The engine liter math problem: Is 13 > 15?
Trading power for fuel?
Some fleets still prefer large-displacement engines for a variety of reasons, and there still are a large number of applications that depend on them to get their work done.
Larger-displacement engines remain popular for a variety of reasons, Baney says. “For a long time, the residual value on the back end of a 15-liter engine remained very strong,” he says. “You simply could get more money for them on the used truck market, but we’re starting to see that change as well because 13-liter engines have carved out a niche for themselves in the horsepower ‘sweet spot’ that fleets desire.”
However, 15-liter engines can push up into the mid-500 horsepower range. “That’s an attractive proposition for fleets in vocational applications,” Baney says. “Each displacement is in a ‘sweet spot’ of its own.”
While 12- to 13-liter engines are stretched to their mechanical and thermal limits, larger-displacement 15-liter engines have increased capacity and margin relative to cylinder pressure capability and mechanical and thermal limits, Rumsey says. “Additionally, lower power density may enable a simpler engine platform or may provide the flexibility to optimize the engine and engine subsystems to achieve the best blend of desired performance, fuel economy and emissions characteristics.”
Larger-displacement engines also can offer improved drivability. Despite the recent market shift, 15-liter engines remain an attractive option for long-haul fleets due to performance and fuel economy, stronger clutch engagement and better braking horsepower.
“While we’ve made some strong inroads with customers and convinced them that their needs can be met by a 13-liter engine, there’s still a legitimate need and demand for 15-liter power – especially with higher-horsepower users,” says Gilligan, explaining Navistar’s reason for relaunching the Cummins ISX15 in its ProStar+, with plans to offer the engine soon in both its 9900 and PayStar tractors.
Making the right choice
Fleets considering moving either up or down in terms of engine displacement need to review their operational history before making a decision.
Looking at the engine’s Logged Vehicle Data can tell a lot about how it is performing relative to the work required and if there’s too much or not enough power on hand. “It can tell you if there’s extremely high demand for power at very low vehicle speeds,” McKenna says. “Likewise, you can learn if there’s lower power demand at higher vehicle speeds.”
Despite the 13-liter’s reputation for fuel economy, Rumsey is adamant that modern larger-displacement engines can compete in the fuel economy arena with their smaller siblings. “For equivalent ratings, a higher-displacement engine has more flexibility, allowing it to achieve greater fuel economy,” she says.
Bore and stroke optimization, valve size and turbo efficiency are all components of optimizing fuel economy. “At the same time, horsepower and torque offerings typically increase with engine displacement because the mechanical limits of an engine are reached as power density increases,” says Rumsey. “A higher-displacement engine provides the flexibility for optimal gearing, allowing the engine to operate at lower engine speeds at cruise – which also translates into good fuel economy.”