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How It Works: Diesel Engines

RIP Gasoline

Text By Jason Thompson

The diesel engine was one of the most ambitious engineering undertakings in history. It was so radical and high-minded that it still hasn’t been totally perfected—even after 150 years of development. Rudolf Diesel was a perfectionist, and his patent described an engine more than twice as efficient as the ones we run today. His working prototype ended up being only about 1⁄3 as efficient as today’s diesels, but this was still nearly three times more efficient than other engines running at the time. The reason Diesel was so successful was because he possessed an enormous amount of knowledge regarding thermodynamics, and he built his machine to take advantage of those fundamental rules. His ideals were ahead of his time, but today we’re almost ready to fully realize his dream.

Turning Fuel Into Work
We’ve discussed the diesel engine from a thermodynamic perspective in past articles, like “Cummins Steam Engine” (Sept. ’08). In a nutshell, the brake thermal efficiency (BTE) of an engine is equal to fuel in, divided by work out. The more work an engine does for a given amount of fuel it consumes, the better that engine’s BTE is said to be. As you no doubt have come to expect, diesels have excellent BTE ratings. For example, the new Ford 6.7L diesel consumes 239.9 grams of fuel per kilowatt-hour of energy it produces. Yet, as good as our diesel engines are, they still only send about 30 to 40 percent of the fuel’s energy to the wheels.

Another way to think of the diesel engine is from a chemical reaction perspective. Complete combustion looks like this: C13H28 + O2 = CO2 + H20 (diesel + oxygen = carbon dioxide and water). In the real world, the reaction of diesel combustion is much more messy. We think of this messy reaction as a vehicle’s emissions. Particulate matter, NOx, unburned fuel, and many other compounds form within an engine because the combustion event is not 100 percent complete. These complex emissions compounds (represented by ever-changing letters and numbers) exit the vehicle’s exhaust pipe. If the vehicle is equipped with a catalytic converter, diesel oxidation catalyst, or urea injection system, these chemical numbers and symbols change places to form different (and hopefully safer) chemicals before entering the atmosphere.

Compression and Expansion
The diesel engine is really many machines in one. It’s a compressor and a combustor that work together to turn heat and explosive energy into work. The first step of a diesel engine is to take in air, which contains oxygen. As the piston travels down the cylinder, the intake valve(s) in a four-cycle diesel open and let unthrottled air fill the entire cylinder; this cools the cylinder. As the piston returns to the top, the volume of air in the cylinder is decreased and the air gets compressed and heated. Fuel is then injected directly into the cylinder, and the heat ignites the fuel. The resulting explosion pushes the piston down. The downward motion of the piston moves the connecting rod, which turns the crankshaft. The power the engine produces equals the energy it took to pump the gases, minus the energy extracted from the explosion.

Split-cycle engines separate the compressor and combustor into two separate cylinders so each can be maintained at its optimal temperature. In these engines, the combustor can be sized bigger than the compressor by about 33 percent—this translates to more power in the piston. Also, one compressor can feed multiple combustors. Some see this development (controlled air injection) analogous to digital fuel injection, but with more potential.

Ignition
In some descriptions of a diesel engine, ignition is covered in a sentence or two. The truth is, these microseconds can be analyzed like a novel, and hundreds of hours of research have been dedicated to this small window of time. In traditional diesel engines, there was only one heat release spike, and this occurred before the piston was ready to accept it since it wasn’t even at the top of its travel yet. To make matters worse, the diesel fuel and oxygen ratios are always mixed heterogeneously (unevenly) throughout the cylinder. The air-fuel ratios vary greatly, from pooled fuel on the top of the piston and cylinder walls to no fuel in the crevices.

This translates into waste heat, pollution, noise, and other things besides work at the wheels. Homogenous charge compression ignition (HCCI) is a combustion scheme that results from the precise control of multiple fuel injection events. In these engines, an early small shot of fuel primes the chamber and makes for a more equal and uniform air-fuel ratio throughout the combustion chamber. Up to seven firing events are possible in a modern diesel. In this way, the combustion event can be shaped to better work on the piston. Another factor that can be manipulated is the amount of oxygen in the cylinder via exhaust gas recirculation (EGR). Without oxygen, the heat of the combustion is reduced and therefore NOx is reduced as well.

Exhaust
There are no free lunches when it comes to engines. Even the act of pushing the exhaust out the engine takes work. Since the power stroke is only able to extract a certain amount of energy, the turbocharger scavenges what it can and helps the piston with its job of compressing the gases again—completing the cycle.

By Jason Thompson
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