Here's the bottom line for the diesel engine: Its operating efficiency is significantly higher than that of a gasoline engine. The old rule of thumb was that a spark-ignition gasoline engine was roughly 25-percent efficient, which meant that about 25 percent of the energy in the fuel was translated into power at the crankshaft. For a diesel that number was over 30 percent. Modern gasoline and diesel engines improve on those figures, but the common belief is that diesel is always better.
There are a couple of major reasons for that advantage. First, instead of a spark plug the diesel uses compression ignition. The air enters the cylinder, is compressed to a very high level -- typical diesel engines have compression ratios around 17:1 -- that high compression heats the air to the region of 1,500 degrees Fahrenheit (depending upon the compression ratio), the fuel is then injected under very high pressure into the cylinder, and the heated air ignites the fuel. Compression ignition, and the very high compression ratio required to initiate it, increases what is known as the engine's "thermal efficiency." Second, the diesel has no throttle; the air intake is always "wide open" and engine speed is governed by the amount of fuel injected into the cylinder. The lack of a throttle reduces the "pumping losses," a description of the forces necessary to move air into the engine, and is another enhancement to efficiency. Since the diesel engine is inherently more efficient, it can deliver better fuel economy in a vehicle.
But diesels have traditionally been heavy, expensive and noisy, their exhausts sooty and smelly, their power bands narrow, their operating speeds low and they have been hard to start in cold weather. Many of these issues are less problematical with large trucks, heavy equipment, big boats, ships, power generators and stationary well pumps, but together they have kept diesels out of most cars, at least in America.
Technology is changing that. Extensive development, spurred by demand for greater fuel economy in the European market -- where fuel costs are at least twice what they are in the United States -- is finding new answers to traditional diesel issues. There are modern automotive diesels that are essentially clatter-free. Exhausts no longer spew out black soot and smell bad. Maximum engine speeds are around 4,000 rpm or above. Today's best diesel-powered cars start, run and accelerate just like customers expect a car to operate. In many cases, occupants are surprised to find they've been riding in a diesel vehicle.
Just as spark-ignition engines will become smaller to enhance fuel economy, so will diesels. Manufacturers are working on pickup truck diesels that will be in the range of four liters, and the European market has seen numerous automotive diesels of 1.5 liters or less. Smaller engines are lighter, so overall vehicle weight is reduced. To retain power levels, these smaller diesels utilize a collection of technologies. Common-rail fuel injection systems will deliver fuel more precisely. Variable valve timing provides the same benefits of higher efficiency and enhanced power band that it provides in gasoline engines. Refinements in turbochargers, perhaps in concert with engine-driven superchargers, increase power and broaden the operating ranges.
Diesels can run on a variety of fuels, and biodiesel has its proponents. It's made from vegetable oils, animal fats and recycled restaurant greases, so you can literally take the grease from cooking fries at the local fast-food joint and turn it into something that can power your diesel engine, though the most modern diesels must be specially tuned to accept some forms of biodiesel. Running an engine on biodiesel can result in lower levels of some emissions, and it's biodegradable and non-toxic, but it delivers poorer fuel economy and less power and can be more expensive. And, there are probably not enough fast-food joints to single-handedly keep the nation's trucks on the road.
An important benefit of smaller engines and reduced fuel consumption will be reduced emissions of carbon dioxide, which some link to global warming. The future will see more diesel engines in all kinds of vehicles, and they will be smaller, quieter, cleaner and more efficient and economical than ever before.
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