January 2008, TruckingInfo.com - Test Drives
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.
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.