The additional cooling capacity Freightliner has designed in for EPA '07 engines, coupled with the higher coolant temperatures allowed by the DD15, ensures the fan will come on from engine requests only in the most extreme conditions to further improve fuel economy. The fan-on temp will likely be set somewhat above 220-degrees F, I'm told. In early fleet-test trucks, the fan-on temp was set somewhat lower, but the company is still working on optimizing the final setting.
Of course, any use of the AC or cab defroster may cause the fan to actuate, but that would happen at any operating temperature.
Fuel And Air Management
There's lots new going on inside the DD15, but two particular features give the DD15 its personality and performance characteristics: the Amplified Common Rail System (ACRS) fuel injection system, and turbo-compounding.
ACRS develops injection pressures up to 32,000 psi in a two-stage process. A gear-driven high-pressure fuel pump produces pressures in the common rail of 13,000 psi, while the final pressure boost is achieved hydraulically within the injector itself. The system is capable of up to five injection events per cycle, optimizing combustion efficiency while reducing noise dramatically.
All this enables better combustion management at any rpm or engine load, and it's all managed by the next-generation DDEC VI electronic controller. Of course, injection pressures of that magnitude require extreme fuel filtration, so there's an easy-to-service two-stage fuel filter and water separator mounted on the left side of the engine. Both use cartridge filters, so replacement is accomplished without filter wrenches.
And last but certainly not least are the turbochargers. There are indeed two of them, but they're not series turbos as we see on Caterpillar engines. The DD15 uses a simple primary turbo to boost manifold pressure for intake air management. I say simple, because it's similar to the proven pre-1999 designs with none of the complex waste-gating or variable-geometry vanes we've seen in recent years.
The second turbocharger is the interesting one, in that it doesn't play much of a role at all in intake air management. Rather, it creates the manifold pressure differential required for exhaust gas recirculation, and it pumps an extra 50 horsepower or so back into the drivetrain (see sidebar).
The combination of ACRS, turbo-compounding,and the rapid response of the primary turbocharger means that the DD15 exhibits up to 75 percent better torque response than the current Series 60 engine - meaning it spools up very quickly to peak torque in 1.5 seconds as opposed to more than 4 seconds as is the case on many other engines.
Lifecycle and Operating Costs
Detroit Diesel says the DD15 has a B50 life - an engineering term that means the point at which half the engines produced will still be running - of 1.2 million miles. That's the highest in the heavy-duty industry, the company says.
Service intervals have been extended to 50,000 miles, thanks to a large 47.5-quart sump, and high capacity oil filters. The head was designed with an access plate to make valve-lash adjustment easier
As for maintenance and serviceability, we got a few words in with the folks who have been tending to the reliability growth test fleet in Portland, and they tell us it's a pretty easy engine to work on. There will be a few new procedures to contend with, and there will be a bit of training awaiting your service techs.
I'm told that all the exterior components, such as alternators, AC compressors and the like are readily accessible - though the starter might prove to be a bit of a challenge. There have been several design enhancements on the vehicle side that have opened up the engine compartment for access, such as engine-mounted air cleaners and radiators, radiator-mounted surge tanks and engine-mounted fuel filters.
But enough about the nuts and bolts. It looks terrific on paper, and it works every bit as well - and then some - out on the highway.
The Sum of DD15's Parts
When Detroit Diesel held the DD15 roll-out event late last year, all the journalists got about 10 minutes of wheel time with the DD15 on the test track at the Chrysler Proving Grounds in Chelsea, Mich. That short time proved to me that there was much more to this engine that a few laps around an oval track would reveal.
So I cashed in nearly all of my chips with my friends at Freightliner and Detroit Diesel and got permission to take the Cascadia out on my own for six hours the following day.
While the nuts-and-bolts stuff I just mentioned describe the ways this engine differs from the Series 60 and other North American engines, it hardly speaks to the way they all work together out in the real world, or how drivers will feel about the package.
Driver acceptance plays a big part in design considerations today, and I think drivers are going to be really, really, happy with the DD15 - once they get over the fact that it's different from anything they have ever driven before.
The DD15 has a personality - if I can use that term - that drivers won't be accustomed to, and therefore might mistake for flat and unresponsive. I can promise you, the DD15 is anything but flat and unresponsive.
What's missing is the turbo lag present on virtually every North American engine currently in service. On other engines, it takes a moment for the turbo to spool up and build boost pressure after an upshift. When the turbo is up to speed and the manifold pressure reaches 20-30 psi, the driver hears the whine and feels a surge of power. Turbo lag is virtually non-existent on the DD15. It spools up to full boost in about 1.5 seconds, compared to three or four seconds on other engines. Drivers will barely hear the turbo whine, and they won't feel the post-lag surge they're used to because the turbo comes up to speed almost immediately.
That means that after an upshift, with the rpm down around 1,000-1,100 rpm, the engine comes up to 90 percent of peak torque before the hammer is halfway to the floor. The pull is there immediately. So while the power is obvious from the way it pulls, drivers won't feel it coming on they way they do now - hence the possible suspicion that the engine is unresponsive.