For the first time ever, HDT had the chance to view the internal processing of carbon within the diesel particulate filter of an '07 engine installation. The DPF was behind a 2007 Cummins ISX and we could see, via computer, varying soot levels with passive regeneration during a drive of close to 200 miles. The icing on the cake was that the ISX in question was a 600-horsepower-rated engine – peaking at 615 horsepower – powering a brand-new Peterbilt 389, due to be delivered to a customer .

The truck, of course, is not yet in series production. It will be the first in Peterbilt's line-up for 2007 that will get the new engines, though every indication is that the early builds will all be Cummins. And because they will get only 2007 engines, all the 389s will be sold as 2008 models.

But that's another story. Here we have a drive of a complete '07 setup: second-generation cooled exhaust gas recirculation; updates to the ISX for improved reliability and durability, such as the relocation of the EGR valve to the cool side of the engine; and the DPF with combo dashboard warning light and regeneration switch.

These driver interface pieces of the '07 puzzle are provided on the Peterbilt dash, newly updated last year and fully multiplexed. It is a handsome instrument panel on the new 389, with engine information and the tach and speedometer in the panel directly ahead. Supplementary gauges are arranged to the left and above the information display/navigation system screen in the center of what is effectively the wing dash. The new DPF lamp sits to the right side of this display panel, prominent in what is quite a busy dashboard.

The regeneration switch, along with other switches, including the cruise buttons and the engine brake, are low and to the right of the steering column, a position I found took more familiarity to use than I had time for in the short morning's drive.

Two new lamps are part of the combo: a DPF lamp and an exhaust temperature lamp (HET). The DPF light will only illuminate in the rare event that a service-initiated, stationary regeneration of the DPF is required. The HET lamp may illuminate when an active regeneration is in process to warn of slightly higher exhaust temperatures than normal. The dual function switch can initiate an active regen, though only if soot loading within the DPF requires it, and even then, the switch must be depressed for a number of seconds. The switch also can be used to override a regen if the driver considers the conditions unfavorable, such as a fuel hauler under a fuel rack. Cummins says normal operation of the DPF does not require any intervention by the driver.

A recap of the function and operation of the DPF may be in order. The filter is a large ceramic "monolith" about the diameter of a standard muffler and maybe 12-15 inches long. It has many tubes through which the exhaust flows, but alternate tubes are blocked at one end or the other. As the exhaust flows, it has to pass through the walls of the input tubes into the exit tubes, depositing the fine carbon particles – micron-sized and invisible to the naked eye – as it goes.

Within the DPF are a couple of noble-metal catalysts. In the first stage, ahead of the filter, nitrous oxide (NO) out of the engine is oxidized to nitrogen dioxide (NO2). Within the filter element, the NO2 combines with the carbon to form NO again and CO2, which both exit to the exhaust stack, leaving the filter clean of carbon.

These reactions go on, in the presence of the catalysts, in the hot exhaust stream without any further interaction. However, under certain operating conditions, the exhaust does not get hot enough for the reactions and needs help with an active regeneration. In this process, a little mist of diesel fuel is introduced into the exhaust stream by a "doser" mounted just downstream of the turbocharger. When it impinges on the oxidation catalyst it heats up, raising the temperature of the exhaust to allow the reactions to proceed and purge the filter of accumulated soot.

This has been explained to us in theory, but the setup in the Pete allowed us a "window" on the DPF's operation via a laptop computer.

During the test, we started out from Cummins headquarters in Columbus, Ind., with a relatively easy few miles to pick up I-65 southbound at Junction 68, the intention being to run down toward the Kentucky border, swing west on I-265 to I-64 to take in the grades through there and give the ISX 600 something to work against.

As we joined the freeway, the DPF started to load up, with something like 11 grams of soot showing in the filter. This is only a trace, because it is sized to accommodate as much as 250 grams of soot, with active regeneration normally initiated at much less than half this level. But even with the relatively easy running of the southbound leg towards Louisville, the exhaust passively regenerated this soot. Climbing the up to 5 percent grades on the westbound leg cleared out the remaining soot, so by the time we turned north on Indiana 135 toward Salem it was all but cleaned out.

