The Reactivity Controlled Compression Ignition concept for dual-fuel engines has multiple challenges of which some can be overcome using Variable Valve Actuation approaches. For various fuel combinations, the engine research community has shown that running dual-fuel engines in RCCI mode, improves thermal efficiency and results in ultra-low engine-out nitrous oxides and soot. However, stable RCCI combustion is limited to a certain load range, depending on available hardware. At low loads, the combustion efficiency can drop significantly, whereas at high loads, the maximum in-cylinder pressure can easily exceed the engine design limit.
In this paper, three VVA measures to increase load range, improve combustion efficiency, and perform thermal management are presented. Simulation results are used to demonstrate the potential of these VVA measures for a heavy-duty engine running on natural gas and diesel. First, TNO’s multi-zonal combustion model is introduced and validated using experimental data from a multi-cylinder heavy-duty engine operated in RCCI mode with variable intake timing. This combustion model is used in conjunction with a commercial zero-dimensional engine simulation tool to examine three different VVA strategies, each with its own advantage:
- 1
Early intake valve closing for extending high load range
- 2
Late intake valve opening for improved combustion efficiency at low load
- 3
Exhaust valve double lift for improved combustion efficiency and thermal management (better after-treatment efficiency).
The latter two are especially important for dual-fuel engines using natural gas since they offer an effective means for CH4 slip reduction. It is shown that thanks to these measures, a heavy-duty engine can run with natural gas - diesel RCCI in the full load range and that significant CH4 reduction towards Euro VI limits is possible. The paper is concluded with an outlook on options to achieve these goals successfully on a real engine.