BeginnerTopic 31 of 105Thermodynamics & Fluids
Diesel cycle
Ideal compression-ignition cycle with heat added at constant pressure.
In depth
Constant-pressure heat addition: η=1-(1/r^{γ-1})·[(r_c^γ-1)/(γ(r_c-1))] with cut-off ratio r_c. Compression-ignition engines run r≈14–22 for high efficiency and torque. For the same r, Otto is more efficient, but diesel can use higher r.
Key points
- No knock since only air is compressed
- Dual cycle combines both
- Turbochargers add power
ExampleA truck diesel with r=18 achieves around 40–45% brake efficiency.
The Diesel cycle models compression-ignition engines, which are common in trucks, ships and generators because of their high efficiency.
What is Diesel cycle?
Ideal compression-ignition cycle with heat added at constant pressure.
Why is it important to understand diesel cycle?
The Diesel cycle models compression-ignition engines, which are common in trucks, ships and generators because of their high efficiency.
How does the Diesel cycle differ from Otto?
Heat is added at constant pressure instead of constant volume, and the engine compresses air to a higher ratio before fuel injection.
Why are diesel engines efficient?
They run at high compression ratios, so the ideal efficiency is high, and they operate with lean mixtures that reduce throttling loss.
How is Diesel cycle used in practice?
Typical applications include comparing the efficiency of a diesel engine to a petrol engine at the same compression ratio, designing a generator set that must run at high efficiency for many hours and choosing a compression ratio for a heavy-duty truck engine.
What should I learn before Diesel cycle?
Study "Otto cycle" first (topic 30 of 105 in Thermodynamics & Fluids). It covers the groundwork this topic builds on.
What are the key points of Diesel cycle?
No knock since only air is compressed. Dual cycle combines both. Turbochargers add power.
Can you give an example of Diesel cycle?
A truck diesel with r=18 achieves around 40–45% brake efficiency.
What comes after Diesel cycle?
The next topic is "Brayton cycle", which builds directly on this one.
How difficult is Diesel cycle?
It is classed as Beginner in Thermodynamics & Fluids (topic 31 of 105). It is an entry-level topic. It relies only on basic school mathematics and the ideas introduced earlier in this discipline.