Indiana 135 is a tight little two-lane that does not encourage high speeds nor lots of accelerator pedal. However, there are a couple of good hills along the route, so the DPF alternately loaded and unloaded soot levels as we proceeded. Through the pretty old town of Salem, with its tight turns and crowded town center, we loaded up again to 6 grams, but cleaned out again with application of throttle on the road sections, particularly the divided route 50. The final few miles on I-65 northbound again cleared out residual soot. We returned the Pete to Cummins R&D center a lot wiser about the DPF functionality.

It's worth mentioning that the ISX 600 performance is truly outstanding. The 600 is the most powerful of a hardware engine group that starts at 525 horsepower. It has the Holset variable geometry turbocharger, new with electronic control for 2007 engines, and boasts 1,850 pounds-feet peak torque. But that peak is absolutely flat from 1,200 rpm all the way to 1,800 rpm. So the performance feels amazing, especially with the throttle response of the VG turbocharger.

On the hills, despite running close to 80,000 pounds, we never had cause to drop below 8-low in the 18-speed transmission unless slowed by other traffic. On the major pull as I-265 merges with I-64, the 600 was so strong we were picking up rpms on the 5 percent climb, so even before we reached the summit, I was able to grab 8-high to sail over the top at a legal 65 mph and 1,500 rpm. (gearing included 3.70 rears and 22.5-inch rubber).

The engine is incredibly flexible. To check out luggability, we chugged down to 950 rpm on one of the grades on the northbound leg up Route 135, and it remained smooth as silk with the VG turbo keeping boost up. During the lunch-time debriefing later, we concluded that you really don't need the 2,050 pounds-feet of the earlier Signature 600 for this sort of performance, though maybe it has to have the rating purely for bragging rights against the competition.

Cummins sees the market for the high horsepower engine increasing, despite the rise in fuel prices, as more fleets expand with owner-operator divisions. Where it used to sell only around 2,000 engines a year, the current feeling is that these big power engines (525-600 horsepower) may account for 6,000 or more sales annually.

Other points raised at the debriefing were the quietness of the ISX, something I had noticed earlier in the week with the ISX 435 in the new International ProStar (see road test on page 110 of this issue). In the Pete, cruise noise levels were only 67db(A), though the engine did makes its presence felt pulling hard in the intermediate gears, probably because the torque was compressing the engine mountings and transmitting more of the engine signature back through the frame.

An incidental benefit of the particulate filter is that it replaces the conventional exhaust with a very effective muffler, so operation of the Intebrake retarder was almost silent. The downside is that without the distinctive rap of the engine brake, it feels like it is not performing as well, but the 600 has close to the same retarding horsepower as its rated output, with the usual three-stage switch.

The other surprising thing about this installation was the lack of fan-on time. Earlier in the week, driving the ProStar, I had noted significant fan time. But on the Pete, the fan kicked in only once with the air conditioner, and then only for about 40 seconds. That obviously is attributable to the generous cooling package with the included fan ring for the big conventionals (which is, incidentally, shared with Kenworth for the '07s).

The other notable feature of the ISX is how clean it looks in the chassis. The inlet arrangement looks more refined and the bigger EGR cooler is tucked in unobtrusively behind the exhaust manifold and VGT. You even have to hunt around for the DPF, which on the 389 Pete is packaged into and behind the passenger side step/toolbox. Exhaust stacks run up both sides of the cab with conventional-looking heat shields. (The downside is, the toolbox is tiny.)

One of the reasons for the on-dash exhaust temperature lamp combined with the DPF light is to indicate an event in progress and alert a driver or passenger that the exhaust system may be hot. It never gets hotter than it does running the engine under wide-open-throttle, but it may not get the cooling blast of air flowing over the exhaust as the truck goes down the road if it's a slow-speed active regen.

Of course, we never saw either light illuminated, as soot levels were low or zero throughout our drive. And it is possible an over-the-road driver may never see these lights come on or ever have to hit the regen switch – at least for a couple of years. The driver and fleet will notice a more frequent light-on as the system approaches 200,000-400,000 miles. That's why DPF cleaning is part of the scheduled maintenance.

Closing the door at the end of this short evaluation, I said to my Cummins passenger Mario Sanchez that this Pete has to be one of the nicest trucks I have ever driven. The combination of the updates to the 389 make it even more civilized than the 379 it replaces, and the powertrain makes it relaxing and fun to drive.

Moreover, there's absolutely nothing to worry about in the operations of the aftertreatment system for the '07engines. The only downside I can see is that they will inevitably cost more, but that's the price of progress.