According to Chris Moran, medium duty program manager of sales and marketing for Detroit, the company's new DD5 medium-duty engine is merely the company's first step in filling out its diesel engine portfolio. A larger displacement 8L DD5 is slated to launch next year, which he said will give Detroit a powerful and fuel-efficient diesel to meet any need for fleets ranging from Class 4 up to Class 8. 

The DD5 is a four-cylinder engine – a design that has been traditionally suspect in the eyes of North American fleet owners from longevity and power points of view. But Moran said the engine's parent-bore cylinder design has removed complexity from the engine, while a cast iron, deep-rib block has increased overall durability and the engine's overhead cam gives it plenty of low-end power to get a truck rolling and keep it both cruising and passing at highway speeds. 

So we headed to Chicago to see how well it lives up to that promise.

Our Freightliner M2 test truck was equipped with an Allison 2500 Series automatic transmission, which gave crisp and instantaneous response when I accelerated out of the dealership into heavy Chicago traffic. In fact, had I not been briefed beforehand that the DD5 is a four-cylinder design, I doubt I would have guessed it, so brisk was the overall acceleration from a dead stop. 

The initial stop-and-go traffic also gave Moran the opportunity to show me DD5's extremely effective engine brake, which is activated by a dash-mounted flip-switch to the right of the steering wheel. The brake switch has three settings, Off, Low and High, which gave me an impressive amount of stopping power without putting my foot on the brake pedal.

The engine brake engages immediately after you let off the accelerator pedal and it is aggressive even at highway cruising speeds. Moran said Detroit is so confident in the design it believes fleets running the DD5 will see increased brake life and a decrease in associated maintenance costs if drivers use the engine brake regularly. 

Even with a fully loaded box, the DD5-equipped M2 showed consistently crisp throttle response and brisk acceleration as I worked my way through a long series of traffic lights heading toward the entrance ramp to I-290 West toward the Chicago suburbs. 

I really put my foot into it as we took off down the freeway entrance ramp and was impressed to see that the M2 was already going 55 mph by the time I needed to merge into traffic.

Once I was at cruising speed, I had to be mindful of the speedometer as the DD5 had no trouble quietly and quickly moving past the posted 55 mph speed limit to much higher speeds. Traffic flow on I-290 was about 65 mph, which the DD5 had no problem maintaining. 

A bit later on, in a passing situation, the engine quickly got the M2 up past 75 mph, with the engine brake in "High" setting, quickly decelerating the vehicle once I was past the offending car.

Overall, the engine performed admirably with impressive acceleration and cruising characteristics. Interior noise levels are low throughout the engine's power curve, and the Detroit engineers deserve a special tip of the hat for the engine's integrated engine brake and its extremely effective performance.

Read the full Test Drive in the January issue of HDT.