2.4: Diesel Engines

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2.4 Diesel Engines

Rudolf Diesel first developed Diesel engines in the 19th century. He did so because he wanted to develop an engine that was more efficient than an Otto engine and that could use poorer quality fuel than gasoline. The Diesel engine also operates on a four-stroke cycle, but there are some important differences. Diesel engines have a high compression ratio (CR)- a small Diesel engine has a CR of 13:1, while a high-performance Otto engine has a CR of 10:1. Upon the compression stroke (stroke 2), there is a high increase in temperature and pressure. In the third stroke, fuel is injected and ignites because of the high temperature and pressure of the compressed air. You can see an animation of this at How Stuff Works (Brain, Marshall. 'How Diesel Engines Work' 01 April 2000. HowStuffWorks.com). Diesel engines use fuel more efficiently; and under comparable conditions, a Diesel engine will always get better fuel efficiency than a gasoline Otto engine. Essentially, Diesel engines operate by knocking. The continuous knocking has two consequences: 1) a Diesel engine must be more sturdily built than a gasoline engine, so it is heavier and has a longer life - 300,000-350,000 miles before major engine service, and 2) fuel standards are "backwards" from that of gasoline; we want fuel to knock.

Cost of crude oil diagram, see caption — Figure 2.12: The schematic shows how the cost of crude oil, the cost of refining, the cost of distribution and marketing, and the cost of taxes contribute to the overall cost of diesel in October 2015. The breakdown of diesel prices for October 2015, Retail Price = $2.52/gallon. Taxes = 21%, Distribution & Marketing = 19%, Refining = 18%, Crude Oil = 43%.

*Credit: by US DOE EIA (Gasoline and Diesel Fuel Update) [Public domain], via Wikimedia Commons*

Diesel Fuel

Diesel fuel has a much higher boiling range than gasoline. The molecules are larger than gasoline, and the octane scale cannot be used as a guide. The scale that is used for diesel fuel is called the cetane number. The compound, cetane, or hexadecane, C₁₆H₃₄, is the standard where the cetane number is 100. For the cetane number 0 (the other end of the scale), the chemical compound used is methylnaphthalene, an aromatic compound that doesn't knock. Most diesel fuels will have cetane numbers of 40-55, with the value in Europe on the higher end and the value in the US at the lower end of that range. In a refinery, diesel fuels are processed in the same fashion as jet fuels, using hydrogenation reactions to remove sulfur and nitrogen and reacting aromatics to hydroaromatics and cycloalkanes. Dewaxing also must be done to improve viscosity and low-temperature problems, particularly in colder climates. Therefore, Figure 2.11 applies to diesel fuel as well as jet fuel. Except in airplanes, Diesel engines dominate internal combustion engine applications. They are standard for large trucks; dominate railways in North America and other countries; are common in buses; and are adapted in small cars and trucks, particularly in Europe.

intake and compression stroke of piston: intake stroke piston moves down (air only); compression stroke piston moves up valves closed — Figure 2.12a: For a Diesel engine, the compression ratio (x/y) is higher and only air is injected in the first stroke, so only air is compressed in the second stroke, and to a higher pressure and temperature than with an Otto engine.

*Credit: Dr. Caroline B. Clifford*

ignition and exhaust strokes of pistons: ignition piston moves down valves closed; exhaust stroke piston moves up exhaust valve opens — Figure 2.12b: Fuel is injected in the third stroke at high pressure and temperature, which is what makes ignition occur. There are no spark plugs in a Diesel engine.

*Credit: Dr. Caroline B. Clifford*