NEED AN OPINION
Thank you for the magazine. As a new diesel owner, I appreciate your efforts to bring me up to speed. I am approaching retirement and will likely become a full-time Rver. Having resided here in
Eau Claire, Wisconsin, for most of my life, I’m ready for warmer winters, and Arizona sounds like a nice place to spend Christmas. To help me with those plans I recently bought my first diesel pickup, a new 2017 Dodge Cummins. The Dodge is a wonderful truck with power to spare, but the fuel mileage is not what I was hoping for.
There’s a lot of information about upgrades that promise to increase fuel economy, but I would like your opinion. I recently read in Diesel World about installing a 5-inch exhaust with a cold-air intake system that could help improve fuel economy, and wanted to know what else might provide the most benefit. And finally, would performance injectors help with fuel economy? Being located in central Wisconsin I am familiar with cold winters. I have to spend a couple more years here before I retire, so I don’t want to make the Cummins harder to start. William Kerst
Via Email With the newer Dpf-equipped vehicles, installing an aftermarket exhaust after the DPF will increase flow, but not drastically. You’ll see a gain, but
it might not be very noticeable. Same with the intake—you’ll gain more flow but you may not notice a huge increase in economy. A tune from one of our advertisers will likely give you the largest boost in mpg and horsepower per dollar spent. But if increasing fuel economy is your primary objective, we’d begin by installing a set of boost pressure and exhaust temperature gauges. This should not affect your warranty or vehicle emissions and will give you good insight on how the engine is running. The gauges will help you drive your truck for best fuel efficiency. If you pay attention to the boost and EGT gauges you may notice a range of engine speeds (and boost/egt) that appear to help the truck to perform at its most efficient. As a general rule, your Dodge Cummins will be most fuel efficient in the 1,800 rpm range, and will develop the best hill-climbing power while towing above 2,000 rpm, because turbocharger efficiency doesn’t peak till the engine is operating above 2,000. You can think of the turbocharger and its boost pressure as a torque adder. A number of factors come together to explain the torque peak/rpm/fuel efficiency rule. Diesel engine pumping losses (or efficiency) is one determining factor in fuel economy. A diesel engine is more or less just a big air pump, with most having an 80-85% efficiency rating for moving air in through its intake and out through the exhaust, and which varies as a function of rpm. Diesel fuel also has a specific burn rate, which limits the upper rpm capability. Diesel burns more slowly than gasoline, and this partly explains why most diesels operate at about half the dynamic rev range of a gas engine. Fuel atomization (droplet size), cetane rating (combustibility), viscosity, and injection timing and combustion swirl all contribute to diesel fuel efficiency. A few years ago we spent the better part of an afternoon measuring diesel fuel consumption with a 6.5L diesel engine on an engine dyno and a calibrated digital fuel consumption device. Each of the following data points were collected after allowing the engine to operate at the indicated rpm and power/torque levels for 1 minute: 1,400 rpm, 29 hp / 110 lb-ft - .54 lbs/hp/hr 1,800 rpm, 53 hp / 155 lb-ft - .40 lbs/hp/hr 1,800 rpm, 82 hp / 240 lb-ft - .39 lbs/hp/hr 2,100 rpm, 50 hp / 125 lb-ft - .45 lbs/hp/hr The above data shows how engine revolutions and load affect fuel use. We have shown that 1,800 rpm with a moderate load, approximating an unloaded run down the highway, produces the best fuel efficiency. This rule has more or less proven itself over and over to apply to nearly all on-road, light-truck diesel engines.