Dear Straight Dope:
In this age of extremely high gas prices, I've noticed that diesel is quite a bit cheaper than regular gasoline ($1.70 compared with $2.10 in Providence, RI). What would happen if I put diesel into my gas tank?
SDStaff Una replies:
Let’s start with the basics. Aside from additives such as ethanol, which is made from corn, the gasoline and diesel fuel commonly used today derive from the same source — crude oil. Although they share the same parentage (just like motor oil, some plastics, and Velveeta), they have very different properties.
Gasoline is often thought of as being a single chemical, but it’s actually a complex mixture of hydrocarbons that generally have 4 to 12 carbon atoms in their molecules. In the case of gasoline made from Oklahoma crude oil, “5 compounds make up 29% of the gasoline fraction, whereas 20 compounds make up 59% of the gasoline fraction” (reference 1). Diesel fuel is a light oil that is heavier than gasoline, composed of hydrocarbons generally having between 10 and 20 carbon atoms in their molecules (2). The point is, gasoline and diesel fuel aren’t the same.
Gasoline is typically burned in spark-ignition internal combustion engines, in which a source of energy (a spark) is required to start the ignition process. Simply put, these engines mix gasoline vapor and air, compress the result, apply a spark to initiate combustion, and then force the byproducts of combustion out as exhaust. Gasoline is well suited to this process, although sometimes either it or engine operating conditions can cause “knock,” an unwanted and destructive condition in which a portion of the fuel ignites spontaneously due to compression alone, without the aid of the spark.
Knock is a serious problem for gasoline engines. The resistance of a particular gasoline blend to knock is given by the octane number, with a higher value indicating greater resistance. Over the years the public has come to think of higher octane numbers as indicating higher energy/more power. That’s not really true, but a detailed discussion of the topic is worthy of its own article. Suffice to say that self-ignition is a pivotal difference between gasoline and diesel engines. In a gasoline engine you don’t want the fuel to self-ignite — that is, to burn without benefit of a spark plug. In a diesel one you do.
Diesel fuel is typically burned in compression-ignition internal combustion engines, which draw in air and subject it to high compression (much higher than in a gasoline engine), causing it become very hot. At peak temperature and pressure, diesel fuel is injected into the cylinder and self-ignites (autoignition). After combustion is complete, the combustion byproducts are exhausted out of the engine. Sometimes, such as on cold days, electrically heated devices called “glow plugs” are used to provide extra heat for starting, but these play no role once the engine is warmed up.
Diesel fuel is rated not by octane number but something called a cetane number. The cetane number gauges the ease with which the diesel fuel will autoignite when compressed. Higher cetane numbers indicate easier self-ignition and better engine operation, whereas a higher octane fuel would be more resistant to self-ignition under compression.
That’s the crux of the problem with mixing gasoline and diesel fuel. In a diesel engine you want controlled self-ignition, whereas in gasoline you want to avoid self-ignition, and have the fuel ignite only when the spark fires.
What happens when you use gasoline in a diesel engine? Either something expensive or something very expensive. Since gasoline is designed to be resistant to self-ignition, gasoline in a diesel engine either won’t ignite or will ignite at the wrong time. Some diesel engines run leaner than gasoline engines (meaning that the air-fuel mix has a higher proportion of air than a gasoline engine). That increases the chances that the gasoline won’t ignite and that unburnt fuel will be sent into the hot exhaust system — where, ironically, it could ignite, leading to possible exhaust damage. Even if you avoid that disaster, you can expect to pay $500 to drain the fuel tank, clean out the fuel lines, and refill the tank with diesel.
Some types of diesel engine use the diesel fuel as a lubricant for the fuel pump (remember, it’s a fuel oil). It’s said that running gasoline through such a pump could lead to serious damage or failure, turning a $500 repair into a $750-$1,250 one.
Trying to use diesel fuel in a gasoline engine also has unpleasant consequences, but generally not as dire in terms of damage to the fuel system. Depending on the proportion of diesel fuel relative to gas in the tank, a gasoline engine will either run poorly or stop altogether, necessitating another $500 trip to the repair shop for draining and flushing. A couple of old grease monkeys at the shop I used to work at claimed that their “super high-compression” gasoline engines of the late 1960s could easily switch between gasoline and diesel fuel with no problems, but I tend to think they spent too much time inhaling fumes from the carburetor cleaning tank.
Some old-timers and shade-tree mechanics will add small amounts of diesel fuel to the gasoline in their cars in the belief that it will lubricate the valves, in the same way that people will add such items as “Marvel Mystery Oil” and automatic transmission fluid to their gas tank. I can find no reputable studies that show any merit to adding diesel fuel to gasoline. While small amounts of these items aren’t likely to kill the engine or cause permanent damage, their use is not recommended. Some of these same old-timers advocate adding small amounts of gasoline to diesel engines to “burn out” carbon deposits and “clean the injectors and valves.” This is not recommended by any diesel engine manufacturer I know of, and due to gasoline’s resistance to burning from compression ignition, it’s possible it might make matters worse by lowering combustion temperatures. In any event, I found no support in any of my combustion and automobile resources for the idea that gasoline and its additives would “clean” diesel injectors or valves better than the additives already in the diesel fuel.
In short, don’t put the wrong fuel in your vehicle. If a significant amount of the wrong fuel does wind up in your tank, have the vehicle towed to a repair shop lest you ruin the fuel pump or get stranded by the roadside. If you want to use inexpensive diesel fuel, the solution is simple. Buy a diesel car.
- Speight, James G, The Chemistry and Technology of Petroleum, 1991.
- Adler, U, editor, Automotive Handbook, 1986.
- Heywood, John B, Internal Combustion Engineering Fundamentals, 1988.
- Kmetz, Brian, “Mixing Gasoline and Diesel,” Turbo Diesel Register, issue 26, pages 14-15; online reprint.
SDStaff Una, Straight Dope Science Advisory Board
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