Maybe it was the day I was passed for the hundredth time on my favorite section of seven percent grade, our big block V8 revved to the point that I was playing tag with the right foot and temperature gauge. Or perhaps I wanted to find out for myself if the diesel owners were telling the truth about these claims in fuel economy. Regardless, I decided to take my first recreational vehicle test drive in 1990. Although I had experience piloting commercial diesel powered vehicles on and off for over twenty years, I was not prepared for what I was about to experience.

The only way to describe the event was excitement and frustration at the same time. 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, certain acceleration characteristics would leave some thinking they just left home plate after a third called strike in the ninth inning, dragging the bat behind. While it’s true that diesels produce gobs of torque and respectable horsepower at relatively low rpms, their driving characteristics are nothing like their gas counterparts. So many of us find the initial introductory diesel driving experience slightly frustrating.

This frustration and fear is not mandatory. Traveling our country is a relaxing and spiritually enhancing experience. If we adjust our thinking and operating habits just a bit, diesels can provide a positive, relaxing and rewarding driving experience.

Start Your Engines

Modern compression ignition engines (diesels) 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 like 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 may be mechanically or electronically controlled, the latter tuned by emissions, durability and economy requirements.

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 are commonly linked to a “wait to start” dash light which allow for the time necessary for the cold start aid to produce the required hot air. The use of “starting fluids” is generally not recommended in the modern automotive diesel, the main reason being increased cylinder pressures leading to catastrophic engine damage and environmental regulation compliance.

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 show 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 higher. 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. However, once the desired speed is obtained, the diesel is capable of maintaining that speed (momentum) with little effort, assuming the vehicle’s weight, transmission gear splits, and final drive ratio are properly coordinated. 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 smoke-free 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 knowledge that we have harnessed gobs of torque (some in excess of 1,000 lb. ft.) that will propel our rig to the summit in record time. The mischievous grin becomes difficult to conceal from our co-pilot. 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. We are fearful of taking the engine to the governed cutoff rpm because somewhere (from our gas operating days) we learned that “redlining an engine” was something that was to be avoided. In a diesel, the engine’s speed is governed either mechanically or electronically to prevent over revving and potential catastrophic damage. By manually selecting a gear that allows the engine to maintain the highest rpm possible, we are allowing the diesel to function in a way that will provide the maximum hill topping speed while maintaining efficient operation. Driving in a higher gear will only ensure “lugging” which will cause excessive exhaust gas temperature (EGT) and almost guarantee 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 selecting 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. By applying throttle pressure prior to disengaging the cruise control the lag can be eliminated. Remember, diesels gather momentum, and once that momentum is lost it takes even more time to regain
speed on the grade as it takes on the flat.

Descending With Grace

So topping the grade was a piece of cake, but what about the descent? The grip on the steering wheel tightens as if that will slow the coach. As our speed increases, we begin to apply the brakes more and more; possibly a transmission downshift is executed as well. What if the speed of our home away from home keeps building? Our gas powered vehicles never behaved like this – so why the difference, we ask.

Remember, we said that 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? Many of today’s diesel coaches are equipped with an engine braking system. Some utilize a Jake brake, which alters valve timing to create backpressure within the engine. The rest use exhaust brakes. In exhaust brake operation, 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 in the exhaust pipe trick you may have played on your friends in days gone by.

In the proper use of most exhaust brakes, 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 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 are not the equivalent of “pulling the chute” at the end of a 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, will extend the life of the service brakes and provide every 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. 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 which will give you important information regarding to your engine’s general performance and health.

Enjoying The Ride

Once we have let go of our gas powered thinking and have made the transition to thinking in a diesel-powered mode, we begin to relax and enjoy our ride. As with any gas or diesel powered motorhome, thinking ahead, allowing more time for lane changes, and passing and stopping will 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 for, 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. Sanctuary on the road. You and your mate sharing the day as it was meant to be.

Scott Dalgleish is a freelance journalist who has enjoyed the Western outdoors and RV lifestyle for over 40 years. He is a contributor to MotorHome magazine and for the past six years Scott has been working closely with Dodge and Cummins in areas of new product development and customer relations. Scott and his wife Debra are the owners of a new Monaco Windsor.

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