Detroit Diesel's DD15 Debuts

Jim Park
Contributing Editor

The clean-slate-design DD15 will power all Daimler, Freightliner and Mitsubishi Fuso HD trucks the world over. North Americans get it first in 2008, followed by Japan in '09, and Europe in 2010.

We're witnessing a huge change in the way trucks and engines come together. We saw it first with Volvo's D-platform engines (shared with Mack and Renault). We'll soon see International take a similar approach, sharing an engine platform between International trucks in North America and MAN chassis in Europe. Paccar is moving forward with the North Americanization of the European 12.9-liter Paccar MX engine that powers DAF trucks in Europe and elsewhere.

Now, Daimler Trucks has unveiled the DD15 engine. It's the first in a four-displacement family that will eventually power all Daimler Group trucks worldwide.

It's become too expensive to design, test and integrate separate engines into multiple chassis and to meet multiple sets of emission reductions regulations. Detroit Diesel, through its global parent company, Daimler Trucks AG - based in Stuttgart Germany - just pumped $1.5 billion into the development of this new global engine platform.

Formerly known ignominiously as the "Heavy Duty Engine Platform," we got our first official look at Detroit Diesel's DD15 in October at the company's Redford, Mich., assembly plant.

Developed from the get-go as a global engine platform, the DD15 will replace the four distinct engine series used today by Daimler Trucks brands around the world. Fully 90 percent of the componentry will be common. As the engine rolls out in different markets over time, they'll be tailored to local demands and to meet local emissions requirements.

The first model we'll see here - with full production set for next April - is the 14.8-liter version of the DD15. In 2010, we'll see a larger 15.6-liter version, which will be essentially the same engine save for a longer stroke, giving it the larger displacement. Detroit Diesel says we'll see 630 horsepower and 2,050 pounds-feet of torque out of that one.

We'll also get the 12.8-liter DD13, which is again essentially the same engine, but scaled down, and with a different block, crank, head and smaller liners. That one will launch in 2009, as the MBE 4000 heads for retirement in 2010.

The DD15 will be offered in variants from 455 horsepower to 560 horsepower and 1,550 to 1,850 pounds-feet in its initial 14.8-liter trim.

Everything's New

This engine isn't at all a repurposed and repackaged Series 60. It's a brand new clean-slate design that was born EPA-'07-compliant. It meets the next round of EPA regs by way of cooled EGR and a particulate filter, and it's ready to meet EPA '10 with the addition of just a downstream selective catalytic reduction (SCR) exhaust aftertreatment system to meet the lower NOx standards. It'll go out way beyond 2010, too, because engineers went with a long-term emissions reduction strategy.

Cylinder pressures will need to be vastly higher in the future, and that sort of thing would require a complete redesign of the engine. So DD15 has the cylinder pressure capability for today, and the ability to grow for many years to come. The single-piece compacted graphite iron head boasts over a million pounds of clamping pressure to the block.

The DD15 is a 14.8-litre, in-line-six with four valves per cylinder and two overhead camshafts. It's built on a stiffer, stronger cast iron alloy block that dramatically reduces noise and vibration. It's got a rear gear train, and a forged steel, counter-weighted crankshaft with huge main- and rod-bearing journals for long life and high load-bearing capacity.

The camshafts are hollow, not solid as is usually the case. This, Detroit Diesel says, lowers the weight and inertia of the camshaft, improving throttle response. Each cam is dedicated: one for the intake valves, the other controls the exhaust valves and the integrated Jacobs engine brake.

The camshafts play no role in building pressure at the fuel injector, so the cams can be tubular and still have more than adequate strength. A gear-driven high-pressure pump maintains pressure in the common rail, with the pressure amplification accomplished inside the injector within the cylinder head.

The cylinder liners feature a unique bottom stop that allows the bottom of the liner to rest in the block. This mounting method also allows for improved coolant flow around the liner. The liners are thick too, which leads to longer life and minimizes vibration.

Liner cavitation shouldn't be an issue at all with the DD15. This engine was designed with higher coolant flow and optimized coolant galleries to avoid hotspots, which could lead to cavitation, and the rad header features a very efficient surge tank to maintain cooling system pressure while de-aerating the coolant. This also minimizes the potential for liner wall cavitation.

Insiders at Freightliner tell me they've torn down several high-mileage test engines and the liners show no signs of liner cavitation whatsoever.

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.

We've become accustomed to the way a variable-geometry turbocharger feels under foot, and this one is decidedly different.

Given that torque is largely misunderstood by drivers, many will miss the key element in the DD15's new power profile. I lugged the engine down below 1,000 rpm on many occasions and the pull is still there. Unfortunately, many drivers will be tempted to downshift long before the engine drops to 1,000 rpm, so a bit of training would be in order here. To get full efficiency out of this engine, you've got to let it drift down to the lower rpm range.

