Diesel versus Gas
The Choice is Yours

Motorhomes have evolved over the years and, while there are exceptions, but they can pretty much trace their roots back to one of two sources - either trucks or busses. Gasoline powered chassis are derived from heavy duty truck chassis. These units generally have gasoline powered engines mounted in a front location and use leaf springs and other suspension parts common to large delivery trucks. Today's diesel pushers are more closely related to a bus chassis. They utilize rear mounted diesel engines and pneumatic suspension systems. All motorhomes have evolved quite a bit and the chassis are now (for the most part) dedicated RV chassis. However, the basic design philosophy hasn't changed radically.

Engines:

While there is much more to the diesel versus gas debate than just the engines, most discussion generally centers around the engine. So, we'll cover that area first.

Gasoline engines run off of gasoline. Big surprise, huh? Gasoline is commonly available at pumps all over the country. These engines utilize an ignition system to fire the spark plugs at the right time in order to ignite the air-fuel mixture inside the combustion chamber. Gasoline engines develop their peak horsepower at higher RPMs than diesels and put out less torque than diesels. For a better description of horsepower, torque, and RPM please check out the document on the Engines Category called Horsepower versus Torque for a better understanding of that.

Diesel engines burn diesel fuel. Another earth shattering revelation, eh? Diesel isn't as easy to find as gasoline but it's not all that hard either. Diesel fuel has much more energy (BTU) per pound than gasoline. Therefore, it has greater potential in the power and fuel economy areas. Diesel fuel is heavier and denser than gasoline and it takes more heat to get it burning but, once it's burning, it produces more energy than gasoline. For this reason, an ignition system used on gasoline powered engines just won't work. Instead, diesel engines rely on higher compression ratios to "fire" the cylinder. These higher compression ratios product more heat in the cylinder. The fuel is squirted into the cylinder at just the right time and amount and it just plain explodes from the heat of compression. It's up to the engine management system to determine how much fuel and when to trigger the injector.

This design gives the diesel greater torque. It also applied that torque at a low RPM compared to a gasoline powered engine. A gas engine revs to over 4,000 RPM. It develops it's horsepower at these higher RPMs. The power gradually climbs there and at lower RPMs the actual horsepower is substantially less. Diesel engines also develop their maximum rated horsepower at close to the peak RPM. However, that RPM is slightly over 2,000 RPMs. The power graph is curved better in a diesel engine than in a gasoline engine so you will have much more power at lower RPMs and you won't have to kick the transmission down a gear or two every time you need more power. Also, the torque rating of a diesel is higher. Following are some of the specs of popular RV engines:

From the above chart you'll see that diesel engines have a substantial improvement in torque and that it comes in at the lower RPM range, which is typically where the revs are as you are driving. Diesel engines also are built stronger to contain the higher compression ratios and extra torque. This strength results in a longer engine life for a diesel over a gas engine. Other considerations at this point are maintenance costs. Diesel engines have greater service intervals than a gasoline engine and there are less parts that need replacing. However, the larger engines, such as the Cummins ISL, do hold as much as 28 quarts of oil. If you are a shade tree mechanic with a couple of large buckets you can easily change your own oil in the driveway. If you rely on others for service you will undoubtedly be taking it to a larger service center. While it is less often, it generally does cost more in labor costs to have the "professionals" service your engine and chassis.

Brakes:

Gas powered chassis generally use hydraulic brakes, which are the same thing as on your every day vehicle, except much larger. In that way, the service requirement are pretty much the same except the parts are bigger and cost more. Because these brakes are used to stop a fair amount of mass going down the road they can tend to get hotter, especially when driving in the mountains. For this reason it is recommended to flush and replace your brake fluid every 3 years in order to keep any moisture or contaminants out so that you won't develop brake pedal fade when you least need it.

Diesel pushers have an air compressor mounted to the engine. This powers the air brakes, which are a more effective way to stop that much weight. Because air brakes are constantly fed a fresh supply of air, there's no issues about hydraulic fluid failures. On the other hand, the air dryer needs to be serviced every 3 years to ensure that the air system is providing clean filtered and dry air. The clamping power of air brakes is superior to hydraulics and better suited for heavier vehicles.

In addition to the above service brakes, there is also engine braking to be considered. On a gas engine you back off the throttle when descending a grade. The throttle plate closes and the engine tends to drag back because it can't get any air. Diesel engines don't have throttle plates because the fuel is direct injected to each cylinder. Therefore, they need an additional method to help slow the engine down. The smaller diesels, such as the Cat C7, Cummins ISB and ISC, use an exhaust brake. This is a butterfly valve that is inserted in the engine's exhaust stream that closes down in similar manner to a gas engine's throttle plate. Because it's restricting exhaust gas, rather than intake air, it is slightly more effective than a gasser's throttle at retarding engine speed. On the larger engines, such as the Cummins ISL, there is a true engine compression brake (commonly referred to as a Jake Brake) that actually modifies the valve arrangement of the engine itself, turning it into a compressor rather than an engine. This means rather than "making" power, it "consumes" power. This is the best method of engine braking and provides the most braking power of any system.

Suspension:

RV suspensions come in two basic flavors - leaf springs and air-ride. Leaf springs are less expensive than air ride systems and are typically found on gas powered coaches while the more costly air-ride suspensions are generally found on the diesel pushers chassis.

Leaf springs are best suited to gas powered chassis because they have no on-board air system. It's also more cost effective. Leaf springs date back to the Conestoga wagon days and are a pretty simple system. In addition to supporting the weight of the coach, they also locate the axle front to rear to keep it underneath the coach. Leaf springs do locate the axle laterally as well but there is more flex and give in the bushings and that does allow heavy winds and bow wakes from large trucks to move the rear of the coach around quite a bit. There are ways to help out in that area if you read the How to Improve Handling topic you'll get a better feel for that.

Air-ride suspension are more sophisticated. However, they don't have the Conestoga wagon technology of engineering either so they give you a more refined ride and better handling. They are well suited to a diesel pusher with on-board air as the basic air system is already there for operating the braking system. The coach is supported by a set of (generally 4) air bags. These bags are filled by the on-board air supply system. The amount of air that fills these bags is determined by height sensing valves. If the coach is too heavy and wants to ride too low, the valves will add more air to return you to the proper ride height. In the same manner, if the coach is lighter, it'll bleed off air to lower it. It can also compensate for roads that are crowned and other out of level conditions. Because the air bags have no ability to locate the axles, either laterally or longitudinally, a system of control links is used to link the axles to the frame. This solid system of links eliminates the slop and flex of leaf springs so you'll find yourself driving straight down the road in heavy winds with far less effort. The smoother ride of the air-ride suspension also means you won't experience the bucking of a leaf spring coach on rough roads. At the end of the day you'll be less tires when driving an air-ride coach.

Summary:

There are a ton of considerations when choosing gas versus diesel. Diesels generally have a much higher cargo carrying capacity due to their greater GVW than a gas chassis. The improvement in ride, handling, and power are all serious improvements. However, a gas powered motorhome will still get you to the same place as a diesel pusher. Further clouding the issue is the ever popular FRED chassis, which features a diesel engine up front in a gasser chassis. So you get the benefits of the diesel engine but no air brakes are air ride suspension system. The secret is to do some comparison drives, know what your budget is, and be knowledgeable enough to understand the differences between the two. Then choose accordingly. And be sure to wave when going down the road.

Submitted by Mark Quasius - 2/10/06
 

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