Fig. 10 shows the comparison of the BSFC between the 4RB2 and the test engine working on the 4-stroke secondary expansion mode in different operating conditions. The fuel consumption of the test engine was lower than that of the original 4RB2 engine. To explain the fuel-saving reason, Fig. 11 shows the p-v diagram comparisons verified GT-Power simulation model. The curves of the compression and combustion strokes are nearly overlapped in Fig. 11(a), which means the expansion cylinder makes little impact to these two strokes for the combustion cylinder. However, the curves of the intake and exhaust strokes are comparatively different. To observe the p-V diagrams of these two strokes clearly, Fig 11.(b) and (c) shows the enlarge p-v diagram comparison. The fuel-saving reason of the test engine can be concluded as the following three points.