April 1, 2005

TT #43 Improving Fuel Economy Part 2 - Diesel Engines

By: Rob Lowe

Technical Tips #43 


Tips to Improve your Fuel Economy (Part 2)

In the previous Tech Tip, I gave a number of examples that if applied will save you fuel whether you have a gas or diesel rig. There are some options and additional methods to improve fuel economy on diesel Coaches. Typically, diesels have been more fuel efficient than gas rigs, however recent emission standard revisions have resulted in a noticeable drop in fuel economy compared to earlier models.

There are dramatic differences between a modern diesel and gasoline engine. Before discussing ways to improve diesel fuel efficiency it is helpful to understand a little bit about the diesel engine There are two measures of performance frequently referred to in sales literature: horsepower rating and torque. Horsepower generally determines the (maximum) speed that a coach can attain and maintain under most driving conditions. The trend over the past decade has been to increase horsepower (hp) from 190-230hp in the early 1990's to 300-330 on entry-level coaches today. The latest top line coaches have diesel engines capable of producing 550hp and the trend is for more. With all of this horsepower, large heavier coaches can maintain the speed limits (and more) on all interstates. The diesel engine also produces an abundant amount of torque or pulling power. This pulling power is the force necessary to move the coach from a stop, up to road speed. The more torque available the quicker the coach will accelerate. Diesel engines maximize their pulling power (torque) over a relatively narrow RPM range. In addition, they produce their peak horsepower lower in the RPM range than a typical gasoline engine. This narrower torque and horsepower range usually requires more gears in a transmission to keep the engine in this range at the road speeds at which we like to travel.

Diesel coaches are geared much more to maintaining maximum torque at typical highway speeds so often the lowest rpm level in high (6th) gear is the "sweet spot" for fuel economy. On our Bounder with the Cummins 5.9L ISB the optimum speed in sixth gear is 62-65 miles per hour. We have averaged 10.1mpg (US) over 10,000 miles with our Toad on.

Modern, turbocharged diesel engines require incredible volumes of air, anything that can be done to increase the airflow through the engine will increase the efficiency and ultimately the fuel economy. The cotton fiber air filter that I installed early in my coach'slife has added to the efficiency of this engine. Most diesel engines utilize a turbocharger, which is an exhaust driven air pump to increase the volume of air pumped into the engine. Turbochargers historically produced a lag on acceleration but improvements in technology has reduced the size of the turbo and increased the speed it at which it operates, making turbo lag on acceleration much less noticeable. Intercoolers are radiators placed in the radiator stack of the diesel rig to cool the intake air thereby improving the engine efficiency.

One other addition that I have found useful is the Silverleaf System Monitoring system. If you have a computer system and you use it for GPS functions while traveling, then the addition of this VMSpcmonitoring system is simple. An adapter connects to the under dash Onboard Diagnostic Connector and your computer. It will allow you, after installing the software, to monitor many engine and transmission functions. I have learned to use this program, set up as the left vertical sidebar on the computer screen, to track ongoing fuel economy, actual transmission gear in use, engine load, RPM and road speed (among others). If the engine is operating on a long uphill grade and is tracking at 100% load, then depressing the throttle further adds nothing to the performance, since the engine is "maxed" out. Knowing this I find the point between 95 and 100% and maintain that engine and road speed knowing that there is no more. I no longer try to add more fuel by pressing on the throttle. (For purists I know that the throttle controls the airflow and that the engine computer controls the fuel however the more throttle the more fuel added). You can see the fuel economy on this trip (since the last time the program was started), the current tank and the historical fuel economy since new. This system may assist you in adjusting your driving habits to improve your fuel economy. I am sure there are other ways to reduce fuel costs, write to me with them and I will compile them and include in a Tech Tip article.


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