---
title: "Fuel-air cycle and actual cycle losses"
subject: "Internal Combustion Engines"
chapter: 1
key: "semester_6/internal_combustion_engines:fuel-air-cycle-assumptions-losses"
source: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/fuel-air-cycle-assumptions-losses
site: BCT Engineering
---

# Fuel-air cycle and actual cycle losses

Internal Combustion Engines · Chapter 1: Overview of thermodynamics of fuel-air cycles and real cycles

- Asked in: year not given, 5 marks
- Video: [Differences Actual Cycle VS Air Standard Cycle Actual Cycle VS Fuel Air Cycle](https://www.youtube.com/watch?v=PauwmqZA9tY) — MUJI TEAM WORLD
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/internal_combustion_engines/fuel-air-cycle-assumptions-losses
- Whole chapter: https://www.bctengineer.com/md/semester_6/internal_combustion_engines/chapter-1

## Question

State the assumptions of air-standard analysis and explain how the fuel-air cycle differs from it. Why does the actual cycle give less power and efficiency than the fuel-air cycle? Name the main losses. Also state what the cylinder gases consist of at the end of combustion.

## Short answer

Air-standard: pure air, perfect gas, constant specific heats, reversible adiabatic compression/expansion, heat addition from external source. Fuel-air cycle uses real gas composition, temperature-dependent specific heats, dissociation, and variable moles, lowering peak temperature and efficiency. Actual cycle losses: finite combustion time, heat loss, exhaust blowdown, pumping, incomplete combustion, blow-by, friction. Exhaust gases contain CO2, H2O, N2, O2 (lean) or CO, H2, unburnt HC (rich) and residuals.

## Answer

#### Air-standard assumptions
- The working fluid is air only, treated as a perfect gas with constant specific heats.
- Combustion is replaced by heat addition from an outside source, and exhaust by heat rejection.
- All processes are reversible; compression and expansion are isentropic.
- No heat loss, no leakage, no valve flow loss.

#### Fuel-air cycle
The fuel-air cycle uses the real composition of the charge (air + fuel + residual gas) and the following:
- Specific heats of the gases **increase with temperature** ($c_v$ rises, so $\gamma$ falls).
- **Dissociation**: at high temperature CO$_2$ breaks into CO and O$_2$, and H$_2$O into H$_2$ and O$_2$. This absorbs heat, so the peak temperature falls.
- The number of moles changes during combustion.
- Fuel quantity affects the heat released (rich mixtures cause incomplete combustion).

Because of these, the peak temperature and pressure of the fuel-air cycle are lower than those of the air-standard cycle, and the efficiency is about 80-85% of it.

#### Composition of cylinder gases
Burnt gases contain CO$_2$, H$_2$O, N$_2$ and excess O$_2$ (lean mixture), or CO, H$_2$ and unburnt hydrocarbons (rich mixture), plus residual gas from the previous cycle.

#### Losses in the actual cycle
1. **Time loss** (finite combustion time): combustion is not at constant volume, so peak pressure is lower and falls after TDC.
2. **Heat loss** to cylinder walls and head during compression, combustion and expansion.
3. **Exhaust blowdown loss**: the exhaust valve opens before BDC, so some expansion work is lost.
4. **Pumping loss** in the induction and exhaust strokes.
5. **Incomplete combustion**, leakage past rings and **friction**.

The actual indicated efficiency is only about 75-85% of the fuel-air cycle efficiency.

| Cycle | Typical efficiency ($r = 8$) |
|---|---|
| Air-standard | 56.5% |
| Fuel-air | 45-48% |
| Actual indicated | 35-38% |

## Check yourself

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

1. Which is an assumption of air-standard analysis?
   A. Air behaves as a perfect gas with constant specific heats
   B. Specific heats rise with temperature
   C. The fuel composition affects the heat released
   D. Dissociation occurs at high temperature
   Answer: A. Air-standard analysis treats the fluid as air with constant specific heats; the others belong to the fuel-air cycle. (Key point: Air-standard assumptions)

2. What does dissociation at high temperature do in the fuel-air cycle?
   A. Releases extra heat, raising peak temperature
   B. Absorbs heat, lowering the peak temperature
   C. Increases gamma
   D. Has no effect on peak temperature
   Answer: B. CO2 and H2O breaking down absorb heat, so peak temperature and pressure fall below the air-standard values. (Key point: Effect of dissociation)

3. What causes the exhaust blowdown loss?
   A. The exhaust valve closes after TDC
   B. The inlet valve opens early
   C. The exhaust valve opens before BDC, so some expansion work is lost
   D. Piston friction in the exhaust stroke
   Answer: C. Opening the exhaust valve before BDC lets gas escape early, losing part of the expansion work. (Key point: Blowdown loss)

4. What is the time loss in the actual cycle?
   A. Heat is lost to the walls during compression
   B. Fresh charge is lost during scavenging
   C. The valve opens late
   D. Combustion takes finite time, so it is not at constant volume and peak pressure is lower
   Answer: D. Finite combustion time means heat is not added at constant volume at TDC, which lowers peak pressure. (Key point: Time loss)

## Related in Internal Combustion Engines

- [Efficiency ratio and specific fuel consumption](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Arelative-efficiency-sfc-numerical) — Internal Combustion Engines, chapter 8
- [Combustion of octane with excess air](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Aoctane-combustion-air-fuel-numerical) — Internal Combustion Engines, chapter 3
- [Volumetric efficiency from orifice air measurement](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Avolumetric-efficiency-orifice-air-consumption) — Internal Combustion Engines, chapter 8
- [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
- [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
- [Brayton cycle; Atkinson and Stirling cycles](https://www.bctengineer.com/question.md?key=semester_6%2Finternal_combustion_engines%3Abrayton-cycle-atkinson-stirling-notes) — Internal Combustion Engines, chapter 1

## Related in other subjects

- [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
- [Cost variance analysis from standard and actual](https://www.bctengineer.com/question.md?key=semester_6%2Fengineering_economics%3Acost-variance-analysis-9000-units) — Engineering Economics, chapter 1
- [Actual gas turbine cycle with component efficiencies](https://www.bctengineer.com/question.md?key=semester_7%2Fturbo_machines%3Aactual-brayton-cycle-numerical) — Turbo Machines, chapter 3
- [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