Diesel Engine Discovery

Intake Air For our purposes, we’ll use a four-stroke, turbocharged and intercooled diesel engine to illustrate the flow of air and fuel throughout a modern diesel power plant. Fresh air enters the compressor housing (intake side) of the turbocharger and is compressed in the compressor wheel, where boost is created. This makes the air denser, but also much warmer. To cool off the compressed air before it enters the cylinder head(s), it is routed through a charge air cooler (also known as an intercooler). The most frequently used intercooler type is of an air-to-air style, and is essentially just a simple heat exchanger. The intercooler significantly lowers the temperature of the intake air along its journey to the engine, and does so with a very minimal loss in boost. Compression Ignition Things get interesting once the compressed air is forced in-cylinder. During the intake stroke—where the piston is dropping to the bottom of its range—the intake valve(s) opens, allowing “unthrottled” air to fill the cylinder. This is different from a gasoline engine in two ways: 1) gas engines introduce a mixture of fuel and air during the intake stroke, and 2) in a diesel, air is only drawn in during the intake stroke. Next, the intake valve(s) closes and the compression stroke begins. As the piston travels upward, the air that once filled the cylinder now consumes just 6% of the area it did before. This air, under tremendous pressure, instantly becomes super-heated to more than 400 degrees—which is more than enough heat to make diesel fuel ignite on its own. And, at the top of the piston’s stroke that’s exactly what happens. The previously mentioned super heated air is met with a shot of diesel fuel (released into the cylinder by its respective fuel injector) during the perfect window of time before the piston reaches top dead center, and combustion occurs. Because a diesel engine uses the heat of compression to ignite its fuel, no aid to begin the combustion process is needed (i.e. spark plugs, such as in a gasoline engine). Turbochargers Make Diesels What They Are: Great The last order of business is the exhaust stroke, where spent combustion gasses are forced out of the exhaust valves, through the exhaust manifold, and into the turbine (exhaust) side of the turbocharger. In your average gasoline-powered engine no turbocharger exists, which means once out of the engine, exhaust gasses immediately head for the tailpipe. This is not so in a diesel, as the turbocharger, which is in charge of forcing fresh air into the engine, actually uses the exhaust gasses leaving it to drive itself. Because a turbocharger consists of a turbine (exhaust) wheel sharing a common shaft with a compressor (intake) wheel, exhaust gasses are always required in order to bring air into the engine. One depends on the other. We’ll break down the importance of the turbocharger like this: You throttle fuel (sending diesel into the engine), combustion occurs, exhaust gasses leave the engine, spinning the turbine wheel on the way out, which turns the compressor wheel, introducing air into the engine. An endless cycle, if you will.