By Scott Dalgleish
Adjusting your habits for ascending and descending steep grades, following proper start-up and shutdown procedures, and understanding diesel acceleration are just a few of the simple things you can do to aid a smooth transition to a diesel-powered RV.
With fuel prices at record highs, finding an economical and efficient method of enjoying an RV lifestyle has become important. While there are many pro’s and con’s to debate when selecting the method of powering your next RV, gasoline-powered vehicles are familiar to most, so the decision to purchase a diesel-powered vehicle for the first time may be somewhat intimidating. Modern diesel power is still the overwhelming choice to move large heavy vehicles efficiently down the highway, and given the technological advances available on today’s diesel engines, owning and operating a diesel-powered RV is easier than ever.
While the lure of the diesel’s rumble and the high pitch of the turbocharger’s whine is clearly enough to wake the boyish fantasy in most, the acceleration would leave most thinking they just left home plate after a third strike called in the ninth inning, dragging the bat behind. Fortunately, those days are long gone thanks to the features found in the modern diesel of today. Things like Variable Geometry (VG) Turbo Chargers, High-Pressure Common Rail Injection Systems (HPCR), and the advent of the computer-controlled engine system. Gone is the black smoke, and they don’t even smell like the diesel of old.
While it’s true that diesel produces gobs of torque and horsepower at relatively low rpm, their driving characteristics are nothing like their gas counterparts. So many of us find the initial introductory diesel driving experience frustrating. Double that if you are attempting to control 30,000 lbs and 40 plus feet for the first time. This can give the appearance of incompetence to our fellow highway travelers, not to mention our co-pilots. This frustration and fear are not mandatory. Traveling our country is a relaxing and spiritually enhancing experience. I don’t know about you, but I have never had much success at relaxing and enjoying anything that I was struggling to control. If we change our operating habits, diesel can provide a positive, relaxing and rewarding driving experience.
Start Your Engines
Modern compression ignition engines (diesel) utilize the same basic principal that Rudolf Diesel patented back in 1892. Relying on a high compression ratio (15 and 22 to 1) to heat air so that fuel ignites as it is injected, true diesel engines do not utilize spark plugs and the associated primary and secondary electrical systems. Working as a large air compressor, a diesel moves huge volumes of air continuously. The combustion process is timed via the engine’s high-pressure fuel injection system, which today is electronically controlled via an onboard computer referred to as the engine control module (ECM). Developed out of a need to meet higher emission standards, the benefits we derive are cleaner, quieter, and more user-friendly diesel engines.
Starting is normally a simple procedure — follow the engine manufacturer’s recommendations. Cold starts are sometimes aided by preheating the air entering the engine via a charge air heating grid in a direct-injected engine or glow plug in an indirect injected engine. The use of these devices is commonly linked to a “wait to start” dash light, which allows the cold start aid to produce the required hot air. The use of “starting fluids” is generally not recommended in the modern automotive diesel, mainly because of environmental regulation compliance and because increased cylinder pressures can lead to catastrophic engine damage.
Following the owner’s manual recommended starting procedure and maintenance interval for air and fuel filter service, along with the use of the recommended fuel (diesel # 2 in most cases), will ensure trouble-free starts and demonstrate to your co-pilot that you know what you are doing.
Gathering Momentum
Diesels typically do not accelerate in the sense that a gas engine operates, but rather they gather momentum. Diesels do not have the same accelerator pump and ignition advance systems as gas engines, and rotating mass is substantially heavier. Since diesels traditionally provide power in a narrow rpm range, it is necessary for the diesel’s transmission to change gears more frequently in order to keep the engine in the desired “power band.” The ECM found in the modern diesel engine of today “talks” to the computer controlling the automatic transmission. While constantly “reading data” provided by the onboard sensors, the onboard computers make the proper gear selection, ensuring optimum engine performance and efficiency. Once the desired speed is obtained, the diesel is capable of maintaining that speed (momentum) with little effort, since the vehicle’s weight, transmission gear splits, and final drive ratio is properly coordinated via computer. Planning ahead, being mindful and courteous of other motorists, slowly applying more throttle as the turbo’s boost begins to rise will generally net smooth, deliberate and effortless departures.
Making the Grade
Allowing sufficient time to “gather momentum,” we settle in and enjoy the ride. Driving out west, we encounter a steep mountain grade. Observing the new obstacle, we tend to beam our (overly) confident knowing that we have harnessed gobs of torque (some in excess of 1,800 lb. ft.) that will propel our rig to the summit in record time. The mischievous grin becomes difficult to conceal from our co-pilot. But as we ascend, our speed begins to drop and as the transmission downshifts for the proper gear selection to maintain our forward progress, that grin is wiped away as the “big rigs” pull to the left in an effort to position themselves to pass. How can this be?
