When a change in fuel emission standards reduced demand for its N-14 truck engine, Cummins Inc. (Columbus, IN) used Witness simulation software from Lanner Group (Houston) to improve the post-assembly of the engine, and in the process realized $268,000 in annual savings.

As important as it was to find these savings, however, Cummins made an even more valuable discovery through the simulation. The company learned that Witness could play a key role in its growing Six Sigma initiative. "The N-14 project opened the doors for using simulation in all our Six Sigma projects," explains Maria Pasqualli, head simulation manager at Cummins. "It showed our master black belts that Witness could help them realize significant savings in the Design of Experiments phase of Six Sigma."

Simulation and Six Sigma

Cummins, makers of engines and related technologies and electrical power generation systems, has used Witness since 1991 and has simulated processes such as engine assembly, testing, post-assembly, supply chain and inventory. According to Pasqualli, Cummins chose Witness as its simulation system for two reasons: "We needed a simulation package that was flexible enough to model a variety of different processes and products. Plus, we wanted to make sure that we were using software that our biggest clients, DaimlerChrysler and Ford, also used. We were looking for a solution that would meet our needs 10 years down the road."

This road includes Six Sigma. In the 2 years since Cummins started its Six Sigma initiative, the program has grown rapidly, and today there are 20 master black belts, 90 black belts and 200 green belts?and the numbers are growing.

Pasqualli says that with the growth of Six Sigma at Cummins and the statistical, process-driven nature of simulation, it was only a matter of time before the two methodologies were used together. "The software helps Six Sigma by providing an inexpensive way to conduct the Design of Experiments without disrupting production," she explains. "And, Six Sigma helps Witness by providing a wealth of accurate data. Building a simulation for a Six Sigma project takes me half the time that a normal simulation takes just because the data is all there."

Aging engine spurs interest

Despite the mutual advantages of simulation and Six Sigma, Witness did not catch the attention of Six Sigma experts until Pasqualli got involved in a project to modify the post-assembly process of the N-14 truck engine.

Cummins needed to produce the engine for countries where the emission standards had yet to be changed and demand was still strong.

Still, production of the N-14 was scheduled to decrease from 160 to 80 engines a day. This presented a problem in the N-14 post-assembly process?the stage of the operation where engines were washed, primed, painted and finished. With the PLC geared for high volumes, there were large buffers between each of the stations. Because the process employed a "push" flow system, the engines would not get released until the buffers filled up. When the volume was reduced, it took longer for those buffers to fill, creating delays in releasing engines at each station. The result was a considerable drop in throughput and an increase in work-in-progress (WIP). In fact, Cummins had more engines in WIP than they were producing each day.

The lower volumes also created a problem with the overhead carriers, an expensive set of capital equipment. When the plant was producing 160 engines a day, it needed 92 overhead carriers to move them through the system. But with the decreased demand, post-assembly actually had more carriers in the operation than the number of engines that needed to be produced each day.

Six Sigma experts at the plant initiated a project to modify the N-14 post-assembly process to cope with decreased demand for the engine. In previous projects, they tried to conduct some of the Design of Experiments without simulation. This proved to be difficult for the workers and too disruptive to production, explains Pasqualli. "One of the big issues was to figure out the right number of carriers to use. Without simulation, we resorted to trial and error. The operators would have to take these carriers off, then put them back on to see what the optimal number should be. But these are heavy pieces of equipment. It was becoming time-consuming and tiring work."

With the simulation, the team learned how the system would behave with fewer carriers. "So every time they took a carrier off, they knew they weren?t going to have to put it back on again," adds Pasqualli. "That was one of the important values of simulation?to create confidence that the changes we were making would work. Another benefit of simulation was that it enabled the Six Sigma team to evaluate months of performance for every scenario in a matter of minutes. This ability to conduct what Pasqualli calls "rifle-shot" what-if experiments gave the Six Sigma team a better understanding of what could be achieved.

Witness showed the Six Sigma team that the best option for optimizing throughput and reducing variation would be to modify the control logic and change the process to a "pull" flow employing 42 carriers. The team learned that the system could handle up to 60 carriers with minimal impact on throughput time.

Based on the findings from Witness, Cummins implemented several changes over a 4-month period. The company reduced the buffers in the drying ovens, eliminated two workstations and removed 27 carriers from the system. Extra carriers were kept to handle any unexpected increase in demand.

Throughput time was reduced 14%, WIP decreased 29% and productivity improved 11%.

The improvements in process flow brought a corresponding improvement in a key Six Sigma quality measurement. The Cpk improved from 0.43 to 1.42 for the daily average process time of the N-14 engine.

John Marshall, the quality engineer on the Six Sigma team, explains, "The simulation enabled us to look at how engines were flowing through the process and to identify problem areas that caused us to take engines off line. Whenever you take an engine out of the process, you leave yourself wide open for defects to occur because they?re outside of the process flow, where all the tooling has been tuned, the build sequence set and the training invested. Witness helped us increase capacity, keep all the engines on the line and avoid the possibility of off-line defects."

Lanner Group Inc.

(713) 532 8008



  • The software integrated Six Sigma and simulation to improve processes.
  • Throughput time was reduced 14%, WIP decreased 29% and productivity improved 11%. Defects found at final inspection decreased 65%.
  • The Cpk improved from 0.43 to 1.42 for the daily average process