The Diesel Engine

The diesel internal combustion engine differs from the gasoline powered Otto cycle by using a higher compression of the fuel to ignite the fuel rather than using a spark plug ("compression ignition" rather than "spark ignition").

Air standard diesel engine cycle 空气标准柴油机循环

In the diesel engine, air is compressed adiabatically with a compression ratio typically between 15 and 20. This compression raises the temperature to the ignition temperature of the fuel mixture which is formed by injecting fuel once the air is compressed. 在柴油机中，空气通常在15到20之间的压缩比下进行绝热压缩。这种压缩使温度升高到燃料混合物的点火温度，该燃料混合物在空气被压缩后通过喷射燃料形成。 The ideal air-standard cycle is modeled as a reversible adiabatic compression followed by a constant pressure combustion process, then an adiabatic expansion as a power stroke and an isovolumetric exhaust. A new air charge is taken in at the end of the exhaust, as indicated by the processes a-e-a on the diagram. 理想空气标准循环被建模为一个可逆绝热压缩过程，随后是定压燃烧过程，接着是绝热膨胀作为动力冲程，最后是等容排气过程。在排气结束时，会吸入新的空气，如图中过程a-e-a所示。

Since the compression and power strokes of this idealized cycle are adiabatic, the efficiency can be calculated from the constant pressure and constant volume processes. The input and output energies and the efficiency can be calculated from the temperatures and specific heats: 由于这个理想循环中的压缩和动力过程是绝热的，效率可以从恒压和恒容过程计算出来。输入和输出的能量以及效率可以从温度和定容热容计算出来：

It is convenient to express this efficiency in terms of the compression ratio r_C = V₁/V₂ and the expansion ratio r_E = V₁/V₃. The efficiency can be written

and this can be rearranged to the form

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For an air standard engine with γ = 1.4 , compression ratio r_C = 15 and expansion ratio r_E = 5, this gives an ideal diesel efficiency of 56%.

The diesel cycle depends upon this temperature being high enough to ignite the fuel when it is injected.

*psig is pounds per square inch gauge pressure. Common pressure gauges in the U.S. measure pounds per square inch excess over atmospheric pressure.

Diesel Engine Theoretical Efficiency

Air standard diesel engine cycle 空气标准柴油机循环

Since the compression and power strokes of this idealized cycle are adiabatic, the efficiency can be calculated from the constant pressure and constant volume processes. The input and output energies and the efficiency can be calculated from the temperatures and specific heats: 由于这个理想循环中的压缩和动力过程是绝热的，效率可以从恒压和恒容过程计算出来。输入和输出的能量以及效率可以从温度和定容热容计算出来： It is convenient to express this efficiency in terms of the compression ratio rC = V1/V2 and the expansion ratio rE = V1/V3. 用压缩比 r_C = V_1 / V_2 和膨胀比 r_E = V_1 / V_3 来表达这种效率更为方便。

The efficiency can be expressed in terms of the specific heats and temperatures.

Now using the ideal gas law PV=nRT and γ = C_P/C_V, this can be written

Now using the fact that V_a=V_d=V₁ and P_c=P_b from the diagram

Dividing the numerator and denominator by V₁P_c

Now making use of the adiabatic condition PV^γ = constant,

the efficiency can be written

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Diesel Engine Cycle

The term "compression ignition" is typically used in technical literature to describe the modern engines commonly called "Diesel engines". This is in contrast to "spark ignition" for the typical automobile gasoline engines that operate on a cycle derived from the Otto cycle. Rudolph Diesel patented the compression-ignition cycle which bears his name in the 1890s.

空气标准柴油机循环

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