---
title: "Air standard cycle; diesel vs Otto"
subject: "Fundamental of Thermodynamics & Heat Transfer"
chapter: 6
key: "semester_2/fundamental_of_thermodynamics_and_heat_transfer:air-standard-cycle-diesel-vs-otto"
source: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/air-standard-cycle-diesel-vs-otto
site: BCT Engineering
---

# Air standard cycle; diesel vs Otto

Fundamental of Thermodynamics & Heat Transfer · Chapter 6: Thermodynamic Cycles

- Asked in: 2068 Bhadra, 6 marks
- Video: [Summarize and compare Otto and Diesel Cycles](https://www.youtube.com/watch?v=jED9LbvSCUA) — Randall Manteufel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/fundamental_of_thermodynamics_and_heat_transfer/air-standard-cycle-diesel-vs-otto
- Whole chapter: https://www.bctengineer.com/md/semester_2/fundamental_of_thermodynamics_and_heat_transfer/chapter-6

## Question

What is air standard cycle? Differentiate between diesel cycle and otto cycle.

## Short answer

Air standard cycle: air as ideal gas with constant specific heats, reversible processes, external heat addition/rejection. Otto cycle adds heat at constant volume, η=1−1/r^(γ−1); Diesel adds heat at constant pressure, η=1−(rc^γ−1)/[γ r^(γ−1)(rc−1)]. Diesel engines use higher compression ratios (14-22) than Otto (6-10), giving higher real efficiency.

## Answer

#### Air standard cycle
An air standard cycle is an idealised cycle for an internal combustion engine or gas turbine in which air is the working fluid and the following assumptions hold:

- A fixed mass of air behaves as an ideal gas with constant specific heats.
- All processes are internally reversible.
- Combustion is replaced by heat addition from an external source, and the exhaust by heat rejection to a sink that returns the air to its initial state.
- Kinetic and potential energy changes are neglected.

Examples: Otto, Diesel, Dual and Brayton cycles.

#### Diesel cycle versus Otto cycle
| Point | Otto cycle | Diesel cycle |
|---|---|---|
| Engine | Spark ignition (petrol) | Compression ignition (diesel) |
| Processes | 2 isentropic + 2 constant volume | 2 isentropic + 1 constant pressure + 1 constant volume |
| Heat addition | At constant volume | At constant pressure |
| Compression ratio | 6-10 | 14-22 |
| Efficiency | η = 1 − 1/r^(γ−1) | η = 1 − (1/r^(γ−1)) (r_c^γ − 1)/(γ(r_c − 1)) |
| Efficiency for same r | Higher | Lower |
| Depends on | r only | r and cut-off ratio r_c |
| Ignition | Spark plug | Compression heat |
| Mean effective pressure | Higher for the same r and heat input | Lower for the same r and heat input |
| Peak pressure | Higher for the same r (sudden heat release at constant volume) | Lower for the same r |

For the same compression ratio the Otto cycle is more efficient, but the diesel engine can use a much higher compression ratio, so in practice it gives higher efficiency.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. An air standard cycle is
   A. An idealised cycle with air as working fluid and reversible processes
   B. A cycle with real combustion products as working fluid
   C. Any cycle that uses atmospheric air in an open system
   D. A cycle with friction and heat loss included
   Answer: A. It is an idealised cycle using a fixed mass of air as an ideal gas with internally reversible processes. (Key point: Definition)

2. How do the processes of Diesel and Otto cycles differ?
   A. Diesel has constant-pressure heat addition; Otto has constant-volume heat addition
   B. Otto has constant-pressure heat addition; Diesel has constant-volume
   C. Diesel has no isentropic processes
   D. Both add heat at constant pressure
   Answer: A. Otto: 2 isentropic + 2 constant volume. Diesel: 2 isentropic + 1 constant pressure + 1 constant volume. (Key point: Process difference)

3. On what does the Otto efficiency depend?
   A. Compression ratio only (and γ)
   B. Cut-off ratio only
   C. Maximum temperature
   D. Compression ratio and cut-off ratio
   Answer: A. η_Otto = 1 - 1/r^(γ-1) depends only on r; Diesel also depends on r_c. (Key point: Efficiency dependence)

4. Why can a diesel engine be more efficient in practice despite lower efficiency at the same r?
   A. It uses a spark plug
   B. It uses a much higher compression ratio
   C. It has higher peak pressure
   D. It has less heat rejection at constant pressure
   Answer: B. Diesel engines use r = 14-22 compared with 6-10 for Otto, which outweighs the penalty from the cut-off term. (Key point: Practical efficiency)

## Related in Fundamental of Thermodynamics & Heat Transfer

- [Air standard assumptions; diesel cycle; Otto vs diesel](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aair-standard-assumptions-diesel-vs-otto) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Gas vs vapor cycles; Otto cycle efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Agas-vs-vapor-cycles-otto-efficiency) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Air standard diesel: efficiency and mean effective pressure](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cycle-cutoff-2-efficiency-mep) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Otto cycle CR 8: state points and efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Aotto-cycle-cr8-2000kj-heat) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Diesel cycle: cut off 8% of stroke, CR 16 to 20](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-cr16-cutoff-8-percent) — Fundamental of Thermodynamics & Heat Transfer, chapter 6
- [Diesel cycle: cut-off percentage, expansion temperature, efficiency](https://www.bctengineer.com/question.md?key=semester_2%2Ffundamental_of_thermodynamics_and_heat_transfer%3Adiesel-1500c-cr16-cutoff-percentage) — Fundamental of Thermodynamics & Heat Transfer, chapter 6

## Related in other subjects

- [Diesel cycle efficiency compared with Otto](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Adiesel-cycle-versus-otto-numerical) — Internal Combustion Engines, chapter 1
- [Otto cycle efficiency and mean effective pressure](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aotto-cycle-efficiency-numerical) — Internal Combustion Engines, chapter 1