1. What is diesel?
A diesel is a type of internal combustion engine. Though similar to gasoline engines, diesels use a different type of fuel and a different method of burning it. Diesel vehicles are popular in Europe due to their high fuel efficiency and lower fuel taxes. In the United States and Canada, diesel cars and SUVs are far less common, though government-mandated increases in fuel economy standards mean that the number of vehicles offering a diesel option is rapidly increasing.
2. What are the advantages of diesel engines?
Diesel vehicles are significantly more fuel-efficient than comparable gasoline-powered vehicles, and because most diesel cars use the same size fuel tanks as their gas-powered counterparts, diesels usually have significantly longer range: While gas cars will typically go 300 to 400 miles on a tank, diesels will often cruise for 700 miles or more. Diesel engines are mechanically simpler (they don’t have an electric ignition system) and tend to be more durable, typically running for a half-million miles or more before requiring an overhaul. Some experts regard diesel vehicles as safer, as diesel fuel is less volatile than gasoline and therefore less likely to ignite if spilled or leaked.
3. What are the disadvantages of diesel engines?
Diesels require a unique type of fuel; they won’t run on gasoline. Not all stations carry diesel fuel, and when they do it’s often more expensive. Filling up a diesel car can be a messy affair, as the fuel is greasy and smelly, and since spilled diesel fuel doesn’t evaporate like gasoline, it’s easy to track it into your car (a potential safety hazard; it can make the pedals slippery). Diesels are noisier than gasoline engines, particularly at low speeds, and even more so when they are cold. Older diesels could be difficult to start in cold weather; newer engines are greatly improved, but extremely low temperatures can still cause problems. Diesels used to require more frequent oil changes than gasoline engines, but most modern diesels have service intervals of 10,000 miles or more.
4. How do diesel engines differ from gasoline engines?
Mechanically, diesel and gasoline engines are very similar; what differs is the type of fuel and how it is burned. Gasoline engines work by drawing in a mixture of fuel and air, compressing it, and then igniting it with a spark. Diesel fuel only burns under intense heat and pressure, so the diesel engine draws in only air, compresses it to a high ratio, and then injects the fuel, which spontaneously ignites. Most modern diesels are turbocharged by an exhaust-driven turbine that feeds additional air into the engine to increase power output. Nerdy fact: While gasoline engines regulate their power output by changing the volume of fuel and air (which is delivered in a fairly constant ratio), diesel engines always use a full charge of air and vary the amount of fuel to regulate power output.
5. Why do diesels get better fuel economy than gasoline engines?
Diesel fuel contains about 10 to 15 percent more energy per drop than gasoline. Additionally, the higher temperature and pressure of the diesel combustion process allow more of that potential energy to be released.
6. Will I save money buying and driving a diesel-powered car or truck?
This depends on many factors. Diesel cars are usually more expensive than gasoline cars, and the fuel is also generally priced higher. (There are exceptions; some Mercedes vehicles offer a diesel as the entry-level engine, and in states that comply with California emissions, diesel fuel prices are often comparable to gasoline.) When comparing fuel economy numbers, it’s important to note that many diesel cars deliver significantly higher fuel economy than their EPA estimates, especially on the highway. Buyers of large pickup trucks will almost certainly save money with a diesel, while car and SUV owners who drive more miles than average (especially on the highway) are most likely to save money in the long run.
7. Is diesel fuel difficult to find?
Not all gas stations carry diesel fuel, which leads to the perception that it is difficult to find. Diesel stations are common in big cities and along major highways, but they can be fewer and farther between in rural areas. Keep in mind, however, that diesels tend to go farther on a tank. In our experience, a diesel car’s range is often close to double that of a similar gasoline-powered car, hence the need for less frequent fill-ups. We’ve made several long-distance diesel trips and never had a problem finding fuel.
8. What is diesel exhaust fluid?
Diesel exhaust fluid (DEF, also known by trade names such as AdBlue) is a urea-based mixture used in the emissions systems of many late-model diesels. Vehicles that use DEF have a tank that must be filled on a regular basis, often corresponding to the car’s oil change interval. Though the diesel engine can run just fine without DEF, EPA regulations require that the engine be disabled if the DEF tank is empty. (The engine won’t suddenly stop if the tank runs dry, but once shut down it cannot be restarted. The vehicle provides plenty of warning when the tank runs low.) DEF can be refilled by the dealer or the owner; it is relatively inexpensive and is sold at most auto parts stores and truck stops. (NOTE: DEF always has its own tank. Putting DEF directly into the fuel tank can cause extensive damage to the fuel system.)
9. Can I use renewable biofuels in my diesel vehicle?
Unlike gasoline engines, diesel can run on renewable fuels with little or no modification; in fact, Dr. Rudolph Diesel originally designed his engine to run on peanut oil. The most common alternative diesel fuel is biodiesel, which is made from vegetable oil and methanol. While it is possible to run 100-percent biodiesel (called B100) in many diesel engines, most automakers limit their vehicles to blends, either B20 (a mix of 20-percent biodiesel and 80-percent regular petroleum-based diesel) or B5 (5-percent biodiesel). Running higher concentrations may void the vehicle’s warranty. Some diesel engines can be converted to run on pure vegetable oil (including waste oil, such as used oil from restaurant fryers), though doing so usually requires modification to the fuel system.
10. Who is the ideal candidate for a diesel car or truck?
If you are buying a vehicle to tow a large trailer, you will almost certainly want a diesel; they provide significantly better pulling power and fuel economy than gasoline engines. For car buyers, owners who do a lot of driving on the highway are most likely to see significant savings, and drivers who do more than 15,000 mostly highway miles per year should definitely consider a diesel. That said, there are other reasons to buy a diesel, including strong low-end pulling power that gives them a unique acceleration profile many drivers enjoy.