It is a common misconception that torque alone is what gives us the speed to top the grade. While torque is a measurement of power and its ability to move a stationary object, it is RPM (revolutions per minute) that gives us the speed. Many drivers are fearful of taking the engine to the governed cutoff rpm because somewhere (from gas operating days) they learned that “redlining an engine” is something to avoid. However, in a modern diesel, the engine’s speed is governed electronically to prevent over-revving and potentially catastrophic damage. That means, in order to avoid an extremely slow ascent, a diesel driver should manually select a gear that allows the engine to maintain the highest rpm possible. This allows the diesel to function in a way that will provide the maximum hill-topping speed while maintaining efficient operation. Driving in a higher gear to avoid high rpm will only ensure “lugging” which will cause the transmission to “hunt” (jump back and forth between gears) and excessive exhaust gas temperature (EGT) which will almost guarantee the last place in the race to the top of the hill. The magical combination can be found in the “cross over point” of your engine’s horsepower and torque curves (another topic in itself).
Some of the coaches available today have fairly sophisticated cruise control systems, that when coupled with the proper engine and transmission combination, are capable of selecting the proper gear and throttle position to ascend the grade. However, good judgment and caution should be exercised when using this option. Traffic and road conditions may not permit the selected speed to be maintained through the entire assent. Once the cruise control has been disengaged, the momentary lag of power will make it difficult to regain speed (remember diesel gather momentum not accelerate). By applying throttle pressure prior to disengaging the cruise control, the lag can be eliminated. Again, diesels gather momentum, and once that momentum is lost it takes even more time to regain speed on the grade than it takes on the flat.
Descending the Grade
So topping the grade was a piece of cake. But beginning the descent, that sinking feeling in the stomach begins to take hold. The grip on the steering wheel tightens as if that will slow the coach. As our speed rapidly increases we begin to apply the brakes more and more; possibly a transmission downshift is executed as well. But the speed of our RV keeps building. Our gas-powered vehicles never behaved so poorly, why won’t this thing maintain speed, we ask?
Remember we said diesels, by design, are giant air compressors? No obstruction, such as a gas engine’s throttle butterfly, exists to hamper the airflow in this reciprocating mass of aluminum and steel. How then do the big rigs use their engines to aid in slowing for downgrades and, for that matter, stopping? Most use a system known as the JakeBrake (engine compression brake), which alters valve timing to create backpressure within the engine. They are both noisy (as evidenced by road signs prohibiting their use) and costly. Most of today’s coaches equipped with smaller displacement diesels utilize exhaust brakes, or in the latest diesel versions, the VG turbo chargers can function as a braking device. The larger displacement diesel engines utilize an ECM controlled engine retarder. In exhaust brake operation and VG turbo applications, no internal engine functions change, but rather a restriction is created in the exhaust stream which allows pressure to build in the engine, which aids in slowing the vehicle. In the simplest of terms, think of an exhaust brake as the potato up the exhaust pipe trick you may have played on your friends in days gone by. The engine retarder is a much more complicated internal engine device but is easily operated thanks to the onboard computer.
In the proper use of most exhaust brakes, VG turbo braking or retarders, the brake is turned on at the top of the hill prior to beginning the descent. The same gear, or one lower (numerically smaller), is selected and your foot is removed from the throttle. With the exhaust brake, VG turbo brake or retarder engaged and the proper gear selected, the coach will maintain its speed the entire descent with little or no input from the service brakes. The most common mistakes made using exhaust brakes are waiting until the vehicle has exceeded a safe speed to descend the grade, and then attempting to engage the brake. Exhaust brakes, VG turbo brakes or retarders are not the equivalent of “pulling the chute” at the end of drag racers run, or an emergency brake for down hill control, though many do attempt to use them in that manner.
Planning, forethought, and proper use of an exhaust brake, VG turbo brake or retarder will extend the life of the service brakes and provide confidence for safe descents.
Shutting Down
At days end, or perhaps at a break to stretch, we come to a stop and want to shut the engine down. Excessive idling should be avoided unless there is a need for an engine power source such as heat, air conditioning, or air. In most instances, it’s as easy as coming to a stop, shifting to neutral, applying the parking brake and turning the key to “off.” There are, however, a few exceptions. If, for instance, we have just topped a long grade, or perhaps we have been pushing into a headwind for some time, we may have built our exhaust gas temperature (EGT) to a level that requires some turbocharger cooling prior to shutting the engine down.
The reason is to allow ample time for the temperature in the turbocharger to cool so that it does not “coke” (burn and form a carbon deposit) the remaining lubricating oil in the bearings.
This is done by allowing the engine to idle for just a few minutes or until the EGT reaches 300 degrees prior to shutting the engine down. Some vehicles are equipped with a pyrometer, which displays EGT. The positioning of the gauge’s sensor is key to accurate readings. If your vehicle did not come with one, most diesel service centers can install one for you. You will find that a pyrometer and “boost gauge” (which reads the manifold boost pressure) are two very handy instruments that will give you important information regarding your engine’s general performance, efficiency, and health.
Enjoying the Ride
Once we have let go of our old gas-powered thinking and have made the transition to thinking in a diesel-powered mode, we begin to relax and enjoy our ride. Thinking ahead, allowing more time for lane changes, passing and stopping, give us more time to enjoy the very things we took to the road for in the first place. As America’s landscape unfolds before us, the pressures of the day-to-day rush leave. The casual, but connecting conversations that we never seem to have time to begin to flow between loved ones and friends. Gradually, we are removed from life’s pressures that we have created and the simplistic order of things naturally falls in place. You find sanctuary on the road: you and your mate sharing the day as it was meant to be.