turbochargers
Two-stage turbocharging can reduce emissions
For ease of installation into ship enginerooms, Caterpillar Motoren’s promoting an attractive alternative to the two-stage turbocharger module approach
by Dick Amos
Riviera Maritime Media 2011 World Turbocharger Conference was the likely requirement for two-stage turbocharging of marine engines to enable IMO Tier III limits to be met. The general opinion was that this would be necessary, but different concepts and layout arrangements are currently being pursued by different manufacturers. Ulrich Hopmann, engineering manager for
A Caterpillar Motoren
said, “We were concerned about the effect of separating the turbochargers by the length of the engine, but testing has now proved to us that the arrangement is entirely satisfactory and offers a number of potential advantages for marine applications.”
The new concept from Caterpillar, of Kiel,
Germany, is in response to demands for an additional 75 per cent reduction in NOx emissions. A leader in medium speed marine propulsion engines, Caterpillar has built on the success of its M32 C low emission (LE), now meeting IMO Tier II limits. This engine design includes increased piston stroke, giving greater flexibility for changes allowing emissions reduction whilst maintaining engine efficiencies. Caterpillar flex cam technology is incorporated, and increased use of Miller timing facilitates a reduction in smoke emissions. Fuel
injection is common
rail and the VM32CLE uses only single-stage turbocharging. Performance and emissions levels are achieved without the need for wastegates or other similar devices, making for a simple and reliable turbocharger installation.
Whilst LE engines were being brought to production standard, Caterpillar was also working towards requirements for IMO Tier III and the technologies required to meet 2016 targets. Having initially looked closely at options for exhaust gas after treatment, Caterpillar developments have now moved away from this and towards the following: • two-stage turbocharging with intercooling
www.mpropulsion.com major discussion point at the cooler cooler cooler
Oxl- Cat
X X X X X X
The exhaust gas bleed for the Caterpillar Motoren EGR system is cooled both before and after its compression by a third turbocharger
• cooled high-pressure exhaust gas recirculation • a high-pressure common rail injection system • early closing of the inlet valves with FCT valve timing control • exhaust system control devices for NOx flexibility and optimised engine operation. In the new two-stage design, the compressor of the first turbocharger delivers intercooled air along the engine to the second turbocharger. The second stage delivers a pressure sufficiently high to charge the engine, even with the early inlet valve closure and the new exhaust gas recirculation (EGR) system. Exhaust gas is first used to drive the high-pressure turbocharger before returning along the engine to the low-pressure turbine stage, which drives the first stage of compression. The high-pressure turbine has a bypass to assist with control of critical parameters such as firing pressure. Bypassed exhaust gas is fed back into the system, upstream of the low-pressure turbine. The EGR system, feeding exhaust into the engine charge air, is another key feature employed to reduce engine NOx emissions. This works through increasing the specific heat capacity of the charge air and lowering maximum cylinder temperatures during compression and combustion. EGR also decreases oxygen content in the charge air, impacting on combustion characteristics and reducing local peak temperatures. The
turbocharger
system involves to
compress
the use of exhaust
a gas
third for
re-injection into the engine system downstream of the cooler. Exhaust gas abstracted upstream of the high-pressure turbocharger turbine is split into two streams. The recirculated gas is cooled both pre- and post-compression and the second stream
is used to drive the turbine before returning to the engine exhaust upstream of the low pressure turbocharger. Caterpillar has tested recirculation of up to 15 per cent of intake mass flow but believes a figure of 20 per cent may be required to achieve the required reduction in NOx emissions. The design of the production IMO Tier III engine is modelled on retaining the basic single- stage turbocharger configuration and adding the high-pressure turbocharger at the non-drive end. This is included in a module with all EGR equipment which can be readily mounted onto an existing design engine with few modifications. Despite initial concerns, Caterpillar has proved that this arrangement is satisfactory, and has carried out extensive analysis to confirm that operational vibration characteristics will be acceptable. Laboratory tests and extensive engine cycle calculations have shown that IMO Tier III emissions can be met using two-stage turbocharging and EGR without a negative impact on other engine operating parameters. Caterpillar believes that without EGR operating, the two- stage turbocharging system will deliver reduced fuel consumption compared with current systems. Caterpillar considers the concept will offer other advantages, compared to the exhaust after-treatment approach, including lower space requirements and ease of installation into standard ship designs. The system also avoids costs that would be incurred in fluids consumption of selective catalytic reduction (SCR) systems. The company is, however, planning to continue test work to validate all aspects of designs, in particular, the temperature and operational aspects of the EGR gas path. MP
Marine Propulsion I February/March 2012 I 43
cooler
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