The Chelsea test track has a few steep grades to play on, so we ran the DD15, first, on a seven-percent grade. That grade wasn't much of a challenge, so I took a second pass, deciding to try an upshift on the hill. From a near-stop at the bottom of the hill in fourth gear, I made my way up through two gears before cresting the hill. The DD15 was still powering up the hill even as low as 900 rpm.

On the 15-percent grade, the engine held its own in fourth gear all the way up.

Out in the real world the following day - on the Zilwaukee Bridge in Saginaw Mich., the only substantial grade within 250 miles of Detroit - I hit the hill on the first pass at cruise speed (60 mph, 1,300 rpm) and dropped only 5 mph at the top. The bridge is only a four-percent grade, so it's nothing to get excited about. On my second pass, I hit the hill at 50 mph (1,000 rpm), and the engine dropped only 2 mph going over the top. It drifted down to 950 rpm and stayed there all the way up.

At this point in time, Detroit hasn't published the torque and horsepower curves for the DD15, but I'd bet I was getting very close to the full compliment of a 1,750 pounds-foot engine, but at 1,000 rpm, which is where you want an engine running for optimum fuel economy.

Everyday Driving

When I picked up the truck at Livonia on Saturday morning, the Detroit Diesel guy, Jason Martin, warned me that the truck had gone into "thermal management" mode - a fancy new term for an active regen event - as he started it up prior to my arrival. He asked to see if I noticed anything unusual while I was driving, or if I could even tell the regen was taking place.

Had he not told me, I never would have known. But since he did, I was paying particularly close attention.

Oddly, because the engine is so quiet, I could hear the regen event, rather than see or feel it. I was aware of a swishing sound coming from the left side of the engine, and this, I'm told, was the sound of the exhaust gas passing through the throttling valve on the intake manifold. When the engine goes into a "thermal management" mode, the intake throttle starts to close on its own to make the exhaust hotter. Then, when the exhaust and filter are hot enough, the dosing system starts to add fuel to the exhaust stream so that the soot will oxidize.

Freightliner has opted not to include a warning light on the dash to advise drivers that a regen is taking place. They will get a "high exhaust temp" warning light at speeds less than 5 mph when a regeneration is in process. The intention is to advise a driver that exhaust is hot when the truck is near a standstill, hopefully keeping the driver from stopping under a tree with dry leaves hanging on the exhaust or from stopping over a pile of gasoline-soaked kindling wood.

Did I mention how quiet the engine is? Aside from a pleasant low rumble, and a bit of diesely cackle when the engine is heavily loaded, you can barely hear the thing. Virtually no whistle from the turbo. And that's not all. There's Cascadia's superb sound attenuation. It's really quiet outside too. Chalk that up to the advanced high-pressure injection system and an engine block designed to minimize vibration - which is just a fancy word for noise.

Cruising at highway speeds, the foot feed is very responsive at any point in the useable engine speed range. The turbo reacts almost instantly, giving the truck an almost automotive feel. There's just power there all the time.

At the low engine and road speeds of an urban environment, shifting becomes much less of a challenge than I've found with some VGT engines. You just don't need any more than 1,000-1,200 rpm in any gear short of eighth and up, so you don't have the torquey shifts and finicky pedal response common to those VGT engines.

It seems to have a min-max-style governor on it, meaning that the throttle pedal position relates quite precisely to the revs you want to make a downshift. It was ridiculously easy to make synchronous when downshifting. I surprised myself more than once by hitting almost every gear the first time.

The truck I had was equipped with a 13-speed transmission behind a 455 horsepower, 1,550/1,750 engine. Given the breadth of the peak torque range of this engine, a lighter, cheaper 10-speed transmission would be an ideal match.

I can't report accurately on fuel economy at this time because I didn't fuel the truck after the trip. There was a prototype fuel economy read-out on the dash, but Freightliner tells me it hadn't been properly calibrated yet. Too bad - it was showing 9.67 mpg for most of the trip.

I can say, though, that the fuel usage meter was working, and I monitored the fuel through-put over a 31-minute period. The meter showed exactly four gallons consumed. That's four gallons over 30 minutes/miles, or 7.75 mpg at 65,000 pounds GVW.

The DD15 is the dawn of a new era for Detroit Diesel and its parent company, Daimler Trucks. This engine will see service all over the world in more or less the same configuration I drove in Michigan that Saturday in October. European drivers will be more familiar with its flat torque curve than North American drivers, but it won't take our guys long to see the value in a setup like this.

I predict this engine will be very well received by drivers, mechanics and fleet owners alike. There's a lot to like about the thing, and if the B50-life and the extended maintenance intervals live up to the billing, this will be an engine the market is ready for.