” But it's broader than that: for marine's medium-speed engines, “concern centres rather more on efficiency and peak pressure”, says two-stage turbocharger development lead for PBST, Sebastian Spengler. Here, it's not just an enabler for high power densities, it's also aimed at NOx and SFOC reduction. However, there's a conundrum: “A greater mean effective pressure in the engine requires more mass coming through the air intake,” says Johannes Kech of Rolls-Royce's Power Systems business unit. This doesn't necessarily sit well with combustion clean up techniques aiming to meet IMO Tier III and EPA Tier 4 NOx requirements (which this year reached the Baltic and North Sea ECAs). All of which puts yet more demand on the turbos. Take Exhaust Gas Recirculation: this diverts some of the exhaust flow back into the intake manifold, diluting the charge, reducing available oxygen, combustion temperature and NOx formation on the flame front. However, large engines require considerably higher EGR rates to achieve the same NOx reduction level as their smaller automotive counterparts - the fraction sometimes reaching over 50%. Therefore, the EGR can be asking for more while the turbo itself might be feeling the pinch. Another technique is playing with the timing to change the combustion characteristics: on a Miller cycle, the charge is limited by either late or early intake valve closure. This can be tweaked to reduce the work required by compression, stretch out the intake charge to cool combustion temperatures from NOx-formation levels, and harvest more energy from the cycle. While a turbo can mitigate the consequent power loss, stronger Miller timing necessary to meet advancing NOx
14 | MARCH 2021
8 Both power density and emissions are concerns
requirements also means “there's much less time for bringing the air through into the cylinder”, explains Kech. He adds that even relatively straightforward particulate filters “put a higher back pressure on the exhaust system, so to ensure the exhaust flows out of the cylinder, again you need more pressure on the intake side”. Initially, EGR, filter pressure and Miller cycle demands were supported through single-stage turbochargers. These are comparatively simple and cost-effective, so they've gained a good slice of development: ABB's product has lifted the pressure ratio ceiling from 6.0 to 6.5, while PBST has been investigating how to move towards 7.0 with package, cost and performance benefits for medium speed and high speed gas engines. But these single-stage units have shortcomings. While they can deliver emissions savings, above a pressure ratio around 5 or 5.5, “the topological map of the compressor tends to narrow, and in this situation, the engine's overall performance becomes rather limited”, says Kech. In other words, it can be set up either for a wide speed range or a high boost pressure - but there's generally a trade-off between the two. Just adding another TC for increased speeds will fix a number of issues. “A sequential system means starting the engine with one turbocharger: accelerating to a higher power triggers a second, then a third and so on,” he explains. That in turn allows more boost at low engine speeds and faster transient response. However, sequential turbocharging has a different focus: broad performance maps and greater surge margins remain the highest priority for compressor development. But as these systems still operate within the confines of single-stage TCs, Kech points out “this setup can't reach the pressure ratios required for emission reduction technologies”. Instead, the answer lies in two-stage TC arrangements
For the latest news and analysis go to www.motorship.com/news101
