What are The Difference Between Diesel and Gasoline Engines

Today, diesels are being purchased in huge numbers by Europeans, with diesels estimated to represent more than half of all new vehicles being sold there. Contrary to what you might think, these are not just small, entry level diesel vehicles from VW, Renault, or Fiat, but also upscale diesel-driven rides like the BMW 7-series, Jaguar S-TYPE, and Alfa Romeo 156.

This is in stark contrast to the diesel scene here in America, where nationwide diesel choices are almost nonexistent and limited to diesel pickups or passenger vehicles like the Mercedes-Benz E320 BLUETEC, VW Touareg TDI, and Jeep Grand Cherokee CDI. That will change dramatically in just the few short years ahead. Many models are in the pipeline to meet what’s expected to be a growing demand for high fuel economy vehicles of all types...an area where diesel excels.

Even if you are a diesel fan, you probably have had little chance to explore this option if you live in California, Massachusetts, Maine, New York, or Vermont, where tighter emissions regs have kept diesels largely off the market in recent years.

The Diesel Difference

A gasoline engine runs on the Otto cycle, in which a vaporized mixture of gasoline and air is delivered to the combustion chamber, where it’s then compressed and ignited by a spark plug. In the Diesel cycle, air is compressed during the compression stroke and fuel is injected into hot, compressed air in the cylinder, spontaneously igniting the fuel. Because of this, the diesel is sometimes referred to as a compression ignition engine in contrast to a spark ignition engine.

There are also significant differences between diesel fuel and gasoline. For starters, diesel fuel requires less refining than gasoline and is similar to kerosene, jet fuel, and heating oil. Indeed, the military now uses a single fuel, JP-8, in its jet engines, helicopter turbine engines, and diesel engines. Diesel fuel is heavier, oilier, and evaporates much more slowly than gasoline. Because of greater use of diesel fuel across the Atlantic, European refineries are more oriented toward refining crude oil into diesel rather than gasoline, the opposite of what occurs here in North America. This preference is then reflected in how fuel is marketed.

A diesel engine gets more miles-per-gallon than an equivalent gasoline engine. With gasoline costing up to $6 a gallon in Europe, fuel economy has historically been more important to Europeans than Americans and a key factor in their preference for diesel power, even when diesels were still noisy and dirty. Diesel fuel has a higher energy density – on average, a gallon of gasoline contains 85 to 87 percent of the BTUs (British Thermal Units) of energy of a gallon of diesel fuel.

Diesel engines are also more efficient than their gasoline counterparts because more power is produced as a result of the higher compression of the air/fuel mixture. Today’s gasoline engines have compression ratios of about 10:1 to 11:1, while the compression ratios in diesels can be as high as 25:1. The higher the compression ratio, the more power generated.

The much higher compression ratio means diesel engines have to be heavier and more robust. This means they are more expensive to build, but the higher cost is offset by much longer lifetimes. For instance, we’ve seen Mercedes-Benz diesels with 350,000 or more miles on the odometer running great on the original engine. Because of higher component weight and high compression ratios, diesels operate at lower rpms, producing lots of low end torque but less horsepower.

The Early Diesel

Rudolf Diesel obtained a German patent for the Diesel Cycle engine in 1892. Early diesel engines were relegated to stationary, industrial, and shipping applications because of their great weight and very low rpm operation. While Diesel invented the diesel engine, an American named Clessie L. Cummins can be credited with developing, and most importantly, marketing the diesel for highway use in the U.S. in the early 1930s.

Cummins set a diesel speed record at Daytona Beach in a converted Duesenberg and drove a Cummins-powered truck coast-to-coast on only $11.22 worth of diesel fuel. In 1931, a Cummins team set a new endurance record of 13,535 miles at the Indianapolis Motor Speedway. This, plus the introduction of the Cummins Model H Truck engine in 1933, caught the attention of truckers and began the virtual “dieselization” of the North America heavy-duty trucking industry.

The popularity of diesel cars was much slower in coming even though Mercedes-Benz offered the first diesel-powered production car, the Type 260D, in 1936, and usually has had at least one diesel car in its catalog ever since. There was heightened interest in the late 1970s and early 1980s as a result of the 1973 and 1978 oil crises, but this really turned out to be a false start. When Americans began buying Mercedes-Benz, Peugeot, Isuzu, and VW diesel cars in significant numbers, American automakers, especially GM, decided it needed to start building diesel cars, too.

Unfortunately, rather than using the vast expertise of its own Detroit Diesel division, GM assigned the task of developing a diesel engine to Oldsmobile, which essentially converted its 350 cubic-inch (5.7 liter) V-8 into a diesel...with disastrous results. Among the many problems resulting from this approach, the higher compression of diesel combustion caused cylinder blocks to crack, head gaskets to blow, and crankshafts to wear out quickly. The Oldsmobile-built diesel was offered between 1978 and 1985 and installed in everything from Chevrolet Impalas to Cadillac Eldorados. In all fairness, the engine was quite good by the time it went out of production, but the damage was already done: More than anything else, this engine soured the thought of diesels in the eyes of Americans for decades to follow.

Environmental Challenges

Environmental concerns have also impacted America’s image of diesels. Many Americans still envision a diesel-powered vehicle belching out clouds of black smoke. That’s an image we’ve seen all too often from transit buses, big-rig trucks, and older diesel passenger cars over the years. But if that’s your view of diesel vehicles now, then you have a lot to learn.

Computerized, turbocharged diesel engines equipped with emission control devices have pretty much taken care of this very obvious pollution problem. If you see a modern diesel smoking, it probably can to be attributed to poor maintenance.

Besides carbon monoxide, carbon dioxide, unburned hydrocarbons, and oxides of nitrogen emissions that are common to all internal combustion engines, diesel engine designers also have to contend with significant amounts of particulate matter, or soot. However, because carbon dioxide emissions are directly related to the amount of fuel consumed, the fuel-efficient diesel engine does better than its gasoline counterpart when it comes to this greenhouse gas.

With the growing attention being devoted to C02 reductions these days, not to mention legislation that’s sure to drive the auto industry toward higher efficiency and lower C02-emitting vehicles, diesel is sure to have a promising future.