Touring Coaches

Under the Hood:
Gas vs. Diesel

Airstream Touring Coaches

Weighing the differences between gas and diesel when shopping for your next touring coach

There are many things to consider when shopping for a motor-home. Size, floor plan, safety features, and decor are often the first things you consider – after all, they are the easiest things to see for yourself when shopping on the dealer lot. But what’s under the hood matters too. If you’ve spent your life around engines, studied them, taken them apart, and built them back up, then the debate between gas and diesel might seem old hat. But for those of us not in the know, we’re here to break down everything you should consider when deciding between a gasoline or a diesel engine in your RV.

 

Gas vs Diesel

Let’s just pull the band-aid off quickly and dive into one of the most hotly debated and discussed topics when it comes to motor-homes. Each has its own benefits and drawbacks, and they share many things in common.

Diesel engines and gas engines are actually quite similar. Both are internal combus­tion engines designed to convert the chemical energy in fuel into mechanical energy. This mechanical energy moves pistons in an up-and-down motion inside of the cylinders. The pistons are connected to a crankshaft, and that up-and-down motion of the pistons creates the rotary motion needed to spin the wheels and propel your vehicle. Simple, right?

Both diesel engines and gasoline engines convert fuel into energy through a series of small, contained explosions. The major difference between diesel and gas engines is the way in which these explosions happen. In a gasoline engine, fuel is first mixed with air, then compressed by the pistons and lastly ignited by sparks from the spark plugs. In a diesel engine though, the air is first compressed before the fuel is injected. That’s because air heats up when it's compressed. Compress it enough, and the fuel ignites.

 

Fuel Injection

This is one of the main areas where a gas and a diesel engine differ. Most gas engines today use what is called port injection technology, while older cars often utilize a carburetor. A port injection system injects fuel just before the intake stroke just outside the cylinder. A carburetor mixes air and fuel prior to the air entering the cylinder. So in a gas engine, all of the fuel is loaded into the cylinder during the intake stroke and is then compressed. The compression of the fuel and air mixture limits the compression ratio of the engine – if it compresses the air too much, the fuel and air mixture will spontaneously ignite and will cause knocking. Because it causes excessive heat, knocking can damage the engine.

Diesel engines, in comparison, use a direct fuel injection process – the diesel fuel is injected directly into the cylinder. The injector on a diesel engine is arguably the most complex component. Using direct injection, diesel engines allow the computer in the engine to control to a far greater extent the where-and-when regarding how fuel is injected into the cylinder. This, coupled with direct injection being a newer technology, means that a direct injection engine will typically get better gas mileage.

Port Injection vs Direct Injection in engines

 

 

Environmental Factors

The burning of any fossil fuel emits CO2 into the air, however, diesel emits roughly 40% less CO2 into the atmosphere when compared to gasoline. Also, CO2 is not the only thing emitted from the burning of fossil fuels. A harmful pollutant known as carbonaceous particle matter, or PM, is also emitted.

PM is known to damage lung tissue when inhaled. And studies show that gas emissions in normal weather contain 10 times more PM than diesel emissions. In cold weather, it gets even worse. At temperatures below freezing, gas engines emit as much as 62 times more PM than diesel engines. This is due to the catalytic converter in gasoline engines which can take longer to come on when the cold engine has to warm up.

There are differences in the refining process as well. Gasoline refinement is a more complicated process that takes considerably longer than diesel refining. The refining process itself releases greenhouse gasses, which means even at the refining stage, diesel is greener fuel.

You can learn more about Airstream's efforts to protect the environment here.

 

Fuel Differences

Speaking of the refining process, petroleum fuel starts off as crude oil which is naturally found in the Earth. When crude oil is processed at refineries, it can be separated into several different kinds of fuels, including gasoline, jet fuel, kerosene, and diesel.

If you have ever compared gasoline and diesel then you know that they are different. Diesel fuel is heavier and oilier. It evaporates much more slowly than gasoline. In fact, the boiling point of diesel is actually higher than the boiling point of water.

Diesel fuel has a higher energy density than that of gasoline. On average, 1 gallon of diesel fuel contains approximately 147,000 BTU, while 1 gallon of gasoline contains 125,000 BTU. This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines. A diesel engine is about 20% more thermal efficient than a gas engine. This directly relates to a 20% increase in fuel economy. Increased thermal efficiency also translates to more torque and more overall power. Diesel engines provide a better bang for your buck in relation to the amount of fuel put into them. A diesel engine’s high torque is also beneficial for hauling, as it helps with carrying heavier loads.

 

Life Expectancy

Another big differentiator between gas and diesel engines is overall life expectancy.

When gasoline engines hit the 120,000-150,000 mile mark, the cylinders will start to show wear, which decreases their efficiency. Diesel engines, on the other hand, are known for their long life cycles – boasting many engines on the road today with well over 500,000 miles in total. The cylinders on a diesel engine are designed with a removable liner, enabling you to replace the liner, instead of the entire engine.

Diesel engines are low RPM, while being high torque. A gas engine will turn twice as many revolutions per minute going down the highway as a diesel engine, meaning the diesel engine will wear at half the rate of the gas engine, resulting in a longer life expectancy. Diesel engines are also built to be stout, which allows them to function longer. The castings are built thicker, the cylinder walls are built to be more robust, and the oiling system has a higher volume.

Interstate EXT Silver Exterior

The Winner is Clear

So, diesel engines use newer technology, which promotes better fuel economy. Diesel fuel is a greener option as compared to gasoline due to fuel economy and the refining process creates lower emissions. And diesel engines have a longer life expectancy. Ultimately, diesel engines are built for the long haul, which is why Airstream exclusively builds motor coaches with diesel engines.

 

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