Powertrain

Gasoline Engine vs. Diesel Engine

The main differences between the gasoline engine and the diesel engine are:

• A gasoline engine intakes a mixture of gas and air, compresses it and ignites the mixture with a spark. A diesel engine takes in just air, compresses it and then injects fuel into the compressed air. The heat of the compressed air lights the fuel spontaneously.

• A gasoline engine compresses at a ratio of 8:1 to 12:1, while a diesel engine compresses at a ratio of 14:1 to as high as 25:1. The higher compression ratio of the diesel engine leads to better efficiency.

• Gasoline engines generally use either carburetion, in which the air and fuel is mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected just prior to the intake stroke (outside the cylinder). Diesel engines use direct fuel injection -- the diesel fuel is injected directly into the cylinder.

Diesel fuel evaporates more slowly than gasoline because it is heavier. It contains more carbon atoms in longer chains than gasoline does (gasoline is typically C₉H₂₀, while diesel fuel is typically C₁₄H₃₀). It takes less refining to create diesel fuel, which is why it is generally cheaper than gasoline.

Diesel fuel has a higher energy density than gasoline. On average, 1 gallon (3.8 L) of diesel fuel contains approximately 155x10⁶ joules (147,000 BTU), while 1 gallon of gasoline contains 132x10⁶ joules (125,000 BTU). This, combined with the improved efficiency of diesel engines, explains why diesel engines get better mileage than equivalent gasoline engines.

When cars burn gasoline or diesel, they would ideally burn it perfectly and create nothing but carbon dioxide and water in their exhaust. Unfortunately, diesel and gasoline engines are not perfect. In the process of burning the fuel, the engines produce:

• Carbon monoxide, a poisonous gas

• Nitrogen oxides, the main source of urban smog

• Unburned hydrocarbons, the main source of urban ozone

• Particulate matter, (for diesel fuel) cause of many respiratory diseases, creates smog

Carbon is also a problem. When it burns, it turns into lots of carbon dioxide gas. Gasoline is mostly carbon by weight, so a gallon of gas might release 5 to 6 pounds (2.5 kg) of carbon into the atmosphere. The U.S. is releasing roughly 2 billion pounds of carbon into the atmosphere each day. 

If it were solid carbon, it would be extremely noticeable -- it would be like throwing a 5-pound bag of sugar out the window of your car for every gallon of gas burned. But because the 5 pounds of carbon comes out as an invisible gas (carbon dioxide), most of us are oblivious to it. Therefore, new technologies that reduce the amount of carbon emissions have become increasingly important.